R compound sizes for autocross

[cgroppi]

I just got a 2007 Z4mc that I will be running in the new AS class. I've already taken care of the alignment, an extra set of stock wheels, shocks (Koni 8711 DA), and I'm getting a custom hitch made next week for a tire trailer.

The next thing is the tires. Since it seems not many people autocross Z4m's, I am having trouble finding info on what tires will fit under the car without rubbing badly. The two main choices for tires are the Hoosier A6 and the Kumho V710. For the A6s, I could get 245/40R18s in front and 275/35R18s in back. For the Kumhos, the available sizes would be 245/35R18s in front and 285/30R18s in back. Never mind if they will fit on the wheel. That can be done. The question is will they fit under the car?

I'm pretty sure the Hoosiers will make it, not sure about the Kumhos. Anyone have any experience actually getting these under a Z4, or know of anyone who has?

-Chris

[The HACK]

Whatever size fits under E46 M3s will fit under an MZ4, except the inside front has about 10mm less clearance because of the strut design (the MZ4 uses E36 M3 style struts). So if you're in stock class with stock rules, you can probably safely run 245s up front and 255s in the rear (I assume stock rules stipulate that you can only run the widest size tire that came on the car).

Does AS allow for camber shims on the bottom of the strut?

[cgroppi]

Originally Posted by The HACK

Whatever size fits under E46 M3s will fit under an MZ4, except the inside front has about 10mm less clearance because of the strut design (the MZ4 uses E36 M3 style struts). So if you're in stock class with stock rules, you can probably safely run 245s up front and 255s in the rear (I assume stock rules stipulate that you can only run the widest size tire that came on the car).

Does AS allow for camber shims on the bottom of the strut?

The rules allow any tire you want, as long as you use the stock size wheel. I know that 275/35s will fit in the back. The main question is whether 285/30s will fit in the back. Both tires will fit on the wheel easily. Most CS Solstice and RX8 drivers run the 285/30 on 8" wide wheels (!).

AS does not allow for camber shims. Only procedures allowed in the factory service manual are legal. Removing the centering pin and maxing out the camber with the strut tops (-1.5) is legal, because that's the only method in the factory service manual for adjusting camber.

[Spreadsheet]

Has anyone else but me tried the new Hankook Rs-3 tires? They are AWESUM with tremendous grip and not a bad ride on the interstate. Recently took a 130 mile trip and would grade them about 10% noisier than stock, so really a non-issue for me. Maybe 10% stiffer, hard to decide a grade, so not that big a change.

I have a loop I take everyday on the way to work and IF I can get a clean run, used to take it at 50-55mph with the stock Contis and ride a drift all the way around. I do not have a clue as to how fast I can take the same loop with these tires. I popped in at 65+ the last night and it was rock solid. Having to re-calibrate my brain as this is a tight 25mph loop and attempting to hit it at 70+, which I believe these tires will handle, it messes with you at the turn in. Once you;re in the loop it's okay, just the enterance messes with me. I will get over it

Anyway, awesum tire and 245/40/18 and 265/35/18 fit just fine on the stock rims with zero rubbing. Car does have a little "head shake" over some roughness with the tires, but that's it. If you've cycled, not an issue, might be for someone who hasn't.

Take a chance and go for it.

[Trev]

Hankook RS2's & RS3's are a stonking tyre. They'll dare you to push & push & push until you suddenly fall out the other side.

As a street tyre they're darn good, plus they're quite reasonable price wise especially compared to Michelin, Bridgestone or any the other big brands.

As a race / ax tyre they'll disappoint. Hoosier A6's are the gold standard. TBO it boggels my mind that these are actually legal in a stock class. But them's the rules & tyre's are my #2 most important improvement.

[Spreadsheet]

Thanks for the feedback, I'll have to watch it as I'm pushing them to the edge.

Right now I'm not racing and only ax once a year or so.. Your assessment of them as a street tire is on the mark.

[Trev]

cgroppi, in answer to your question on whether a 285 will fit on the stock 18x9 out back. The official answer from Hoosier is "not recommended". The 275's will fit a 9"rim no problem. The 285's really need an extra 1" of rim width. However as you've pointed out other cars at AX Nationals do run the biggest tire possible. IMHO the wider tire's will balloon the sidewall, allowing more flex than the 275 which will reduce feel & reduce contact patch. If i remember years ago tirerack & grassroots motorsport magazine did an article on this very subject. But you pays your money & makes your choice. Let us know how you get on I'm very interested in your real world experience.

Tire Size. TreadWidth. Recom Rim. Measured Rim
P275/35ZR18 10.3" 9-11" 9.5"
P285/30ZR18 10.9" 10-11" 10.0"

Data from Hoosier's site https://www.hoosiertire.com/rrtire.htm