Race Cars Tire Size: Rear Vs Front ?

Hello,

My Grandson asked me the following question, and as I am not, frankly, into auto technology I thought I would take the liberty of asking here.

"Why, so often, on many different types of race cars, are the rear wheels so much bigger ?"

Didn't know what the right answer is.

Is it simply that the front wheels have to be smaller so they can turn easier, or a better engine/torque match with bigger wheels, or bigger wheels have more rubber on the road, or... ?

What's the right explanation ?

Much thanks,

Bob

Reply to
Bob
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Depends on the type of race car.

Straight line drag cars run slicks on the rear. They also don't have transmissions, just a multi disc clutch. The tires start out around 34-36". When the light turns green and the driver nail the throttle the tires absorb a HUGE amount of power and squat down. That increases the contact patch and changes the effective gear ratio at the same time. (Just like starting out in a passenger car in first gear increases the torque transfer to the tires) As the run progresses down the track the centrifugal force starts to build and the tires start to grow. Farther still they will grow to around 43-46" depending on the tire. During that change the overall gear ratio is also changing, just like in a passenger car. The effect is to keep the engine turning at it's peak power. The front tires are small to limit the amount of weight up front and limit the actual effect that a twitch of the wheel could cause during the run.

On a dirt track car like a sprint or modified the larger rear tires also help keep the engine in the power band. They are also used to help turn the car using "stagger" Most race cars have a solid rear axle and don't run a differential. By altering the diameter of the rear tires they can make the car always push toward one side or the other. You will also notice that one side usually has wider tires than the other. That is because of the weight transfer at speed. On a sprint type car especially this is VERY pronounced. They don't run a clutch or trans at all. Just a simple sliding coupler. With the rear tire stagger and short wheelbase they are VERY difficult to control, especially at low speeds. You basically steer them with the throttle.

On a street/strip car the idea is still gearing and contact patch. (Unless it's a trailer queen, then it is simply "the look")

You want the tires to be able to handle all the power the engine can make and transfer it to the pavement. The larger the tire the farther it pushes the vehicle for a given rpm. BUT those larger tires also take more power to turn. It becomes a balancing act. Get it right and you win races, wrong and you sit on the trailer a LOT...

Reply to
Steve W.

Here is a child-grade answer. This glosses over a lot of important things, but it is short.

  1. With a rear-wheel drive car, all of the power is on the back wheels, and your main goal is to accelerate quickly without losing traction, and to do that, you need as much contact areas as possible. So big, wide tires are a win for rapid acceleration.

You'll notice that cars designed for short runs like dragsters tend to have the biggest rear tires, because they are the cars where acceleration is most important.

  1. Again with a rear-wheel drive car, the front tires are used for turning, so what is most important about them is that they can handle side to side forces. A big wide tire doesn't help with that; a smaller skinnier tire will have less friction. You'll notice that racers that are designed for rally work will tend to have bigger front tires than those that are designed to go in a straight line.

Take a look at a modern rally car, a NASCAR car, and one of Don Garlits' vehicles and compare how the size ratio changes as the amount of turning and amount of time spent accelerating changes.

--scott

Reply to
Scott Dorsey

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