John, you state that:

"A street motorcycle is capable of stopping so hard that the entire mass is held by one tire. When this happens, the vehicle has acheived maximum stopping power based on mass as the limit (harder braking would flip the bike forwards)."

This statement is false, and it doesn't mean that this bike will always brake harder than a car. The fact is that the ease with which a bike will lift its rear wheel under braking is a function of :
1. the mass of the bike and
2. the distance of the centre of gravity from the front tyre.

It is easy to imagine that two bikes of equal mass, one which is twice as long as the other. The shorter bike will lift its rear wheel under braking far easier, for the same amount of braking force. And it is the braking force that dictates the rate of decceleration and therefore time it takes to stop.

The reason for this is again simple kinematics. Nature is smart and lazy. Under braking, the bike is constantly making a decision : whether to deccelerate linearly (slow down), or rotate (lift the rear wheel). It chooses whichever is easier (whichever requires less power input). In other words, once a certain amount of braking power is applied through the front wheel (and tire), it becomes easier for the bike to rotate than deccelerate linearly. So it lifts its back wheel.

The point : today's sports bikes, with short wheel bases, will lift their rear wheels quite easily. At this point, they cannot brake any harder. This LIMITS their braking ability, not maximising it. Having said that, they still stop pretty damn hard (despite this fact, not because of it).

I think your pages are excellent information, with a great point of view, and I am sorry to bring to your attention two minor but significant errata.

1) It was not Sir Isaac Newton who first demonstrated that all bodies fall at the same velocity, regardless of mass. That was Galileo Galelei, throwing heavy and light objects from the tower of Pisa.

2) The same article states that "sticky" motorcycle tires are wholly responsible for for the superior acceleration/deceleration of motorcycles, as compared to that of automobiles.

The article states that vehicular mass is not a significant factor in vehicular acceleration/deceleration. This is false, and could conceivably lead to dangerous rider judgement error.

Vehicular mass is THE significant factor in vehicular acceleration/deceleration. More massive cars have more inertia, and are harder to stop than less massive motorcycles, a simple fact of physics.

The article errs first in mistaking VELOCITY with ACCELERATION.

While all falling bodies of whatever mass ACCELERATE (increase velocity) at the same rate (32 ft/second per second), that does not mean that they all have the same VELOCITY.

The article errs further, concluding that because objects fall at the same rate of acceleration, they all have the same inertia - that heavy objects have the same inertia as light ones.

That is easily proven to be false. Have a heavy guy run at you, and then a light one. When you get up, tell me which hit you harder.

Or (less painful) run a 100-yard dash against a horse. While the horse is capable of higher sustained speed, it is more massive, and takes more time (therefore DISTANCE) to reach that speed. (It also takes much longer to stop.) You should beat the horse to the finish line easily. (I am not just talking here, I won a $30 bet last weekend doing just this.)

Motorcycles can accelerate/decelerate faster than cars because they are lighter in weight, and therefore have less mass, and therefore less inertia.

It is harder to speed up or bring to a stop a heavy object (car) than a lighter object (motorcycle). So inertia translates directly into STOPPING DISTANCE.

A bike has half as many brakes as a car, but only a fraction of the mass and inertia, so the brakes don't have to work nearly as hard, and the stopping distance is shorter.

Also, cars and bikes use BRAKES (not tires) to convert motion into heat, to slow and stop. Unless the tires are actually SKIDDING, they are not doing the motion-to-heat conversion - the brakes are. If the tires are sticky, that helps transfer the motion to the brakes better, but the brakes are doing ALL the deceleration work.

Please correct these two errors, to continue providing your high standard of reliability.

I enjoy these pages immensely. Keep up the good work!

I am writing regarding a short response in your contrary opinions section.

I think you have spelled it out elsewhere, but people reading this contrary opinon may get the wrong impression. Sam Longoria is correct in asserting that inertia is a significant factor in the acceleration/deceleration equation.

However, what he fails to realise is that the mass of the vehicle figures directly into the amount of force that can be applied to slow the vehicle.

momentum = mass x velocity frictional force (read stopping force) = coefficent of friction * mass * gravity

Therefore we can eliminate mass from this equation which leaves firctional force being equal to coefficent of friction * gravity. This figure is the amount of deceleration that a motorcycle (or car) can apply.

Arguements that brakes dissipate all the energy are false. The brakes apply force to the wheel, which is controlling the bikes speed. As Newton's Law states for every force there is an equal and opposite force. The opposite force is applied to the surface of the tyre by the road. This force means that tyre surfaces are rubbed away. The kinetic energy of the bike is dissipated through a number of different systems, including the brakes, the axle of the wheel and the tyre.

So what is the end result of this:

Motorcycles do not stop faster than cars because they are lighter (and if you check new car braking distances you may find they don't stop faster period).

If Motorcycles do stop faster than cars (I'm yet to get definitive information), it is not enough to make that big a difference. What do I mean. If you were doing 55mph on bike next to a modern (max 10yrs old) car doing 50 mph you would find that the car would brake within a shorter distance.

Please, let your readers know not to be deceived. Motorcycle brakes will not help you if you are going too fast.

Cheers,

Clem Colman

The argument is based is an ill conceived notion. Motorcycles do stop faster than cars and that is entirely accomplished by their tires because a motorcycle will have stickier tires than the average car.

I agree with your definition of momentum, and in the case of a motorcycle skidding to a stop and a car skidding to a stop with the same contact area and the same tires they would indeed stop in the same distance because in this case momentum and potential friction are proportional.

However, a street motorcycle is capable of stopping so hard that the entire mass is held by one tire. When this happens, the vehicle has acheived maximum stopping power based on mass as the limit (harder braking would flip the bike forwards). Both the tire and brake had more friction.

In a car, this almost never happens. A car will lock the wheels before it will lift the back end in the air. In this case the friction between the tires and the road is the limiting factor, the brakes could still apply more friction.

Here we have a case where momentum is not proportional to potential friction. The motorcycle is using more potential friction than the car. The difference is the tires. The reason you need better tires on the motorcycle is that it will only stay upright while the wheels are spinning. Consequently, locking the wheels it not an option. Conversely, locking all four wheels on a car is the fastest way to stop it.

John