Tire Traction

What are the limits between the tires and road when turning?


An equation is not needed to tell you that given enough force on the tires as you are turning that you bike will start sliding out from under you. In this section, we want to qualitatively describe what happens and quantitatively provide some numerics as to when that happens.

What are the road coefficients of resistance?

When moving in a straight line, we know we used the Coefficient of Rolling Resistance to describe road resistance. But when our cycle begins to slide out, it is not rolling, rather it is slipping. We started out discussing resistance in general in terms of static and sliding coefficients, and then added in rolling resistance. When a cycle begins to slide out, we have return to these first two road resistance coefficients found in Physics 101 text books.

The Coefficient of Static Friction measures how much force is needed to start an object sliding from rest. The Coefficient of Sliding Friction measures how much force is needed to keep an object sliding once it has started. In terms of cycling, the static coefficient is appropriate to when the tire starts to slide and the sliding coefficient is appropriate to the tire continuing to slide. Here are some values on dry concrete: Static= 1.0, sliding = 0.8, and rolling = 0.002.

What is moving the cycle in a cornering circle?

We said earlier that anything moving in a circle is being “pulled” towards the circle center, but we know nothing is pulling on the cycle. So where does the cornering force come from?

The answer is a force can be either a “pull” or a “push.” When cornering, the force turning the cycle is the road pushing back against the tire which is acting upon the cycle. As simple as that sounds, turning involves more than the road pushing on the tire, we have an issue of balance as the cycle wants to continue to move in a straightline resisting the turning motion. We will talk more about lean in a bit.

How much traction do I have when turning?

How much traction can you count out when cornering? By traction we mean how much force is needed to cause the tire to start sliding? We know that road resistance forces are determined by multiplying the object by a fraction determined by the appropriate coefficient.

Let’s get a feel for the tire slip point in a static situation. Assume a 166 lb, block of rubber sitting on a dry concrete road and want to start it sliding. How much force would we need?  The coefficient of Static Resistance is 1.0.

F = 1.0 * 166 lbs = 166 lbs

This gives us an upper limit to how much force can be applied before the tires slide.

What happens to traction when a cyclist is leaning?

We will have a lot more to say when we get to why a cyclist must lean into a turn. But for the moment, remember this. As  a cyclist leans into the turn, the relative portion of their weight into the road contributing to the friction decreases. You might think a cyclist who is at nearly parallel to the road. Virtually no weight is pulling the cycle into the road.

Tire Slip Limits for an Elite and Recreational Rider as a function of Lean Angle. Reagan Zogby

Traction Takeaways

Here are the key takeaways:

  • Traction is a function of the Static Coefficient of Resistance. Once the wheel starts slipping, it gets easier to continue.
  • The amount of traction increases the heavier the weight of the object.
  • As a cyclist leans, the amount of force pushing the tire into the road decreases reducing the tire traction.

Next Topic:   Cornering Lines

Cycling as seen through the eyes of elite cyclists.