265 Rear Tyre?

[MackemPete]

I've got a set of spiders Tuscan fronts and Rears (yes I know I should have fronts all round) I've currently got 255/35/R18's on the back but I'm replacing all the tyres and was wondering if I could get away with fitting a wider 265 rear for a bit more traction? Anyone done this?

[JensenA]

You wont neccasarily get more traction, and possibly a lot 'slidier' in the wet. I'd be interested to hear what the optimum 'width' is for a Cerb (for use on on 'normal' roads).
Any of you handling Guru's any thoughts or answers?

[Cyclone1]

JensenA said:

Any of you handling Guru's any thoughts or answers?

Not a handling Guru although I am a driving God , but I think 255 is about as far as you can go on the rears before they stat rubbing on the arches under load. Not sure if someone tried 265/30's a while back and got away with them though.....

Best bet would be to stick to 255's ad go for a stickier tyre - R888's etc

Jules.

[brakedust300]

Iv got 265/35/18 on mine with no problems... although interesting to hear that 255's will be better as i will need some new ones soon

[optimax sniffer]

When I had some Michelins (225/35/18) on the back they rubbed the arches when the car was loaded, but swapping back to toyo's of the same size there was not a problem!!! The M's had squarer shoulders which caused the rubbing.

[JensenA]

I didn't mean an optimum width for not rubbing the arch.
I meant that a point will be reached when an increase in the width of the tyre results in a decrease in Grip. What is that optimum.?

[bosscerbera]

I run 225/35 18 fronts with 265/35 18 rears.

The front end it is light and pointy, the rear has enough grip to follow it - perfect balance on a Cerbera IMHO. The 225s only work properly on an 8" wide rim, the 265s are mounted on 9" rears.

[Cyclone1]

bosscerbera said:

I run 225/35 18 fronts with 265/35 18 rears.

The front end it is light and pointy, the rear has enough grip to follow it - perfect balance on a Cerbera IMHO. The 225s only work properly on an 8" wide rim, the 265s are mounted on 9" rears.

So on a rear Spider the 265 would be too big? Hence sticking with 255.

Jules.

[Nefarious]

Just a technical note:
Changing the width of tyres only changes the shape of the contect patch not the size. It'll get wider and shorter, but the total area in contact with the ground with remain the same.

[plasticman]

So are you trying to say my spiders with 225 on the front and 255 on the back have the same amount of tread on the road ?

[morebeanz]

I'm trying to revive my A-level physics, but is that something to do with having the same pressure in each tyre, and the surface required to exert an equivalent force to counteract the weight of the car?



Or have I just spouted yet more pseudo science tosh!?

Edited by morebeanz on Thursday 21st February 10:29

[volospian]

Nefarious said:

Just a technical note:
Changing the width of tyres only changes the shape of the contect patch not the size. It'll get wider and shorter, but the total area in contact with the ground with remain the same.

Although tyre pressure will effect that.

There's a good thread about how wider tyres affect grip on the Porsche forums here:

http://www.pistonheads.com/gassing/topic.asp?f=48&...

[Nefarious]

plasticman said:

So are you trying to say my spiders with 225 on the front and 255 on the back have the same amount of tread on the road ?

No, because they'll have a different weight acting upon them. If, however, you put your front wheels on the back, at the same pressure as the back ones were, then the area of the contact patch would be exactly* the same.

• to pre-empt any pedants out there, its not exactly because there will be a *fractional* difference in the elasticity of the tyres as a whole due to the different tread/wall ratio.

[quote]I'm trying to revive my A-level physics, but is that something to do with having the same pressure in each tyre, and the surface required to exert an equivalent force to counteract the weight of the car?

Basically, yes. The collective tyre pressure multiplied by the collective contact patch must be equal to the total weight of the car (or, strictly, normal force excerted by the ground, but its the same thing). Therefore, for constant weight and tyre pressure, total area over which the normal force is excerted must remain the same.

Edited by Nefarious on Thursday 21st February 08:58

[volospian]

plasticman said:

So are you trying to say my spiders with 225 on the front and 255 on the back have the same amount of tread on the road ?

For the same pressure, yes, technically, as the tyre will deform due to weight, so the 225 will have a narrower (side to side), but longer (front to back) contact patch, while the 255 will be wider (side to side), but shorter (front to back)

"Contact patch area is a function of vertical load on the wheel and the tyre internal pressure, the tyre deforms to achieve this area (pressure x area = force)."

/EDIT Nefarious has answered better than me. I should have said "for the same relative pressure" or something

Edited by volospian on Thursday 21st February 08:58

[Brummmie]

My 255 Pirellis are 1" wider than the Toyo's i took off, even though the numbers are the same. So 265 in are another brand will probably go as i dont have rubbing issues.

[MackemPete]

I've read the thread on the Porsche forum and its convinced me to stick with 255's. Why risk a bigger tyre that could cause rubbing problems when it could potentially make the handling more unpredictable.

Probably going to go with a set of Goodyear asymmetrics rather than the R888's. They seem to win every independent tyre review so they must be good. I dont feel I'll do enough track work to justify the extra money and sacrifices that come for the R888's.

Cheers for the help guys.

[bosscerbera]

According to the ETRMA guidelines, a 265 shouldn't be put on an 8.5" rim anyway.

[Brummmie]

bosscerbera said:

According to the ETRMA guidelines, a 265 shouldn't be put on an 8.5" rim anyway.

Like i say which brand???

[JensenA]

Any brand thats a 265 section.

Edited to add ..........(and when I'd finished Brummie had already replied)

Tyre manufacturer aasosciations have created a set of standards. When you fit a tyre to a rim, as rubber is flexible, its dimensions change. So to ensure that a tyre meets the designated meausurement specifications, a tyre's dimensions must be measured when fitted to a rim of the correct width as specified in those standards.
In other words if you fit a 265/32x18" on a 4" Rim, you wont actually have a 265/35 it will be distorted into someting like 195/66

Phew ! Does that make sense??

Edited by JensenA on Thursday 21st February 17:27

Edited by JensenA on Thursday 21st February 17:32

[Brummmie]

JensenA said:

Any brand thats a 265 section.

My 255 PIRELLI IS 1" wider (30mm) than the Toyo i took off.