Autocross wheel and tire size question for '06 Z4 3.0i...

[dudekma]

I had a question about what size front and rear tires I can fit on my 2006 Z4 3.0i. I do not have the sport package and the standard tire size is 225/45R17 all around. As you may know, the sport package comes with 225/40R18 on the front and 255/35R18 on the rear. My question has a couple parts to it:

1) Can I fit the sport package tire sizes on my non-sport packaged car?

2) Even if I can, should I try to stay with 17s as they may handle better (you tell me from your experience please) and try to fit a wider 17 on (please tell me what size if you know what would fit.)

3) Also, depending on your recommendation, what are the optimal sizes rim I should place them on? I will be purchasing the rims and tires together.

I already plan on purchasing the Dunlop Dirreza Star Specs as tires, but I want to know what I can fit on the front and rear for autocrossing. I am already at the bottom of class C with 107 points, so I have a lot of room to play with and the only other modification I will be doing, eventually, is a sway bar. I do not want to cause any damage or unnecessary wear on my car by putting on tires that are too large for the suspension, etc... I would appreciate your input if you have the time.

Thank you!

[montaillou]

Not much help in this response, but I'm curious about your "class D", I'm assuming that's not a stock category? If it is, and you put on a sway bar, you'll take yourself out of stock. If it isn't, maybe you should look at getting into a stock class instead of trying to compete with all the modified cars.

[vachss]

Not much help in this response, but I'm curious about your "class D", I'm assuming that's not a stock category? If it is, and you put on a sway bar, you'll take yourself out of stock. If it isn't, maybe you should look at getting into a stock class instead of trying to compete with all the modified cars.

Umm... last I looked SCCA stock classes allow free choice of FRONT sway bar, just no modification to the rear. So I would assume the OP is considering the addition of a stiffer front bar (which may or may not be a good thing depending on how the rest of the car is set up).

Also I thought that changes in rim size weren't allowed in SCCA stock classes. If you go from 17 to 18 inch rims I think you need to change over to a complete Sport package suspension at the same time to remain stock.

Physically though, you can fit 18" sport package rims on a non-sport package car with no problem. The main argument against this is usually aesthetic, as the higher suspension of the non-sport Z4 can make the empty space in the wheel arch look objectionably large when lower aspect ratio 18" tires are used instead of the stock 17's.

[dudekma]

I must clarify that I made a mistake in saying class "D" I actually meant class "C" I will adjust my original response to reduce confusion for new readers, sorry. Also, I am referring to the BMWCCA classification (I realize there is a big difference between that and SCCA, so I apologize for not making this clear earlier either). I will be doing some SCCA and Renegade Miata events as well, but I am not concerned with classification in those clubs for this year.

Vachss- Thanks for your input! You are right about the stiffer front sway bar. In saying "eventually," I am vaguely referring to the fact that I do need to understand this car before I start making changes, such as a sway bar. I will probably do that halfway through the season if I feel the need. I just bought this car last week, so this will be my first season with the car. Once I am very competitive, then I will worry about national events, etc... For now, my main interest lies in my local BMWCAA autocross events where some friends of mine are also racing and the events have been run smoother than other clubs, in my experience.

Here is an overlook at the classifications for this club if you are interested (at least the Boston Chapter): http://boston-bmwcca.org/EventDescri...oss-rules.aspx

That said, if I can't go for a larger rim size in SCCA, can I go wider? If you have a link to the SCCA classifications and allowable modifications, I would appreciate it.



Thanks for your comment as well montaillou!

[Z3SpdDmn]

1) Can I fit the sport package tire sizes on my non-sport packaged car?

YES

2) Even if I can, should I try to stay with 17s as they may handle better (you tell me from your experience please) and try to fit a wider 17 on (please tell me what size if you know what would fit.)

Yes. 17's are much lighter. 17x9 with 245/40/17 should be a hot ticket for autox, but there might be a 255/40/17 available.

3) Also, depending on your recommendation, what are the optimal sizes rim I should place them on? I will be purchasing the rims and tires together.

If you run the 245's, you can get away with 17x8 wheels, but that would be the minimum. Contact patch would be better with a little more wheel width in the 17x9's. See what the other Z4 guys are running.

I already plan on purchasing the Dunlop Dirreza Star Specs as tires, but I want to know what I can fit on the front and rear for autocrossing.

245 (maybe 255 will fit). Don't staggar. Keep same size front/rear.

Here's a helpful tool:
http://www.rims-n-tires.com/rt_specs.jsp

That said, if I can't go for a larger rim size in SCCA, can I go wider?

I think you can go 1" wider than stock, but don't quote me on that.

You might opt to put autox tires on your stock 17" rims and get blingy 18 or 19" rims/tires for the street

[dudekma]

Z3SpdDmn- THANK YOU! That's exactly what I needed to know. The website is helpful as well. If I do the 17x9 rims with 245/40R17s, will I have to worry about the wheel hitting the wheel well when I am at maximum lock, or at any other time I guess?

Also, not that I disagree, why shouldn't I staggar? I'm just curious...

Thanks a million!

[Z3SpdDmn]

Z3SpdDmn- THANK YOU! That's exactly what I needed to know. The website is helpful as well. If I do the 17x9 rims with 245/40R17s, will I have to worry about the wheel hitting the wheel well when I am at maximum lock, or at any other time I guess?

Also, not that I disagree, why shouldn't I staggar? I'm just curious...

Thanks a million!

Not if you get the offset right. You'll have to play with the link I showed you. Put your stock wheel and tire size on the left and the new wheel and tire size on the right. The offset will be critical to figure out where the extra tire is going. Briefly, I can say that if you use the stock offset, I think it'll clear fine. You may want a bit more offset to space the wheels out, though. Be careful that you don't then run into the fender!

The car's balance and rotation is better on a non-staggared setup. Your current setup is not staggared, right? Also, you can rotate your wheels/tires. Bonus.

FWIW, I run 17x8 BBS RK's with 235/40/17 RA-1s on the track.

[dudekma]

Thanks again! I just figured out what I was doing on this link haha I'll need to call the dealer and find what the stock offset is as it seems to make a world of difference.

What are your thoughts on your current wheel and rim size? Do you wish you went larger with anything? Do you happen to have a close up of what the tire looks like on the rim? You've been a huge help!

BTW, 3.0si coupe: BEAUTIFUL CAR! *thumbs up* I really wanted one of those but couldn't afford it. Maybe next time...

[Z3SpdDmn]

Thanks again! I just figured out what I was doing on this link haha I'll need to call the dealer and find what the stock offset is as it seems to make a world of difference.

NP man.
www.real-oem.com
You can find your offset there. Or, on the backside of the wheel. It'll be written something like "ET42", which would be a 42 mm offset.

What are your thoughts on your current wheel and rim size? Do you wish you went larger with anything? Do you happen to have a close up of what the tire looks like on the rim? You've been a huge help!

Not sure on the current rim size. If they're 17x8's, I'd consider using your stock wheels for autox. Actually, I'd just put the StarSpecs on them and run them Spring thru Fall. No need for another set just for autox.

I don't have any real close ups. Here are a couple pics of the car on track, though. The angles might help you see what you're looking for. I'm not sure who's driving in the first pic, lol. In the second one, I'm instructing my dad. That was an experience!

[dudekma]

Two more questions:

1) According the the tire size calculator you sent me, there seems to be no difference in the contact patch size between a 225 vs 235 under the "recommended" inflation. My question is would I then run a lower pressure than normal to obtain the advantage, or does the advantage come when the tire flattens out under load (i.e. hard braking/acceleration/cornering)?

2) Also, would you recommend going with 235/40R17's or 235/45R17's and why? Again, my stock size is 225/45R17.

[Z3SpdDmn]

1) According the the tire size calculator you sent me, there seems to be no difference in the contact patch size between a 225 vs 235 under the "recommended" inflation. My question is would I then run a lower pressure than normal to obtain the advantage, or does the advantage come when the tire flattens out under load (i.e. hard braking/acceleration/cornering)?

I believe the "contact patch" you're referring to is actually the rim width. The contact patch should increase proportionally to the increase in section width. That would be ~10mm. In the real world, though, not all tires are exactly the same. A 245 Dunlop Star Spec may actually be wider than a 245 Kumho SPT. Its a way they can sneakily create a performance advantage. Its very common in race tires. Tire Rack MAY publish the actual section widths under the tire details section.

2) Also, would you recommend going with 235/40R17's or 235/45R17's and why? Again, my stock size is 225/45R17.

I would suggest 245/40/17 or 255/40/17's, honestly. But between the 235/40's and 235/45's, I'd go with the 235/40's. The 45's look too tall (I've gotten them before), but the shorter 235/40's will give you a slight gearing advantage. If 255's will fit all-around, I'd get them. 245/40/17's may be more plentifully available, though.