jmartpr

Those of you that have run the DH Slicks on their GT4 have you tried the 275/675-19 on the front? I'm running the 255/650-19 but they do lower the front almost a full 1" and the 275 would be almost perfect while gaining some width.
Wheels are Forgeline in 10 x 9", -3 deg front camber using plates and shims.
While the diameter looks Ok...I wonder if it would be too wide?????
Going by pure specs they would be a full 1" wider than stock (OEM Michelins). 0.8" more than the DH 255s I currently have and 0.4" more than a Hoosier 265/35 which I know some of you guys run.
Any feedback is welcome.

911therapy

sorry, I am no help but I'd like to know as well. Judging by how close my 265/23/19 R7's are, I'd guess the 275 slicks would be to wide

what size rear Pirelli DH slick are you running?

jmartpr

I'm running the 305/690-19 on the rear and fits just fine...same diameter as OEM and 1" wider in section width...but I think this Pirellis don't run as wide as the specs mention.
I have been running that tire on the rear on 11" Forgelines and -2.5 camber.....no problems so far. Perfect fit.
Would hate to order 2 275s and then not being able to use them but looks like I will be the guinea pig on this one.
Per the spec sheet the front 275 will be 0.4" lower than OEM compared to 1.2" lower on the 255...

911therapy

Well, always fun to do R&D ....and expensive usually...good luck and let us know how it works out

Thx for the info

jmartpr

Will get the car on the lift, mount the 255s and take measurements....that may give me some idea of the space I have.
I can get two DH scrubs at a fair price and test those...worst case I'm out $300.
People using the Hossiers 265 usually trim the front brake duct so the 275 DH if they are 0.4" wider than Hoosiers in fact would require more trimming, which is OK.
On the GT4 CS Pirelli offers a 265 but in 18"...for the GT3 Cup a 285 in 18" again.....I wonder how similar those two could be to a regular GT4 in the front wheel area.

jmartpr

A bit more info on this....Did a quick search in the GT4 forum and found reference to a GT4 owner using RE71R 265/35-19....those are 10.7" wide basically the same as the DH in 275. Also found other using RE71R in 255 which are 10.2.
I think I will get two fronts in 275 to test...only way to be 100% sure.

911therapy

Im on pins and needles...I'd love to run that 275 on the front if it will work. I am tired of my R7's spinning/slipping on the wheel and getting out of balance (plus, they are pretty tough to balance from the jump

jmartpr

Tires are on their way to me right now.....Todd from GT Racing Tires took a quick measure of the 255 vs 275 and as expected they come well below the advertised spec by Pirelli. We will see how wide they end up once mounted in 9" Forgelines.

jmartpr

275 DHs Tires are here and unmounted measurements confirm what GT Racing Tires mentioned...about 0.25 to 0.35" wider than the 255 DHs.....So far looking good as they came well under the advertised specs.
I should have them mounted on 19 x 9 forgelines later this week and do the actual static and road test during the weekend.

scott40

Question for the guys using the DH slick on their GT4s...are you using this tire with the OEM suspension? I am curious to know how well the GT4 stock suspension can handle this tire on track.

Bill Lehman

I've run them on my track prepped 987.1 but not on my GT4. They need a lot more camber. For the front you can get it by shimming the GT3 LCA, In rear you need adjustable toe links as well as some shims. The GT4 also needs attention to rear bump steer under braking.

jmartpr

Besides what Bill mentions I would also do camber plates up front to tuck the tire in....you need -3 degrees up front and around -2.5 rear. On my GT4 I use DSC, TPC rear toe links and front Tarrett camber plates + shims on the lower LCAs (F+R). The DSC really helps control the car under braking while the TPC links round up the package minimizing toe change. I could use a bit better front caster....have the adjustable RSR trust arms and LCA solid bushing but haven't come around to installing them since I'm selling the car early next year. Car feels great and it was a big jump from MPSC2/RE71/Trofeo Rs to the DHs...even that I'm using scrubs. If the new front ones work well I could consider buying a new set of DH as we don't have that many track days and are single; day with 4 15 minute hits.

Yargk

How do you adjust the rear bump steer?

Bill Lehman

Lots of toe help, and the TPC link works with less toe.

Yargk

Ah I see. I thought you meant the bump steer itself can be adjusted like the front can. I see it as the toe curve itself is the bump steer. So adjusting bump steer would be changing the shape of this curve. Adjusting the toe shifts the curve over (in the toe variable), but doesn't change its shape. (Although this is still important, I run 3/16 total toe in). Switching to the TPC link should change the bump steer curve to something else, so perhaps that could be considered a bump steer adjustment. However, strictly speaking, I don't think the rear has a bump steer adjustment in the sense that people usually mean, that is, a continuous adjustment of the shape of the toe curve.