[Silverspeed]

Was wondering why turbo's have such huge rear tyres?
Even the new mk2 has gone from 305 to 325 right? Why?

The GT-R has only got 285 and is it because it's more efficient or has it more to do with the design of a porsche and their weight distribution?

Thanks.

 

[kpkpkp]

I would say it is part of the 911s design and some of it must be due to the placement of the engine in the 911. I had 997 turbo wheels on my 997 C2S and considering the 2S is RWD the levels of mechanical grip are amazing. I would say that this applies to the turbo as well.

I think the GTR is more efficien at distributing it's power so does not need such wide rubber.

You really need one of the techies on here to give you the rocket science explanation.

Kp

 

[Guy]

The Turbo has 235 fronts and 305 rears. This is wider at the rear (by 20mm), but narrower at the front (by 20mm) as compared to a GTR, simply because the 911 has such a rear weight bias, with about 65%/35% front/rear. The total tyre width for both cars is actually identical.

The 325s are only fitted to the GT2 and GT3RS, but they are both rear wheel drive.

 

[speedyK]

In a mid-engined or rear-engined car, the rear tyres have to cope with the weight of the engine too instead of just panelwork.

This weight means the rear is more inclined to pull sideways in a corner. A wider tyre means more resistance to it slipping.

I've had several mid-engined or rear-engined cars and the only one that had similar sized tyres front and back was a Fiat X1/9 (mid-engined and a fairly small engine so it probably didn't need wider rears that much).

 

[paul__k]

Yes, its primarily the large tyre weight difference front to rear which is why Pork can't compete on the track.

http://press.nissan-global.com/PRESSKIT/NISSANGTR/0710/ENGLISH/index.html

 

[Zed Ed]

Is there scope to put wider rubber on the 35?

Would there be any benefit?

The car is hard on tyres, would this change with wider rubber?

All these questions

 

[kpkpkp]

I think wider fronts would stop some of the under steer but I think the rears are fine. To be honest the only time I have ever changed the tyre sizes on a car and good results was my E46 M3.

It had 235s up front and 255s at the rear. I changed them to 255s up front and 275 at the rear.

The car turned in sharper with less wash and the rear had more grip. The down side was the rear rather than sliding out with a bit of progression and then it letting go it held on for much longer but when it broke away it did with venom.

I am not sure if the compound helped as well because the car came with Continentals and I changed to Toyo Tr1 Proxes which were softer IIRC.

Kp

 

[Frosty]

Due to the extra weight at the rear of the 911, the wider tyre just changes the shape of the contact patch. The total contact patch will be the same no matter how wide the tyre, but the shape of it will be wider which is better for a performance car as it causes less tyre deformation, and therefore heat. A 255 tyre will have no more rubber on the road than a 305 tyre, but the breakaway characteristics will be different, and so will the heating characteristics.

The wide tyre is as much to do with heat management as it is anything else. As the weight increases, so does the tyre deformation, but a wider tyre will reduce the deformation again by fattening and shortening the contact patch.

There are a variety of other reasons why a manufacturer will arrive at a specific tyre width though, but the above reasons are probably the biggest factors.

 

[R33_GTS-t]

Due to the extra weight at the rear of the 911, the wider tyre just changes the shape of the contact patch. The total contact patch will be the same no matter how wide the tyre, but the shape of it will be wider which is better for a performance car as it causes less tyre deformation, and therefore heat. A 255 tyre will have no more rubber on the road than a 305 tyre, but the breakaway characteristics will be different, and so will the heating characteristics.

You're assuming that the tyre pressures remain the same. With larger tyres pressures can be reduced, increasing the contact patch without squashing the walls too much.

 

[Frosty]

You're assuming that the tyre pressures remain the same. With larger tyres pressures can be reduced, increasing the contact patch without squashing the walls too much.

You're absolutely right. Tyre pressures would come under "other factors" as if you were running a particularly low pressure, a wider tyre would be beneficial as it has the same effect as increasing the weight on the wheel with regards to the size of the contact patch. This won't affect grip directly though. The fact that there is more rubber on the road still won't gain any more grip in itself, but it will change characteristics slightly.