Changing in 1" increments I totally agree, but it is more significant in going from 16" to 18".

 

an often overlooked gem of suspension knowledge here.


.....remembering driving a racecar on the road 500 miles on high profile tires and wishing that I had some stiffer sidewalls during the bouncy concrete patches......

 

The added width I see with 18" over 17" on my NB car 993 ...is about 10mm. On the rear I run 275, with 18" I could go to 285. On the front I run 235, I could go to 245...10 silly millimeters. In tire sizes with my ABS, I need to keep tire rolling radius within about 5%, front to rear...if mismatched greater than about 5%, the ABS turns off, not good. As a result, my tire selection in 17", 18" and 19" is not as large as one would perceive.

Couple that with the note from Hoosier about 285/30/18 coming off the wheel...I am going to stick with 17" tires on my 993 NB coupe.

If I could slip anything bigger than a 285/30 into the rear wheel wells, I would consider it....but then..I have to consider front to rear rolling radius.

 

Martin, I run the 285/30s on my car and talked to a Hoosier tire engineer about it. He said that there have been problems with that size in the past and that they have been solved on the R6. They were a little concerned about running them at Daytona, but did not tell me not to. I checked them after every run and put about 3 hours on them with no problems at all. A setup that most 993 guys run is the 245/40s in the front and the 295/30s in the back. No rolling radius problems, and the 295s are only slightly wider than the 285s, so they should fit.

 

Thanks Larry..............

 

You're running 275s on 17X9 rears and 235s on 17X7 fronts? Cup IIs?

 

By adapting the sidewall height from say 45 to 40 when you go from 17-18 the rolling diameter stays the same thus no real impact.

 

That is what I am doing...I also think the 245 Kumho 710 would work on a 7" wheel, although "officially" the 7" wheel would be too narrow. I have seen 245 Kumho 710 on a 7" front...the sidewalls looked nearly vertical....but I choose to err on the side of caution..235 is as big as I'll go in the front, and 275 is it for the 9" rears.

 

Are you using any spacers and did you roll the fenders? Any rubbing issues? You must have some serious negative camber.

 

I do have some serious negative camber, about -3 up front and about -2.5 in the rear. I have ERP monoballs front and rear too. My fenders are not rolled and I have 0 rubbing issues.

My car was set up by Jae Lee at Mirage International in San Diego, CA. He also set up my kinematic toe....my car is on rails, I swear....

 

If your car is on rails it really means you are driving too slow

 

Tom that is my take on it too. I do like the looks of the lower sidewalls on the 18s though

 

I did some back to back runs on 17's and 18's, both with Hoosiers. I took some laps and had a pro do some laps on each. We were both faster on the 18's. His changes were smaller than mine because he was able to push the car on the 17's more and deal with the rolling over better than I was. Bottom line is the 18's worked better on my car with its stiff suspension.

 

Martin's not slow. He may describe the handling as like rails, but his laptimes say there's some decent slip angle involved.

 

This is what i was looking for at the start of the thread with this kind of back to back, comparsions .
I wanted to see with no other changes than the wheels .
The 18" should be superior because of the smaller shoulder they tend to put more rubber to the ground for the same rolling radius, along with being stiffer. you will also have more leverage against the brakes , so hence the need for bigger brakes , not the other way around . i have worked on a BTCC car , and they use 19inch wheels , at the track i was at a BTCC volvo with 2.5L displacement was only 1 sec back of a 996 GT3R , could be driver difference i was not able to tell , but it was one hell of a High tech monster , and way too fast for a 2.5L.