Stealthy Bullitt Pump Compartment

There’s a fair amount of usable space under a Bullitt’s cargo deck, space which I don’t think is often utilized to it’s full potential.  A good candidate for making use of this space is small / thin objects which you want to have with you all the time – like your pump and puncture repair kit.

In this article I’ll summarize how I’ve designed and implemented a stealthy compartment for holding such things.

This solution aligns with my guiding Bullitt principles:

  1. The Bullitt should be carrying stuff, not me: E.g. I should never need to wear a backpack.
  2. Peak performance: the Bullitt is a lean machine, so unless restricted by a specific cargo load, the overall performance of the Bullitt should be maximized.  That means nothing that limits speed, handling or cargo carrying.
  3. Elegance: the Bullitt’s fit-out (locks, pumps, bike bags, etc) should be elegant.  Elegance should be the product of following the first two principles, and likewise, by aiming for elegance I should achieve the first two principles well.

General Concept

The concept is to have a hinged compartment that runs along under the cargo deck.  The obvious location is along the side opposite the steering arm – for easier access.  Having it hinged at one end provides a good balance between accessibility and strong mount.

By utilizing the space under the cargo deck:

  1. The exterior of the bike is not cluttered with stuff, which frees up space for other things (both on the frame in general, but also in terms of cargo carrying.
  2. It is not directly visible in most scenarios, so not so obvious to casual thief’s.
  3. It is protected from the worst weather.

Compartment Design

In my case, I only have a thin space available since I already used the central space under the cargo deck for my speaker system (see: Bullitt Boom Box).

I decided my compartment only needs to hold the pump and puncture repair-kit.  I didn’t build it to hold my tools as well – for a number of reasons, one of which is that this is bit of an experiment: start off small and see how it goes.

Above is the compartment, detached from the Bullitt.  On the left is the forward attachment point, through which runs a single bolt with a wingnut; this is how you release the compartment for loading/unloading.  On the right is the top of the rear mounting hinge.  This is permanently screwed to the cargo deck.

The long wooden board provides the main structural part of the compartment, joining the two attachment points, and provides something to secure the pump against.  The rear box-like area holds the puncture repair-kit.

You’ll notice the gap at the rear, between the hinge and the side board, this is to accommodate the Bullitt’s front wheel brake and light cables that run along under the frame. 


The rear mounting point is immediately in front of the cross-bar, on to which the kick-stand presses when the Bullitt is parked.

Be aware that the sweep of the kick-stands pads will likely scrape the back of the pump compartment if it’s too close.  Also make sure there’s enough room for any cables to run past.

In terms of the cables, at the front of the Bullitt these should be located to the side, before the run up to the front forks.  This means that there should be enough room for the front mounting bolt on the inside of them.  I then have mine running down the side of the speaker enclosure, in such a way that they are secure but with enough flex to run comfortably along the outside of the compartments long side board.  They then exit through the rear of the compartment.  Make sure that the compartment doesn’t foul on them with opening or closing.

The front attaching position is well forward – just behind the front cross-bar.

Images: left – the rear mounting bolts.  Centre – with the compartment removed, showing the bolts sitting in position ready to take the compartment.  Note the location of the kick-stand (in down position) and the cables running around the back of my custom speaker enclosure.  Right – photo taken from a similar angle, showing the compartment attached.

There are three mounting bolts, all of which are flat-head and counter-sunk.  When mounted, this type of bolt head sits flush with the cargo decks surface, so there’s zero impact to cargo.

My cargo deck is made from 7mm plywood.  This isn’t especially thick, but it will safely hold the counter-sunk bolt head as long as the bare minimum of counter-sink is used (so that as much wood as possible is retained, to hold the bolt head).  Part of the reason I think this is safe is because the load the compartment carries is very light.

Note: depending on your design, I suggest you periodically check the holes, and the wood around the bottom of them, to ensure there’s no wear or weakening – you don’t want the compartment to come loose whilst riding.

The forward bolt:

  • The hole in the deck only needs to hold the bolt in position, the bolt is held fast by the wingnut used in combination with a flat washer and a spring washer.
  • The diameter of the hole in the compartment is such that it slides easily on/off the bolt.

The rear mounting bolts – I’ve used two slightly different approaches for these:

  1. The holes in the deck only needs to hold the bolts in position, the bolts are held fast by the compression due to the compartment mounting.  The same diameter is used for both bolts.
  2. The compartment hole for bolt-A is tight, so that the bolt’s thread bites securely into the wood.  Be very careful not to over-tighten otherwise you risk striping out the wood that holds the bolt secure.
  3. The compartment hold for bolt-B is not tight, the bolt slides through and is fastened by a hex type locknut.

Why the two different approaches?  Basically it’s bit of a hack and an experiment. making the wooden hinge was much trickier than I anticipated – the one you see is about the third attempt.  The wood used is strong enough but I managed to break the first one whilst working on it – so I was mindful not to over-stress the wood.

I also wanted to maximize the strength of the hinge, which meant having enough thickness to secure the metal hinge, side panels and so on.  Using this combination seemed to work the best based on my first two attempts.

Bolt-A provides the most timber for strength of the unit, whilst bolt-B provides the most secure fastening due to the locknut.

In Closing

The position of the front bolt is such that it’s fairly easy to access the compartment, but it is a little bit fiddly to re-attach.  It’s definitely convenient enough for occasional use but it’d be nice to have a slicker design if you wanted really frequent access – and don’t forget to manage the Bullitt’s cables.

The pump is secure sitting in the compartment, but I also use a piece of rubber to tie it to the side board so that there’s no rattle whilst riding.

I’ve had this in operation now for around 6 months and all’s going well.

If you don’t have a speaker system under your deck: (a) you should get one :P, and (b) you will have more flexibility in terms of your compartment design and what you can accommodate in it.




Bullitt Boom Box

Bullitt’s are awesome, but sometimes awesome can be more awesome when you’re playing your favorite tunes… and what better way to do that than by building a stealthy speaker system for your Bullitt, of course!

Images: Various views of the speaker enclosure.  (Right) a stealthy pump and puncture repair-kit holder, next to the speaker enclosure.

The premise of the design is that because we tend to ride bikes on hard surfaces (i.e. sealed road) the sound waves will bounce off the road and be audible. I can confirm through experience that this is definitely the case – more on that in a later section. As such, the speakers are mounted under the cargo deck, facing directly at the road.

Profile View

This location is significant: it means the cargo deck is virtually* unconstrained by any sound system paraphernalia – so it won’t interfere with any load; the largest part of the system – the speakers and their enclosure – is neatly tucked in under the cargo deck, meaning there’s no impact on the bikes performance; and it’s visually obscure, so not an obvious target for theft. So you can essentially leave it on the bike all the time: simply jump on, turn up the vibes, and turn heads.

* The only impact is the bolt-heads that protrude above the deck, and even then there are mitigations.  For me these are very small so there’s no practical impact on load carrying.  See the “Speaker Enclosure > Bolt-Heads and Their Impact on Load Carrying” section below for more info.

Disclaimer: the ideas and information provided here are provided “as-is”, no warranty is provided or implied.  Building a system such as the one described here involves various risks, both during implementation and operation.  If you damage yourself, someone else or any property, through directly or indirectly being influenced by this content, that is entirely your responsibility.

88x31  This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.  Want to commercialise this?  I’m open to discussion – get in contact.



Above, the main on-Bullitt components, including speakers, amplifier and battery.

The Recipe

You will need:

  1. Amplifier.
  2. Speakers (water resistant).
  3. Speaker cabinet / enclosure, with mounting nuts & bolts (assumes you have appropriate mounting holes in your Bullitt’s cargo deck).
  4. Battery.
  5. Battery Charger.
  6. Battery charge/voltage meter.
  7. Speaker cable (approx. 1.5 metres x 2).
  8. Speaker cable bungy (x2).
  9. Old bicycle inner tube (speaker cable protector).
  10. Various cables including: battery to amp, battery to charger.
  11. Carry pouch for amp & battery.
  12. Connectivity between the amp and your phone / music player.
  13. Speaker rear cover-plate (optional).


The heart of the system is the amplifier – so choosing this first is recommended. You want an amp that has good power output, and is relatively small and light-weight. Another critical consideration in amp selection is how you’ll power it (i.e. what it’s power requirements are – more on that later).

You may want to consider amp and speaker in combination, since it’s important to ensure these are well matched in term of power per channel and speaker impedance. In my case I bought the amp first, based on guidance from a friend and colleague of mine, Steve*, and selected other components based off that.

* He used to work for Sony fixing electronics. One day a VIP client brought their favorite Walkman in for repair – my colleague was selected to repair it. The client was Princess Diana. So, needless to say, I trust his judgement. 🙂

Steve recommend a digital “D-Class” amplifier, because the overall characteristics of these amps are well suited for use on a bike: they are lighter and more robust that a valve-based amp, and have very efficient power usage (which is important as we’ll be running off a battery).

The actual amp I use is: “TPA3116 Mini Power Amplifier ISSC Bluetooth HIFI Stereo Audio Digital AMP 50Wx2”, purchased from China via e-bay.

Images: (Left) Promotional photo.  (Middle) Rear of the unit, with hand for scale.  (Right) Mounted on spacer-board, and with power on/off switch.

Amp specifications:

  1. Work efficiency: 90%
  2. Rated output power: 2 x 50W
  3. Frequency response: 20Hz to 20KHz
  4. Operating voltage: DC18V to DC24V
  5. Maximum output current: 3A
  6. Bluetooth receiving range: 10 meters
  7. Size: depth 108* width 90* height 39mm (without antenna)



I discovered that Pioneer make “Marine” speakers – i.e. for use on boats etc. These are designed specifically to operate in wet conditions; the manual says “Marine use, water-proof design … UV and corrosion-resistant design”.

I have a pair of these, specifically: TS-MR1640’s, which are a good match for the amp.  I had intended to get the TS-MR1600’s but StreetSoundz (where I bought them) kindly offered me the next model up for the same price 🙂

Images: (Left) Spec’s.  (Inner-left) Rear of a speaker, as mounted in the enclosure.  (Inner-right) The business end of the speakers as mounted (note, enclosure unmounted in this photo).  (Right) Speaker with cover removed, showing grime build-up after several months use.

Another great thing about these speakers is that they are a decent size – about 6 inches, so not too small yet fit nicely under the Bullitts deck, which is key to the overall design.

You could use non-marine speakers, but it would be somewhat limiting; if you’re like me, and operate your Bullitt in all weather conditions, then the marine speakers mean you can still pump out the vibes even in the rain.  I’ve painted mine using some oil paints I had lying around (didn’t have black), mainly to make them less visible.

TS-MR1640 Speaker Specifications:

  1. Maximum music power: 100W.
  2. Nominal power: 25W.
  3. Impedance: 4 Ohm.
  4. Woofer diameter: 160mm (~6 Inches).
  5. Sensitivity: 90dB.
  6. Frequency response: 30Hz – 20kHz.
  7. Weight: 790g (per speaker).
  8. Depth: 56mm.
  9. Speaker cover: 28mm high, 168mm diameter.


Speaker Enclosure

I made the speaker enclosure myself, mostly using pine and plywood. The enclosure you see here is my second version.

Images: Various views of the speaker enclosure (unmounted).  Note the additional ply (also 7mm) around the speakers to provide a secure anchor for the speaker mounting screws.

The essentials of the design are:

  1. A plywood (7mm) plate to mount the speakers. Note I have used a second layer of ply inside the enclosure to provide extra strength for the speaker mounting screws – the last thing you want is for them to come loose through all the shocks and vibrations associated with riding.
  2. Light-weight and strong sides/ends. I’ve used untreated 60mm x 10mm clear dressed pine.
  3. Bolts running vertically through the enclosure, so that the open / rear side of the enclosure is held flush against the bottom of the main cargo deck – which is also 7mm ply.

My design is like this:

Speaker Enclosure Design

You’ll notice that the middle bolt hole is off-center.  The reason for this is the lack of space between the speakers along the center-line, plus the presence of the Bullitt’s first central cross-bar.  Initially I had a symmetrical design – 4 bolts with two in the middle, but I have since dispensed with one – three bolts in total is enough.

I have a custom 7mm thick plywood cargo deck on my Bullitt, plus the enclosure’s 7mm speaker plate, plus the 60mm high sides, which is an overall depth of 74mm. The enclosure is bolted to the deck using 80mm M6 bolts and Stainless Steel Nyloc Nuts.  The initial assembly is fairly tight on the bolts, but you’ll find that the plywood is somewhat malleable, so you will get secure assembly.  (You’ll want to keep the bolts short as possible so they don’t protrude down too far and catch on anything).  After 8+ months use I find that the bolts will happily extend past the top of the nut, as the plywood molds to the pressure of the washers/nuts. Initially you might find they end of the bolts only go to flush with the top of the nut – which is fine if you use lock-nuts. Just check them regularly to make sure they are not coming loose.

Plywood is an excellent material for the plates, as it’s light-weight and strong; clear pine is fine for the sides and ends – again because it’s light. 10mm provides enough strength to maintain integrity, and because the design has the bolts passing through the plywood – and taking all the weight – the pine is not really load-bearing.

The Bullitt has two central horizontal cross-bars running across the cargo deck – the length of the speaker enclosure means you’ll need to cut parts of the sides away, so that the enclosure fits snug around this cross-bar, and flush against the bottom of the cargo deck.

You’ll notice the angular joinery at one end; this isn’t part of the acoustics, it’s pure decoration, I thought a jet engine air intake type of look might be cool.  All the joints are sealed internally with a wooden sealant from the hardware store.

Images: Various views of the speaker enclosure.  (Middle) Note the recess that accommodates the frame’s cross-bar, allowing the enclosure to be flush against the bottom of the cargo deck.  I’ve wrapped recycled inner-tube around the steering rod for protection from scratches.  (Right) note the inner-tube protecting the speaker cables.

Bolt-Heads and Their Impact on Load Carrying

The bolt heads are only a few mm high, which (in my experience) aren’t an issue for any loads.  I often use large fish bins; the design of these includes a small rim around the bottom, meaning there’s a thin recess underneath them – which is enough to accommodate the bolt heads.

You might find them an issue though in some specific cases.  There’s a few mitigations you can use, some of which affect the design:

  1. Use packing blankets or some other soft material to protect your load.
  2. Make a solid spacer that acts as the cargo deck, which has holes cut out for the bolt heads.  This could be a separate piece of wood which you use as needed or you could build it into/onto the deck as a permanent fixture.
  3. Use bolts / screws that result in a flush finish with the top of the deck.  Note: you’ll want to make sure there is enough strength to hold the bolts even after months of use / wear & tear.  I don’t think 7mm ply is safe to do that.
  4. Use a different mounting system – e.g. screwing the enclosure directly to the bottom of the cargo deck.  Note: this will make maintenance a lot harder, but not necessarily protect you from thieves.


I’m not an audiophile, so you may want to do your own further research the overall acoustics and the designing of speaker enclosures.

There’s no doubt that some types of music sound better than others. I tend to prefer Techno, Ska and Roots/Reggae/Dub; some Classical can work well too (a spot of Rostropovich, perhaps), and some Pop.  Interestingly I’ve found House music doesn’t fair quite as well.  Your mileage may vary.

The sound is generally clear, but some wavelengths can get a bit lost.

Because the speakers face into the ground, sound is emitted in 360 degrees. This means that you provide some fun and vibes for those around you – whether they like it or not. Interestingly, it also lets people know your coming, which can be useful form a safety perspective – more on that in the “Operation” section.


Battery System

This is a huge and complex subject, so I’ve posted the details separately here: DYI 18650 Battery Pack

Speaker Cable

You’ll need two lengths, around 1.5m each.  This runs from the amp, mounted in a pouch hanging off the back of the cargo deck frame, down the frame and along under the main frame to the speakers.

Do yourself a favor and get good quality cable.

Speaker Cable Bungy (x2)

Use these, or something similar to lash the speaker cable to the frame so that it doesn’t foul on anything.  I have one on the side vertical, and one under the deck.

Old Bicycle Inner Tube

I use an old bicycle inner-tube to protect the speaker cables from getting scratched / cut, sunlight, and it’s a lot less obvious than speaker cable (i.e. theft) – mine was bright copper in a clear rubber seal, so bit of an attraction.

Various Cables

As with everything in do-it-yourself systems like this, you’ll need to ensure you can connect everything together.  These are also covered in the DYI 18650 Battery Pack post.

Carry Pouch for Amp & Battery

Army surplus stores are great for random bags and pouches, usually at good prices. The bags I have are old rifle ammo pouches – the dimensions are perfect for mounting on the read cargo frame of the Bullitt, and holding the amplifier and battery. The canvas is nice and thick (protective). They are rated as “shower proof”, the canvas lids have sides that provide decent protection and clips for holding them down.

How you mount the amp and battery is entirely up to you. What I like about the pouches is that they fit perfectly at the rear of the frame – which means the volume knob on the amp is easy in reach, so I can change volume even when riding.

Images: (Left) Army surplus ammo pouch.  (Inner-left) The amp and battery, as it sits within the pouch.  (Right) The amp in its operational position; volume knob removed to avoid accidental volume change.  (Right) Battery and amp.

For my design, I have attached the amplifier to a light-weight wooden board, which serves two purposes:

  • As the ammo pouch is bigger than the amp, it pushes the amp up towards the top, so that the amp is easy to access, even when riding.
  • By pushing the amp up, it also helps create a space where cables can sit without getting crushed.

The amp is basically just taped on to the broad, but if you look closely at the photo’s you’ll notice a small piece of wood bolted to the main board, which holds the weight of the amp when it vertically in the pouch.

Lastly, think about how you’ll mount the pouch to the Bullitt – in terms of managing shocks and vibrations.  I’ve used bungee cord to suspend the pouches in place, to help minimize vibrations and shocks.

Speaker Enclosure, Rear-Plate

This is optional. It’s purpose is to cover the back of the speaker enclosure, if you want to use the speakers off the bike. The plate serves two purposes: protects the speakers, and acts as a baffle so that rearward generated soundwaves don’t distort those emitted by the front.

Amp-Smartphone Connectivity

This really depends on the amp; I had intended to simply use an old-school style lead: 3.5mm headphone jack to twin RCA connectors – RCA connectors are very common on amps. The amp I got also had Bluetooth, and I’ve ended up just using that exclusively. If you need them, cables like 3.5mm jack to twin RCA should be easy to buy off the shelf.



The most interesting point I’ve noticed is how people around you react, because the idea of audible music coming from a bike is not one most people are familiar with.

In one case, I was cruising along the waterfront in a zone which is mixed pedestrian and cyclists, next to this is a road. I was coming up behind an older gentleman, and as he heard the music (some techno) he instinctively looked towards the street – no doubt expecting some “youths” to go cruising past.

I’ve also sat at the lights with pedestrians nearby looking around, unable to sense where the music is coming from.

There’s also an interesting safety angle. There’s been a few times, especially around the CBD where pedestrians have been just about to step out in front of me – and they’ve heard the music and stopped.

Battery Management

Managing the battery charge is pretty critical – naming not letting it run down too far.  make sure you check the behavior of your amplifier – as I mentioned above, mine has a small current draw even when switch “off” – approximately 0.025A.  If you leave them connected to long you can risk discharging the cells to unsafe levels.  Having some sort of protection is a good idea.  I use an additional switch to completely break the battery-amp circuit, and keep in the habit of using it.  Whilst I haven’t done extensive research on this, it’s likely you can find or build basic BMS’s that would cut-off in low voltage.  The basic voltmeter shown above has a siren built into it, which will go off once you configured voltage is reached – but it has to be plugged in all the time (itself a very small power drain), and it’s only useful if you can hear it (so if you’re not in earshot…).

More info in the DYI 18650 Battery Pack post.


One aspect to carefully consider with the design of your system is security. You will need to judge for yourself what the risks are, how likely they are, and how to deal with them.

The speaker enclosure design is a key component of this as it’s easily the single largest component. How it balances convenience and robustness with security is a key point to consider. My line of thinking is that the space under the cargo deck is perfect because the speakers are effectively out of sight; and out of sight = out of mind. You may want to be careful in how you operate – e.g. having music blaring out, and then turning the sound system off in a really obvious way, may attract unwanted attention.

I like the straight-through bolt design as it’s very strong; having the speaker enclosure come off whilst you’re doing 40Km per hour downhill would be… sub-optimal.

The other bonus is that it’s relatively straight-forward to remove the speaker enclosure – i.e. for cleaning and maintenance, or to use in the traditional sense – pointing directly at you and your friends rather than bouncing off the road.

My assumption is that as long as most people don’t know exactly where the speakers are, it’s relatively safe. There’s also the issue of needing the right tools, and time, to remove them.

Same logic applies to the amplifier and battery. You can design a system that is less obvious, and relatively less secure once it has been detected, or a system that is more secure but perhaps less convenient.   Its really up to your personal preference.

Big Shout-Outs to:

  • Pete for building many of the version 1 electronics – battery, cables, etc, and advice on the battery design and charging approach.
  • Steve for the amplifier selection and other advice.
  • Street Soundz for providing a good advice and price on the speakers, speaker cables and RCA connectors:


DYI 18650 Battery Pack


This article focuses on how to build a battery pack using 18650 cells, for use with a small digital amplifier; and is intended to support my post on how to build a Bullitt Boom Box.

Disclaimer: the ideas and information provided here are provided “as-is”, no warranty is provided or implied.  Building a system such as the one described here involves various risks, both during implementation and operation.  If you damage yourself, someone else or any property, through directly or indirectly being influenced by this content, that is entirely your responsibility.

The battery pack is one of the most complex parts of the system, because you need to find a battery system that:

  1. Matches the power requirements of your amplifier.
  2. Is mobile – can be mounted on your bike with relative ease, and cope with the demands of being on a bike (vibrations, weather, and so on).
  3. Can be managed (i.e. power level, when to charge).
  4. Can be charged.
  5. Is safe.

After a lot of research, the solution I settled on was to build a custom battery pack out of five lithium-ion 18650 cells, supported by a small voltage meter and a versatile hobbyist charger.

Let’s look at what’s involved.  First we’ll cover the 18650 cells, and then we’ll look at the whole battery pack solution including its design, implementation, and operation.

Images: (Left to Right) The battery pack next to the amplifier.  The SkyRC B6 Nano charger.  Small voltage alarm/meter showing the battery pack’s total voltage.  My second battery pack, with 18650 holders at allow cells to be changed.


Part I: 18650 Lithium-Ion Cells

Please note that the following information is based on my understanding and research, but I’m not an expert in this field, so you may want to verify with other sources.

18650’s are cylindrical lithium-ion rechargeable battery cells.  They are physically larger than AA batteries, and have a higher nominal voltage of 3.7V, compared to an AA’s 1.5V.  18650’s can be bought new, or – if you are careful and understand what you’re doing – recycled from old laptop battery packs.

In terms of recycling: there’s plenty of video’s explaining how to recycle 18650 cells, and if you plan to do this I’d suggest watching a few to get a better idea of what’s involved.  Treat the following info as an introduction,  If you plan to actually do it, do some further research so that you have a better understanding – or work with someone who has the proper experience.

The following content assumes you’re recycling 18650 cell’s from a laptop battery pack, but its still mostly applicable if you’re using new cells.


18650’s are somewhat dangerous – when treated badly they can swell and rupture, causing fire.  “Bad treatment” can be physical (knocks, puncturing) as well as electrical (incorrect charging, unsafe-discharge, use beyond safe lifespan, etc). The Samsung Galaxy Note 7 is a famous example of what happens when Lithium-ion batteries are mistreated through incorrect charging.  See: Washington Post’s: Why those Samsung batteries exploded.  You’ll also note airlines have travel restrictions for lithium-ion batteries, usually depending on how potent they are.  E.g. Air new Zealand: Travelling with lithium batteries.

That said, like anything electrical or mechanical, these cells are safe to use if you understand the relevant factors involved and don’t stray outside the safety constraints.

Technical Specifications: Voltage

Not all 18650’s are the same.  Check the manufacturer’s specifications to avoid fire and personal injury.  A reference that may help you identify the your recycled cells is Second Life Storage’s Cell Database.

A rough guide to a typical 18650 cell:

  1. Most will have a nominal voltage of 3.7V (but you may find some that are 3.6V).
  2. Maximum charge is typically rated at 4.2V.  In practical terms, my charger’s default setting is to charge lithium-ion 18650 cells to a maximum of 4.1V.
  3. Maximum discharge is typically 3V, and may be lower (say 2.5V – check the cell’s specifications).  Remember, over-discharging can be as dangerous as over charging.  In practical terms it may be advisable to discharge to a lesser extent, e.g. to 3.5V rather than 3V – if that is the cell’s stated maximum.

To put that in layman’s terms:

  • Cells can typically be charged up to ~4.1V, and discharged down to ~3.5V.
  • A cell’s “nominal” voltage is the voltage measurement used for specifications.

Note that the nominal value is not a simple average (e.g. 4.2V + 3.2V / 2 = 3.7V) because the voltage discharge is not uniformly flat – it curves:

Capacity-0.5AThe chart above shows the behavior of a number of different lithium-ion cells, in terms of how their capacity drops over time:

  1. Starting at the top left, voltage drops from the maximum fairly quickly, but then starts to plateau.
  2. The long plateau is where the voltage drops at a much slower rate, providing the majority of the cells useful power.
  3. Towards the right, the voltage starts to drop dramatically.

This curve can be observed when charging: you may find that when charging a relatively “flat” battery, the initial charge up to about 80% is relatively fast, whereas the last 20% seems to take much longer.  That’s because you’re walking up that steep (top left) curve in reverse.


Advice on how far you can / should discharge the cells depends on who you talk to.  general advice is to never discharge beyond 3V, and I’ve seen suggestions to not discharge beyond 3.5V.  As you can see from the chart above, the voltage curve varies between different manufacturers and models.

The idea is to utilize the plateau, and not let the voltage drop too far over the cliff on the right – as the discharge rate is faster it’s easier to over-discharge.

Ultimately you’ll need to do the research into the specific cells you have to determine what the safe discharge limit is, and then decide how far you want to discharge your cells so that they maintain a good useful life.

Be aware that some cells may have protection mechanism’s in-built to protect against operation outside of their safe voltage range – but many will not.

Side Note: Capacity vs Voltage

The capacity of cells will decrease as they age, but the voltage range they provide will remain constant.  Therefore, as your cells age, you’ll find that they still cover the same voltage range – but discharge faster than they used to.

Example Specifications

Let’s take an example: the Sanyo UR18650A, using information provided by the Cell Database:, and then compare it to the official specifications.

Unofficial specifications:

Caution, this information is provided for reference only and is not guaranteed to be accurate.

  1. Capacity: 2100mAh.  mAh stands for Milliampere-Hour, a unit of electrical discharge.  As a rating, this is how long the battery will last (whilst remaining within it’s safe voltage range, and assuming a consistent discharge rate of 1 amp.  1 mAh = 1,000th of an hour, so a 2100mAh should last for just over 2 hours.
  2. Voltage: 3.6V nominal.  Slightly less than the more common 3.7V found in a lot of 18650’s.
  3. Charging: 4.2V Maximum; 1510mA standard.  Cells should never be charged above 4.2V.  1510mA standard represents the normal charging current.  Charging at a lesser current will be slower – which can be better for improving battery longevity.
  4. Discharging: 2.5V* cutoff; 420mA standard.  Should never be discharged below 2.5V and appears to have an in-built cutoff to protect the cell from discharge below 2.5V.  420mA standard refers to the expected discharge current.  Some cell specifications may also publish a maximum discharge current.  See the side note below.

* Note this appears incorrect when compared to the official specifications (see below) which state and end voltage of 3.0V.  If in doubt, use the more conservative figures or official specifications.

Side Note: Discharge Current

Measuring the current draw on my battery pack, using the sound system described in the other post, reveals that: when playing techno at maximum volume, “Doofs” in the music cause current spikes of up to 1 amp (1000mA), with the average draw being in the 500-700mA range.

Reviewing the thread we see that someone has found the official specifications…

Official Specifications:

  • End Voltage: 3.0V.  Would suggest not discharging beyond 3.0V, despite what the unofficial crowd-sourced info above suggested.
  • Capacity: 2100mAh nominal, typical capacity 2250mAh.  Here you can see the difference between nominal ratings and what you might find in the wild.
  • Discharging Current (Std) 2.15A.  suggests that the current drawn by my amp on maximum volume is well within the cells expected usage.
  • Discharging Current (Max) 4.30A.  presumably for intermittent spikes only.


Here’s some further reading:


Part II: Battery Pack Design, Implementation & Operation


Battery Pack Design: Power Ratings & Circuitry

The battery pack (i.e. collection of cells) needs to be designed around the power requirements of the device it’s expected to power, so you should start by understanding what those are.

For exmaple, my amp’s power requirements are an input range of 18.0V to 24.0V DC.  Five 18650’s (assuming a 3.7V nominal voltage) wired in series provide:

  1. A combined nominal voltage of 18.5V.
  2. A combined maximum voltage of 21.0V.
  3. Based on the amp’s minimum input range of 18.0V, we can run the cells down to an average of 3.6V, which is comfortably above the minimum voltage of most 18650 cells.

The thing about the 18650’s is that they have excellent capacity, and are well suited to an application of this kind (i.e. in terms of electrical discharge rate, and so on). They are also relatively small – so a pack of five of them are really easy to place on the bike. They are relatively more volatile than other battery types – you definitely will not want to pierce them, or give them any massive shocks. This is definitely relevant considering what can happen on moving bikes. That said, the batteries should be safe to use as long as you’re sensible.

I have two battery packs.  The first was made by my friend Pete, who is familiar with doing this – five 18650 cells wired in series (5S), with a balance lead also attached (giving two separate circuits – the balance lead is used for charging, the other circuit is straight output, which connects to the amp.  The battery cells have metal plate spot-welded to them, which serve as an anchor for the wires to be soldered to.  This leads to a very compact design, but fixed (not easily modifiable).

Images: various views of the first battery pack, with protective cover removed.

I also built myself a second battery using five 18650’s, but utilizing 18650 battery holders (shown below).  This means I can swap out cells with ease.  I’ve also added a switch so that I can fully power off the amp, and ensure the battery doesn’t get run-down.

Images: various views of my second battery pack. Left image shows the balance-tap lead on the left, main power lead on the right.

The top cover on my pack is a recycled shoe inner-sole.  This specific design is not overly water-proofed (it’s only seen summer operation so far), and I may yet modify it ahead of winter.

Even if you don’t want to build your own battery there will be other battery options out there, depending on what you’re after, and how much you’re prepared to spend – it’s just a matter of doing the research.


The actual battery packs have two circuits – the main power output circuit, and balance charging circuit – as illustrated in the following diagrams:

Images: (Left) The overall circuitry.  (Middle) The main power circuit.  (Right) The balance circuit for the 3rd cell, specifically.

Battery Charger

Hobby stores will have lots of electronics gear on offer, including battery chargers. I got this one (SkyRC B6 Nano) because it supported balance charging up to 8S (so can handle my 5S configuration), and a variety of chemistry types – including lithium ion (Li-Ion), so I assume I’ll be able to use it for various random charging needs in the future.

One thing to look out for is the power supply for the charger – check that it comes with one.  If the one you plan do get doesn’t have a power adaptor, just see what sort of power connector it has and what it’s power requirements are.  In my case I needed to fit an XT60 plug on to an old laptop power supply – which had the right power output range for the charger.

Images (left): SkyRC B6 Nano hobbyist charger. (Right) in action, main battery power lead top-right, battery balance-tap lead bottom-right.

Battery Management: Charge/Voltage

Assuming you use a battery without a BMS, you’ll need someway of reading the batteries voltage, so you know when to recharge it. For example: 18650’s can be run beneath their intended safe voltage range. If you do this you risk damaging the cells and/or causing a serious fire.

The amp I run draws a small amount of current – even when switched off. If the battery is left connected to the amp for a prolonged period, it will drain the battery to an unsafe level. At such levels your battery charger (like mine) will not attempt to charge them because the low voltage will fall outside the normal parameters of the 18650.

Fortunately there’s plenty of ways to manually measure battery voltage.  The cheapest way is to get a voltage meter like the one referenced below, which connects to the balance lead. It provides the overall voltage, and the voltage for each cell. Because it draws a small amount of power you may just want to periodically plug it in to check the voltage, then unplug it.


Images: (left) Lithium-Ion Lipo Battery Voltage Tester Alarm, 1S to 8S.  (Right) Reading the overall voltage.  This little meter will also read the voltage of each individual cell, cycling through each in sequence.

Battery Management: Supply Control

You might find that your amp draws power even when switched off.  Mine certainly does – around 0.025A, which seems to be due to Bluetooth receivers going into standby mode as soon as power is supplied.  This might not seem like much, but, it’ll be enough to fully discharge your cells if left long enough (trust me, I know from experience – more than once).

Accidently draining your cells is at best a pain, denying you from amplifying your vibes, but its also likely to retard your cells performance (reduced capacity) and may lead to other even less desirable damage.

To control the supply of power to the amp, I fully recommend building a simple switch that completely breaks the battery-amp circuit, such as the one shown below.


This simple system is basically just a switch integrated into the battery-amp power cable.

Below is a prototype of a more advanced control system, one that includes a built-in voltmeter.  This specific unit uses a pair of XT60’s to fit in-line with any circuit that uses XT60’s between the power source and device.  This specific prototype does not have a power switch (I’ll be using this one with my on-board camera, so no switch required), but the next one, that I’ll use with the amplifier, will.

The idea of the button is to activate the voltmeter on demand, so it’s not a constant power drain.  It’s wired in parallel to the main power circuit.



That’s pretty much it – good luck with any DIY projects you attempt.  Remember, using 18650 cells can be safe as long as you do the research and exercise some caution and common sense.

Cargo Bike Guide – Camera Mounting Positions #2: Frontal Selfie Stick

I’ve recently been experimenting with a new idea, whereby a camera is mounted out the front, looking back at the Bullitt:

Aro 2

How it works

All you need is a light-weight, but very stiff and rigid, length that you can strap securely to the side of the cargo deck, and mount the camera out the front.  I’m using an old carpenters spirit-level which I found in a skip.  But you could use anything else that was appropriate – I was looking at new broom handles: the 28mm ones looked like they might be long enough and stiff enough.

Above left: the spirit-level lashed to the side of the Bullitt using rope.  The white cable is a power cable, as I’ve made a custom battery pack for the camera so I can shoot longer takes.

Above right: the camera as mounted, which luckily is just large enough to securely fit around the spirit-level.  Rubber bungies secure the power cable.

Above left: the inside padding from an mattresses is excellent for packing and protecting delicate objects; here I’ve recycled some and have used it in two ways:

  1. To protect the frame from damage / scatches.
  2. To pad out the spirit-level so that it’s on a very slight angle, angling out away from the from the front wheel.

Above right: the amount of clearance.

You may notice the rear lashing-point has no visible padding.  On this very first experiment I just used the rope itself to keep the spirit-level off the frame, but in the first road-test I ditched that approach in favour of an approach very similar to the front-lashing: using some protective cloth, but not as thickly.


I’m really happy with the results – it seems to work fairly well.  As expected there’s a fair amount of vibration – especially when going over uneven surfaces.  I’m interested to try it again and see if I can more rigidly mount the spirit-level to the frame.



Cargo Bike Guide – Camera Mounting Positions

This post explains some of the detail behind the various mounting positions shown in this video:

The positions are listed below in the order they appear in the video: starting from underneath and working our way upwards.


Low, Mid-Deck (LMD)

This position works off the one of the Bullitt’s cross-bars, under the cargo deck.  To make it work, I’ve fabricated a custom deck out of 7mm plywood.  You can see two holes for camera mounts – the one closet to you is this position, the mid-deck position.  The similar hole at the front (just under the lock) is the next position.

This position is great for a sense of speed and having the deck above provides a lot of visual context of the bike in action.

Low, Front-Deck (LFD)

As shown in the photo above, this position is on the front cross-bar of the cargo deck.

This is one of my favorite camera positions as it provides and balances three things:

  1. It’s low so provides a good sense of speed.
  2. The front wheel is clearly visible which adds visual context of the bike.
  3. Provides good visibility of the view ahead – the wheel doesn’t overly obscure it.

Forks (FK)

This was a surprisingly interesting position.

Because it’s directly on the front forks, vibrations from the front wheel directly transfer to the camera – where as positions on the frame tend to be just that little bit more cushioned by the flex of the frame, and by being less directly affected by vibrations.

The front wheel provides a really interesting visual reference point, and stays very steady – but the view off the bike will be more shaky.

Front Frame (FF)

For this position simply using big rubber bungies gets the best results in terms of stability.  It’s high enough to provide a good view of the road ahead, whilst also being low enough to feel some speed.  The front wheel provides some visual context of the bike.

The issue with the Load 75 is that the lack of space between the panels and frame – the only you’d get a solid clip around the frame would be to remove or modify the panels.  The small hole in the frame (left photo) goes all the way through the frame and should be useful for mounting also.

Front of Load (FL)

This was the first position I ever tried.  Bascially I used cargo straps and bungies to mount the camera on top of the fish bins, at the front, to one side.

I like how the frame provides a good point of reference, as you get a feeling of how the bike is tipping whilst cornering.

Upper Rear Deck (URD)

This position is at the top of the main frame, immediately below where the steering stem starts. This is probably the best position for “scenic” rides, where you want the viewer to get a good sense of the surroundings vs a sense of speed from some manic downhill.

You’ll need to be very careful of any cables, to ensure they don’t fowl on the camera – or obscure the camera’s field of view.

Note – in case you’re wondering, the bend on the brake cable was there already and isn’t because it’s up against the camera.

Head Set (HS)

This position is off the head set – i.e. where the frame accommodates the front forks.

On the Load 75, the shape of the frame is such that there’s enough frame to safely mount the camera using it’s bracket – without interfering with the very top part of the head set which turns with the forks.  On the Bullitt the clearance is slightly less but looks like it would work.




Quick Review: Riese & Müller, Load 75 e-Cargo Bike

Pepper has been invited to go to Auckland for a trade show, so I’ve been given a Riese & Müller, Load 75 e-Cargo Bike* to use in the interim.  So, time for a quick review.

(The bike shop didn’t actually say which model it is, and I’m assuming it’s a the Load 75.  For more info from R&M, see

Essentially the Load 75 is like the big comfy family car of cargo bikes – compared to the leaner Bullitt which feels more like a sports car.  The Load 75 is certainly a nice smooth ride, feels very stable and is a great option for anyone wanting relative convenience and comfort.


Smooth Riding

The full suspension really helps smooth-out the ride, which is great for precious delicate cargo – such as the kids, electronics or fruit.

The other neat thing about this specific bike which enhances its smoothness is the Enviolo step-less hub.  Essentially there’s no perceptible steps between gears – because there aren’t any.  Changing the pedal-to-wheel ratio is easy – you simply twist the gear selection grip (which is the inner section of the handgrip) and the gearing increases/decreases as you like.  This shifting is perfectly continuous and smooth.  You can also change up or down whist still putting pressure on the pedals.   Because the gear selector is part of the handle it’s very safe in that you don’t really need to change your grip.

For more info, see:

Quick Comparison: Load 75 vs Bullitt

Please note, the baguette is not factory supplied.

  • Physicality: the Load 75 is longer, slightly wider, slightly heavier and feels larger.  The extra width may be an issue for anyone who needs to get through very narrow doors, or the more aggressive rider who likes to weave through traffic.
  • Ride: the Load 75 is a comfortable ride, whereas the Bullitt feels more racy.
  • Load carrying: the Load’s fixed side-frame cargo space is great for convenience but not as flexible as the fully open Bullitt.  The cargo space is longer and wider making it excellent for most scenarios, assuming a closed cargo space fits your needs.
  • Load Securing: The factory supplied side panels are nice and sturdy, but there’s an absence of gaps and holes useful for securing loads.  It does have some narrow slits useful for flat cargo straps.  Without side panels the frame would provide plenty of lashing points.


The nature of the frame does not offer many anchoring points if you have a D-Lock.    The best option for securing to vertical posts is to loop through the small bracket that strengthens the seat post. However, the physicality of the bike is such that getting close enough to secure objects to lock the bike to is finickity.  The easiest method requires a post that is relatively freestanding as the protruding nature of the rear cargo deck frame will get in the way of anchoring points that are part of a flat wall.  This won’t be an issue if you don’t have the factory side panels attached, or if you cut suitable holes in the panels, or if you make custom ones with such gaps.

Alternatively you can position a D-Lock through the rear suspension arm and wheel.

Camera Mounting

One of the implications of the factory supplied panels is that there’s a dearth of mounting points for cameras.  I ended up lashing my camera to the front frame with bungies – which mostly worked ok.

Above, a slightly reserved descent in the wet, down through Brooklyn; below, some off-road single-track action.




Cargo Bike Lifestyle

The Cargo Bike: Bullitt by Larry vs Harry

Larry vs Harry is a cargo bike maker based in Denmark, and Bullitt is the name of their flagship cargo bike.  They offer e-assisted and ‘manual’ versions (I’m fortunate enough to have the former).  You can see lots of great pictures that give you a broad sense of the bike on Instagram – links at the bottom.


How Does It Feel?

It handles really well.  It’s essentially the longboard of bicycles.  The front-loading cargo deck configuration means that the center of gravity is kept low (stable) and loads are visible (safer).  The cargo deck is still relatively narrow (less than the width of the handle bars) so it’s relatively streamlined and can still manoeuvre through tight traffic.

Basically the Bullitt is a great example of great European design: functional, elegant and balanced.

It is rated to carry 180 Kgs including the rider, which equates to you and a small truckload of groceries and or children.  My heaviest regular load is the weekly market shop for fresh fruit & veg (~20-30 Kg’s worth?) – more on that later.

As you can see, it’ll accommodate a wide variety of load types and can be used in a number of ways.

It goes fast.  Bullitt’s are equipped with the Shimano STePS system, which will assist you up to ~25 Km/h, after that it’s up to your legs and/or gravity.

Battery & Motor Performance

To understand this some local context is needed.  My daily commute is just over 4 Km, of which 3.3 Km’s of that is a steady 150 m vertical ascent (heading home).

The battery lasts me 5-7 days, based on approx. one return trip to town a day (x7), and depending on how much additional riding I do.  The journey home usually drains ~10-12% charge depending on how charged it is.

The STePS system has 3 assist modes: eco, normal, high, as well as no assist.  I tend to ride with the e-assist off except for when going up-hill or into a string head-wind.  Unless my load is especially heavy (or my knees are feeling particularly weak) I’ll stick to Eco-mode assist for the normal commute home.

According to the specs a full charge will assist you for 145 Km on the flat, for comparison High will assist you for 65 Km.  I tend to use eco to get better endurance, and frankly the extra power achieved by the more powerful assist modes isn’t something I find I need – but it’s good to have it in reserve.

Loads & Configuration

My goal is never to carry anything on my back ever again; I want to maximize the bikes ability to carry stuff and my own comfort.  I’m also keen for my configuration to be as flexible as possible.

My solution to this is:

  1. Flat open deck – no permanent / fixed side walls.
  2. A pair of large fish bins (approx. 41 x 64 cm) with lids for general cartage.
  3. A pair of army surplus ammo pouches for small items.

The bins have the same width and length but slightly different heights (28 cm & 19 cm), the idea being that the variety might come in handy.  They both have (interchangeable) lids which provide adequate protection from rain.  Depending on their relative orientation they can stack on top of each other or sit nested inside each other.

I have sliced up a foam camping mattress for padding – you can see it lining the front bin in the picture above.  A single layer of this seems to protect fruit adequately, as well as laptops, and you can always use a double layer (I got three sections from the one mattress).  The foam also helps raise your items off the very bottom of the bin, so if any moisture does make it inside your stuff is less likely to get wet.

The bins can be used a number of configurations:

  1. Stacked on top – as seen in the first picture above.
  2. Stacked within each other.  If the taller bin is placed in the lower one you get one big bin with the ability to expand out if you need the extra capacity.  f you stack the lower bin in the taller one you get a split level; e.g. you can put laptops and other ‘nice’ things in the bottom and other items in the top -like wet raincoats, dirty boots or whatever.


I have two cargo-straps: 1 x 3m and 1 x 4m.  The 3m will comfortably loop around the cargo deck and both bins top-stacked.  (Note: the strap needs to pass between the frame and the steering rod as you don’t want to impede the steering rod).

The 4m strap will comfortably to a double “n” loop over both bins high-stacked.  This is the configuration I am most using currently.  Imagine the loop starting at the top of the load – it passes down one side, goes under the frame and back up again; down the opposite side, under the frame and back up again, where it completes the loop.  This technique give s nice firm double strap.

Above: showing the double “n” loop, and an ammo pouch for small items.

I also carry two lengths of 6mm rope from the local marine supplies shop, which is great for random stuff.

Above: for comparison – the shorter fish bin inside the taller one; and “high stacked”.

Finally, I also have a few rubber bungies, these are made from recycled car tyres with a bit of dowel for a handle.  Basically you just need to carefully cut an intact cross-section so that you have a nice strong loop of rubber, and then loop one end around the dowel and back through itself (a little bit like the first step of tying a cats-paw knot).  You can extend these by hitching a second loop onto the first (like the longer of the three bungies, below).


The only minor pain is that with strapping stuff in I’m frequently forgetting things – either that I should have out, or need to put in, so I find myself messing about with straps and so on a bit.  But on balance its a pretty minor problem to have.


You’d think the longer wheel-base was a challenge – actually it’s not too bad, but you’ll want to be a little more conscious when planning stop-offs so that you can avoid having to do U-turns on the footpath.

The biggest challenge I find is inner-city stops where you want to change direction – you can pick up the rear of the bike and pivot on / manoeuvre with the front wheel easily enough, the only real challenge is available space.

As you can see in the video below, slow cornering around tight corners is no problem provided there’s enough space.

In terms of slow riding, I can stay upright doing as little as 4 Km/h.  The relative size of the wheels to the overall bike means the ratio of gyroscopic force the wheels produce is not as great.  But in general the Bullitt is really well balanced and great to ride.

Have I Ever Fallen Off?

Just the once.  It’s hard to pin-point the cause exactly.  It was early on in my Bullitt days, doing a sudden lane change on a wet smooth road.  There was a gap in the traffic and a car was offering me entry into the far lane.  I had both bins high-stacked with a bunch of gear.  I turned into the lane and rode across it, then sharply turned to straighten-up and the bike flipped over on it’s side and slid.

Did I hit a patch of oil?  Impossible to tell.  I think it was partially the sharpness of the turn, the wet conditions, and perhaps overall speed – not super fast but not hanging around either.

Once I fell I was pretty much just tobogganing along, the nature of the frame meant I was somewhat lifted off the ground by the mid-frame “h” shape off the handlebar stem, so apart from some minor scuffs I was unhurt.

Interestingly, I had the fish bins lashed down with a single cargo strap and they were totally intact – didn’t really move at all, so picking the bike up again and clearing the road was pretty easy.


Front-loading cargo bikes are super versatile, possessing good speed and manoeuvrability but with prodigious carrying capacity.  The Bullitt is an awesome example of this architecture of bike, and makes for a great car replacement.


  1. – designers and manufacturers of the Bullitt.
  2. – more information on the Shimano STePS system, 6000 series, which I have mounted on my Bullitt.  Various Bullitt models use different versions of the STePS system.
  3. – local Wellington Bullitt dealers.  They also carry other types and brands of cargo bike.
  4. – The Adventures of Pepper the Cargo Bike; me & Pep’s cruisin’ around Wellington and beyond.