Do we want the sidewall straight or stretched on a road race track car?

[darkostoj]

Hey guys, I never really learned the science between the proper width of a tire on a rim for a all out road racing/track application.

Do you want the rim a little wider so the sidewall is slightly stretched?

Or do we want it where the side wall is straight up and down?

Most tire sizes are usually 35 or 40 aspect ratio, does the sidewall height affect this?

I am putting larger tires on our Rolls Royce race car and want to try and fit a 295/40/18 RS4 for the endurance racing and a 305/680/18 pirelli slick. My thought was try and get a wheel width that can fit both.

Thanks!

[mc84_zz4]

Tire sizes:
So a 295/40/18 translates to 295mm width, 40% of that is sidewall height, and last is diameter of your wheel, so you can calculate the height by multiplying 295 x .40 = .118 mm
If you want inches 118mm / 25.4 = 4.64 “

Or you can use a nice tire/wheel calculator.
Here is one that has all in one shot, great when going +1, +2 in size..
https://www.rimsntires.com/specspro.jsp

I have read some in the discussion of tracking your car you want a slightly stretched tire, so to avoid tire ‘rolling over’ the sidewall.
I would leave to others more experienced to share their knowledge on that.
I would guess a 1/2” to 1” narrower tire, which is 12-25mm
Also depending on tire brand width and sidewall, you may need more pressure in your tires.

The other component is wheel size, an 18x10 wheel is 10” at the mounting surface, but is around 11” from external wheel edge-to-edge.
usually a 245 tire is used on a 8” wide wheel,
a 275 on a 9.5” wheel
a 295 on a 10.5-11” wheel
a 305-315 on a 12”

so now the hard part is fit as wide a wheel/tire combo into your fenders and can still turn.
Usually a ‘square’ tire setup (same width front/back) has best behavior.

HTH

[darkostoj]

Tire sizes:
So a 295/40/18 translates to 295mm width, 40% of that is sidewall height, and last is diameter of your wheel, so you can calculate the height by multiplying 295 x .40 = .118 mm
If you want inches 118mm / 25.4 = 4.64 “

Or you can use a nice tire/wheel calculator.
Here is one that has all in one shot, great when going +1, +2 in size..
https://www.rimsntires.com/specspro.jsp

That is a really nice tire/wheel calculator.

However, my questions is related to the wheel rim width in comparison to tire width and sidewall height. What is the most optimal rim width compared to tire width? Should the sidewall be stretched (wider wheel) or straight (wheel matched sidewall with perfectly)

[mc84_zz4]

I hope the updated post has something more useful, I usually end up timing out, even losing posts if I take too long.

HTH

[Flash68]

This is what you're looking for.

https://motoiq.com/how-to-properly-select-and-size-tires-for-performance/

[stab6902]

This is what you're looking for.

https://motoiq.com/how-to-properly-select-and-size-tires-for-performance/

Great link!

[mikedc]

I am putting larger tires on our Rolls Royce race car and want to try and fit a . . .

Wait . . . your what?

[darkostoj]

This is what you're looking for.

https://motoiq.com/how-to-properly-select-and-size-tires-for-performance/

Wow! This was extremely helpful and exactly what I needed! Huge Thanks!


Wait . . . your what?

Hah...yeah. www.instagram.com/rollsroycedoesntcare

full build thread here, it's a pretty wild and entertaining read. I built the whole car in 2 months with no previous experience.
https://forums.24hoursoflemons.com/viewtopic.php?id=36831

[JustJohn]

Are you assessed a lap penalty for that? Clearly not a $500 car.


Wow! This was extremely helpful and exactly what I needed! Huge Thanks!



Hah...yeah. www.instagram.com/rollsroycedoesntcare (http://www.instagram.com/rollsroycedoesntcare)

full build thread here, it's a pretty wild and entertaining read. I built the whole car in 2 months with no previous experience.
https://forums.24hoursoflemons.com/viewtopic.php?id=36831

[darkostoj]

Are you assessed a lap penalty for that? Clearly not a $500 car.

They thought the car was so crazy, they told us we could do whatever we wanted...

[TheJDMan]

My experience with wheel width vs tire tread width goes back to my kart racing days. Back then we were required to run a spec tire but there was no rule on the wheels we could use. So we had a selection of wheels in widths ranging from narrower than the tread to wider than the tread. We used these different wheel widths as one of our tuning tools depending on the grip level of the track and the feel the driver wanted. A narrow wheel would give a softer response because it allowed the tire sidewalls to flex more where a wide wheel would make the sidewalls stiffer and give you a faster more precise response. But in general, a wheel that is the same width as the tread will allow the sidewalls to flex more than a wheel that is wider than the tread. Both will work but they will feel different.