[bigd_SD]

I just picked up a 1994 968 coupe this month (80k miles, stock suspension, 16" wheels, no LSD). I am an AX guy (will do some DE's this year as well) with occassional track days. I thought I would report on my current project - street & track set-up.



I want to have a really well set-up car without going crazy with the mod dollars. I am dialing-in the car in 2 phases as budget and driving skills improve.



Phase I:



> install 17" 993 cup wheels 9” all around with biggest tires I can fit.

> install strut brace.

> M030 sway-bars with sway bar braces.

> airbox mod & KN filter.

> 5 points belts w/ stock power seats and harness bar.

> corner balance, camber and aggressive alignment.





The BIG Question



The big issue was how much room up front for wheels, tires, negative camber and, with the help of others (thanks Bill and Jae), we've answered that question. I thought I would share what we found here.



First, we put a 9" 993 Cup rim with 255/40 tire... too wide. Although about 1/2 inch clearance (between the wheel and spring) with zero camber, at 2.8 degrees of neg camber the tire then rubs the stock spring. So, in Phase I, we're going with 2-2.5 degrees and a 245/40 tire. This should give me plenty of clearance.



In Phase II, I may get 8.5" rims on all 4 with different offset. The new springs (and Koni adjustables) will have smaller diameter and allow for a lesser offset and hopefully a 255/40 tire. The car will be lower and additional neg camber will be required to "tuck in" the top of a 255 tire (and not have to attempt rolling the fender).



Anyone else been through this exercise of trying to max the rubber up front? I am wide open to suggestions on this.

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[RS Barn]

These cars don't really like too much tire up front. If you are doing AX you want to stay with 16" wheels, 8" all around with225/45/16 up front and 245/45/16" in rear. This keeps the weight down and has tons of grip with small rolling diameter. The biggest factor is keeping the gearing correct. 17" wheels screws up the gearing.

Pete

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[bob blackwell]

Well, I have been running 18 inch wheels for track use and autocrosses and I don't think they mess up the gearing at all. I have 8 and 1/2 inch wide wheels on the front with 245/35x18 tires mounted and run one and 1/2 degrees negative camber in front. In the rear I am running 10x18 wheels with 275/35x18 tires. Two degrees negative camber in the rear. The best tires I have used for track events are the Hoosier R3S04. since I corded my front tire last event I have ordered Hoosier A6 replacements all around. I think the widest wheel with the softest tire will give you the best times. Good luck, Bob Blackwell.

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[flash]

i would agree with running 16" rims in AX - the weight benefits are huge - in AX, the speeds are relatively low - acceleration is best with lower rolling mass - braking is also improved



my car is not set up for AX, but rather more for street and some light track stuff - i have an 18x9 up front with a 255/35/18 S0-3 (one of the widest brands) - no rubbing at all, even lowered over 1.5", with 1.75 degrees of negative camber



i presume you have a strut tower brace - making big camber settings without one makes for some rather odd results in the geometry



your clearance issues are due likely to offset problems - also, since the spring is at an angle, the 17" rim puts the tire closer to the spring - the same size in an 18 would be almost .5" farther away

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[TheMirror]

Hi Bob -



What offsets are you running on those wheels again?



Cheers,



-Mirror

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[TheMirror]

Can't remember who it is offhand, but someone on the boards here is running 9" wheels up front for AX. I distinctly remember the stipulation that the car needs a stiff front spring setup for it to work well with that size wheel and tire. A search will probably bring that topic up.



Best,



-Mirror



[quote name='bigd_SD' date='Jul 19 2006, 05:10 PM']> install 17" 993 cup wheels 9” all around with biggest tires I can fit.

[right][post="24312"]<{POST_SNAPBACK}>[/post][/right][/quote]

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[bigd_SD]

For the record, the 17"/9" 993 cup wheels are 55 offset.



Bob, given you've done 1 or 2 mods to your suspension, <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" /> I'll assume you are not running the stock (non-MO30) springs which are wider than most. If I had smaller diameter springs, a 255/40 might fit (if we tucked it in tight).



The next time around, with 8.5" wheels, smaller diameter springs (more than 1/2" smaller! ...we measured today), and smaller offset... 255/40 should be a nice fit.



I'm no expert... however, with regard to the gearing getting screwed up, wouldn't that be a factor of Outside Diameter (OD)? For instance, if a 225/50 on 16", or 245/40 on 17", or 255/35 on 18" rims all have approximately 25" OD, wouldn't the gearing be the same b/c the revs per mile are about the same?



If the OD was significantly smaller or larger then stock, then yes, I think there would be some effect.



Wheel diameter would affect sidewall height which would affect stiffness (and ride comfort), right?

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[bob blackwell]

Mirror - the front ET is 50mm and the rear is 54mm.



Yes, my front springs are H&R Progressive springs which are narrower than the stock springs were. They are 190 - 230 pounds and lower the front 3/4 inch. You are correct about the outside diameter and the effect on gearing. as long as the overall diameter is not changed there will not be any change in the gearing.



Flash, I agree about the added weight being a disadvantage but the better traction seems to overcome the weight penalty. My personal best times have been on the 18 inch wheels with the Hoosier tires. Good luck, Bob Blackwell.

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[333pg333]

When I was researching for new race tyres I found that the 18" 285/30 were actually smaller in rolling radius than my current 255/40 17" so I would think my gearing will be roughly the same. The weight may indeed be a bit different but you would hope that it would be more than compensated by better traction of the wider tyres?

Also what are the events you guys call autocross? Is that where you go in and out of the orange witches hats on a skidpan or flat area? If so, we do those and call them Motokhanas. We seldom get out of 1st gear, but then they flood the track half way through the day which makes it fun. It's harder than you think. Especially remembering your way around the course. If you hit a cone it's a 5 sec. penalty and if you go the wrong way it's 10 secs. It seems to be worse if you've had a late night. Funny that...

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[flash]

mirror - my front offset is 57



big sd - my rims fit with standard spring diameter - i only recently changed to 2.5" - on an AX car, i would run the smallest diameter i could find, with the lowest profile, and not worry about trying to maintain 25"



bob - perhaps you're a better driver now? that would be an interesting test though - on lighter cars, we always found a huge advantage to smaller wheels - this car may be too heavy to do that, because perhaps it gets down below the minimum size to be effective - i wouldn't think so, but i suppose it's a possibility - the lower sidewall on the 18 may be the reason too - that's why i constantly recommend against running an oversize tire - sidewall flex robs you of time in an AX



333 - just a little math - going from a 17 to an 18, even on a very light rim, you gain 15% more mass, but in going from a 255 to a 285, only 11% more tread - it gets worse with a 16 - i haven't tested traction and control limits, but that sounds like fun - bob?

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[bob blackwell]

Yeah Flash, I am just like fine wine - getting better with age (NOT!) With my poorer depth perception, overall worse vision, slower reaction times and more cautious driving.........oh well, you get the picture. The added traction from much wider tires and the lower profile as well have made the difference. Prior to getting the 18 inch wheels I was running 8x16 wheels with 245/45 tires all around with Michelin Pilot sports tires. Good luck, Bob Blackwell.

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[flash]

hmmm - then i'm guessing that stylistically, you enter pretty hard on corners, and snap out - the sidewall flex was probably working hard against you - you might have even been feeling some understeer before

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[333pg333]

[quote name='flash' date='Jul 20 2006, 08:43 AM']mirror - my front offset is 57



big sd - my rims fit with standard spring diameter - i only recently changed to 2.5" - on an AX car, i would run the smallest diameter i could find, with the lowest profile, and not worry about trying to maintain 25"



bob - perhaps you're a better driver now?  that would be an interesting test though  - on lighter cars, we always found a huge advantage to smaller wheels - this car may be too heavy to do that, because perhaps it gets down below the minimum size to be effective - i wouldn't think so, but i suppose it's a possibility - the lower sidewall on the 18 may be the reason too - that's why i constantly recommend against running an oversize tire - sidewall flex robs you of time in an AX



333 - just a little math - going from a 17 to an 18, even on a very light rim, you gain 15% more mass, but in going from a 255 to a 285, only 11% more tread - it gets worse with a 16 - i haven't tested traction and control limits, but that sounds like fun - bob?

[right][post="24355"]<{POST_SNAPBACK}>[/post][/right][/quote]

Fair point. Lightness is a very real way of gaining speed and control. With turbos we may have extra power to overcome some of this extra weight and I won't be running these wheels and tyre on the M-khanas. Track only.

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[flash]

no worries - it's all compromise - what works best for one use, isn't necessarily best for another - and then there's driving style that enters into the equation



it's that whole "symphony" thing - how i set up my car, with the combination of things that work for me and my driving is not how the next guy will set up his - that's what makes it a sport

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[bigd_SD]

[quote name='flash' date='Jul 20 2006, 07:43 AM']... just a little math - going from a 17 to an 18, even on a very light rim, you gain 15% more mass, but in going from a 255 to a 285, only 11% more tread

[right][post="24355"]<{POST_SNAPBACK}>[/post][/right][/quote]





Flash - hmmm. I might be missing something here because the fastest guys I know are always looking for ways to put more tread on the track. There may be more science here than just personal preference. Not sure about your math, maybe some racers could shed some light on this...



Yes, bigger wheels mean a gain in mass but only from a wheel/tire perspective, not the entire car. That is, if 16" wheels + 225/50 tires weigh 45lbs ea (for example, Falkens Azenis) then that's 180lbs total for all four. If you move up to a 17" wheel + 255/40 tires, the weight increases to about 50lbs ea, or 200lbs all around with an increase of 20lbs overall.



That's a weight increase of about 11% in wheels/tires, however, in a 3,100 lb car, that equates to an increase in weight of less than 1.0%.



Now, in this example (moving from 225 to 255) the tread width increases from 8.2 inches to 9.4 inches (according to the tire specs). That's a 15% increase in tread! That additional tread equates to more traction (roughly 15%, all other things equal).



But here's the point, any additional traction applies to the ENTIRE car, not just the wheels/tires. So, even if you wheels/tires weighed 15% more and you only picked up 10% more traction, wouldn't that be a huge ROI?

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[flash]

ok - we can go here for a minute



here's one thing you're missing - most tuners will tell you that 1 pound off the wheels is the same as 4 pounds off the car - that mass is extremely important, and effect things more than just the added weight - the effective difference between a 16 and an 18 is 120lbs to the car - that's a lot



also, not to nitpick, but i WISH i could get an 18 that won't break, with a tire that weighed a total of 50lbs - it's more like 51 front and 54 rear, and i have very light wheels - most of them are a lot heavier - but, even at that, it makes for a total effective difference of 40lbs over a 50lb setup



more traction is good, no doubt, but throw in more mass, and it may not be best, depending on the track - short tight courses are best served by smaller diameters and less weight - big tracks with higher speeds are better served by wider tires, larger diameters, and the weight is less of a factor



i guess it can best be explained in thinking about what gear you are in - if you're running 1st and 2nd, ounces can make a difference - if you're running 4th and 5th, you'll never notice them - to make the point - go do a 0-40 run with a passenger, and then without - you'll be amazed



i'm down around 3000lbs now, and these things really start showing up in the numbers



on the math of tires, i was using the metric numbers - 255 to 285 - this came out to 10.5% (i rounded up) - 225 to 255 is 11.76% - the real widths vary from brand to brand, so i chose the nominal



as for the traction issue, that frequently comes down to how you drive - but, it's a lot like setting up swaybars - you wouldn't use the same setup on a road race course as you would on an AX - you would generally set the bars up a lot softer on AX



if i could get a 17" tire in a 30 or even a 35 profile, i would quickly dump my 18s for the weight benefits - the sidewall flex is a problem area for this heavy car though, so i'm stuck with the compromise

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[thingo]

[quote name='flash' date='Jul 21 2006, 09:09 AM']ok - we can go here for a minute



also, not to nitpick, but i WISH i could get an 18 that won't break, with a tire that weighed a total of 50lbs - it's more like 51 front and 54 rear, and i have very light wheels - most of them are a lot heavier - but, even at that, it makes for a total effective difference of 40lbs over a 50lb setup





[right][post="24403"]<{POST_SNAPBACK}>[/post][/right][/quote]

They aren't that hard to find, try Enkei NTO3+M with Michelin PS2

8.5x18 245/35 42 lbs

10x 18 295/30 48.5 lbs

not the lightest wheel available, but value.

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[333pg333]

[quote name='flash' date='Jul 20 2006, 02:16 PM']no worries - it's all compromise - what works best for one use, isn't necessarily best for another - and then there's driving style that enters into the equation



it's that whole "symphony" thing - how i set up my car, with the combination of things that work for me and my driving is not how the next guy will set up his - that's what makes it a sport

[right][post="24388"]<{POST_SNAPBACK}>[/post][/right][/quote]

Exactly. If you have synchronicity you may indeed create a "symphony".

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[flash]

actually, they were pretty hard to find, and i had to have these made



the enkeis don't fit - offsets way wrong - the front is only an 8.5 wide too - also, the tires sizes you specced were wrong, however a good indicator that there are lighter tires out there



i wanted the most rubber under the car i could get - i also wanted to maintain 25" OD - that meant the wheels and tire sizes i chose



yes, the brand of tire i chose is quite a bit heavier (i'll be fixing that next go-round), but they were the only ones that made the sizes i wanted



yet another compromise

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[thingo]

[quote name='flash' date='Jul 22 2006, 01:34 AM']actually, they were pretty hard to find, and i had to have these made



the enkeis don't fit - offsets way wrong - the front is only an 8.5 wide too - also, the tires sizes you specced were wrong, however a good indicator that there are lighter tires out there



[right][post="24445"]<{POST_SNAPBACK}>[/post][/right][/quote]



Offsets might be wrong for your car but don't think bob would have a problem with 8.5 et50 10et 50 or 60, guess the tyre sizes are wrong for you, no problem with a 968

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