The stock brakes on many e-bikes may be inadequate. What changes would make them adequate, and at what cost?

Question

Note that some links in this question go to sites with a metered paywall, i.e. you can access something like 4-5 free articles per month. While the question mentions a bike manufacturer, the problem very likely spans all manufacturers of low-cost e-bikes.

Recently, Molly Steinsapir died while she and a friend were riding a cargo e-bike downhill. This was covered in mainstream and cycling media outlets, e.g. on Bicycling magazine's site. Her friend was unable to stop the bike, because the brakes did not engage. Why the brakes failed to engage during the accident sequence is not known. This Outside magazine article says that Molly's friend, who was controlling the bike, applied the front brake, but it didn't slow the bike and caused the wheel to wobble. As covered in both articles and this Tweet by cycling journalist Peter Flax, there may be widespread problems with the brakes on lower-cost cargo e-bikes. These may stem from any combination of the following:

• Cheaper disc brakes may generate less power
• This may be exacerbated by lower-quality cables and/or poor routing
• Low-quality brakes may come out of adjustment faster, wear faster, or otherwise be unreliable
• Novice consumers may not realize they should keep adjusting their brakes, or adjust them wrongly, or be dissuaded from taking them to a store repeatedly because of cost

For the record, cable (aka mechanical) disc brakes are specced on lower-end bikes because they're cheaper than hydraulic disc brakes. Hydraulic brakes self-adjust, and the system requires little maintenance if set up properly and the bike is in tolerance (e.g. disc brake mounts are flat). This Cyclingtips article after a 2022 bicycle field test argued that you could get good braking with cheaper mechanical disc brakes, although many bikes have low-quality cable housing or routing that robs braking power.

E-bikes in general are heavier and faster than unpowered bikes. This translates to more kinetic energy to dissipate while stopping. Cargo e-bikes are considerably heavier and can carry much more load than their peers. For the record, the Radpower model involved in the accident claimed a capacity for 300 lbs total weight. It has Tektro mechanical discs with 180mm rotors.

Question: What spec changes would suffice to ensure adequate stopping power on e-bikes? Would these raise the price of such bikes enough to affect demand?

Note that the original motivating issue was e-cargo bikes, which can carry significant loads. However, it seems warranted to discuss both cargo and non-cargo e-bikes if posters feel it's warranted.

Answer 1

The problem I see constantly is lack of maintenance. It's very common for ebikes of all varieties to come into the shop I work at with brakes that the user has treated essentially as an automotive brake, i.e. with the expectation that maintenance and wear needs to be addressed only very occasionally. These users often fail to advance the pads on mechanical brakes, and wear through the pads to the back plates or pistons on hydraulic ones.

The power level when most ebike brake systems are in sound mechanical state is acceptable.

Fixing the situation on the behavior/education end is impractical. Ebike buyers can be hard to reach, since direct-to-consumer and other low-interaction business models are so predominant. Beyond that is the fact that ebikes to date have needed to largely adopt normal bicycle brakes and the mounting/spatial standards that go along with them, which were not designed for the purpose. The braking needs of an ebike are much closer to that of a moped, which have more overbuilt brakes. The best avenue to pursue is brake systems that have much longer life on their wear parts at the expense of weight and/or cost.

Getting to much longer wear life for brake components (pads and rotors) has both material and form factor considerations. There's a further question here that deals with where the point is that new or different frame spacing, rotor mounting, and/or caliper mounting standards may be necessary to accommodate needed form factor changes, and at what cost in tradeoff factors like wheel strength and minimum q-factor if making rotors and pads much thicker was needed, such that either the left hub flange needed to be closer to the frame centerline or the frame spacing needed to be increased respectively. This topic has a lot of ins and outs as it applies to both future designs and/or retroftting.

At this point most disc brake pad makers have ebike compound offerings on the market designed for longer wear life, but some seem to be getting there at the expense of power (friction). This suggests there are tradeoffs involved in the formulation of the compound where more thickness and/or contact area is the only solution for applications that need both power and longevity, as is often the case for hilly, fast, and/or load-carrying ebike applications.

Many ebikes with hub motors have a rear caliper where the inner pad adjuster can't be accessed in the intended manner because the hub is in the way. Mechanics usually can figure out simple enough workarounds for this issue, but average users often cannot. If mechanical brakes keep appearing on hub motor bikes (ideally they wouldn't, but they probably will), it would be ideal to abandon the conventional design completely and replace it with an external hex bolt so that, for example, a simple small combination or adjustable wrench could be used to make the adjustment.

Some may propose replacing all the cheap mechanical brakes in the world with good hydraulic models. This would be better than what we have, but doesn't fix everything. Automatic pad advancement and extra power are welcome, but it's common for users working with the same low-maintenance expectation to simply go through all the pad material on hydraulic brakes, until they're running the back plates against their rotors.

Answer 2

Problem with all E-Bikes generally - from the article the only people riding down those hills pre -E-Bike would be people who rode up, and to ride up, you have enough cycle time to know how to descent safely. E-Bike takes novices to places and at speeds, that novices should not be going. Focusing on one thing (in this case forcing better brakes on cargo bikes) does not address the root cause of the problem.

User education is the key here. It does not matter how highly speced the brakes are, poor maintenance and/or poor braking technique and lack of understanding of how brakes work and what their limitations are will lead to failure, accidents and deaths. Better brakes will give bigger margins for errors and reduce these accidents to a minimum. In the case of children, the parents are the ones who should be educated.

Manufacturers have all the responsibility though here. They are selling their product to a target audience that is naive, who want a utility item, rather than the enthusiast. This should be reflected in the product they offer which should be safe and robust. Why is it acceptable that bikes can be delivered with crap brakes that always need adjusting when car brakes 'just work, all the time, every time'.

If people cannot afford a bike with good enough brakes, they cannot afford a bike. If that affects demand, so be it. Is it not better to have fewer bikes and fewer accidents (as a percentage). (If cars were cheaper, more people would buy one, but we don't let the manufacturers sell unsafe cars so more people can buy them.) If the manufacturers are trying to meet a price point that means their product unsafe, and do so anyway, it is 100% on them. Often regulation is needed to keep people safe from these unscrupulous manufacturers, although (in the US particularly) the civil legal risk may be enough that regulation is not needed. (I am removing from my discussion any idea that morals will prevent a for-profit selling an unsafe product)

Answer 3

Storytime:

Years ago I bought a bike (in France) and was surprised to find it had very weak brakes. After almost rear-ending a car with it on the way from the shop to my home, I disassembled the brakes and discovered they had been fitted with a braking limiter, some sort of spring assembly that got squished when pressing the brake handle, thus limiting pulling force on the cable. Once this was removed, the brakes worked as they should.

I immediately assumed lawyers and/or lawsuits involving someone flying over the bars, or worse... regulation... had been somehow involved in the otherwise unexplained apparition of this murderous device on my brand new bike.

They are no longer selling bikes equipped with this sort of self-destruct device, so I presume someone sued the manufacturer because they crashed.

So yeah, it's risky to be alive. And quite often when cowards panic about something they're not doing anyway, the result is over-regulation which makes the regulated thing impossible to do. Or in the case above, "safety features" that make it a lot deadlier.

Anyway. First, regulation! I will translate choice bits about brakes from the French code, it is quite simple:

• Bicycles must have two independent brakes acting on two different wheels
• Brakes must stop the bike under reasonable circumstances including in wet conditions
• Brakes shall be designed to not lock the front wheel if they break
• Usual maintenance operations (changing a tube, putting on some lights, fenders, rack, etc) must not force the user to disassemble or unset vital safety parts of the bike (braking, steering, etc). (this mandates quick releases on V-brakes so you can pull the wheel out without having to unscrew the screw holding the cable).

So, for this cargo bike, "reasonable circumstances" would an emergency stop, at reasonable speed, with the rated load weight plus some safety margin, on the steepest slope available, which should be about 20-25%, since it's not a mountain bike. Either it can or it can't, and if it can't, it's illegal to sell it, and the manufacturer is liable.

What spec changes would suffice to ensure adequate stopping power on e-bikes? Would these raise the price of such bikes enough to affect demand?

I have very little information about the accident you're talking about, so I'll hand-wave it: 13 year old girls are not very heavy, so it is extremely unlikely they overloaded a bike rated for 300lbs.

However a 13 year old girl does not have the grip strength of a grown adult either.

So, unless the accident was caused by a mechanical failure, defective assembly, etc, it would be interesting to know if the brakes have enough leverage to stop the bike at its rated load, under reasonable circumstances, taking into account the low grip strength and small hands of a young rider.

The fact it's an ebike actually matters here because it allows a child to ride it with a lot more weight on it that they would would be able to without the help of the motor.

So it might make sense to overspec discs and brakes on a kid bike, or at least check if the levers fit their hands and they can stop properly with an acceptable level of hand effort, not too much.

I would also like someone to invent brake pads that screech loudly when the pads are worn out, to warn the user it's time to change them.

Of course training is essential, but no-one does it.

Answer 4

It may be possible to design brake pad wear indicator of one or another kind as some motor vehicles have. It could simply make lots of noise when braking if worn, or, having advantage of electronics readily available, show the warning on the control panel when metal base of the brake pad touches the rotor. I would be happy to have such.

Answer 5

Uh oh, rant ahead...

Disc brakes work just fine, even affordable mechanical ones. They do of course consume pads though. It's not a big problem, but as Nathan remarked the average E-bike rider doesn't exactly maintain their bike well.

There is one way this aspect could be rendered far less relevant: if manufactures properly implemented regenerative braking, as they should. It makes so much sense if you're anyway going to have an electric motor anyway to also use it for braking, because this both extends battery range (or allows for a smaller/lighter/cheaper battery) and gets rid of pad wear. There's a lot of nonsense talk regarding that regen wouldn't work on bikes because the motors are too weak or that it wouldn't be useful because bikes are too light. Those arguments are particularly nonsense when it comes to cargo bikes, which are a lot heavier (which is bad for friction brakes but actually good for regen), strongly powered (which means there would also be more torque for regen braking), and usually ridden at low speeds (which means less energy is consumed by drag, despite the non-great aero).

The regen braking wouldn't be enough for coming to a stop or for emergencies, but should be plenty for 50-80% of everyday use, which would mean the disc brake pads stay in good shape for longer. Plus, the regen could even make the front braking ABS, which by itself would be a large safety boon for inexpert riders. And front-wheel drive could be quite handy for maneuvering a heavily loaded long-wheelbase bike up curbs as well.

Unfortunately of course, even most cargo bikes nowadays use mid motors which aren't suitable for regen braking at all. Mid motors have some merit for full-suspension e-MTBs, but for cargo bikes they don't really make sense at all – a strongly geared hub motor would work just as well; for a cargo bike it wouldn't matter that it can't cope with high speeds and incurs a lot of drag when turned off.

Cost of this? Very little actually. Any standard motor controller can also be used in reverse to feed back energy into the battery. The main changes needed is to get rid of any freewheeling mechanisms between wheel and motor, to double-check that the gear mechanics are sound also under reverse load, and include some safety circuitry to avoid overloading the battery. (The last point is possibly the main reason manufacturers aren't doing this... they're afraid that batteries might go up in flames and create a publicity disaster, and shy of the testing effort to ensure it doesn't happen.)