Based on what exactly?

 

based on the limitation of trying to stuff the widest tire and width at the cost of losing adjustability.

 

Heavy,

8''is skinny but In my experience a 9.5" tread width is about the limit of front tire width without serious fender liner rubbing. A shorter tire ie 25" tall will allow more width than a 25.7" tall tire. Since Porsche has app 65% of its weight on the rear having a skinner front tire is not necessarily bad.

Your Alignment, Suspension, Wheels is 100% correct order. I would add @ the end Power. I realize 996TT came with really skinny tires and going to 245 front and 315 rear will make a big improvement. I run 9" fronts and 12" rears on the GT2 and these sizes will handle any tire width I care to use.

Peter

 

Adjustability to what and where? Let's talk specifics. Toe? Camber? Caster? The only thing affected by a wheel that size is caster and much of that is offset by the bigger front foot print controlling where the car is going instead of the caster effect. Toe adjustment is too insignificant to pose a problem to the overal wheel position.

For street, yes, impractical, for track, where you'd be running the same amount of camber and toe anyways, no not impractical.

 

Peter, I have to disagree, a skinny tire in the front is bad for a Porsche. For the simple reason that back end traction is not a problem, especially for a 911, and especially when you can also then pile 800-1000 more lbs of down force with a huge wing. Front traction is the problem.

Skinny front tires only lead to more mid corner push which has been a 911 shortfall almost forever until the 08 RSR came out. How much more would it be a problem for our street 911's? A lot considering the RSR's are years ahead of our cars in technology and developement.

How do we remedy this? Let's see how the RSR's did it.

- RSR's have wider wheels in front 11 inches to be exact and the rears are 13 inches. We can't get 13 inches in back or 11 in front, but we can get 10 in front and 12 in back for the same ratio.
- RSR's now have way more downforce in front than you'll ever get in a street 911 with the dive planes, and aero passing through the fascia and up through and over the hood. But we can do a front splitter that extends well beyond the front bumper, something the RSR's can't.
- A flat bottom undertray, more advanced than ours, but ours is pretty sufficient.
- Rear DF, huge wing, which we can get.

After that, speaking from a logistical standpoint we are limited in ride height due to suspension geometry, gearbox, chassis stiffening, throttle response etc. But to make the best street/track car we can, those aren't practical or even remotely easy to acheive.


So you see the answer to making your 911 handle better is to take a chunk out of as much of that as possible. And there are some easier ways to do it, though it may take a little effort in the fender well area. Probably not good for a street car, but if you're racing, 10 inches is definitely better.

The only reason I'm not set on going with a 10 inch wheel is that the tire selection is really crappy across the board. I intend on running A6's for NASA and a 10 inch wheel screws up the height differential for AWD. I agree that 9.5 is the best compromise, but there are limitations there too. But I've seen with my own eyes the 10 inch wheel work on a NB 996 so I know it can be done and is more effective in corners. but a 9.5 and 11.5 may not be so bad either. But the 8.5 fronts and 12 rears that come on the GT2 are designed to make the car push.

 

Very true since the GT2 inherently oversteers as opposed to the TT that understeers due to the awd. But when it comes down to it, for the track, the more rubber you can put to the ground the better. Suspension adjustments can be made to neutralize some of the GT2's propensity to oversteer as well as the TT's understeering issues.

 

more rubber better? hmmm.... to the expense of other more critical perhipherials?

there's a pragmatic limit.

 

More critical perhipherials? I don't even know what that means. But again, please more detail as to what you're talking about. You haven't mentioned any specifics only loosely based generalizations. What adjustments are being hampered and how is that going to hurt you on the track?

 

heavy, don't get upset. I'm just saying there's a practical limit that's all.
if you can't understand then I'm sorry.

 

Oak, I'm honestly not upset. I'm just trying to figure out exactly what you're trying to say, that's all. I don't see the unpracticality you mention with the setup. In racing and even just tracking there are always compromises, be it tire size, wheel size, alignments vs suspension vs aero setup (finding the fine line between longevity and speed) gear ratios, hp, and on and on. I completely agree that the setup is compromising for the street and have said that, but unpractical for the track is something I can't understand without details. (and even the street issue can be rememdied with a second set of wheels and alignment changes)

If you have tracked your car a lot, you'll know that most people find small window of an alignment range, and hardly ever change much from it, youd be pressed to see a guy who runs -2 camber at one track and -3 at another. Most times they run with a few tenths of a degree at fast and slow tracks alike. Same with toe which can't be adjusted much anyways. Caster, like I said is only effected a little. And ride height is limited by the suspension geometry regardless of wheel size, but you can run the same ride height on a 10" as an 8".

That pretty much covers all the possible adjustments, so I'm trying to figure out what you're saying in terms of it's impractical for the track........ A little frustrating, but I'm not trying to make this an argument, just trying to see if there is something you have experienced/seen that I haven't.

 

cool no worries. no reason to get frustrated.

 

Heavy,

Skinnier Front Tire not skinny I stand on what I said. Going to an extremely large front tire is a band aid. You can't make a 996TT/GT2 into an RSR but you can greatly improve the handling with proper suspension upgrades without needing to gut the front fenders to run ubber big tires.

When we ran my 996GT3 in SCCA Super Stock Class AutoX everybody said we were crazy to to run 245 front 285 rear tires, instead of running the 315's on the rear, this was in 2005. In 2007 when Ian won the Solo Nationals in Super Stock the fastest GT3's were on 245/285.

The reality is there is no proper answer to suspension/tire size, etc. Everybody drives differently and no setup works for everybody, Some like a little push, some like to rotate the rear, trail braking rears its head, Momentum or point and shoot, Do what works for you and your driving style.

peter

 

It's not a band-aid if you have done all of the other mods you would have likewise done anyways. You would essentially be saying going from the stock sizes of 8.5 to say a 9.5 is a band-aid as well. A bigger WHEEL is not a band-aid, it's a proven measure to increase contact patch and handling. Changing tire sizes on the same wheel width is a different story.

And this has nothing to do with putting the biggest tire on the car that you can because you can run the same 245 tire on a 8 inch wheel that you can run on a 9.5 inch wheel or a 255 on a 8.5 inch wheel and a 10 inch wheel. This is about contact patch dynamics and simply changing tire sizes does not have any significant effect on contact patch and it certainly compromises sidewall integrity under load. So that 245 on the 8.5 in rim wouldn't be nearly as effective as the same 245 tire on the 9.5 inch wheel concerning handling because the contact patch on the 8.5 inch wheel would be significantly more narrow.

 

Oak, I understand what you're saying and I agree, you can't just put on the biggest tires on that'll fit. I mean don't take off your fenders and throw on giant Top Fuel racing Slicks at all 4 corners. Obviously, the bigger the tire, the more adjustments you're going to have to make to the suspension. Eventually you will meet the point of diminishing returns. However that being said, with proper adjustments to correct and re-align the suspension geometry, you will have faster lap times with bigger contact patches and a greater level of adhesion. Obviously, it differs between tire makes and models. For example a Conti 315 street tire isn't going to handle as well as a 285 MPSC or R888 on the track. But all things being equal, the bigger tired car will out handle the smaller tire car, so long as it fits the driver's style, bigger tire wins everytime. Like I said, everything else being equal except for contact patch sizes. As Heavychevy said, once you start talking about differing tire sizes and wheels, the sidewall becomes part of the equation, but in my above statements, this was not part of the variable. Don't even get me started on Tire pressures, because obviously, this could also change the dynamics of how a car handles every bit as much if not more so than the actual sizes of the tires/wheels being used. I used to race motocross and even on the dirt, tire sizes and sidewall flex made a huge difference depending on the particular track. Soft loamy tracks did better with a large sidewall and a slightly larger rear tire/wheel so you could get bite in the soft dirt rather than dig into it. Whereas, hard packed supercross type tracks, did better with smaller rear wheels where you could better transfer the power to the ground especially in low speed tight corners. Ok, I guess I'm drifting off topic, but I'm just trying to point out all the variables. But like I said, ALL THINGS BEING EQUAL, bigger contact patch wins in most situations. The only exceptions would be things like Bonnieville top speed runs or big meats on the front of drag cars where drag comes into play Hope that clears it up a bit.

 

hmmm....this turned into an interesting debate. Thanks for all the perspectives, I've actually quite enjoyed reading them. =)

But, to shed a little background here, I'm not taking a bone stock commuter C4S, and trying to shove 10" front wheels on it for looks. =) I Time Trial aggressively (and competitively) with the car, and am at the track at least 30 days a year. I have GT3 control arms stacked with 13mm of shims, a Moton Motorsport Tripple adjustable suspension, monoballs, etc... I data log every event and analyze the data religiously, including chassis slip angle. On my relatively low powered (N.A. X51) wide body car, it is really clear that the traction limited end of my car is in front. The story on a GT-2 or highly modified Turbo would be the opposite. Hence Al Norton's battle with the 12.5" rear wheel, while I comfortably use an 11" rear with nothing more than a wider offset. In my case, I also have several other setup compromises that are made with respect to spring rates, sway bars, and camber and toe settings, since I also drive the car on the street daily. As a result of a lot of tire temperature data collection, load data logging, slip angle data logging, etc... I can see that what I need is more front end grip. =)

There are a lot of ways to increase front end mid-corner (steady state turn) grip: Stiffer rear springs/sway bar, softer front springs/sway-bar, larger front contact patch, for example. But, also useful in improving front end grip - according to most modern tire models - is an increase in front tire sidewall stiffness. This reduces the slip angle required to generate a specific lateral force, while increasing the maximum force that can be generated. When a stiffer sidewall is achieved by using a wider rim, the contact patch is also widened. Even if the same hot tire pressure is used, the wider/shorter contact patch will have more tire in adhesion, and less in slip. This increases absolute grip. And, a wider wheel increases grip while also improving response and feedback. It's one of those very rare win/win situations on the front end. It's almost impossible to find accurate, current data on tire load/grip models from manufacturers, but this book does have quite a bit of borrowed data in it: http://www.amazon.com/Racing-High-Pe...3066503&sr=1-1


The drawback of course is fit. I've done a lot now to help improve that situation, but I still have liner scorching occurring under heavy trail braking, and very slight fender rubbing under heavy corning. CCW is going to send me another set of fronts with slightly more backspacing to test. I happen to have about 10-11mm of clearance at the closest point between my wheel and strut, and John feels I can safely take this down to about 4mm, or 1/8". So, we're going to try another 1/4" of backspacing. That will eliminate my fender touches and may solve the liner rubbing. (The liner has been pushed forward/back as far as possible at this point, through heat treatment, and some other more invasive work on the mounting points behind.)

On the issue of camber - the more shims used in the control arm, the worse the fit of the tire. The more camber acheived at the strut top, the better the fit. So, it's difficult for us AWD guys w/o the reversible camber plates the 2WD cars can use. But, I don't actually need more than -3.5 of camber, and I'm a little beyond that right now, so I'll be pulling some shims out of the control arms shortly. That will help improve the fit further.

We'll see... if the new 10" 7.25" backspacing wheel doesn't work - I'll just wait for CCW to get their 9.5" forgings in. John is being super cooperative with the testing - since they are making the recommendations based of their experience, and all our cars have slightly different clearances. If the 10" wheel does work - I'll be lucky to have 1/8" of clearance (dynamically), I suspect.