The effects of wheel diameter

[AU N EGL]

Originally Posted by Notso Swift

AU N EGL,, Depends on the circumstance some classes have rim width limits, so a 255 would be worse on an 8" or even a 9" wheel than a 225.

Absolutly. Rules dictate most everthing.

[dtype38]

Changing to a big rim with low profile at the mo. I have a 1000kg rwd car with near 50/50 balance. I run my current 225/60R15's at 23-24psi. Anybody got any idea on what sort of pressure I might try as a starting point on 235/40R18's on the same car?

[trikesrule]

dtype what sort of psi are they pumping up to when hot (the 15')? And do the bigger rim tyres have the same circumference?......trikes

[dtype38]

I've never got a "hot" tyre pressure reading on my current tyres, its on my (very long) "to-do" list. If it helps, I also run some Yokohama 008Rs of the same size and I start them off at 26-27psi cold. Yes the 888s are the same circumference, and I'm only really looking for a first guess that won't cause anything silly to happen. I've never used tyres below 60 profile and I really have no idea where to start on a set of 40 profiles. Soon as I'm up and running, I'll then do some tyre temperatures to tune the pressure. Thanks.

[Chris Y]

I would assume (perhaps incorrectly!) that a lower profile tyre would need less pressure, as there's less sidewall to support and/or flex, potentially less heat buildup, etc.

Gut feeling, rather than scientific reasoning

[dtype38]

From my little understanding, I believe that the low profile means that the side wall is relatively stiff compared to the wide flat tread face. This would suggest that they are much more critical on tyre pressure to prevent the tread face becoming convex or concave and distorting the contact patch. Hence my question.

Oh and my tennis raquet turns me into Ken-do Jag-asaki :-)

[norman-normal]

F1 uses 13" wheels this tells me that wheels need only be large enough to clear the brakes. Very large wheels have a large rotating mass and limit braking as well as handling and accelerating, not to mention fuel mileage.

There is a very common misconception about "foot print". A pneumatic tires foot print is dependent on only two variables: the downward force (weight) and the air pressure. If those two variables are the same, the area of the foot print will be the same, with any tire. But, the foot print will vary: a narrow tire (tyre?) will produce an elongated FP and a wide tire will produce a wider FP which is desirable for road racing because it offers more resistance to sliding sideways.

The low profile tires have very little side flex (making 4 wheel drifts almost a thing of the past) they will however, help keep the entire with of the tread in contact with the road.

As I see it the value of the larger diameter wheels, as Al said they "look soo coool" .........I got Racing hart C-5'S 17X8 F and 18X10 on my supra.

[Al Weyman]

The only thing wrong with the F1 argument is its in their rules if not I bet they would go to bigger rims like Le Mans type cars.

[norman-normal]

I think the reverse would be true: if the Le Mans cars could Had the means to use F1 brakes on their cars they would go to smaller wheels.
In all top line road racing the wheels are big enough to clear the brakes, no more. The big wheels started as "bling" here in Southern California by the street racers, mostly Honda owners who were self conscious about their "baby buggy" wheels, nearly twenty years ago.

[Notso Swift]

Originally Posted by Chris Y

I would assume (perhaps incorrectly!) that a lower profile tyre would need less pressure, as there's less sidewall to support and/or flex, potentially less heat buildup, etc.

Gut feeling, rather than scientific reasoning

From my experience you are basically right. Most of the "core" heat in a tyre comes from sidewall flex, not from friction on the tread surface, that surface temp dissipates as quickly as it gets there!

When I went from 60s to 50s that is what I did, however, I had a hot pressure and temperature goal. With experience (wear patterns and optimising temps) I found that I could run a lower hot pressure as well.


Dtype, I think you really do need to have a hot pressure in mind, temperature requires another person to help you (to be accurate), but you can do pressure by yourself as they don’t drop off that quick (not with road based tyres, at least) If you have a good (motorsport) tyre supplier they should be able to point you in the right direction as to a goal.

With Yoko AO32’s in 60’s, for instance I can say aim for 36 to38 (psi) hot, while AO48 in 50’s around 34 to 36 does the trick. Combine that with a 70 degree C tyre temp and you will be getting the most out of them.

[Al Weyman]

Right well if thats the case and it makes sense then maybe this is the real advantage of the lower profile tyres, less tempreture fluctuation. I remember running a 255x50x16 Falken road tyre (control tyre) which had quite a tall sidewall and I had to set the tyre pressure to be either good in the first half of the race and go off towards the end or visa versa. Now the pressures would rise 15 psi in the race and if I fitted a 35 or 30 series 18" then maybe there would be less heat generated so the rise in pressures would be nominal and thus mean I would start the race more or less as I finished without the pressure fluctuation causing the tyres to over inflate and go off after about 7 laps.

[greenamex2]

This is also why a lot of tyre manufactures have a 'softer' compound for the lower profile tyres in a given range.

[Al Weyman]

Even more performance advantage then.