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Autocross tire and wheel sizing

Discussion in 'Autocross' started by SpeedOften, Dec 19, 2010.

    • Member

    SpeedOften

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    I'm looking at a set of Team Dynamic Pro race 1.2 wheels: 8.0 x 18 e11 - 45 and/or 9.0 x18 e15 - 35 but I wasn't sure what would be the best backspacing for an 07 335i w/ ZSP.

    My other question is about how the car is staggered in OEM form, 225 up front and 255 in the rear. Do you recommend staying staggered or squaring the car up for autocrossing and the eventual move to more hpde's? I was thinking about getting some 265/35's on 18x9's in the rear and some 235/45's for the 18x8's in the front but I just was not sure.

    I am coming from autocrossing a momentum car (SVT Focus) so my driving style is geared towards that type car. Any recommendations on what I should do differently with the 335i?
    • Member

    CRKrieger

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    Part of the answer may lie in where you want to be classified. Read the wheel rules carefully before making a choice that would bump you into a class where you might not want to be.

    Another part of the answer lies in what your car does now. I would assume that it understeers (since all BMWs do, in stock form). We all know how much fun THAT is on an autocross course. To move the handling more toward neutral/oversteer with wheel and tire sizes, you'd reduce rubber in the rear/add rubber in the front or, as you suggested for this already-staggered setup, use four same-sized tires, which has the same relative effect.

    Even if you don't get enough oversteer effect from running the same size tires all around, you'd be closer to being able to power oversteer if you haven't 'over-tired' the rear and can break it loose as you please. With your car, that should work, opening up a new 'tail out' autocross style to develop - if you wish.
    • Member

    SpeedOften

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    If I understand it correctly, per SCCA rules I have to be within 6mm of the effective OEM backspacing to stay in B-stock. If I went to STU, the largest tire I could go to would be 265, along with a 140 treadware. I ran ST class this past summer.
    • Member

    conechaser

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    The rule for variance in wheel offset in SCCA Stock class is +- 0.25 inch which would be 6.35 mm.

    From the 2010 Solo rulebook:
    http://www.scca.com/documents/Solo_Rules/2010_scca_solo_rules.pdf

    Stock class wheel rules:

    13.4 WHEELS
    Any type wheel may be used provided it complies with the following: it
    is the same width and diameter as standard, and as installed (including
    wheel spacers if applicable) it does not have an offset more than
    +- 0.25 inch from a standard wheel for the car. The resultant change in
    track dimensions is allowed.

    Wheel spacers are permitted, provided the resultant combination complies
    with the offset requirements of this section. Wheel studs, lug nuts,
    and/or bolt length may be changed.


    Street Touring tire rules let you go up to 285 section width in STU:

    14.3 TIRES
    Tires must meet the eligibility requirements of the Stock category (excluding
    13.3.F), with the following additional restrictions:
    A. Tires may have section widths up to and including the following:
    ST, STS, STR (AWD) - 225 mm
    STX (AWD), STU (AWD) - 245 mm
    STR (2WD) - 255 mm
    STX (2WD) - 265 mm
    STU (2WD) - 285 mm


    Street Touring wheel width rules are unrestricted in STU:

    14.4 WHEELS
    Any wheels are allowed with widths up to the following:
    ST, STS, STR (AWD) - 7.5"
    STX (AWD) - 8.0"
    STX (2WD), STR (2WD) - 9.0"
    STU - unlimited


    I think your choice may be more driven by a couple things.
    What tires are available in the sizes you are looking at? Different brands of R comp tires may not come in the size you want/need. (Same with ST tires)
    Do you want to modify your car more than what's legal for Stock class?
    • Member

    CSBM5

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    The 335i is in F-stock. If you want to stay in stock class, you have to run stock wheel widths, so 8" F and 8.5" R. The stock offsets for the E9x 18" wheels are 34mm front and 37mm rear.

    You can easily mount 265/35 all around on stock wheels widths and have no clearance issues. However, for stock class, there isn't a V710 or A6 in that size. The 255/35-18 A6 is a monster 255 (actually it is 275 in width, measures something like a 275/28-18), so it comes down to either the 245/35 A6, the monster 255/35 A6 or the 245/35 V710.

    The car will eat the daylights out of the front edges even with the pins tapped out for max negative camber (you can get about -1.2 which isn't enough) and a large front bar. A6s will only last about 20 runs before you have to flip the fronts (don't let them cord that outer edge which they will do QUICKLY on this car).
    • Member

    SpeedOften

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    My plan is to modify it slightly, but not excessively. I was thinking about getting H&R sport springs to go with either Koni Yellow's or Koni FSD's. I have not researched enough yet about sway bars to make a decision. I also plan on getting an intake as well as a cat -back exhaust of which I'm also still researching. As of right now I do not plan on getting a tune for a few years.

    My plan also is to learn the car as much as possible with street tires before moving to R-comps.

    Thank you for the advice on the tires!
    Is that a new change for 2011 of the 335i going to F-stock?


    My best showings from last summer: 4/11/10 http://www.odr-scca.org/index2.php?option=com_docman&task=doc_view&gid=101&Itemid=29
    and on 5/2/10 http://www.odr-scca.org/index2.php?option=com_docman&task=doc_view&gid=104&Itemid=29
    and a video of one of the 5/2/10 runs: http://www.youtube.com/watch?v=InHga1gRIR8
    • Member

    CSBM5

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    Nope, it's been in F-stock for a couple of years, I think since 2008.
    • Member

    SpeedOften

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    Thanks, I think when I used the search function of adobe I must have misread BSP for BS...:eek:
    • Member

    conechaser

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    If you are thinking of adding lowering springs, sway bars (though Front is allowed in Stock) intake, and wider wheels... then you don't need to watch the Stock rules anymore. Any of those mods will take you out of Stock classes. It seems that you are well on your way to STU with an eventual arrival in BSP.
    • Member

    SpeedOften

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    Thanks!

    The Team dynamic wheels I'm interested in only go to et of 35. Will this be sufficient to run in the rear? Also, to run a square set up, using the same 18x9 wheel, should I keep the stock backspacing of 34 or should I use the same backspacing front and rear?
    • Member

    CSBM5

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    You'll need to do some measurements on the car with your existing wheels/tires and then do the calculations necessary. Note that backspacing changes with wheel width (if you keep the offset constant), so you can't refer to backspacing by the offset value.

    The stock rears are 8.5" wide with 37mm offset, so they have a backspacing of (8.5/2) + (37/25.4) = 5.7". A 9" wheel with a 35mm offset has a backspacing of (9/2) + (35/25.4) = 5.88", so it is about 2/10ths of an inch closer on the inside.

    I've run 8.5" rears with 44mm offset on the E90 with very wide for a "265" r-comps, 265/35 (BFG R1), with no rubbing (6" backspacing), so you should have sufficient room.

    I would run the same size wheel and tire all around -- eliminate the stagger, balance out the car, and allows for rotation to even out front edge wear. Since you're going with springs, etc, absolutely use camber plates up front so you can get some serious negative camber.
    • Member

    SpeedOften

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    Thank you very much for all this advice! Really appreciate it!
    • Member

    conechaser

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    And with a square wheel/tire package you can also rotate them front to back to equal wear without any issues. If you need to gain a little more track you can always use a small spacer to move the wheel away from the strut on either end of the car.
    • Member

    CRKrieger

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    If it's legal to use spacers in your class ...
    • Member

    conechaser

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    Correct. Since we've been talking SCCA classes, I can't think of a SCCA autoX class that doesn't allow spacers of some kind. Even Stock class allows up to a 0.25" spacer as long as you don't increase your offset and track beyond the +- 0.25" allowance. You can also change your wheel stud, lug nuts, and bolt length if needed.
    • Member

    CRKrieger

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    I'm from the Club & Pro Racing scrutineering area. Track has maximums, regardless of how you get there. One of the 'noninvasive' post-race checks we often do is track width - and I have seen competitors disqualified for it.
    • Member

    conechaser

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    Yep, you are correct, it's only OK if the rules for the class allow it. In Stock class, if some one used OEM wheels they could use a 0.25" spacer and be legal. If someone used an after market wheel that the offset was so that it made the wheel stick out an extra 0.125" then that 0.25" spacer would make them over the limit but a spacer of up to 0.125" would be OK. (0.125+0.250=0.375, 0.125+0.125=0.250) If another aftermarket wheel tucked the wheel IN by 0.125" then a spacer of up to 0.375 (-0.125+0.375=0.250) would be OK. It's all about the sum of the total of the offset and spacers if used for Stock Solo rules.

    I drove an SSC Neon from 2002 to 2004 and then moved it to ITA for a couple of years so I know the drill for rolling thru Impound. The Impound during Runoffs qualifying was always a treat (I never finished high enough to go to impound after the race and get the invasive treatment at the Runoffs). Now my wife and I go to the races to flag and work timing.
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    Satch SoSoCalifortified

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    Simple rules for simple minds

    When I got to San Diego, home of mile-long autocross courses, I found one very effective local rule: R-compound tires put you in the Modified class! Whoop!

    Lately the SD Chapter has revised its rules to make for more equitable groupings---not that it's going to help a certain geezer in his roadster.

    I say they should knock a tenth of a second off our time for every year we've held a license!
    • Member

    mattm

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    SpeedOften, I'm thinking you will want to look at the ST (X, S, R) rule sets to do some of the add ons you are looking at. I am not at all versed on the ST classes and classing so it would be inappropriate for me to recommend a place to play in SCCA. If you wander over to sccaforums.com there are many knowledgable folks there (certainly smarter thanI!) who can assist you in classing and things that would make sense to do to the car. On that note, do a driver's school like a Evolution performance Driving School http://www.evoschool.com/ That'll be best bang for your buck right now. Most of the instructors are multi-time National Solo Champions. Fix the "nut behind the wheel," is what most seasoned foilks say.

    As Satch noted, many regions toss "R" compound cars into sort of a catch all (I'll beat 'em up in my 318i anyway), so keep that in mind. There are some great choices for a "street" tire (Bridgestone, Kumho, etc.). If you stay with an R tire, Hoosier has great tires, but they are not really designed for driving to and from an event. FWIW, I've been running DOT legal Hoosiers since 1996. :)
    • Member

    TomF BMW EnthuZiast

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    Resurrecting a really old thread . . .

    With my 335is Coupe I currently run in the Turbo Stock class in San Diego club autocrosses. This class allows any size tire/wheel combo (but a minimum of 140 treadwear rating) so I want to get a dedicated non-RFT tire/wheel combo. Stock is the staggered 18" setup, 225/40ZR18 on 18x8 ET34 front and 255/35ZR18 on 18x8.5 ET37 rear. I want to use a square 18" setup and the question is what width wheels and tires will fit.

    I've been told that the rear wheel and tire will fit on the front with no problem so that seems to favor a square 8x8.5 255/35ZR18 setup. The question is whether an 18x9 ET42 or even 18x9.5 ET35 wheel with a 255/35ZR18 tire will fit. The tire will probably be the Hankook Ventus R-S3 or the Bridgestone Potenza RE-11, both of which come in a 255/35ZR18 size.

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