Hypothetical question time (okay... how long is a piece of string question ).

Assuming a vehicle weight of around 600kg and around 400 ponies under the bonnet, then are there any general rules regarding the required tyre width at the back?

13" wheels are my preferred choice of rim on a Se7en, but the rubber sizes seem to stop around the 245 mark (Avon CR500's). Assuming a premium tyre is chosen, then would such a width really be all that suitable for such a power-to-weight ratio?

Sorry if this doesn't make much sense... I've been up all night with backache and am doped up to the eyeballs .

With that much power and so little weight, i would probably be looking for a bit more rubber on the rear.

Having said that a well balanced car with narrower tyres would be better than a not so well balanced car with wider tyres.

Depends on camber change at the rear with squat - if there's a lot of camber change then width won't count for much.

Also I would tend to think that the wider you go the lower the tyre pressure you need because of low weight - in fact you might have to be so low to stop tread bulge (and hence negating the width) that you have problems ensuring that it doesnt break bead seal - bearing in mind that road tyres of that size are made for much heavier cars and hence hiigher pressures.

This is where I would talk to the tyre manufacturers technical advisors - make sure you get the engineer who understands the problem and not just the guy on the phone with a manual to page through. Have all your technical info at hand like corner wheight, diff open or closed and camber change when you talk to him.

Regarding tyre widths, much as there is a case for wider tyres to lay down the power, there is also the need to balance the grip to the nature of how the car handles. I'm not saying that there is such a thing as too much grip, but larger tyres can certainly cause a light car to be a worse car to drive. You want it to stay nice to drive,
Have you considered the diff you will use in this scenario ?
In situations such as tight hairpins, no amount of wide rubber will be as effective as a good LSD. Fix the traction issues before it gets as far as the rubber on the road.
Spring and damper rates are equally as important at taming all of that power.

Here's a car with a mere 265bhp from an all alloy ultimate spec SBD Vauxhall engine. Garry used 10" wide slicks on this car. Now, in this pic, he certainly does have traction issues, but he had marvelous traction off the line and it won lots and lots of events (Scottish Champion in the year of that pic.)
Come along to some events bext year, speak to the people that know, the people that are making setups like yours work, in real life. There are lots of them and in my opinion, you will find that laying down 400bhp is not purely down to tyre width, it starts further up the chain.

Cheers guys... some interesting stuff there!

Where can I find event details Zil?

Agree with the Zilmiester except that you can have too much grip on the rear of a car. I have this problem in my race car in certain circumstances where the rear grip pushes me too hard out of corners causing some understeer.

It could on the other hand just be my terrible driving!

It's all about balance!

To get traction in that picture he needs 30" wheels to reach the floor

quote:

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Originally posted by scootz
Cheers guys... some interesting stuff there!

Where can I find event details Zil?

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wider wheels does NOT allways mean more traction! Wider wheels will mean cooler tires, but since our cars are very light, they will heat up less anyway.

Please read this aricle for you make any overhaste decisions.
http://www.carbibles.com/tyre_bible_pg2.html

245's are pretty wide, and as above, too wide can cause understeer.

With the addiditonal traction, youll exert additional stress on the drivetrain.

At least if its wheelspinning, its not trying to wring the propshaft and diff.

I would get 245 R13's, and then adjust the camber, spring rates etc to get the squat, camber and launch correct. Then use the throttle and the clutch to control the wheelspin.

Here's a video I recorded earlier this year.
It's a single seater, so entirely different weight distribution, but I still think illustrates the point.
Engine is a N/A Cosworth XD, don't know the power, but let's assume at least the 400bhp mentioned above. I think it's much in excess of that.
First pass on the video is at full speed, I then slowed it down to show what's happening to the driven wheels under power a the corner exit. These might be 12" wide slicks, not 100% sure.
Give it 5 minutes for youtube to process the video.

http://www.youtube.com/user/zilspeed#p/a/u/0/fX7XW9I0ZM8

Discuss at your leisure.

Once again the LCB faithful do me proud... cheers!

I phoned Avon (edit - twice as it happens as on reflection I could not believe them the first time!) with almost the same query - 40/60 weight balance and 400/400 bhp/ftlb and their advice was 9/21/13 front on 10" rim and 10.5/23/13 rear on a 12" rim.

Single seaters with same engine power run considerably wider rear tyres - I noticed that at Kames the Robbs were running 12" tyres.

However I guess the final choice depends on the roll resistance front/rear and the traction as above really.

Note the aspect ratio greatly affects the traction - the higher the aspect ratio the higher the traction/braking ability.

Also bear in mind that if the c of g is low with a rearward weight bias a significant proportion of the braking is on the rear tyres approaching 50%.

Some say that a car design starts at the tyre availability and progresses from there - not arse about tit as I did!!

Cheers

[Edited on 31-12-09 by v8kid]

Quite right , wheel and tyre availability should always be one of the first things to select before even drawing the chassis .

its worth noting that if you go wide the available contact patch goes wider , but there is less contact patch available fore/aft , so you need to increase the diameter aswell .

the grip also comes from compound selection , suspension geometry , springs/dampers etc .

you need the least amount of spring rate you can get away with , so that weight transfer is gradual and not instantaneous asking the tyre to do all the work (a bit like a go kart) .