November 2000 Safety Thoughts

Tires

by Dave Hansen, Safety Officer

When it comes to maintaining control, what is the most important part (besides the rider) of your motorcycle?

Why is this?

What dynamics make this happen?

Let me give an example of how these forces interact:

Let's see what happens to the tires' contact patches when the brakes are applied in a fast stop situation. You're traveling at a constant speed with a passenger, when a car suddenly pulls out in front of you. You apply both brakes. What happens?

Before applying brakes, the weight of you, your passenger & the motorcycle are distributed maybe 35% on the front tire, 65% on the rear tire. That means there's more downward force applied to the rear tire than the front tire, potentially providing it with more traction. Upon initial brake application, because of the weight distribution or bias toward the rear tire, more braking force can be applied to the rear tire than the front.

But that's only initially, only momentarily, because as soon as braking begins, other forces are also acting that will change how the weight is distributed between front & rear tires. That's why the front forks compress when you stop. The bias shifts to the front tire and off of the rear tire. Now the front tire may have 75% or even 95% of the vehicle mass acting upon it. This means the front tire gains more traction and can support more, most or even all (sport bikes or racing bikes that do "stoppies" - lift the rear tire completely off the road while braking) of the braking effort.

The rear tire no longer has enough traction to maintain the same braking force it could upon initial application. What happens? The rear wheel locks up when the force of the brake pads against the brake disk exceeds the force of the tire against the road.

At this point, one of two things will happen.

  1. You keep the brake applied, wheel locked & ride it to a stop (it won't be pretty, but it shouldn't hurt you)

  2. You can panic, release the brake, regain traction with its' resultant steering input, and high-side (crash hard)

The important point here is to understand the dynamics of what happens when braking in order to correctly apply proper braking technique and avoid getting into trouble by locking your rear wheel/tire during hard braking situations.

What are desirable characteristics of good motorcycle street tires?

What can we do to maintain safe tires?

Ride safe & enjoy what's left of the riding season.

Words of Safety