Wheel offset question - '00 Mustang

Discussion in 'OT Driven' started by javamac10, Mar 30, 2008.

  1. javamac10

    javamac10 New Member

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    I'll preface this with saying this might be somewhat of a noob question!

    Ok... Question for some fellow mustang owners...

    I have a '00 v6 stang with the stock wheels (15x7 with 24mm offset)

    It's going to be used in a couple of HPDE schools this summer as well as schools and lapping days next summer. I am planning on having a swap set of wheels and tires for those days (high performance tires to start with, then R compound possibly later). I am interested in getting 17x8 wheels with 235/40ZR17 tires.

    The problem is, there is such a discrepancy on offset when upgrading wheels... The optional wheels for the mustang of size 17x8 have a 30mm offset.
    http://www.miracerros.com/mustang/ww.htm
    Also, almost every forum agrees and says the correct offset is +30mm. However, most of the manufacturer websites for wheels suggest that the offset is 25mm.... :dunno:

    I am currently looking at 5zigen fn01r-c wheels. They are available in what I want (17x8, lightweight, fairly inexpensive). I just don't want to order them if they are going to be the wrong offset... I don't want them to stick out/stick in too far or rub either... (Also, backspacing?? I want to leave the window open for upgrading rotors and calipers without having to get new wheels in the future...)
    (look under 1780)
    http://www.5zigenusa.com/english/products/wheel/5zigen/fn01rc.html

    I'm not really willing to just order them, and then find out I will need to get spacers or something else to repair a bad decsion...

    Hopefully you guys will be able to straighten me out on this! :x: (thanks in advance!)

    (first non-initiation post, woot!)
     
    Last edited: Mar 31, 2008
  2. CJPA

    CJPA New Member

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    5mm offset difference really isn't worth all of this worrying. You'd have to REALLY look to notice the difference. The +30mm offset will tuck into the wheelwell slightly more than the +24 or +25.

    Backspacing won't (directly) affect your brake options. What is more important when going to larger rotors & calipers is the clearance between the caliper and the wheel spokes (independent of offset or backspace) and the caliper clearance to the wheel local structure diameter. --backspacing is more useful to calculate interference w/ suspension components (the main point of contact in the Mustang will be the strut).
    You can calculate the backspacing as follows:
    [(1/2 width of wheel)+(offset)+(0.5" wheel lip)]

    Likewise the "frontspacing" can be calculated using a similar formula:
    [(1/2 width of wheel)-(offset)+(0.5" wheel lip)]
    The "frontspacing" formula will be helpful for you to compare how far the wheels will stick out as compared to your current wheels.

    Stock Backspacing calculation:
    On a 15x7 wheel, with 24mm backspacing:
    [(1/2 width of wheel)+(offset)+(0.5" wheel lip)]
    [(3.500")+(.945")+(.5)] = 4.945" stock backspacing
    This is the amount your stock wheel will extend into the wheelwell, measured from the mounting flange of the wheel.

    Stock Frontspacing:
    [(3.500")-(.945)+(.5)] = 3.055" stock "frontspacing".
    This is how far outboard the stock wheels protrude from the mounting flange.

    Using these same calculations:
    17x8 w/ 25mm offset:
    Backspace: 5.484"
    Frontspace: 3.516"

    17x8 w/ 30mm offset:
    Backspace: 5.681"
    Frontspace: 3.319"
     
  3. javamac10

    javamac10 New Member

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    Ok, that definitely cleared up some questions. My previous perception was that backspacing was the distance between the mounting flange of the wheel and the inside of the wheel spokes. I didn't know that backspacing and frontspacing were directly related to the offset of the wheel.

    If I were to order brakes in the future, how would I know if they would fit? I hope it wouldn't come to a trial and error kind of fitment. Is there some calculation or measurement from the manufacturer of the calipers and/or wheels that would help determine this before ordering?

    From the calculations you've made, it doesn't look (like you said) that I have much to worry I guess. From those measurements, it looks like the outside of my new wheels/tires would sit almost an additional half inch towards the outside of the car, and the inside of the wheels/tires would site closer to the struts than the current wheels/tires, but not as close to the struts as the OEM 8 inch wheels would (which should have me in the clear!)?

    I'm hoping that the 5mm wouldn't make a dramatic change in the car's handling... I don't want to put any more strain on the suspension than should be. Also, if this wouldn't affect my handling, then 25 and 30mm offsets should both be considered within a medium offset range, correct?

    My only issue after that would be if I decide to go 45zr17 instead of 40zr17... 45 would keep the overal tire diameter almost the same, where as 40 would subtract about an inch.

    Current:
    205/65 15 = 25.5 diameter

    Suggested:
    235/45 17 = 25.4 diameter
    235/40 17 = 24.4 diameter

    Would it be better to go 40 or 45?
    40 pros:
    -less tire weight
    -better acceleration (because of tire diameter AND weight)
    -lower center of gravity

    45 pros:
    -more give in the tire for track irregularities?
    -?
    -?

    Either way I should have room to lower the car if necessary later...

    Again, thanks for all the responses/advice/help on this! :bowdown:
     
  4. javamac10

    javamac10 New Member

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    also, if I were to go 17x9, there shouldn't be any trouble as long as i obide by the 26mm offset for the cobra standard wheels correct? I don't think there are any changes (space-wise) in a standard v6 stang wheel well compared to a cobra's...? My worries are:

    A: I wouldn't want any rubbing. If I got 255 tires for the front I would hope I would have enough clearance to keep from rubbing. I say this because while they will be very wide, the wheels should never be at full lock while on the road track.... I just hope the tires wouldn't flex enough to rub on the anti-roll bar (which I've seen happen before)

    B: I wouldn't want to put too much stress on the car. I don't want to have soo much grip (which I would eventually, if I decide to get R compound tires) that it would put too much stress on the suspension components of the car. I would rather have a limited amount of grip (from a smaller footprint of tire) and not run the risk over over-stressing any parts.
     
    Last edited: Mar 31, 2008
  5. CJPA

    CJPA New Member

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    Dude it's a V6 Mustang--you're overthinking this stuff.

    Get the 40 series tires for lower effective gear ratio and weight.

    The offset isn't going to affect your handling in any way you'll be able to notice, especially since you're just starting out.

    FYI I just bought a set of 95 Cobra-R replicas for my car to use w/ NT-01s, they are +24mm offset, my current wheels are +50mm offset. I don't expect to see any real impact other than being able to have a second set of wheels for r-comps. (although I am running an '06--if you do the math, you'll find out what fits and what doesn't)

    Just go, learn, then get serious. Also a Terminator powertrain swap wouldn't hurt ;)
     
    Last edited: Mar 31, 2008
  6. CJPA

    CJPA New Member

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    If you're worried about suspension rubbing, turn your current wheels to lock and take some measurements of the clearance between the tire, wheel lip, and strut. Then do the backspace calculations for whatever wheels you want and compare the differences to get an idea of what the physical relationship will be.

    Also an easy way to baseline your tire and wheel choices is to see what the largest factory setups are (usually the Cobras) and figure out what the measurements on those are.
     
    Last edited: Mar 31, 2008
  7. javamac10

    javamac10 New Member

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    Yeah I'd just be worried about the tire flexing a lot... I mean tracking it will be pushing those components to the max... I'm not going to be able to calculate if 255 tires on a 9 inch rim in the front will rub while the tire is at the edge of adhesion...

    What car do you have? Those 95 cobra r replicas should be 9 inchers I think?
    Do you track your car?

    And what is a terminator swap?!?! Searched, couldn't find anything... links?
     
  8. Chris951

    Chris951 New Member

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    03/04 cobra powertrain.
     
  9. CJPA

    CJPA New Member

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    255s on a 9" wheel is nothing to worry about. I run 255s on the front of my car in 9.5 wide wheels and they bow in slightly. The 285s I run in the rear bow out slightly :)

    And to answer your question if I track my car:
    [​IMG]

    [​IMG]
     
  10. javamac10

    javamac10 New Member

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    jesus! haha, thats a bit more than I'm looking to do. Plus I'd probably have to swap out the gearbox, axle, driveshaft, computer, for it to be useable, then there's the cost too.... But I can dream it!


    Nice! How long have you been tracking it? Doing schools/lapping days?

    [​IMG]
    Mine's a few years older... Bought it while I was in college
     
    Last edited: Mar 31, 2008
  11. CJPA

    CJPA New Member

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    Doing my 8th or 9th track day on Apr 5th, been doing HPDEs for about 1.5 years now. I drive w/ NASA Southwest.

    Don't forget to upgrade your brake pads and flush your brake fluid w/ a DOT4 hi-temp.
     
  12. javamac10

    javamac10 New Member

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    yeah, someone recommended hawk brake pads.. the brake fluid will definitely be flushed/dot4.

    My first will be in early July. It's a BMW school. A guy I know a little south of me here in IN runs it at Putnam Park every year.
     

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