human909 wrote:uart wrote:Seven to nine pounds (32 to 41 Newtons) of brake force at the levers is not exactly feathering the brakes though. That's close to full on brake application in my opinion (think braking both wheels). With most modern braking setups I'd think that would stop you in under 20 seconds in most circumstances, let alone 20 minutes. Unless of course you have a 1200 watt motor driving you forward.

To a roughly close approximation the brake level application force shouldn't matter. If you have a 1200watt motor putting in 1200 watts then you have roughly 1200watts going into the wheels. That all might be erring on the high side so lets balance it out by ignoring rear wheel braking.

Yes but the speed was almost a constant across all wheels, even when they increased the braking force to 9 pounds. So there had to be some form of

**speed control** on the motor. This means it was actually just putting out whatever power it required to achieve the desired speed of about 20 MPH. The 1200W was just a nameplate rating. If the speed is a constant then the friction is a wildcard. The power is inversely proportional to the coefficient of friction for each rim.

So what do you need to put 1200watts into your front wheel for 20minutes?

According to the bike calculator an 15% grade and an 80kg rider would do it, riding the brakes at a 66kph. So at that speed for twenty minutes you would cover 21.9km and an altitude change of 3285m.

Yeah, the braking power under descent depends

**a lot** on the areo power loss - so on both air density (elevation) and what figure you use for the CdA aero drag term. That 66 km/hr figure must have been under fairly low CdA assumptions. The more aero you are the faster your speed (at the maximum braking loss point) and the higher your braking power for a given gradient and mass, but I can't see any sane rider trying to hold a super tight tuck when they are trying to slow down.

I used CdA = 0.4, Crr=0.005, and an elevation of 2000 metres to get a maximum braking power of just under 1100 Watt (1079 Watt) at 52 km/hr on 15% gradient (m = 80 kg). So in the same ball park, but my simulation obviously had higher areo losses than yours. Going any faster actually decreased the braking power because of aero losses.