Michael W.

I am now looking into new wheels for my car and noted that almost all you with 19" wheels use 19 x 8.5 in front and 19 x 11.5 in the rear. I ran across some pictures of the Hamann wheels and really like them. However, Hamann uses 19 x 9 in front and 19 x 12 in the rear. The larger size looks pretty wild. Is there any reason why nobody else seems to be using this size? Is there any detriment to it?

Here is a link to pics:

<a href="http://wheelpower.com/hamann/carfolder/porsche/996_gt2_turbo.html" target="_blank">Hamann Wheels</a>

KPV

I am running the 8.5 and 11.5 setup. There are two things to keep in mind. First is offset (or complimentary, backspacing) and second is tire size. With the offset, you need to make sure there isn't any interference with existing wheel well mechanicals (struts, sway bars, inner fenders, etc.) as well as any future upgrades (spring perches on a new suspension, sway bars, etc.). With the tires, the ideal setup is for the overall diameters to match the factory specs, the front and rear overall diameters to be equal and the sidewalls to be per the tire manufacturer's spec. If your rim is too narrow for the tire, the sidewalls lean in to meet the wheel (this could lead to tread convexity leading to excessive wear at the tire center not to mention loss of contact patch area). If the rims are too wide for the tires, the sidewalls lean out to meet the wheel (this could result in tread concavity leading to excessive wear at the tire edges). Keeping the tire as flat as possible guarantees the best possible grip. In my opinion, the 315/25/19's and 235/30/19's seem to be near perfect on the 8.5 and 11.5 rims respectively. The wider wheels may require wider tires than these. You should check to see what they are using. Another important thing to remember (probably the most important) is that the overal front tire diameter and the overall rear tire diameter must match. If they differ from one another, the viscous coupling will account for the relative differences in wheel speed. This is bad and results in heat buildup in the VC. Different front and rear wheel speeds on a prolonged basis will potentially cause damage to your viscous coupling. In addition, the PSM, since it is programmed using basic physics, will receive incorrect wheel speed data and would potentially not function properly.

Jack

Ken provided some great info.

As to tire sizes, Porsche runs 315's on their GT2 12" rims -- so I'd feel comfortable running those tires on the Hamman 12" wheels. Also, with regard to the AWD/viscous coupling issue, while it would be ideal to have the overall diameter ("OD")of the front and rear tire/wheel assembly equal, slight variations are acceptable -- typically a differential of 1% is OK. On the stock Turbo, for example, the front Pirelli tires are .2" taller than the rear tires. BTW, Ken, my calculations could be off, but I believe the 19" tire sizes(235x30 and 315x35) you mentioned are not ideal for the AWD system if you're shooting for identical OD's front and rear -- the differential between the front and rear tires is more than 1/2". Whether this difference in OD between the front and rear drives would cause a problem is unclear.

One other important wheel issue to consider is the weight factor, IMHO. Some of these after market wheels are significantly heavier than the stock hollow spokes they're replacing. This increase in unsprung weight will negatively impact the car's performance, both handling and acceleration. Ideally, when you change wheels, it would be nice to keep the new wheel weight close to (if not lower)than the stock hollow spokes. Just my 2 cents.

Lydon

I believe he wrote: 315/25/19 & 235/35/19, those two sizes fall within the tolerance of .2. The 235's measure 25.4 inches, i'll measure the 315's when mine arrive. For my RUF rims 8.5 &11.5, these are the recomedded tire sizes!

KPV

Jack,
I agree with you on all counts sans one. My research (quite a bit prior to tire/wheel purchase) has indicated that my 315/25/19 AVS rear tires have an outside diameter of 640mm and the 235/30/19 AVS front tires have an outside diameter of 646mm. This 6mm difference in overall height equals .236 inches (slightly less than 1/4"). This amounts to 0.9375% (6mm/646mm x 100) which is compatible with your stated guideline.
<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />

KPV

Just for sh_ts and giggles, I decided to do the conversion based upon the data in my previous post (315/25/19 vs 235/30/19) and an assumed perfectly flat road and properly filled tires. Following are the results:

For one mile of car travel distance,

the rear tires make 800.422 revolutions

the front tires make 792.988 revolutions

Hmmmmmm.

Jack

Ken:

I'm sure your tire brand specific calculations are spot on. I'm betting you have 235x35 front tires, as Lydon suggested. <img border="0" title="" alt="[Wink]" src="wink.gif" /> Based on the math, a 235x35 tire (vs a 30 series) would be a closer match with a 315x25 tire. Works out to a difference of 6-7mm.

Each tire manufacturers specs are slightly different even though they all use the same sizing designation. That is, a Pirelli 225x40x18 can actually have a different OD than a Michelin tire of the same size -- they're usually within .1" of each other.

KPV

Oops, you were right Jack. They are 235/35/19's in front. I wrote the size wrong but the overall diameter I stated was correct at 646mm. I agree that the sizing between mfr's varies. That is something that has always bugged me. It would be great if it were all in metric (what's with the metric width and the english diameter in the tire size? ) and if it was standardized with all manufacturers. It would be great to take the width, multiply it by the aspect ratio and be confident that the actual sidewall height matched the calculation exactly. I guess the world is not a perfect place.......

msindi

Ok guys,
My current set of HREs is 19x8.5 in the fornt and 19x11 in the rear. The tire sizes are 235/35 ZR19 and 295/30 ZR19.
I am thinking of getting a new set of HREs with the same wheel sizes but with new rear tire size of 315/25....WOuld this work?? Or should I change the spec to 19x11.5 rear?? I really do not want to roll the fender and I already have big brakes and the PSS-9 is on the way.
Your input is always appreciated.