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Way way way way waaaaaaayyyy too many variables for there to be any rule of thumb.

Everything from suspension stiffness to tyre specifications will have an impact.

 

My advice is to decide on what body modifications you are happy to perform, then fit as big rubber under it as you can if you are expecting serious torque (hp means dick) levels. You have the advantage of a light car, and I have seen 245 semi-slicks successfully fitted under std rear S30 guards.

 

All depends how the torque curve is too. If the engine gains 400ft/lb in 500rpm your tyres are going to be hard pressed to keep up. What engine management do you use? A throttle/rpm based increasing boost limit might help.

 

Dave

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Hey thanks for your comments Dave, you do raise good points, I just remember reading a chart one day that had a list of tire widths to power level. Was just intended to be a general guide. I think it was published by Hoosier or someone.

 

I was just interested as i dont want to end up over tiring the car if i dont need to.

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Nice question :) For circuit work using 8" wide rims I've found 225 medium compound R tyres fine, thats with the S30 and say 250whp and turbo torque. For ordinary road use 7" rims with say 205 tyres would be fine.

 

Taking into account the practical limitations imposed by the authorities and car manufacturers, for circuit work I'd go for the widest that will fit. The wider the tyre the softer the compound you can use the more grip you will get, its that simple. But the rim width has to be appropriate to the tyre size, some people try to run wide tyres on narrow rims, not sensible.

 

If/when my 500whp project eventuates, I'm planning on 10" wide rims for circuit work on probably a 16" diameter wheel. I consider 10" a minimum requirement with that sort of power.

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Nice question :) For circuit work using 8" wide rims I've found 225 medium compound R tyres fine' date=' thats with the S30 and say 250whp and turbo torque. For ordinary road use 7" rims with say 205 tyres would be fine.

 

Taking into account the practical limitations imposed by the authorities and car manufacturers, for circuit work I'd go for the widest that will fit. The wider the tyre the softer the compound you can use the more grip you will get, its that simple. But the rim width has to be appropriate to the tyre size, some people try to run wide tyres on narrow rims, not sensible.

 

If/when my 500whp project eventuates, I'm planning on 10" wide rims for circuit work on probably a 16" diameter wheel. I consider 10" a minimum requirement with that sort of power.[/quote']

 

Thanks for your comments Rich, I have been thinking of running 10" in back and 9" up front. I am thinking that it shouldnt be too far out of the way. I should be well and truely up around that power level and bags of torque without too much trouble. :-)

 

Cheers.

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I have 285/30/18 P Zero Corsa's, (not a Z, but similar weight). I have an LSD and at track days I have pretty good stick coming out of corners in 2nd and if I'm going in a straight line it pretty much stays hooked up in that gear. There are a few places that I can still break loose in 3rd on some high speed corners if I get into it full throttle, but in a straight line I'm fine. I last dyno'd at 357 rwhp, but that was with a smaller turbo. With this set up I think I have around 400 or so and that's what I had at the track the last time out. I can still break loose easily in 1st on boost and you have to modulate the throttle in 2nd if you're not going straight. Still, alot more control than I had with an open diff and 245 street tires. My torque peak is around 3750 rpm's, if you have a V8 with more torque down low results may vary.

 

Anthony

http://www.cardomain.com/ride/674663

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Ideally, you need:

 

Formula A

0.788 mm of tire for every 1 hp

 

Tire Width = .788 X HP

 

 

Fromula B

28.6 mm of tire for every inch of rim

 

Rim width = Tire width / 28.6

 

All formulas according Cyrus, for use on RWD. Remember the first word "Ideally", before anyone responds about what they can get away with.

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BTW... I'm going to start this month on a customer's 240Z that will be a national level autocross car. Our first decision, before any other work, was selecting a tire to build the car around: 285/30-18 Kumho V710s. All other chassis/suspension decision are based on that specific tire.

 

We wanted at least a 275 width tire but overall diameter had to be below 25" because we cannot relocate the suspension mounting points per the class rules. The 30 series sidewall worries me so the suspension wil have to be made as compliant as possible and a lot of money will have to be spent on shocks.

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Nice question :) For circuit work using 8" wide rims I've found 225 medium compound R tyres fine' date=' thats with the S30 and say 250whp and turbo torque. For ordinary road use 7" rims with say 205 tyres would be fine.

 

Taking into account the practical limitations imposed by the authorities and car manufacturers, for circuit work I'd go for the widest that will fit. The wider the tyre the softer the compound you can use the more grip you will get, its that simple. But the rim width has to be appropriate to the tyre size, some people try to run wide tyres on narrow rims, not sensible.

 

If/when my 500whp project eventuates, I'm planning on 10" wide rims for circuit work on probably a 16" diameter wheel. I consider 10" a minimum requirement with that sort of power.[/quote']

 

Hi everyone, I'm on holidays in Europe, currently in Prague on my laptop - what a great world eh that I can keep an eye on everyone and what they are up to as I travel away from Perth W Australia. I have just bought a 1973 240Z ex Melbourne and also a R32 GTR Half cut ex Sydney. I pulled the GTR motor out just before I came on this holiday & plan to get stuck in on my return. The 240Z currently has 15 inch x 7 inch Ultralites (Aust Minilite copy). I was thinking of putting Ultralite 15 x 8 inch on the back with as big as possible tyres without changing guards or anything else - ? 235/245 - have any of you Aussie guys used the ultralite rims - 15 x 8's?

regards Mike

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....................... I was thinking of putting Ultralite 15 x 8 inch on the back with as big as possible tyres without changing guards or anything else - ? 235/245 - have any of you Aussie guys used the ultralite rims - 15 x 8's?

regards Mike

 

Performance Superlites? The new design 15x8 Superlite is quite good, the old design was less so, they are not 'super light' though, far from it. With the right offset they apparently will just fit under the S30, common as muck though, everyone is using them.

 

Take a trip to the US or Japan and get some decent wheels :)

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Performance Superlites? The new design 15x8 Superlite is quite good' date=' the old design was less so, they are not 'super light' though, far from it. With the right offset they apparently will just fit under the S30, common as muck though, everyone is using them.

 

Take a trip to the US or Japan and get some decent wheels :)[/quote']

 

Thanks Richard, its just that I already have superlites on the car, so getting 2 8's for the back is a cheap option - they also are at least period, but I agree there are a lot of nice wheels out there now

Mike

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No width difference. Run the same front and back.

 

Oh ok, I am very new to Rear wheel drive race cars (have been running FWD cars)

 

Would it be fine to run 10" front and rear, 275-285 sounds huge for the front?

 

Please can you explain the theroy behind this, I saw someone else mention they prefer same size wheels and tires all round on sports car in another post recently as well.

 

Thanks John, appreciate it.

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If your car has a 50/50 weight balance then equal sized tires on each end help you achieve neutral handling in steady state cornering. Equal size tires also help reduce understeer under power as your car exits the corner. Some people argue that wider tires in the rear can help you put more power down on corner exit, but in the powerful 240Zs I've built and raced a wider rear tire would not have helped.

 

Its kind of a viscious feedback loop:

 

1. A wider tire allowing more power to the rear tires also increases rear weight transfer.

2. This pulls weight off the front wheels and reduces their grip.

3. Reduced front tire grip under power exiting a corner increases understeer.

4. Understeer on corner exit requires a reduction in power to correct.

 

There's a fine balance between available power and front and rear grip. You want enough rear tire to allow full power before corner exit and enough front tire to keep gripping as they lose weight transfer. One way to know things are setup pretty well is when you just barely lift the inside front wheel on corner exit. Like this:

 

RODatLVMS.jpg

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