tvr-4

I am slowly building my 88 951 to SP3 class specs with an intent of racing 2-3yrs from now, and have talked to several people about wheel / tire sizing so wanted to bring the discussion here to hear the wisdom.

Per SP3 rules, maximum tire and wheel widths allowed are 305mm and 12", while maximum allowed car width is 74.5" so keeping this in mind, what are the recommendations?

Would also appreciate advice on specific wheel choices and offsets that allow for max width of 74.5" not to be exceeded.

PS - some of the folks I talked to didn't recommend going to max tire width due to weight / drag, while actual wheel size recommendations were ranging between 17"-18".

Thanks in advance, any advice greatly appreciated!

TK

rlm328

I would look at a 9" rim in front and a 10" in the rear on a 17" or 18" diameter wheel. 245's in front with 275's in the rear work well. Too much tire and you will lose top in speed due to rolling resistance.

tvr-4

Makes sense - two questions:

- Has anyone tried 275s all around? I have 245s all around now and like how the car feels, but also makes it nice for tire rotation purposes.

- Would you consider going with bigger tire on a higher HP / alreasy heavier car (I.e. Turbo S)?

951and944S

SP3 racing-wise, you'll be on the Hoosier or at a disadvantage, so keep in mind the R7 is generally an inch wider in cross section than another brand's, say 245/45

If it were me, I'd stay with 16" wheels, the tires in 245/45 are $50 ea cheaper than the 17" (that's almost a free tire every time you buy a set) and on a 9" wheel, the 245/45ZR -16 has 10.3" cross section, more than plenty tire for a stock power limited lighter than stock 944T.

Neither Hoosier 245/40 or /45 17 has any more rubber on the ground than the 245/45/16 and the 275/35 and /40 only has about 5/8" wider rubber at +$60 per tire that you will never even need.

Use 4 identical wheels to rotate, much better wear life.

Cheaper......, able to rotate.....less unsprung weight and rolling resistance.

It's a win/win/win and you don't get many of those in racing.

T

tvr-4

Well, when you put it that way... sure sounds like a good idea!

Any specific wheel recommendations? Im on stock phone dials right now, don't think they are 9" wide though so would they hold a 245 Hoosier?

rlm328

While the tires are cheaper I prefer the slip angle of the slightly lager rim. The car seems to react just a little quicker.

944crazy

I run 255 or 265 tyres all round (depending on what deals I can get) on 18x9.5 wheels . I really like this set up. I've only got near stock power though, if I had more power I'd definitely consider going up to 275 or 285 all round, especially on a track only car.

Humboldtgrin

I recommend 951 club sport rims with your late offset. Use 16x9" front and rear. Plus the're forged. Have fun with your build.

951and944S

We run the 8" phone dial F/R. This wheel falls into the recommended wheel width for the 245/45-16 Hoosier R7 which is 7.5-9".

The side wall profile movement/flex is going to play a large part on your race spring setup, so, mileage may vary. Some of these guys have the philosophy of 1000# + spring rates and 18" tires with very narrow sidewall that has little to no lateral of bump flex.
I'm not of that school. The tires take an extra second to set but are way more predictable at limits of grip. Sp3 doesn't allow a lot of aero gimmickry, so you want mechanical grip, not aero aided grip, thus, little sidewall flex tires and super high spring rates that limit body movement to keep aero efficient due to constant relationship between the body and the ground is moot.

The best analogy I can think of at the moment - two identical monohull performance boats, one fixed with outrigger stabilizers.
At identical speeds, you turn the rudder to make sharp turn.
Boat A will have a delay a second as it leans slightly (tire flex/suspension travel) then the hull will dig in as designed and turn the boat in a sharper radius than boat B which remain flat a it turns, i.e., slip angle.

The forged Club Sport and cast D90 have 9" widths and there are some 17" tires available that will have the same circumference as the 16" 245 Hoosier but getting into the 18s, you will be affecting gearing. One of the top Sp3 cars in the country is running 15" rear wheels/tires. Such is the current trend.

T

tvr-4

Super helpful info - thanks for the advice!

Musche

For many years I ran 9" clubsports on all 4 corners with 245/45-16 Hoosiers or RA-1's and really liked that setup. Running square means you can rotate tires to even out wear and get a bit longer use of tires. But to be competitive in racing you will need to replace them because of being heat cycled out as opposed to wearing out tread wise. I have tried 18" wheel/tire combos and 17" as well. So far, my favorite combo as far as feel and compliance with 30 year old suspension technology is the 17" wheel with Hoosier R7 245/40-17 up front and 275/35-17 in the back. These seem to be more compliant and have more of a "bending" feel as opposed to a "skipping" feel of the rubber band 18's in the high G fast corners. You may find the 245's a little too wide for the front and have some rubbing issues and may need to drop back to the 225/45's as the R7 have a really squared off edge to them. Of course this is personal preference based on the setup of my car and your tastes may be different.

JustinL

My experience is almost exactly the same. I really like the 16X9 clubsports with 245-45-R16 tires and that's what I'm running now due to rule limitations at 245 width tires. I prefer the 245,275 combo on 17s and I've tried 275s on 18s square, but that was too much size, and weight, and rubbing at the front. My car puts something around 240RWHP at about 2700lbs and with a torsen LSD I don't have many traction problems with fresh tires.

Max Energy

TK,
For your class with the weight and HP I would recommend 18X10 all the way around and also going square with the tires. Start with 275x35x18
You will need to go with AIR 1" wider fiberglass front fenders, GT also offers them but the fit and finish is much nicer with the AIR Porsche Fiberglass
Call me if you have any questions.
Max
225 573-0664

por944s2

For what it's worth, consider hankooks and run 245 35 18 and 275 30 18, with the following g spring rates, no torsion bars. 700F, 950R. Enkei mt-03 are cheap and strong. The next move maybe 275 or 285 all around. The but who just crushed the sp3 record at the Glen ran 15" wheels, 245f and 285 or 75 rear R7's. So there does not appear to be one right answer. Good luck and look forward to more SP3 racers.

jerome951

A little late to the party, but I'll add my $0.02. Here are my thoughts based upon my experience.

I ran Supercup (precursor to SP3) for a year back in the day. I remember I had to run ~3300# in my TurboS, so pretty heavy.
At the time I ran 245x17 Hoos on the front on 8.5" rims and 275x17 rear on 10". I always seemed to have more rear grip than front. If you're running a heavy car, I suggest you try to maximize front end grip with the widest tire you can fit (either 265x18" or 275x17", though either may be tough to fit without rubbing). My only caveat would be if you run tracks w/ long straights (e.g. Daytona, Road America, etc.) where the wider tire may slow you down.

After that I tried running 245x17 both front & rear and find my car is still balanced, and I also pick up a bit of acceleration due to slightly smaller rear diameter. However, I could still use more front end grip.

So, I suggest you make the decision on front tire/rim width first by stuffing the widest tire possible, then select the rear to obtain balance. You may be able to fit a 10" wheel in front but highly doubt you can go wider.