m3irish

Tirerack's info is spotty and not what I would call a safety guideline. I think you'll find that a XXX mm tire is not created equal in tread width, tire width, and sidewall across the brands or even the models within a brand. The main issue is making sure the bead is well seated and the tire is aired up properly (that's what fortifies the tire sidewall). That being said a 295 on a 12" is ok and the stock 305 is most certainly ok and safe. A 225 on a 10-11" wheels is getting sketchy although I know some NASA racers running those setups for qualifying.

Here is great read on tires if anyone is interested. https://www.amazon.com/Racing-High-P.../dp/0768012414
There's a section in the book that talks about wheel width to tire width and what it does to the tire contact patch. Good stuff

DrJupeman

alignment?

m3irish

From what I've seen the RE71R and BFG Rival S are slightly faster than the NT01 (a 255mm was used in my experience. Ymmv based on surface type and driving style), although the NT01 wears better than the RE71.

Edit: as a side note my Spyder has four tracks days between COTA and TWS on its RE71Rs and I'm about half tread depth.

orthojoe

http://www.prima-racing.com/bridgest...king-part-two/

Everything you need to know

452gt3

I feel my alignment is good I did not use a pyrometer but I think my pressures may have been a little low. May have been able to get one or two more sessions out of the RE71R. I will try them again this weekend with a little more pressure.

SmokinGTS

Joe we just need the 295's and 305's.... RE 71R's

GrantG

While I agree the dimensional variations you mentioned above do exist, all manufacturer info that I have found seems to share the same recommended wheel width for a given size. And if you look at all 26 models of the 295/30/20 tires on Tirerack, I think they all have the same 10-11" rim guideline (should match individual manufacturer recommendations on their websites as well):

http://www.tirerack.com/tires/TireSe...earDiameter=17

Click on Specs for each tire model (295/30/20 will be separate at top of each list)

Michelin's website also lists 11.5" as the max for the OEM GT3 rear 305/30/20 tire (unfortunately, Bridgestone's site is blank for the 295):

http://www.michelinman.com/US/en/tir...tail-sku-table

Click on Right Arrow next to "More Data" to expand.

Don't think there is anything wrong with Tirerack's data with respect to rim widths, unless unavailable.

The one exception to the industry standard rim recommendations is Hoosier which recommends rims to be within +/- 0.5" of tread width:

http://www.hoosiertire.com/images/co...s/TCTR7_A7.pdf

"Wheel Widths
Wheel width dramatically affects wear and performance of the Hoosier P-Metric radial tires.
There is about a one inch window of optimum width. The trick is to figure out that window. A good rule of thumb to use for determining proper width
is to use the tread width of the tire. Measure the tread width. Plus or minus 1/2 inch from the tread dimension will indicate the proper rim sizing. It is
possible to use narrower wheels, but at a sacrifice to shoulder wear and cornering power."

m3irish

Grant, I get where you are coming from and what they post is called "CYA". If you go by Hoosiers mounting recommendations, I'm not sure you could actually mount a Hoosier. Checkout and read that book I posted. Specifically, the section on wheel and tire widths.

mqandil

Hi Joe. Thanks for the update on these tires. Quick question, why did you go with stiff setting on your front bar in the first place? Did you notice any understeer with that sitting? Just curious. Mark

orthojoe

I did it because I had problems keeping the back end planted on corner exit and a stiff front bar helps with that. I then added stiffer springs up front to combat front tire wear, which introduced understeer on turn in, so I went back to the middle setting up front. Then the rear 295 re71r were put on and the back end got loose again. I'm chasing my tail, unfortunately

MaxLTV

Joe, I do not understand why you are having these issues. My set up is fairly simple, and it works reasonably well with no real mods - just shims to get to the right camber and caster pucks to get caster back to stock levels. It's not ideal - entry oversteer/understeer balance is still tricky and rear tires wear on the outside a bit more than inside, but the tire wear is even, it's reasonably fast and fun to drive (oversteer is more fun than understeer).

Front: -3 camber, slight toe-out, stock caster, medium bar
Rear: -2.5 camber, stock toe-in, stiff bar

I'd try -2.9 or -2.8 front and -2.6 rear if I were not too lazy/busy.

could you try plotting pivot angle relative to steering angle to see where the car understeers and where it oversteers? When I did that, I noticed that many of my oversteer events were a result of me fighting mild understeer and actually making car oversteer by that (often without realizing it). So I stiffened the rear bar and the car started to oversteer LESS. It also reduced front tire wear.

Manifold

A while back, I talked with the owner of a local Porsche shop involved in pro racing, and he told me that changing the front and rear bars of the GT4 and 991 GT3 can have effects opposite of what one might typically expect. I didn't understand it (and still don't), but your experience fits what he said.

I moved my front bar to stiff, and didn't notice that making understeer worse. If anything, front grip may have improved, and in general the front of the car feels more responsive, with better turn-in.

orthojoe

Max, the only difference I have in mods from you is the front springs. It was my original setup that you are using. The springs were an experiment to see if I could get the front tires to wear slower and even up with the rear. It didn't work. The car was fine before the the switch to 295 rear. Now I'm trying to make the necessary adjustments to make the 295 work.

stuntman

TireRack has some of the most detailed information on tires across all brands than any other source out there. It's unbelievable that they use a standard measuring practice for the section (and especially) tread widths that they do to EVERY make, model, and size they offer.

This is a MUST-READ to better understand the aspects of a tire and how to properly size them for a given rim width:

http://www.motoiq.com/MagazineArticl...RFORMANCE.aspx


0.02

MaxLTV

I see. Makes sense. I think we have different anti-roll bar set up though - I have medium front and stiff rear. Fronts live 3 fast days (5 sessions @ 20-25 mins) and 1-2 slow days and rears live 4 fast days and 1-2 slow (and scary) days. So I have almost even wear now.

I really wish 71R tires would work well - it would make this car a track bargain, seeing how other maintenance items are already cheaper than they were for E90 M3.

Max