RonCT

I have my Volks from my narrow body 997S that I'm using with my GT3. I checked with Tire Rack's engineers and they agree with me that the wheel / offset will work just fine if not better than the 12" for a variety of reasons...

1. The 305 tire specifies up to an 11.5" wide rim and the measured width is 11". So an 11" wheel is much closer to optimal than a 12" wheel. The 11" Volk weighs less than the 12", so that's a small bonus.

2. The 11" Volk has an offset of 62 where the 12" is 65 (and OEM 12" is 68). That's actually 3mm to the better as the center of the 11" wheel is further out than on the 12". Granted, 3mm is nothing, but 3mm more track is perhaps better than 3mm less. The 12" will have 1/2" more "rim material" left and right of that center, stretching the tire.

The 11s looked and performed just fine...

Anyway, just wondering if I'm understanding this correctly?

todinlaw

I ran into this issue a few weeks back and posted a thread, interestingly, both Tire rack and Pirelli say that the max rim width for the 305 is 11 to 11.5 inches yet they mount it on a 12 inch rim. I think at the end of the day the 12 inch rim with the same tire should give you less sidewall flex and thus more stability and control. It will also allow you to jump up to 315's if you elect to do so. I don't think the differences either way are going to be tremendous so, it comes down to your choice.

Yargk

315s work on 11s, I have them on my 930, so 11 is sufficient. However, you'll usually get the best feel and best grip from the biggest wheel supporting the tire. Sure, tirerack says it's a no go, but if Porsche does it, I'm sure it works (why would the stock setup not be good for the tire) and since it's a bigger wheel it should be better.

305 on 11 is ok
305 on a 12 is better
315 on a 12 is the best imho (maybe even 325)

340Elise

I take it you have the TE37's? If so, I think those are a hidden treasure; an excellent lightweight wheel at down to earth prices!

"The 11" Volk weighs less than the 12", so that's a small bonus."

That is if you are comparing an 11" Volk to a 12" Volk; although they both show 22 lbs, I'm sure there is a difference. Where you are really making out on weight savings is compared to the stock wheels, which weigh just over 30 lbs! So if you stick with the same tires (brand, size, version), then you will lose a total of 8 lbs per wheel unsprung rotational mass just for the rears. I mention the same tires because tire weight can vary just as much as wheels can when talking about the same sizes, or sizes that fit. And the stock 19" tires that Porsche uses in both the Cup or Corsa tire (both excellent tires for street and track IMO) are probably the lightest you can find for that exact size.

As for 11" being optimal, I understand where you are getting that from, but I think it is the recommended rim width to use with this tire within the range of 10.5 to 11.5" rims. Recommended does not necessarily mean best performing. Sometimes just going with a wider rim and the same tire, will allow the tire to sit more flat on its contact patches and more squarely on the edges (without bulging). I know that the measured rim width for the stock tires is 11", and that is probably the best rim size to use for the tire being properly seated, but I'm not sure if it will give the best handling results???

I do wonder that Porsche picked such a wide rim with the 305 tires; a rim that is actually outside the recommended range. Why did they do this? To allow for more tire in the rear for those who want to go wider on the track? If so, then the front should have been a 9" for sure, if not 9.5". So why a 12" rim width in the rear with the 305 tires? The tires are specifically made for Porsche and engineered for this car; both for the Cups and Corsas. I think that the 12" rim gives these tires the best contact patch by really allowing them to sit squarely within the rim. I have two sets of stock wheels with both the Corsa's and Cup's and they are cleaned and off the car. I just looked at the rears and the fit is perfect for the rim, and like I said, they really fit squarely in the rim.

Also, if you look at the Cups, they are asymetcrical and have that outside edge surface without a major groove running through it. I'm sure this helps with tread squirm, because they do feel a bit sharper than the Corsa's (new tire to new tire), and I think it is important that this "shoulder junction" part of the tire sits properly on the rim without being forced inwards by an 11" rim.

This is just my guess, I am by no means a tire or wheel expert. I'm only theorizing.

Stephen

340Elise

To support this view, if you look at GMG Racing new WC GT3, they are running a 265 in the front with a 325 in the rear using the Cup tires. They are using the BBS E88 wheels in a 19" and left the rear at 12.0", but increased the front rim width to 10.5"! And I do not think this is a print error. If you are going to apply the same theory to the front wheels as the rear, then this 10.5" is about right, although I would think 10" would be plenty. But I bet they tested the 10" rim and got better results with the 10.5". Btw, I think the WC GT3 is just amazing and would love to test drive it. I'm guessing the difference between it and a GT3 is much greater than that of a GT3 and an RS. Just a wee bit!

997gt3north

- I have both
- I use the 11" for my rear winter tires 295/35/19 contis
- I will say that your above math is likely off (I will guess why below) but the 11" rear is not more outwardly offset than the 12" volk - if anything i would think you would use a spacer to push it more outward

Is this math correct?
- a 12" is 1" wider than a 11"
- that 1" is splits into 2 parts one part sits inside and one part sits outside
- 1" is 25.4mm, therefore 1/2" is 12.7mm

Therefore is an 11" 62 offset rim really equal to a 62+12.7 = 74.7 offset 12 inch rim in terms of how outwardly it sits?

am i correct or totally wrong?

RonCT

I'd say you are wrong in the math in the way I intended the point. Yes, the rim is wider and the overall maximum "outside" piece of rim in the 12" is beyond where the 11" would sit. What I was trying to point out is that the tire on the 11" with ET 62 will sit outside of where the same tire on a 12" ET 65 will sit by 3mm. So while it may "look" like the track on the 12" is wider, in fact the contact patch is actually 3mm wider per side with the ET62. I posed this question back to TR and they confirmed my theory. As to why Porsche decided on the 12" rim for the 305, I have to wonder why Porsche also puts the 305 on an 11" and an 11.5" oem rim on various 997 products. I don't think there's a right or wrong answer here, just trying to see if there's any motivation or justification to change from the 11" Volk to the 12" and I'm not hearing any because I'm not planning to go wider on the tire. I will also say that I have 305 PS2s on my oem 12" rims and they look terribly stretched and when I look at track photos it looks like the rims are separating from the tires at high G cornering.

todinlaw

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wbrownie

Ron,

Try this calculator...http://marksink.com/tire_wheel_offset/offset.html

Th 12 wheel will set 13mm outward of the 11 wheel therefore giving you the extra 12.7mm of track mentioned above.

Using the 11 wheel you should use a spacer to get the extra track.

TripleM

With 12j +65 vs 11 J+63
U will end up getting more inner clerance by 15mm and the position of the outside edge of the wheel retracts by 11mm

2 disadvantages using the 11
1. The rear looks sunken, so visually it's crap
2. The track width becomes smaller, so cornering speed gets compromised

So if u want to run 11J, u need 10mm spacers and u will hv more innear clearance but ur wheel will now look flushed

As for tires ....305 sits fine on 11-12.....being 11.5 probably looks best. On 12 it's a lttle stretched

Yargk

Stock is 68, but don't you use a 5 mm spacer so stock is really equivalent to a 63 without a spacer. So if you run without a spacer you have slightly more track (only 2 mm more track, one per side). The edge of your wheel is closer to the center of the car by about 13-1=12 mm because you would have narrow wheels, but if you put the same tire on the difference in tire will be less (tire doesn't follow wheel exactly). The stock 5 mm spacers might be enough to make everything look fine for this reason. Then you'd have 12 mm more track...

mdrums

That is the way all the race cars look. I just saw the tire rim seperating look on the Porsche newsletter I got via email on the Spyder and the Lizards car from some Sebring photo's. The GT3's I track with even including me are running 12x18 for the rear and 9x12 fronts.

RonCT

Thanks for the viewpoints. So final thoughts...

Yes, the car has 5mm rear spacers installed at the factory, so ET 68 oems are really 63, ET 62 11" Volks are actually 57 and so on. Yes, the tire track is actually 6mm better (3mm less offset each side) with the 11" Volks than with the 12" Volks and 12mm better than with OEM. Yes, either the PS2s or PSCs in 305 19" are stretched on a 12" rim. How wide the wheel is and whether it "looks" wider at the rear has nothing to do with anything because it's the tire itself that has the contact patch with the track. Sidewall on such a low profile 19" tire is very stiff, so whether on an 11, 11.5, or 12" wheel isn't going to have much impact. Conclusion is it's just fine to stick with the 11" Volks I already have instead of buying 12s because the tires on the 11s fit well, look perfect (they don't look "sunken" in fact they look better than the stretched look on 12s), and they have a 6mm wider contact patch track.

The wheel offset calculator is a great tool. It shows going from say 12" OEM to 11" Volk (the two wheels I have) will move the inside edge 19mm outward while the outside edge retracts 7mm meaning the tire contact patch is in fact moving outward 12mm in total as it is centered between those figures. If I plug in the 12" Volks, it's 16mm less 10mm outside for a 6mm centered tire advantage.

Couple of close-ups in the Esses at Watkins Glen. One shows the OEM 12s with PS2s and the other Volk 11s with PSCups. You have to look past the "rim protectors" on the PS2s to the contact patch.

Thanks everyone for helping me through this.

997gt3north

Ron,

- When I read your above note, I think the one possible mistake you may be making is that when you refer to the inside and outside "edges" make sure you understand that you are speaking about the edges of the rims and not the edges of the tire - clearly the middle is both the middle of the rim and the tire but the "edges" in the language you are using above are the edges of the rim and should not be specifically used as a meassure to calulate contact patch

- speaking less scientifically, i think the 11" volk is a very good rim and a good choice for the car for a few reasons:
1) it is weighs much less than stock
2) it is priced well
3) it is easy to clean
4) RAYS has a fantastic reputation so they should last a long time
5) the 997gt3 vs. the 996gt3 clearly is known to push more and since the 996gt3 is setup with an 11" rim running a 295 - i think this alone may better balance the car and help the car turn better
6) 11" rim fits all the tires you are likely to run (295 winter, OE mpsc, OE PSC, 295/315 hoosier, 305 R888)
7) it is easy to swap spacers if you want a wider rear track

From my multiple alignments on the car, I have found that the 997gt3 on a 12" rim running a 305 width tire would appear to need less rear camber than a 996gt3 running a 11" rim running a 295 tire. My guess as to why this is, is the "lever effect" - i.e. a 12" lever versus a 11" lever. The temp guage doesn't lie and I have found that -2.2 rear camber for a 12" 305 for the 997 provides similar temps that the 996 on a 11" 295 gets running -2.5 (obviously the different chassis / shock differences come into play). The reason that I am mentioning the above is that you may find that running the 11" volk you may need to run a little more camber than if you were running a 12" rim.

good luck


paul

RonCT

Thanks Paul,
I was trying to make a point that to me it all comes down to contact patch and that whether you use an 11, 11.5 or 12" wheel, the center of the tire and therefore the contact patch is dependent more on the offset (center of the wheel) than it is dependent upon the width of the wheel. So on the 11" Volk, the contact patch will actually be 6mm wider overall than the 12" Volk and 12mm wider than the OEM 12" wheels.

When Dan sets up the alignment, he'll do so with these wheels and tires on the car. I don't ever plan to go beyond 235-245 front and 305 rear as for now that's all the tire I can buy for the car (PS2, PSC, Corsa, R888, and Hoosier). If I ever make a switch to 315/325 in 19", then I'll have to think about getting some 12" rear Volks. Granted, I don't know much about the dynamics of a tire that's spread out on a 12" rim vs. one that sits properly on an 11" rim, how that really impacts sidewall flex, etc. Me, I'd rather have a little more sidewall flex than not as I simply don't drive on good pavement - there are always imperfections and as I found when I ran 18s with even more sidewall flex the overall "feel" of the car was better as it wasn't skittering over the pavement (think WGI Flat through the Esses).

I think this is great information and I'll be happy keeping the "free" 11" rear Volks I already own vs. buying a set of 12s.