Which is better a 17" tyre or an 18" all else being equal

[TMcRally]

Hi again

On a tarmac rally car, odviously not as smooth a road as a race track and sometimes quite rough, but more often pretty good roads. 25% of the time wet or greasy.

Assuming tyre OD and width and compound being the same am I better off with a lower profile 18" tyre or a taller side wall 17".

It's an MY05 STi, weighs around 1,600kg with driver and navigator in. Looking at using 255 wide tyres.

Thanks
Dave

 

[GregLocock]

Impossible to say, since you don't specify the OD and width. But, if you aren't going to take advantage of the 18 to fit bigger discs, go with the 17s (it is a crapshoot).

In an ideal world I'd suggest you ask your tire supplier for the cornering stiffness, tire load sensitivity, and wet and dry mu, if they can't tell you, switch tire suppliers.

In the real world that won't work since they won't tell you what the performance of their tire is, even if they know.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ufe777]

That begs the question; how do tyre manufacturers/car manufacturers specify or validate tyre characteristics?

 

[GregLocock]

We give them a spec (basically a bit like a Pacejka model) covering the things I've suggested and many others, they come up with a design, we drive it, and so it goes for 1 or 2 or 20 submissions.

The other alternative is that we find a tire we like and say - we want it like that, but with 5% more dry grip (or something).

That is usually a better approach.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[silverbullet1986]

Greg Locock, what manufactures supply that information? I've tried getting info last year from Falken and Hankook tires but they both told me they don't release that data. I was specifically looking for Temp vs mU, and slip angle vs mU.

 

[GregLocock]

So far as I know the only tire company with the guts to publish actual numbers is Avon, for some of their racing tires.

The rest will all tell you it gets 4 stars for handling and 3 stars for durability, or whatever, completely useless.

Of course at an OEM level we get a proper set of numbers for each tire, but that doesn't help you guys.

I would honestly have thought there was a marketing opportunity there, at least for performance tires. The trouble is the average weekend racer buys one set of tires every two seasons, so he doesn't have much leverage. I'm quite sure the big teams have the data, it costs you 4 tires and about 2 days on a Flattrac rig.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.