Semis - Recommended Size For 8"/9" Rims

[jmac]

Hey guys, I'm looking into getting a new set of semis for the Buddy Club rims I've got for track days. The wheels are 17", 8" width front, 9" rear, the usual deal for a GT-T / GTSt.

They currently have a pretty much rooted set of Dunlop Formula D01Js on them now (fronts down to the wear bars), 235/45 on the front and 265/40 on the rear (which *just* clears the guards).

Now in pricing some tyres today one of the recommendations from a tyre place in Brisbane was to go for 245/45 on all four wheels. I know we're only talking 10 or 20mm here but is this a good idea? A better idea than having different widths front/rear?

This place also recommended Falken RT615s but from what I understand these are a street/race tyre and I was thinking more along the lines of Bridgestone RE55, Dunlop D01J or DZ02G or maybe Yoko A048s - I'm pretty sure those Falkens don't really compare to them!

Anyway should I go diff sizes front-rear or the 245s on all corners?

[phunky_monkey]

personally, I'd be going for maybe a 235/45/17 up front and a 255/40/17 up the back. If you've got room for bigger rubber up the back you may as well use it.

The reason I bought 17x8 and 17x10 algernons for my car instead of the TE37's I was going to get was simply because I didn't want to run 245/255's all round

[Sydneykid]

personally, I'd be going for maybe a 235/45/17 up front and a 255/40/17 up the back. If you've got room for bigger rubber up the back you may as well use it.

But that's not the issue, the problem is the corner entry understeer generated by using narrower tyres on the front. In addition you are compromising the braking performance by limiting the front tyre size. The extra width on the rear means using a lower profile tyre on the rear, exactly what you don’t want. A more compliant sidewall is required on the rear for improved power down. Conversely you want a less compliant sidewall on the front for improved steering response.

The truth is you should be using 9” rims all round with the same size tyres on all 4 corners. Since you already have 8” rims for the front you need to use the right size tyre for that rim, hence 245/40/17. This then means using the same size for the rear to achieve some semblance of balance.

Hope that made sense

cheers

[jmac]

As usual The Man comes through for me.

Thanks SK.

[phunky_monkey]

I stand corrected

will the guards on a gt-t/gts-t accomodate 245's on the front??

[Sydneykid]

I stand corrected

will the guards on a gt-t/gts-t accomodate 245's on the front??

If the wheel offset is correct then they will.

cheers

[jmac]

So... how come it seems to be fairly common practice to put wider wheels/tyres on the rear then?

[AzzurrA]

wank factor?

[ellie]

One would suggest that there just might be a more scientific reason for wider rear wheels than the "wank factor", especially when it is common place on BMWs, Merc., Ferrari, Lambo, porsche etc. etc.

I would imagine the logistics would go along the lines of= A car only wieghs so much and requires a standard of tyre and width of tyre to brake and handle to the manufacturers specifications.

2. the rear tyre has a slightly different roll of handling also but trying to tame 250-300+Kw while doing it. Hence much wider. I Know on an M3 if a skinnier tyre is fitted on the back of the same make (michies usually) you can break the car into wheel spin in a straight line just by planting the throttle but with the correct size rim/tyre on, it won't.

I'm sure some one will correct me but cars that require wider tyres on the rear have a high power to wieght ratio and need the extra traction other wise the car becomes undrivable and useless to the average driver.

[phunky_monkey]

wank factor?

I'd say it has a bit more to do with traction rather than wank factor...

F1 cars run staggered wheel sizes.

[Sydneykid]

Weight distribution, R32/33/34's are very front heavy, the 4wd versions even more so.

cheers

[phunky_monkey]

what about cars that aren't so front heavy, such as 180sx's??

[Sydneykid]

M3, Ferrari, Lambo, Porsche

Those examples have fundamental differences to a Skyline;

M3 = 50/50 weight distibution and high power rear wheel drive

Ferrari = mid engine and high power rear wheel drive

Lambo = mid engine and high power rear wheel drive

Porsche = rear engine and high power rear wheel drive

what about cars that aren't so front heavy, such as 180sx's??

They have MacPherson strut front suspension, not double wishbone. So there are a whole pile of dynamic issues (camber change, strut stiction, SAI etc) that make a direct comparison quite complex.

cheers

[phunky_monkey]

cool, thanks

I'll be running 235's and 255's on my 180, guess we'll see how it goes!

[gazza750]

look into the legality of it .. by law ya sposedto have same size rim and tyre all round on ya car

[Sydneykid]

look into the legality of it .. by law ya sposedto have same size rim and tyre all round on ya car

Yep, that's the ADR. But Porsche, Ferrari, BMW, Mercedes, Lamborghini, Maserati, Honda (NSX) etc don't worry about it. It's one of those "letter of the law" but "we will ignore it" things.

cheers

[phunky_monkey]

I've been defected quite a few times, and have never had the same size tyres front and rear, never had an issue with it.

[mokompri]

They have MacPherson strut front suspension, not double wishbone. So there are a whole pile of dynamic issues (camber change, strut stiction, SAI etc) that make a direct comparison quite complex.

cheers

so what do you recommend for a 180sx with stock guards, in terms of wheel width and tyre sizes ?

[Sydneykid]

so what do you recommend for a 180sx with stock guards, in terms of wheel width and tyre sizes ?

The widest that will fit, same size front and rear and don't lower it too much, it really stuffs up the geometry.

Cheers

[mokompri]

same size ey, interesting cheers for the info