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53rd and 3rd

Tires. Volumes can and have been written on tires. The single point of contact between the car and the road. Round, rubber transmission devices that link the driver to the surface. I like tires. There is so much to be said, but not enough room to say it in. So let’s keep this session to tires and their relation to suspension design.

Before we lay any frame or design any suspension components let’s do what makes sense and choose our tires. Size, load rating and compound. What’s the load range? Are we allowed to run full slicks or only DOT compounds? How wide? How tall? Is there a limitation on size by sanctions? What size wheels offer our biggest selection of tires? Is wheel size limited by sanctions? The list goes on and on. Do we choose the largest tire we can for traction?

Keep in mind this applies to all kinds of cars from Formula cars to your daily driver. It’s applicable to all. For the sake of examples I’ll be limiting discussion to the chassis I’m currently designing.

Lets talk about the dynamics of a tire, that apply directly to the current build status I’m in. I’m limiting this to what I believe are the two main factors in understanding what exactly a tire does. It holds a vertical load and it outputs traction.

Vertical Load: simply put this is the weight on the tire.

Traction: how well the tire sticks to the ground.

As the vertical load changes, traction also changes. The increase in traction lessens as vertical load increases. Vertical load goes up, available traction decreases. Tire loading is a very dynamic thing. It is constantly changing as the car is traveling and weight is being shifted from corner to corner and front to rear. This all relates to a relatively simple to understand curve on a graph.

Remember, tires are a very, very dynamic component. I have only touched on two factors that deal with tires. There are many more.

Choosing a load range for your race chassis is not as scientific as it sounds. Afterall, we are limited to what tire manufacturers offer us in terms of tire sizes. You can’t call Hoosier and tell them you want a 245/45/r17 A7 in a different load range then what they offer. It’s not practical and the tires cost enough as it is.

The point is that understanding the input and output characteristics of your tires can effectively help you set up your chassis to gain the most traction when and where you need it. Doing a little math along with some deliberate weighting of the car can go a long way in increasing handling characteristics.

For the sake of proper design, use the largest tire you plan on running. This way there will be minimal questioning of variables when we switch from a 315 to 295 or smaller.The suspension is designed from the tires inward. That’s for the next episode though.

theDoc View All

Chassis builder, engine builder, cynic

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