Tyre Options..

[Tim]

I'd just fit the same sizes and look at addressing the understeer problem with geometry.

 

[willgbr]

Don't worry about bigger tires on the back to the front, keep to the same size and spend time on your suspension set-up. The combination will help to reduce understeer and you'll find it'll drive better. :thumb:

Played it safe and ordered 4 x 215/45/17, thanks for everyones help

 

[PobodY]

it wont cause centre diff to wind up george
it will only do that if you use different size wheels or tyre depths front to rear
ie: 205.40.16 front and say 205.40.17 rear as the wheels will be turning faster on front than rear.
width will make no difference at all

Yeah, but what I'm saying is that 205/40 is a different size to 225/40 - the width only makes a difference because 40% of 225 is more than 40% of 205. At some point it will make a difference, but 185/50 is about the same as 225/40 so you'd get away with that.

I think that unless you go for a really big difference it's not enough to have an effect. - Probably this is one of those times where the theory doesn't reflect the reality of it... and the little bit of difference is probably enough to preload the dif without cooking it.

 

[MarkTurbo]

The car is mostly for street use

I thought this was going to be a serious track car or something along those lines. Whatever made you decide to build a car with 500-600 bhp to use on public roads? Having that sort of power in a road car is totally pointless in my opinion ;-)

 

[willgbr]

Well its pointless to have anything other than a diesel shopping trolley going by your logic, we are doing this for fun.

 

[Tim]

You'll not need to worry about understeer on the road, just pitch it in correctly.

600hp is pointless on the track too imho but each to there own, building something like this is half the fun anyway so good luck with it

 

[pulsarboby]

Yeah, but what I'm saying is that 205/40 is a different size to 225/40 - the width only makes a difference because 40% of 225 is more than 40% of 205. At some point it will make a difference, but 185/50 is about the same as 225/40 so you'd get away with that.

I think that unless you go for a really big difference it's not enough to have an effect. - Probably this is one of those times where the theory doesn't reflect the reality of it... and the little bit of difference is probably enough to preload the dif without cooking it.

sorry george but you are wrong there!
the width of tyre is totally irrelevant......its the rolling radius difference front to rear that will upset the diff, or should i say differences on each axle front to rear as the wheels will not be turning at the same speed so therefore cause the centre diff to wind up eventually.
this is why the cars must be towed with all 4 wheels on the deck.

its irrelevant now anyway as wills gone for same size boots.

 

[PobodY]

I think we're talking at cross purposes Bob; I agree that the width makes no difference its self. What I was trying to say that as the sidewall is a function of the width, to match the radius you need a lower profile as the width goes up. All I was trying to say is that 40% of 225mm is more than 40% of 215mm so the wider tyre has a bigger side-wall (and therefore a bigger radius).
Another combination that would work is 185/55 with a 225/45 to give just a small difference between them.

As you say, it's academic and I'm not sure it really makes any real difference when added to the 16 or 17 inches of the rim (406 and 431mm respectively).

 

[bracpan]

If you look on say the Toyo website they give the radia of each tyre, as Podody say's the radius is different on the various sizes of tyres. Also if you want to fit wider tyres they should be on the front not the rear as its understeer you want to cure not oversteer.. so the front needs more grip and the rear less...
Personaly I would always fit the same size front to rear...but play with the compounds of tyre if you have a choice..
Phil