Do Wider Rears Produce Understeer?

[Davesvroadsport]

I'm really tempted to fill my rear arches with 8 inch wheels, while leaving 6 inch ones on the front, as I've seem other cars like this and they look great. My question though is this, does this produce any amount of understeer during hard cornering? It would sem logical that the additional rubber pushing from the rear might produce this - any experience out there?

BTW running Toyo R888's if that might make any difference.

Thanks all - Dave

 

[thompster]

I'm interested in the answer to this too. My car came with 6f 8r wheels with cr500s fitted and a spare pair of 6's for the back, but with a measily 120bhp the rears seem unnecessary. I'm tracking it next month so might try both and see what happens.

[Cookie Monster]

Dave

With your reputation for hedge trimming you need to ask yourself if this is wise 

[Davesvroadsport]

Phil - I drive like a nun now!

 

Mostly.

[Golf Juliet Tango]

Dave

How do nuns drive? I never been a passenger when one is driving.

[andy couchman]

I think that, logically, understeer should increase (in the dry). For cars that already understeer (K series?) the effect could be increased, whereas for cars with a more oversteery characteristics it might introduce understeer or reduce oversteer. In the wet, all bets are off...

Anyone tried both routes?

My crossflow runs one set of 6s all round and one 6s and 8s and am planning to go to two sets of 6s and 8s so if anyone fancies a swap from a pair of their their 8s (Caterham 8 spoke anthacite) to my 6s, do shout!

Andy

[Davesvroadsport]

@ GJT - flying nun, naturally, as opposed to Blue Nun, who's always pissed. 

@ Andy - yeah, K-Series....."I think i'd better think it out again" 

[ChrisC]

I would have thought it alters the balance of the car, not the front grip level.  I.e if the car is currently balanced and the front and rear let got at the same time, adding larger rears will change the balance so the front slips first, making it feel like it's udersteering.

[Grubbster]

Ineresting question and the answer is not simple, I just pinched this from another forum -

In terms of simple physics F = V mu... If you double the contact area, then the pressure will halve (per mm2) so the friction per mm2 will halve. However, you have twice the area of contact patch so that cancels out the halved pressure. The conclusion is that basic friction is independent of area, it only needs vertical load and a co-coefficient. You can do whatever you want with the area from a stiletto to a surfboard, the friction force will be the same.

However life is more complex than simple physics. The most obvious point is that tyre rubber is adhesive, so if you double the area you double the amount of adhesion (gross approximation). So wide tyres do have more grip than thin tyres.

Another point is that the stiletto will dig into the surface causing interlock but that is also ignored in F = V mu
 

My view is that the balance will change slightly with wider rear tyres so you might understeer at the limit, but if you can adjust the setup (rollbars, damping, ride heights, camber etc) then you could restore the balance. I run 8" rears but my car is so different to how it was 11 years ago that it can't be compared - but it is very well balanced IMHO.

[andy couchman]

8s will also give you a  wider rear track, which may further affect things.

Oh this is too complicated!

Andy

[F355GTS]

A wider rear track will normally induce oversteer ie make the car more pointy. I suspect it would cancel out any understeer created by the increased grip

[martyn]

Don’t forget that a LSD will encourage understeer as it locks up.  Wider tyres are going to exaggerate that effect.

[prangerman]

My R300K with 8 inch rears has mild understeer which, with the LSD, turns to snap oversteer if one over corrects too enthusiastically...not that I've ever got myself into that situation with a passenger who went very quiet, you understand.

 

Peter

[Davesvroadsport]

All interesting and thanks. I think if I'm going to go down this path then taking the car to get corner weighed and the Nitrons properly setup is key.....

[6speedmanual]

With a 120 hp motor I wouldn't even consider running wider tyres at the rear. Go for 175s or 185s all round and then sort out chassis set-up. It will be a JOY!

[Slipper man]

I have run my R500 with both 7inch rims all round, and the standard 6 1/2 and 8 rims. It was much better balanced with the 7 inch tyres all round, but it is definately faster with the larger rears.

[6speedmanual]

I just fitted grippier (new, more adhesion, so far impossible to unstick, whereas old ones were very loose on wet/dirty surfaces) tyres to the rear of my tintop. Now the understeers noticeably less despite more grip at the back than it had. Work that out ;)