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February 2019

Stopping Distance

by John Allen

Quora is an online service where people can ask questions for other people to answer. I recently answered the question:

“Do people realize that bicycles take longer to stop than a car?”

The question reads like wagging the finger of blame at cyclists. “Do people realize…?” assumes that is premise is true without saying that outright.

The true answer depends on the bicyclist, and on the bicycle. And you might actually not want to have to stop quickly.

It is true that a bicycle with the conventional riding position can’t decelerate as hard as a car. Stopping is limited by the potential to pitch forward due to the bicycle’s high center of mass. But with typically lower bicycle speeds, the stopping distance is short. At half the speed, perception/reaction distance is half as long -- and braking distance, only a quarter as long -- braking distance being proportional to the square of speed.

For a bicycle with dual handbrakes in good condition and an expert rider, on a good, dry paved surface, maximum deceleration is about 0.5g. It is achieved mostly with the front brake. For a car with ABS, maximum deceleration is about 0.8g. (One g is the force of gravity.) So, at any speed, the bicycle has about 1.6 times the braking distance. That translates, though, only to the bicycle’s going 80% as fast as a car for the same braking distance, say 24 mph instead of 30.

(I might also ask: “do you know that motorists speed, and speed kills?” But I’ll hold it there.)

Even a novice cyclist – or a cyclist with only a rear brake – who can achieve only 0.3g, has the same braking distance at 16 mph as a car has at 30.

A cyclist with hands over the brake levers can have a shorter reaction time than a motorist who has to switch a foot from the accelerator pedal to the brake pedal. The bicyclist also has no hood out in front, effectively reducing braking distance when a look to the side is needed for cross traffic.

In over 50 years of riding bicycles with dual handbrakes, I have never run into the back of a car. Avoiding this is simply a question of maintaining a reasonable following distance. I’m struggling to remember any time when I couldn’t stop as quickly as I’d have liked. Well, actually I recall one incident: my front brake cable snapped just as I was reaching an intersection.

On group rides, it’s different. I have overrun a bicyclist ahead of me more than once. Most recently was only last September, when I was arrowing for CRW’s Cranberry Century. The cyclist in front of me slowed abruptly. We touched wheels. I was fortunate not to crash. (I knew to steer toward the wheel I touched.)

Strong anecdotal evidence suggests that riding close behind another cyclist is the largest single cause of crashes on CRW rides. Cyclists who ride in pacelines are always taking a risk, to some degree – though wheel touching doesn’t happen only in pacelines.

Though the leading and following cyclists in a paceline may have the same ability to decelerate in theory, perception/reaction time is the villain. Everything I have said about the advantages of expert braking to avoid a dog that runs out, a car that pulls out from a stop sign, a pedestrian who steps off the curb – comes to nothing if you are close behind the cyclist in front of you. Best I can say is -- even though the cyclist in front of you can’t stop quite as short as a car, keep your distance unless you are willing to take the risk!

More about how to use bicycle brakes effectively:

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