Posts Tagged: brakes

Jul 2010
by don

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!