braking distance

[handlebar]

https://www.youtube.com/watch?v=FJe99BjV8bM

This guy glided down a hill to 22 mph to take the average of three stopping distances for each of several equipment combinations, including a premium, oversized disk with hydraulic brakes and metallic pads. He took about 45 feet, and upgraded brakes made little difference.

I tried my Radmission on a street where I could let the speedometer settle at 20.0 mph. The odometer is accurate, so I assume my speed reading is, too. It took just about 25 feet each time. That would translate to 30 feet if I'd been going 22 mph.

These are stock Radmission brakes with 1,000 miles of wear and little maintenance.

With all his upgrades, why was his stopping distance so much longer than mine?

[Tree]

https://www.youtube.com/watch?v=FJe99BjV8bM

This guy glided down a hill to 22 mph to take the average of three stopping distances for each of several equipment combinations, including a premium, oversized disk with hydraulic brakes and metallic pads. He took about 45 feet, and upgraded brakes made little difference.

I tried my Radmission on a street where I could let the speedometer settle at 20.0 mph. The odometer is accurate, so I assume my speed reading is, too. It took just about 25 feet each time. That would translate to 30 feet if I'd been going 22 mph.

These are stock Radmission brakes with 1,000 miles of wear and little maintenance.

With all his upgrades, why was his stopping distance so much longer than mine?

best i can figure is that he was hauling a wheelbarrow of bricks behind him....

[handlebar]

best i can figure is that he was hauling a wheelbarrow of bricks behind him....

I've thought about it. He rode a Schwinn Boundary. The seat bracket appears to be 5 inches aft of the crank, and the handlebars no higher than the seat.

My Radmission came that way, and I found it unsafe. I used a torch to bend a layback post to get my seat clamp 14 inches aft of the crank. (Naturally, I had to add a plywood support against the rear rack.) With my hips farther back, I am better able to use my legs to brace against the pedals in a hard stop.

I used two risers to get my handlebars 9 inches higher than the seat. That and the aft position of the seat puts the angle from the grips up to my shoulders around 45 degrees. Pushing with 70 pounds would give me 50 pounds forward bracing and 50 pounds trying to lever me up and over the bars.

A seat farther forward and lower bars would mean a steeper angle. If it were 70 degrees, pushing the bars to brace yourself with 50 pounds would produce 125 pounds trying to lever you up and over the bars. You'd better not brake too hard.

I was averaging .56 g deceleration. With his super brakes, he was varying between .35 and .40. Subconsciously, his sense of being lifted over the bars was modulating his braking. (I modulate, too. I figure my center of mass is about 26 inches behind the front contact patch and 45 inches above it. In that case, braking at .58 g would send the bike end over end. Raleigh knew what they were doing when they gave the Roadster a long wheelbase, more than a century ago.)

[handlebar]

The guy who made the video then tried the front brake alone and found that he could stop in 55 feet. He said he had to squeeze his hydraulic brake lever as hard as he could. He switched to a 180mm front rotor and, squeezing as hard as he could, stopped in 45 feet, about what he'd managed with both brakes and 160mm rotors.

I hadn't realized the Schwinn Boundary came with 160mm rotors. My Radmission came with 180mm rotors. Some say a 160mm rotor will give you more braking power than you can use. With equal pressure, the bigger rotor will give you 1/8th more braking force, but usually the guy with the smaller rotor can just squeeze 1/8th harder. 

Yesterday, I found that my front brake alone would stop me from 20 mph in 20 feet. That's 5 feet shorter than I'd stopped with both brakes! I thought about it and remembered that when I'd used both brakes, I'd gone lightly on the back brake because I didn't want to skid a flat spot on the tire. Apparently I'd also taken it easy on the front brake to keep some weight on the back tire.

I had estimated that .58 g would send me end over end, but stopping from 20 mph in 20 feet takes .7 g. I weigh a lot more than the bike, so I'd ignored its weight when I estimated where our center of mass was. I see now that the bike brought it down several inches.

To make that stop, I had to brace against the bars and pedals with a force equal to 70% of my weight. I'm glad I raised the bars and moved the seat back.

Unlike the guy in the video, I didn't have to squeeze hard, so I guess my disk has a higher coefficient of friction. He said he'd bedded his rotors, but he must not have done it well. Manufacturers say you should do one brake at a time. You speed up to 20 or 25, brake moderately to 5 or so but don't stop, and repeat 20 to 50 times.

Leaving my garage, I can go straight or turn left. If I turn left, I go down a hill with a speed bump at the bottom. Approaching the speed bump, I'll use the brakes to slow from 25 to 10. I may do it a thousand times a year. I guess that gives the front brake a high coefficient of friction.