What bicycle design factors affect stopping power?

Question

I've been under the impression that bikes and motorcycles cannot stop as quickly as cars. I'm curious what factors have the largest impact on bike stopping distance.

What comes to my mind are:

• Tire size (more rubber on the road means more stopping power), a higher diameter/lower-pressure tire should be able to apply more friction to stop
• Bike weight (less weight is less intertia)
• Brake types(?) Do rim vs. disc make a difference (it would seem they are negligible as both lock up the wheel when too much pressure is applied)

What impact do these factors have? What other factors affect braking? Which factors can be tweaked for greater stopping power--and at what trade-off?

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Note: My assertion that cars can stop more quickly than bikes originated from my motorcycle rider's ed when I was getting my motorcycle endorsement. It was stated that cars are able to stop more quickly in general, and as such you must learn to follow further back than if you were driving, and also learn to swerve around stopped vehicles (in emergencies) rather than trying to brake in a straight line--as you are most likely going to end up on the bumper if you only brake without additional maneuvering.

Answer 1

The biggest factor in bike stopping distance is the mass transfer towards the front of the vehicle when braking. On an upright bicycle, the limiting factor in how hard you can brake is the point at which the front wheel goes from slowing you to simply throwing you over the handlebars. As you brake, your center of mass shifts forward, lifting the rear wheel. Shifting your weight backwards and down while you brake dramatically increases your braking capability. A human-powered vehicle with a lower center of gravity such as a recumbent trike or recumbent low-racer can also typically stop more quickly.

A bicycle's braking limit is approximately 0.67g, while the limit for a typical passenger automobile is somewhere between 0.8g and 1.0g. Legal requirements in the US are even less stringent than these limits - typically a passenger automobile must be able to brake at 0.53g, while a bicycle is only required to demonstrate the ability to skid the rear wheel, providing at most 0.2g of braking capability! It's certainly the case that there are automobiles driving around with less braking capability than many bicycles.

The factors you mention aren't really significant in the limiting case - friction is related to the normal force over the size of the contact patch, so larger surface area doesn't really help, and even if you did increase friction (sticky tires) you'd still hit pitchover before the front wheel skidded. The bike's weight is usually a tiny fraction of the total system weight. Disc brakes help in the average case since the discs typically have a better interface between the rotor and brake, but clamping a dry and clean rim brake can easily pitch a rider over the front wheel.

Answer 2

• Tire size: not significant because on paved road any tire should provide enough friction (stopping force) to get you to the critical/tipping point.
• Bike weight: not significant compared to your weight (the difficulty is to stop the rider)
• Brake types: I've found disc brakes reliable in wet and snow on hills and in traffic.

The critical factor is the angle between your centre of mass and the point where your front tire meets the road. Being lower and further back reduces that angle, so push your bum off behind the back of the seat.

You also need enough arm strength or alignment to stop you: beware being high with your arms downwards when you hit the brake. Positioning yourself as far back and as low as possible gets you behind (not above) the handle bars, against which you push to brace yourself as you come to a stop.

The rear wheel skids before the front (as the deceleration pitches you forward, the weight goes onto the front wheel and off the rear wheel, and friction is proportional to weight). On the other hand, it's skidding the front wheel that would cause a loss of control. One possibility then is to brake with both wheels simultaneously, and to ease up on both sets of brakes when the back wheel starts to skid (treat it as a ABS system)..

Answer 3

Yep... what the others said. Your deceleration reaches its max limit when the rear wheel becomes totally unloaded and the bike is about to flip over by pivoting on the front axle.

Yeah, you can mitigate the situation significantly by lifting your butt off the seat and pushing your weight backwards as you're braking. Having more of your weight further behind the pivot point (the front axle) will give you even more deceleration before you flip.

These considerations, however interesting, are pretty much just academic.

The REAL determinants of being able to avoid a collision is NOT the theoretical limit of deceleration but whether or not the rider is paying enough attention to not get into a panic braking situation in the first place, the reaction time and judgment of the rider, the physical ability of the rider to properly use "body english" and bike handling to either stop the bike in time, maneuver around the obstacle, or just crash in a way that avoids the dreaded "face-plant."