Cargo bike brakes and safety

Yesterday I took my bike back to the bike shop to replace a brake rotor that was warped and making loud screeching sounds all the time.  For awhile, I tried to convince myself this was a great safety feature: everyone could hear me coming for at least a city block.  But the joy of riding in near-silence was missing, and dogs were more likely to bark as my bike squeaked by.

Disc rotors are prone to warp when they get too hot.  In most cases, my brakes stay relatively cool, because the BionX motor bears the brunt of braking.  There is a little magnetic sensor on my rear brake lever that engages the motor’s highest level of regeneration when I pull slightly on the lever.   Usually, the drag of regeneration is enough to keep my speed in check.  When the hill gets a little steeper (or the load is heavier), I apply my brakes in addition to the regenerating motor, but most of the time I use the brakes lightly and for short duration.

However, there is a problem.  If I sprint up our hill at maximum speed, there is some limitation in the battery or the motor which shuts off regeneration on the way down unless I wait for 10-15 minutes before I descend.  Perhaps something is getting too hot, although I haven’t noticed excessive heat in either the battery or the motor.  I called the motor manufacturer to ask if they were familiar with this situation, but the technical assistant who answered said regeneration shouldn’t turn off unless the battery is fully charged.  I am quite sure my battery is only partially charged when I lose regeneration.

Riding down our hill and carrying two kids (about 150 pounds total) without regeneration, my front and rear disc rotors get very hot.  Hot enough to leave scorch marks on the rotor, actually.

To be fair, this is a steep hill (12-18% grade for about 1/3 mile), and the total weight of the bike, rider, and cargo is around 350-400 pounds.  But the duration of braking is less than two minutes.  It’s hard to imagine a situation where I would worry about hot brakes after two minutes of braking with a car or motorcycle.  Are cargo bike brakes really up to the tasks we’re asking of them?

According to the owner of the bike shop, they are not.  Most cargo bikes come with disc brakes that were designed for single-rider bikes.  A beefy cargo bike carrying a heavy load could easily double the weight the brakes have to stop.  Add a motor and a battery, and you’ve not only increased the weight, but you’ve also increased the capacity to carry big loads up a hill.  What goes up must come down, and the brakes better be ready for it.  I haven’t done rigorous research on this topic, but it seems fairly obvious that the brakes need to be upgraded to handle bigger loads and higher speeds.

Unfortunately, this isn’t a problem that the biking industry has squarely addressed.  To absorb more heat, the mass of the disc rotor must increase.  My rotor is 6 inches in diameter and 2 millimeters thick.  Apparently, there is a company that makes a thicker rotor (3 mm), and that would help.  Instead, I decided to lay out $100 to buy an 8-inch rotor laser-cut from stainless steel (

The first problem we encountered was that my brake cable wasn’t long enough to move the brake caliper mechanism the extra inch or two to accommodate the bigger rotor.  In the process of replacing the cable, I upgraded to “compressionless housing.”  The idea is that very stiff housing transmits more of your braking force to the brake rather than deforming the conduit through which the brake cable moves.  I had already done this for the front brake on a previous visit to the bike shop, and I can tell a difference in braking responsiveness.  The new cable and housing cost $38 for parts.

Bar precludes repositioning of brake calipers

With the brake cable replaced and the rotor bolted on, all that remained was mounting the wheel.  That’s when we discovered that the Hammer Truck panniers are supported by a bar in the exact position where the brake calipers need to be for that 8-inch rotor.  The shop owner said we could crimp the bar to make room for the calipers, but I was concerned about compromising the integrity of the bike.  Also, the caliper adjustment wheel (red in this photo) would be inaccessible if it were nestled into the dent he was proposing to put in the bar.  (Click on photos to see them full-size.)

In the end, I decided to buy another 6-inch rotor.  It’s nice and true right now, and my bike is quiet again.  I’ve learned enough that I might be able to avoid warping this rotor.  First, I should allow more time to let the BionX motor cool, or I should climb the hill at a slower pace (and perhaps a lower assistance level).  With a little experimentation, I should be able to reduce motor/battery strain and avoid regeneration drop-out during my return trip.  Although it might take a little more time or effort, this strategy might even extend the life of the motor and battery.

Second, my daughter just graduated from elementary school.  Now I will have only one kid to carry down the hill in the afternoon, and that should reduce the braking load.  My immediate concerns over our safety have been reduced.

But my concern for other cargo bikers is escalating.  The electric cargo bikes that I have mentioned in earlier blog posts, Kona’s Electric Ute and Yuba’s elMundo, have no motor regeneration and only one disc brake in the rear (both bikes have a rim brake on the front wheel).  I’m concerned that this might not be adequate to stop a heavy load after a moderately long and steep descent.  It wouldn’t take more than a few mishaps to cause legal problems that could restrict the electric cargo bike market before it has a chance to develop.

Sometimes I try to convince myself that this is just an issue of setting expectations appropriately.  When we get really excited about the economic and environmental advantages of cargo biking, we talk about our bikes as being “car replacement vehicles.”  But it has been over a century since Ford’s Model T was first sold.  Since then, cars have benefited from continuous technological improvements and fierce competition between many different car companies.  The electric cargo bike has barely reached a comparable level of development to those first Model T’s.  Manufacturers are still treating the motor like an after-market option rather than an integrated design feature.

Arrows show exposed pressure plate

My safety concerns aren’t just limited to the brakes, by the way.  As the shop owner was re-installing my rear wheel after the rotor replacement, we got a good look at how the BionX motor mates with the Hammer Truck frame.  The critical interface is where the motor axle bolts onto the frame — the C-shaped flanges called “drop-outs.”  The owner pointed out that my drop-outs weren’t deep enough: at least one-third of the pressure plate that the drop-out should be holding is left dangling below (noted by arrows in this photo).  If you’ve been reading my blog for awhile, you might recall that I stripped a hub nut during the first month I owned the bike.  The results were catastrophic: the motor wiring was damaged beyond repair and a new motor had to be installed.  When the nut stripped, the rear wheel twisted a little and locked up against the brakes.  The bike went into a skid, but fortunately I wasn’t travelling very fast at the time.  Even though there were cars nearby, I managed to stay out of traffic, and escaped unscathed from the incident.

If I had been travelling a little faster, or if — God forbid! — I had my kids on the bike when that nut failed, it’s not pleasant to imagine what could have happened.  It seems obvious to me that this problem could have been avoided if the Hammer Truck had been designed with deeper drop-outs to accommodate the motor.  This is what I am looking for: a manufacturer that designs the bike, the motor, and the brakes to work together, rather than putting these pieces together and hoping their designs and capabilities mesh well.  So far I have not seen a bike that impresses me in this regard.  It feels like a classic chicken-and-egg problem: manufacturers won’t spend the time and energy to develop that bike until they see a significant market opportunity, but the market won’t grow quickly until there is a serious bike that people can see, test drive, and feel confident about its safety and reliability.

There are still some breakthroughs required in battery and motor technology (and price) that may be required to make electric cargo bikes attractive to the mass market.  However, those same advances will also benefit electric cars.  The market for an electric car is so much broader than electric cargo bikes — I think Americans are going to jump straight from gasoline-powered cars to electric cars, and cargo biking will remain a tiny niche of ultra-environmentalists and fitness freaks.

Of course, that’s only the most likely scenario.  A massive and sustained increase in energy prices would obviously benefit bikes of all kinds.  A generational shift of mindset is also possible and can’t be underestimated.  For example, if teen heart-throb Justin Bieber decided that cargo bikes were the only way to travel, who knows how fast cargo bikes could sell?  But at this point, most of the people reading my blog are more likely to be the parents of the Bieber generation.

I’m in a strange position.  For nearly a year, I’ve been riding my bike and enthusing over this method of cutting carbon emissions, burning some extra calories, and seeing more of your neighbors, nature, etc.  Now suddenly I’m worried about safety.  I’ve always known it’s a little risky competing with heavy metal boxes for a few feet at the edge of the road.  But if you’re not sure your equipment is fundamentally safe and sound, there’s another set of issues to worry about.

Do you think my concerns are overblown?  Does your cargo bike inspire confidence?  Let me know in the comments below!

  • This is a great discussion, safety perception is a big issue that keeps the average commuter in their car. I agree that with electric bikes and cargo bikes we are at a very early stage of development. Kind of like cars were at the point they did away with the horses. I can certainly understand the problem you are having with brakes, however for me is is not a concern as I don’t ride hilly terrain. It shouldn’t be this way and the bikes should be able to handle any terrain (within reason). It is a price we pay for bikes being essentially unregulated (some basic design restrictions aside).

    If bike safety was regulated like car safety we might be a bit safer, but certainly more restricted. I doubt we can rely on manufacturers to build us totally safe bikes at this point and so personal responsibility and riding within the limits of the bikes are important strategies.

    In terms of a totally integrated ready to ride product I don’t think a one person electric bike like this exists as yet so certainly not for a cargo bike. Sadly I recently talked a friend out of purchasing an electric bike for his mother as I just feel the reliability and after sales service is not there yet.

    Regarding your specific problems,I suppose the rear rotor is not cooling well tucked in next to the hot motor and behind panniers. I wonder if you might get a 185cm disc to fit better? A standard Avid BB7 rotor would not cost much to trial anyhow. Regarding the shallow dropouts, you might see if Bionix have a “torque arm” option instead of that slotted washer, or you might source one from someone else. I think the axle on you motor is fairly standard and could fit any torque arm. Check the electric bike forums here and you may find a local supplier

    Enjoying your blog keep it up!

  • Don

    Good points and suggestions. Thanks, Mark! I will do more research on the torque arm especially.

    I agree that personal responsibility and appropriate caution are the best safety measures we can have, and the price is right! For seasoned cyclists and cargo bikers, this probably isn’t a big concern. But as people start looking for transportation alternatives and cargo bikes with motors become more prevalent, I worry about casualties. I worry about someone like myself just a few months ago, who might have read that a certain cargo bike could haul 400 pounds in addition to the rider. With a motor to accelerate that load to 20 mph or more, the standard brakes will not be adequate to stop that bike in a reasonable distance, even on level ground.

    In this case, the combination of powerful motor and barely adequate brakes could be deadly. Is this the reason that manufacturers seem so reluctant to push electric cargo bikes? It’s hard for me to say it, but it feels like a lawsuit waiting to happen. I would love to hear that my fears are exaggerated.

  • I had a closer look at the “pressure plate” picture and it looks like it might be integrated into the motor. Most cheap hub motors have an axle with flat sides (14mm axle flat sided down to 10 mm usually)and rely on a washer or a torque arm to stop it twisting in the dropouts.. Might be tricker with yours, your axle looks round. Your pressure plate certainly looks beefy enough (better than my hub motor with similar power) so it should be fine as long as it stays in the dropout!

  • Tomio

    There is a fault in your motor/regen system if is cutting out in this way. Have you followed up on this issue? Because I want to experiment w/ cargo bikes and tandems; I was thinking of trying the BionX system because of the well developed regen controller. If they can’t help you with this issue I may want to try another source.
    Tandems and cargo bikes absolutely need a reliable regen system as a primary brake. I was thinking of getting my brother to design a power dump circuit to put the output through a resistance on the off chance that the battery was fully charged.

    The 203 or bigger rotor is a very good idea. If you have a good framebuilder in your area (or if you area near the original builder) you should be able to get some disc mounts inside of the rear triangle. They should fit there.

    Any luck with the torque arm investigation?

    • Don

      Regarding BionX regen cutout: I phoned BionX some months ago, but the tech I talked to didn’t have any answers. Tonight I sent an email query to the company. I’m hoping to get a more definitive response in writing. If/when I do, I will let you know how it goes.

      At this point, I can highly recommend the BionX motor for quietness, functionality, and reliability. However, I have a few complaints. The regen cutout issue is the most serious. Occasionally, after hard braking, the regen feature seems to get stuck on for maybe 5 seconds, producing an unpleasant stuttering as I start to pedal. This passes pretty quickly and does not have any lasting effect, but it caused me concern the first few times I experienced it. Finally, the LCD display which shows how much electricity is being used or generated does not seem to be calibrated for my loads. The graph disappears if I’m climbing a hill too steep, or when I apply regen braking faster than 15 m.p.h.

      I’ve been able to live with these quirks because the motor, the regen braking, the integrated battery/controller, and the torque-proportional activation are so good. I haven’t seen any other motor that I like half as much. It’s a little expensive, but definitely worth the investment for me.

      If I get a timely response on the regen issue, I will ask what BionX thinks about my shallow dropouts, and we’ll see if they have any suggestions.

    • Don

      I received timely answers from BionX regarding both my questions (kudos to BionX for the quick responses). Regarding the interruption of regenerative braking, the BionX technician confirmed that this can happen if the motor gets too hot. It will inhibit regenerative braking to protect itself from heat damage. Unfortunately, there is no indication of this on the otherwise excellent BionX display, so the rider doesn’t know when the motor is in an overheated state. I am pretty sure this is an accurate diagnosis of my situation. It only happens after I sprint up my steep hill for almost a mile. This may be pushing the motor to the limits of its design.

      As I think of it, I think this is a reasonable explanation for the cutout of the motor during my test drive of the Kona Electric Ute. I was riding up a pretty steep hill fairly quickly. It’s possible that the Ute’s smaller motor detected excessive heat and shut down. It recovered during my relatively level ride back to the bike shop.

      The possibility of heat-related shutdown may apply to many hub motors that could be used on cargo bikes. It’s definitely a topic I intend to explore in a future article. (Yes, I know, one more argument in favor of the Stoke Monkey…)

      Regarding the torque arm, BionX does sell one (part number 012038), and I intend to install one as soon as possible.

  • Tomio

    Hi Don,
    I was expecting something to show up in my web-mail so I didn’t realize you had responded to my comment. From the experience you report, I am not going to go with the BionX system.

    I agree that the BionX has the best features of all the systems I’ve seen but as you have experienced it is not fully able to deal with cargo bike duty. I am going to try to put together a system of my own. I will use the BionX functionality as a target but I will focus on better motor cooling. I am also going to see what I can do about the controls response time as all of the e-bike I have tested seem to have a very irritating lag.

    I am wondering how your experience is an argument in favor of the Stoke Monkey? Couldn’t it overheat going up a steep hill?

    Did you think about getting a brake mount put in between the seat stay and the chain stay? I saw a Bike Friday tandem in San Francisco w/ the caliper mounted there. If it works w/ your frame geometry this location is safer due to brake forces pushing the axle into the dropouts. There are a lot of standard 203mm rotors around that you can pick up on sale. No need to spend too much. They are a wear item that need to be changed about every third (or fourth) pad change.

    • Don

      I will be very interested to hear what you discover during your search for a better cargo bike motor. Elsewhere on my blog, the founder of Cycle9 recommended motors such as the 9 Continent or BMC V2. I believe these use internal gears to deliver higher torque at low speeds. However, the gears probably generate more noise than the BionX. In addition to that luxurious silence, I’m also pleased with the BionX regenerative braking on my steep hill (as long as I avoid that overheated state).

      I believe the Stoke Monkey would deliver better performance on steep hills than a hub motor, because it drives the wheel through the chain and the bike’s gears. You can keep the revolutions high and the torque relatively low by shifting to a lower gear. The downside is that you have to remember to throttle back before you shift in order to avoid damaging your components or jamming the chain. The advantage of hub motors is that you don’t have to worry about that: the motor’s power is added after your muscles have put in their effort through the chain.

      I will investigate moving the brake mount as you suggest. I suspect there is an obstacle, or my bike technician would have recommended that when we were stymied during my latest brake overhaul.

  • SeattleElectricBike

    Are you still concerned about the safety issue regarding your drop-outs? If so, please contact    Heidi at

  • Charlie

    As a new cargo biking dad I don’t think your concerns are overblown. I take it very easy on my Babboe and avoid major streets. I will also upgrade the brakes. More standardization would be great. Thanks for your post – even three years later it has some good info that is hard to find.

  • Steven Sykes

    I realize this is a very old topic but I would like to address the problem you encountered with the motor spinning out of the rear dropouts for people who might still be reading this. You probably know this by now but there is a small piece of hardware called a torque arm designed to prevent the motor encountered that wrecked your Bionix motor. It’s used more on front mounted motors but as we can see from your post should probably also go on the back of some if not all bikes.

    Here’s a link to a retailer of torque arms. They have one that will fit most bikes, front and/or rear.