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?
Because the tires are what contacts the road
They transfer your inputs to go, stop, and turn from your motorcycle, a moveable, less than stable object, to the road, a solid fixed surface - the reaction of this results in movement of the motorcycle
What dynamics make this happen?
The tire's contact patch (the part of the tire that's actually touching the road) is held to the road by gravity
The contact patch also has other forces acting on it as well; surface roughness, loose debris, moisture, surface tension, and other forces all factor in, as well as the forces input by the rider (twisting on throttle, applying brake, steering & balance inputs)
What makes it all work is the ability of the contact patch to maintain enough traction (solid contact) with the road to overcome the input forces fed to it
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.
You keep the brake applied, wheel locked & ride it to a stop (it won't be pretty, but it shouldn't hurt you)
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?
The tread material or compound needs to be soft to grip the road, yet hard enough to last more than a couple hundred miles
The tread design needs to be able to channel water away from the contact patch to prevent hydroplaning (when the tire's contact patch doesn't connect with the road surface, instead it contacts a layer of water on the road surface)
The tire needs to be resistant to damage from road debris
What can we do to maintain safe tires?
Check your tire pressures frequently & maintain correct inflation pressures
Inspect your tires regularly for damage and wear
Words of Safety