Rim Performance 17" Vs 18"

[Roy]

OK, i am finally going to pony up and buy some wheels spefically with the goal of getting the widest rubber under my car as i want to try and squeeze a lap time out of it

So i am planning oin running a good quality 8.5" front rim and 9.5" rear rim with 235/40/17 RE55s on the front and 245/40/17s on the rear.

Now the question is that over a single lap, would you expect to see a car running the same tyre and rim width combination, only 18" run a quicker time? Allowing for the fact that the 18" rim/tyre package is bound to be heavier?

My gut feel is there wouldnt be a real difference, only i know of a person with an R33 GTR who changed from R33 gTR rims to R34 GTR rims and dropped over 1.5seconds off his best time. What i cant do is quantify how much of that is the diameter change, just more practice, or the fact he went from 255 to 245 on the 9" wheel????

Ppls thoughts?

[adam-__-]

I've got a good video you can watch that might answer your question. It was some best motoring video using an evo which had 'normal' wheels and wheels which were slightly lighter. There wasn't a big difference.

I'll try to get you the video.

[adam-__-]

I think it was in this video. I tried to look for it on youtube for easier viewing, but I couldn't find it. It's an interesting video.

Edit: It's free to sign up.

Edited February 18, 2008 by adam-__-

[scathing]

According to this comparo by Option, rigidity is more important than weight.

Not only did it punch out a better absolute lap time, but it was better on the tyres and deformed less so the suspension could do its work better, which means you could probably run more consistent fast laps.

Since I doubt you're going to be doing one-lap dashes and pulling over to let the tyres cool before trying to set another hot lap, consistency might be relevant to your goals.

Assuming the rims are the same type and the rigidity is constant, it boils down to whether the smaller sidewalled tyre on the bigger rim makes enough of a difference.

The smaller rim will have cheaper tyres, of course, which means you can afford to get more laps in to try running a faster time.

[Cubes]

If that were the case why doesn't F1 and v8 supercars etc run super low profiles? Or does it have more to do with regulations more than anything?

[djr81]

If that were the case why doesn't F1 and v8 supercars etc run super low profiles? Or does it have more to do with regulations more than anything?

F1 cars are irrelevant.

V8's run 17" because thats what the regulations say. That started in 1993, so it is an old reg based around old tech. No one else of note goes this small on diameter.

If you take a look at the old super tourers the ran big wheels diameters because their regs said with a flat/no tyre the body work couldn't scape ont he ground.

As for Roy's question. Hmmmm.

I suspect the answer would be yes. The weight thing isn't exactly obvious as the tyres themselves weigh as much as the rims. On the up side you get loads of room for rotors/calipers. On the downside it will probably cost you near on $500 each time you change tyres if you go for 18's rather than 17's. Plus the rims themselves will be more expensive.

Money better spent elsewhere?

[Ant97GTR]

Mate, while there would be a tiny difference in lap time if you ran EXACTLY the same lap back to back (which is never going to happen) I'd say in a real world situation it wouldn't make much of a difference. That 1.5sec in the 33gtr (and I know who you are talking about ) would be down to the driver having more experience plus perhaps he was trying different suspn settings.

More of a consideration is cost of tyres, maximizing the track and what brake setup you can fit under the rim.

[Sydneykid]

OK, i am finally going to pony up and buy some wheels spefically with the goal of getting the widest rubber under my car as i want to try and squeeze a lap time out of it

So i am planning oin running a good quality 8.5" front rim and 9.5" rear rim with 235/40/17 RE55s on the front and 245/40/17s on the rear.

Now the question is that over a single lap, would you expect to see a car running the same tyre and rim width combination, only 18" run a quicker time? Allowing for the fact that the 18" rim/tyre package is bound to be heavier?

My gut feel is there wouldnt be a real difference, only i know of a person with an R33 GTR who changed from R33 gTR rims to R34 GTR rims and dropped over 1.5seconds off his best time. What i cant do is quantify how much of that is the diameter change, just more practice, or the fact he went from 255 to 245 on the 9" wheel????

Ppls thoughts?

If only it were that simple.

On one of the Production Cars we went from 16" to 17" tyres this year. I had to make quite a lot of suspension hardware changes (eg; softer springs) and the geometry was quite different (eg; less camber).

On one of the Improved Production Cars we went for 17" wheels to 16" wheels and it was 0.75 secs faster at OP, without changing anything. Why? Because the compound in the 16" A048 is softer than in the 17".

There is no quick/easy answer here, all I can suggest is that you do your homework on tyre compounds first. Then make sure you have sufficient budget to change at least spring rates and allocate the time to optimise the geometry.

Cheers

Gary

[djr81]

If only it were that simple.

On one of the Production Cars we went from 16" to 17" tyres this year. I had to make quite a lot of suspension hardware changes (eg; softer springs) and the geometry was quite different (eg; less camber).

On one of the Improved Production Cars we went for 17" wheels to 16" wheels and it was 0.75 secs faster at OP, without changing anything. Why? Because the compound in the 16" A048 is softer than in the 17".

There is no quick/easy answer here, all I can suggest is that you do your homework on tyre compounds first. Then make sure you have sufficient budget to change at least spring rates and allocate the time to optimise the geometry.

Cheers

Gary

So could you reasonably say that with 18" rims you would look to less camber & softer springs?

[Sydneykid]

So could you reasonably say that with 18" rims you would look to less camber & softer springs?

Generally speaking, yes, assuming the overall diameter is the same (ie; lower profile tyres are used). Tyres are a spring and less sidewall means less spring (ie; higher rate). So when you increase the tyre's "spring" rate you need to decrease the spring's rate to compensate. Camber is not so straight forward, there is less sidewall to flex, so in a straight line (acceleration or braking) you need less camber to keep the tyre contact patch up. We also tend to run lower tyre pressures with lower profile tyres, which affects the rigidity as well. So, as usual, I let the pyrometer tell me how much camber to run.

Cheers

Gary