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


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Bob wrote:
> 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

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...



--
Steve W.

Bob wrote:
> 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.

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.

2. 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

--
"C'est un Nagra. C'est suisse, et tres, tres precis."