[farman]

Hello. I have posted this on Tire/Wheels tread - no response so far.
I am about to order tires (from Tire Rack using link from this site of course ).
My car is 07 328i Sport Sedan and wheels are Type 161 (Front: 17x8 ET34 225/45 R17) (Rear: 17x8.5 ET37 255/40 R17).
I've read whatever info I could find on this site about square setup, but still not sure 100%. Need your expertise.
My question is: should I use original size tires (225/45-17 front and 255/45-17 rear) on Type 161 wheels? I was thinking about running square 245/40-17 Dunlop Direzza Sport Z1 Star Spec. Will it improve handling/understeer? Any potential problems with these? I went thrugh couple HPDE sessions on RFTs and now is time to get something better. I believe 245/40-17 is the biggest size that I can safely put on front wheels as they are only 8" wide.
Please help. Thank you.

[mid-corner fun]

lots of threads on the topic, search for "square setup"

[farman]

I tried hard to find an answer... All posts about square setup assume that all wheels are the same and tires are of the same size. My question is if there will be any advantage in car handling on track while running same size tires on staggered wheels or I shoul stay with OEM sizes even for occasional track days?
Sorry if my question does not sound right, I am learning.

[The HACK]

There's really no way to be sure what the effect would be unless you're willing to test on the track, especially when you say "understeer" there's a lot of different kind of understeer that require different fixes, one of the many being square tire set-up.

Also, you need to keep in mind that putting tires that are too wide for the intended rim, or too narrow for the intended rim, may have negative consequences with regards to how the tires behave. Tires usually have an "ideal" foot-print on a specific width range, and if your rim is narrower than the ideal width range then the result is sidewalls that bow in resulting in squirmy-ness mid-corner, or tires that are mounted on rims wider than its ideal foot-print may result in the sidewall "stretched" out that it doesn't create the ideal contact patch and results in reduced grip.

If you ask me, you're going about this the wrong way. First determine when, where and how you are understeering. Then find ways to fix it. Going "squared" is usually one of the last step necessary, sometimes a simple added camber up front fixes 99% of your "understeer" ills.

I run 245f/275r on a very "balanced" chassis and have no "understeer" issues unless I decide that I want it.

[zsapphire7]

just do it . running square helps a ton. I def would not go back to something close to stock staggard. The most i would do would be 245front and 255 rear if you really wanted to stagger it. Even with a square setup i doubt youll have any oversteer issues unless you modified your suspension to make it otherwise..

[S4to335]

I personally wouldnt try to put the same tire size on two different width wheels....one might be fine..but the other might have tons of tirewall flex...or you will be rolling over on the rim too easily. If you are gonna run square..then run the same size wheel on all four corners.

[farman]

Thank you all for the suggestions. I understand that putting tire on too wide or too narrow rim will affect its performance. If Dunlop 245/40-17 spec says 8"-9.5" wheel range can I assume that tire should perform to spec or that is wrong assumption? I just want to understand exactly what Rim Width Range in spec stays for. Is it Min - Max allovable or something else.

[S4to335]

Originally Posted by farman

Thank you all for the suggestions. I understand that putting tire on too wide or too narrow rim will affect its performance. If Dunlop 245/40-17 spec says 8"-9.5" wheel range can I assume that tire should perform to spec or that is wrong assumption? I just want to understand exactly what Rim Width Range in spec stays for. Is it Min - Max allovable or something else.

If you put 245/40/17s on an 8" front rim..and 245/40/17s on a 9.5" rim in the back..the car will most likely act strangely. Putting that on an 8" rim will bulge the tire out on the side wall and affect how it turns. Putting 245/40/17 on a 9.5" rim might be stretching that tire to the limit. Yes, it will officially fit..but will not be ideal. You usually want to be in the middle of the range of what COULD fit. A 9.5" rim really needs a 255 or 265...maybe even a 275 width tire. Talk to jbass on what he used on his car..since he used the Direzzas.

[dgbuilder]

Echoing earlier wisdom, to run square, you need to match tires and wheels, otherwise you really are not square. The characteristics will change and you may as well still be staggered.

FWIW, i run 235/45/17 Direzzas on 17x8 wheels squared and love the difference it makes. You will not regret it.

[farman]

Thanks for the detailed explanation S4to335. I am not saying that I will do it, just asking: if I put same 245 tire on 8" and on 8.5" rims, and both widths are allowed by tire spec and close to each other, will it make big difference from practical standpoint or not? Is it better to have 255 rear staggered or 245 all around? Does car gain neutral balance with 245 square but loose some traction?

[The HACK]

You're really making this harder than it needs to be. If you HAVE TO RUN SQUARED, go with square rims as well.

To find the "ideal" width rim for the size tire, they're usually listed on manufacturer's spec as the "measured rim width," meaning to achieve the 245mm wide foot-print it needs to be measured on a 8.5" rim:

http://www.tirerack.com/tires/tires....t+Z1+Star+Spec

This means on a 8.5" rim it achieved the 245mm foot-print as measured, any size above and below will result in a smaller contact patch due to tire sidewall stretching out or bowing in. Since the front and the rear of the non-M 3 series has roughly the same clearance running a 8.5" all around (say, buying two more rears to match) would give you the desired result with 245/40/17 tires.

Still, I advocate TESTING to see about getting desired result, because, in my not so honest opinion, you can probably get better results with regards to steady state understeer with added camber up front than with wider/square tires.

[farman]

Thank you guys. This was very informative. Your suggestions are very appreciated. I will look for another pair of 8.5" rear wheels and then put everything square per The HACK's suggestion.