> You upgraded to mechanical disk brakes? What were you running before? V brakes?

Yes, V-brakes.

> I wouldn't run anything less than hydrolic disks on something that fast. Preferably dual piston like the ones Sram are making now.

I actually didn't specify anything about the speed of my bike in the post, but mostly I ride below 25km/h. The 35-40km/h quoted was coasting down a steep hill, and I would have gone the same speed without the motor. It's just that the added load caused the V-brake stirrup to bend open over time, and then it finally popped out in that situation.

I think idea that disc brakes, mechanical or hydraulic, are "more powerful" than V-brakes or canti's is a misunderstanding. If you think about it, the absolute limit to a bicycle's braking power is as much force as possible without locking the front wheel and sending the rider over the handlebars. Any bicycle brake can do that, and if it can't it should be returned to the store as defective. The reason I switched to discs was that the whole assembly seemed more solid than V-brakes (no loose coupling like in the harness). The other big reason to switch is the feel of the lever (better control over braking modulation), and needing to put less force on the lever for more braking power, so your hands get tired less.

The main reason I went for mechanical disc brakes vs hydraulics is the ability to service them on long trips. I'm not going to bring tools to bleed the hydraulic system on touring trips.

EDIT: Another reason I picked mechanical is that it's simpler to wire a motor safety shut-off switch to mechanical brakes. For hydraulics you need to do the reed-switch mod I mentioned which is a bit more finicky, or install a hydraulic sensor which requires bleeding the system.

Disc brakes use different materials than the rubber blocks on cantilever or v brakes. I would exect that more friction can be applied. It will also handle heat differently.

Yes, I know, I'm just saying that the extra braking power isn't useful because you can already apply the maximum safe braking power with any bicycle brake. Anything more will just send you flying over the handlebars. There are other reasons to prefer discs though, as I mentioned.

As an aside, it's weird that in the bicycle world we talk about disc brakes vs rim brakes since they are all disc brake systems (in a rim brake, the rim itself takes the place of the rotor). I don't think I've ever seen a bicycle with a drum brake :)

People go over the handlebars on braking because they use the rear brake, or are not properly braced on the handlebars. If you use the front brake (and brace properly) there is little practical limit to braking force. You are more likely to skid the wheels than go over the handlebars.

It's physically impossible to go over the handlebars using only the back brakes.

I've destroyed two sets of Avid Juicy 7 hydraulic disc brakes with upgraded pads just by mountain biking, and these are almost top tier of bicycle brakes. Can't imagine people are running electric bikes with a lot of power without matching it with appropriate braking system.

I'd only use original pads and discs from the brake manufacturer. If you upgrade your brakes it's best to upgrade the whole system, not just the pads. Overbuilt components with more metal and more surface would normally dissipate heat better, reducing the prospect of brake failure. Also, mechanical brakes are easier to inspect, maintain and fix. You won't see if your DOT fluid absorbed humidity or you won't notice a leak until the brake fails, but you will certainly see a broken cable.

If you're running a >30mph ebike, you probably want something larger than 160mm rotors and something heavier duty than XC disc brakes. I'd suggest some downhill gear, including DH forks, headset, bars, cranks, etc. Shimano has the Saint line: http://bike.shimano.com/content/sac-bike/en/home/components1...

Hope makes some nice DH brakes: http://www.hopetech.com/product/tech-3-v4/

Avid has the Code: https://dirtmountainbike.com/bike-reviews/brakes-gears/avid-...

And other manufacturers also make calipers with >=4-piston calipers.

Again for rotor size, 160mm is typically standard for XC, road bikes run 140mm usually, some "all mountain" bikes have 180mm fronts, and 200mm is typically seen only on DH/4x/trials/dirt jump bikes but likely what you want for a fast ebike.

All that said, competition-level road bikes generally hold up fine at speeds up to 60mph (and the riders hit those speeds going down hills) and their brakes work fine. You obviously don't just mash them, and the rim surface they're clenching matters a lot, as do the pads, but the tech works well and has worked for the past 100 years. Weight distribution and even application of both brakes help a lot.

Competition level road bikes are new and get used for max one Tour de France season, so chances of wearing the rim, or the calioer material are minimal. I doubt they even wear the pads too much.

For >750W e-bikes that go over 28 mph you'd normally need DH brake tech, 180 rotors at least. Shimano Saint Ice tech is good, Magura brakes are also top notch, even the lower end models. Avid Code looks rather flimsy and advertises low weight. I would avoid any calipers w/o metal bodies as it does not dissipate heat well. The Hope caliper looks especially good - lots if surface, lots of metal. The brakes and the steering are the last things to save weight on.

I had some heavy-duty hydraulic brakes fade on me on my 2kW electric mountain bike. Scary times.

I think the failure mode of cable-operated disk brakes might be better (though I've since ditched the bike, so that's speculation).