Can you brake faster than cars?
Good question. During our emergency braking exercises I always ask this question, and I get two answers, both wrong. One is "Certainly not; motorcycle tires have two small contact patches, car tires have four larger contact patches, so motorcycles have less traction." You can read my page on
traction and contact patch area to see why contact patch area is irrelevant, so this answer is wrong. The other answer is "Certainly; you can accelerate faster than cars because you're lighter. You can decelerate faster than cars, because you're lighter." I subscribed to the second view until one of my students, Eli Baldwin, said that the physics in the two situations is entirely different. He's right. Motorcycles can accelerate faster than (most) cars because the ratio of power to weight is greater for motorcycles; there may be less horsepower but it's pushing a lot less weight. But for braking, the horsepower of the engine is irrelevant. To find out whether motorcycles brake better than cars we have to look deeper into the physics of braking. The force on a vehicle during a stop is just the vehicle's mass times the (negative) acceleration, F = ma. That force has to be applied at the tires via their traction. The friction equation is F = μW (where W is the weight of the vehicle and μ is the Greek letter mu, the coefficient of friction — again see
laws of friction for details). The weight of the vehicle is the mass m times the gravitational force g, so F = μmg. The maximum stopping force that can be applied is the maximum frictional force that the tires can sustain, so ma = μmg; and we can cancel the mass which appears on both sides to get the maximum deceleration possible:
a = μg
Now before we go further, let's note some assumptions. One is that the downwards force in the friction equation is actually the weight of the vehicle. Race cars use airfoils to develop a downward force to improve their traction, so their stopping distances would be better than an unassisted vehicle (at speeds allowing the airfoil to work). If street cars ever begin to use this technology then the conclusions would have to change to take that into account.
Another assumption is that the limiting factor in stopping is traction, rather than the ability of the brakes to dissipate the energy. This is true of cars, as their brakes can overwhelm the traction of the tires, causing a slide. But it isn't true of large trucks. Their additional mass provides additional traction, as shown in F = μmg, but the additional energy is more than the brakes can deal with, resulting in longer stopping distances.
The limiting factor in a stop of a motorcycle may also not be the traction, but the stability of the vehicle. We've all seen sportbikes with the rear tire in the air in a stop. The front tire isn't sliding, so there may be still more traction available to slow, but any additional braking will just result in the motorcycle going over the front tire.
But with the assumption that stopping distance is limited by the traction of the tires, a = μg shows that the mass of the vehicle is not relevant; it does not enter into the equation. The only difference might be in the value of μ for car and bike tires. There is some evidence that bike tires have stickier rubber than auto tires: My motorcycle tires last between 10,000 miles (for the rear) and 20,000 miles (front), and the tires on my cars will go 40,000 or more. Softer rubber is stickier than harder rubber, so I think this indicates that motorcycle tires have better traction than auto tires, and thus that motorcycles will stop shorter.
But there is one final complication: Can you use all the superior traction of your motorcycle tires? If you get too hard on the brakes in your car, you slide, you let off the pedal to resume rolling, you get back on the brakes. When the same thing happens on your motorcycle, the slide generally results in a fall. Thus motorcyclists are reluctant to approach the limit of their braking, where the same isn't true of auto drivers.
So where does this leave us? Here's my rule: I figure the guy in front of me might be able to outbrake me, so I leave enough room between us, and look well ahead of him, so that I won't hit him. And I figure I can outbrake the guy behind me, especially if he's too close or not paying attention, so I leave even more room in front (to reduce the probability that I'll have to brake hard), and keep a close eye on my mirrors, and stay in the proper gear so that after braking hard I can escape if needed.
What do you think about antilock brakes?
The word from the experts at the motorcycle magazines, the people who do the 60-to-zero braking tests, is that under test conditions, they can outperform antilock brakes. That is, when their skills are already highly-developed, and if they have three or four tries to get tuned up, and when they know exactly when they're going to start braking, and if there are no traction surprises (like going over a sandy or painted patch of pavement), then they can outperform the machinery. But under real-world conditions, they say that antilock brakes win.
That's good enough endorsement for me. (Furthermore, my guess is that over the next few years even test conditions won't be enough to outperform antilock brakes.) Neither of my motorcycles has antilock brakes. My next motorcycle will have them.
