asbi

All,
I changed my 16" D90s 6"Fr/8"Rear with Conti Sports 205Fr/225Rear to 17" OE Cups 7" Fr/9.5" Rear with Pilot Sport Cups 205Fr/265Rear.

I was on a sprint course today in the 964, very tight and twisty and noticed that the car understeers quite badly compared with the original wheels/tyres. The rear cannot be unstuck at all unless really provoked by lifting off mid turn.

I had noticed this previously when driving hard on the road but it didn't really register.

Geometry is within tolerances (on D90s) wheels are std offsets excpet for rears which are running an 18mm spacer as they are turbo Cups (same wheel setup as JBoggs old 964).

Ideas anyone?
Thanks,
Kevin.
PS I was watching my mate test his car today which is how I got the chance to throw my 964 round the course, he has a single seater F3 Chassis hill climber in the sub 2 litre class running on methanol, boy does that thing go!

CraigyB

Methanol ! Ouch.. Just be careful for that stuff burning, the flame's invisible. In my marshalling days we were instructed to extinguish the man running towards you frantically patting himself, or rolling around on the floor !

rroobbcc

Basically you added 40mm of additional rubber on the rear, but didn't add any additional rubber on the front, so it is no surprise that the front lets go before the rear. You will want to go with a 225 tire on the front to achieve a similar balance to what you had with the stock setup.

asbi

Doh, boy do I feel stupid - is it really that simple?

rroobbcc

Yep, its that simple. BTW, did you mean 255 on the rear or is it really 265? The 255 is a more common width on 17s, but if 265s fit I guess it shouldn't be a problem.

BGLeduc

I would have to 2nd what Rob said......you have a huge set of meats out back there!

My track/AX set up is 245 front and back on 16x8 D90's.......that tends to balance things well, and allows some amount of rotation on the tight stuff. I once tracked my road set up (17 Cup 1 Replicas with 205/255 Kumhos) and she plowed like a farm vehicle. It felt very nice to swap back to my track set up.

Brian

asbi

Rob,
Definitely 265 and pretty close to the oil pipe but OK!

Brian,
I love the farm vehicle allegory, that's just how it felt today. I guess on the road we are so far from the liimt that it's not a problem.

Thanks guys,
Kevin.

ttAmerica RoadsterAWD

Here's another angle. When tires are brand spanking new, the meaty treads are supple, soft and relatively tall. They usually benefit from some heat cycles so they are not too jelly-like. This was explained to me by the guys at Tire Rack. O'l wives tale?? Fact??

Jaime

Computamedic

Kevin,

It's not quite as simple as has been said although the previous comments are on the right track (!!). The car is now over-tyred on the rear which explains the terrific grip you're experiencing and emphasizing the smaller front tyre. Unfortunately you will be struggling to do much with that problem since the 7" front wheel won't take a 225 tyre and be safe. Also the rear would be much better with a 255 tyre. The front needs to go to a 7.5J wheel to get the ideal tyre size (225) but the Cup 1s of that size are pretty rare. You didn't mention if your car is a C2 or a C4 but the understeering issue will be much more prevalent with a C4.

Regards

Dave

ish3

I agree with Dave. I lucked out in buying my wheels and tires from Wheel Enhancement in Ca.

That is the reason they recommended this set-up:

17" Cup Wheels 7.5x17 52mm offset 9x17 w/ 47mm offset
Goodyear Eagle F-1 GS D3 Front 225 45 17 Rear 255 50 17

964C4

ish3: same as my case !

But had to get the car alligned 5 times until it was perfect !

964rh

Dave

Are you sure that a 225 on a 7" rim isn't safe?

I've had 225's now at the front on 7" rims for over 12 months, just under 5k road miles some hard some Miss Daisy and about 6 trackdays (And I like to drive as hard as I can on these) and I've had no issues.
Granted the wheel/tyre combo doesn't look as if they should work, but pics from track days have show the rear to look more dodgy than the fronts.(mainly due to me playing around with pressures)
They have helped in making my C4 handle more neutrally both on road and track.

Rob

Computamedic

Rob,

It's a difficult one. Some tyre manufacturers say that a 225 is OK on a 7J, some don't. It came to mind specifically bevause I know Jon Kent in Luxembourg wanted to go with 225s and was strongly recommended against it because he had 7Js and he reluctantly went with 205s instead. Obviously the tyres actually "fit", but according to most tyre manufacturers a 7.5J is the minimum for a 225 tyre with the 45 aspect ratio. With 50 tyres it's a completely different story.

Dave

964rh

That's prob. it then Dave
I did actually send Dunlop an email as the company selling the SSR's cheap said the 225/45/17 were fine but I wanted backup from the manufacturer.
According to them, for the SSR's, 225's are ok up to 7" rims. Ideally should be 215 on 7's.

IanR

Yeah, Toyo are the same - according to them, their 225/45ZR17 R888 is OK on 7-8.5 inch rims.