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Confused about tire width change points

Discussion in 'Wheels & Tires' started by keetkb, Jun 1, 2015.

    • Member

    keetkb

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    Very new to this and the points calculation for stock vs. new tire width has me confused. The formula reads:
    The points to be assessed by a tire width change are computed by subtracting the increase in width in mm divided by 10 to get the number of cm increase. The points are calculated by the cm difference times a multiplier. The multiplier is 2.0, 2.5, or 3.0 depending on the upgraded tire's aspect ratio.
    Width multiplier new tire aspect ratio.



    2.0 80-70
    2.5 65-55
    3.0 50 - 30
    Points = ( [new width mm stock width for each axle] / 10 ) * (width multiplier)


    Stock tires on my 05 Cooper S were 195/55-16 and my new tires are 205/50-16 all around. Does the formula count both tires for "each axle" or what? Can someone show me the correct points number?
    Thanks!!!:confused:
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    MGarrison

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    Uh, need more context - sounds like you're trying to figure out autocross class due to a wheel/tire change.

    If so, a better forum category for auto-x posts is here: https://www.bmwcca.org/forum/index.php?forums/autocross.115/

    (no need to repost your question, moderators will move the post there if they want to)

    First thing, what auto-x rules are you referring to? O'Fest auto-x rules are here ( http://ofest.bmwcca.org/autocross_class ), and don't have the elaborate points-assessment systems for classing that might be used by the SCCA or local BMWCCA chapters.

    A link to the specific set of rules you're trying to address would be helpful.

    If it helps, plugging your tire change in the formula...

    Points = ((205-195)/10)*3... so, 205-195 = 10, 10/10=1, and 1*3=3, so... Points = 3

    Would need more context from the rules, but they would likely say if it was intended to apply to all four tires, or both axles, or whatever, since competitors will tend to argue vehemently for whatever advantage they might be able to wrangle, in this case, not being bumped into a higher auto-x class due to a tire width change. Auto-x rules quite often are written to be interpreted specifically and literally, and if something isn't specifically stated, quite often there's a reason for that - competitors can't be expected to intuit vagueness or hoped-for intent by the rules-makers.

    The other clue could be the split points for each class jump - if, say, 12 pts would be bumping you up two or three classes from such a relatively minor change in width increase and aspect-ratio decrease, then it's probable that it's only 3 points, in your case. It may be that IF you had a staggered setup, and the change on the front or rear was more than the other axle as far as width increase or aspect ratio decrease, then it would be a double points assessment, but that's very likely moot in your case, since you made the same change all around.
    keetkb likes this.
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    keetkb

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    I thin that "3" makes the most sense, but it's the "each axle" part that has me confused. This is for autocross with the Boston chapter of BMWCCA and here is a link:
    http://www.boston-bmwcca.org/_Upload/EventOverview/imf.pdf

    Thanks for taking the time to look at this.
    • Member

    MGarrison

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    Looking at the .pdf, there's also this below the tire formula -

    "If you are just increasing the width of the front or the rear tires, enter half the points from the formula."

    So, the formula assumes an increase for both front & rear tires. If you only changed both tires of one axle, your points would be 1.5; you've changed all 4 corners, so, 3 points.

    If you've only put the wider tires on your o.e. rims, then there's no points penalty for wider wheels - if you're using different, wider wheels, looks like you're supposed to add-in points for that.

    You can use this to compare tire diameter - http://www.rimsntires.com/specspro.jsp

    Best I could find, the 16" rims for a 2005 Mini Cooper S were/are 16x6.5 with a 48mm offset -

    Plugging that into both comparisons, and 195/55's & 205/50's, it says 8mm less diameter; the .pdf says multiply the decrease in inches times 4, and that takes a metric to inches conversion.

    8mm = .314960629921", so .314960629921 * 4 = 1.25984251968... rounding up, that's 1.26 points for a tire diameter decrease.

    Simple enough, ya? Ok, but...... what about taking into consideration that your car may have come with different o.e. wheel & tire packages? The .pdf doesn't say anything about that, maybe something on your chapter website does. Point being, taking a look at realoem.com, it shows 17x7 wheels with 205/45-17's (you'd have to confirm what were original options). Realoem.com also shows JCW 18x7 wheels with 205/40-17's, but that argument might be a bit of a stretch, unless it were possible to order the JCW 18x7 wheels with the original car. Anyway - the question is whether it's valid to say that your model had an o.e. option of, say, 205/40-17's, and thus, you putting 205's on your 16's shouldn't incur a points penalty because it's not wider than a common o.e. setup. The rationale behind that is if you acquired some stock-sized 17x7 rims and ran o.e.-sized 205/40-17 tires, if that's an option that was available originally, it's not a change from stock that would yield a performance improvement, and wouldn't be subject to a points penalty. If you had those o.e. wheels & tires originally and that's what you run, it certainly wouldn't be subject to a points penalty. Now, whether any of that is worth arguing is whether the points put you in a different class - it's only relevant really if you're on the bubble, and if you feel compelled to argue the point.

    But, you can see what difference such rationalizations can make when calculating points - no points for a tire-width increase or a wheel width increase (if you've gone to different wheels no wider than 7"), and, arguing a comparison of going from 205/40-17's to 205/50-16's, 4mm less in tire diameter is .63 points, vs. your 1.26. I suppose one could rationalize that _you're_ the only one who knows you didn't make the illogical move to ditch an o.e. 17x7 setup going to 16x6.5's (assuming that's your stock size), but auto-x, at least as far as club events are concerned, is meant to be fun, and the rules are there to help ensure some even competition, intentional deceit isn't justifiable regardless. However, I'd say it's fair enough to pose the question of what the wheel/tire comparison base can be if a vehicle came with several setups originally - might sound persnickety on the surface, but if someone else is running a car identical to yours with no points assessments for running an o.e. combo that's advantageous at least for auto-x, it hardly seems fair that you'd be assessed more points using a less-advantaged combo. Again though, if it's not a points difference bumping you up a class, it's a moot point. This wasn't an issue years ago, there was only one stock wheel/tire setup for a given model, so any bigger wheel/tire setup was something that could affect classification, points assessment, etc.

    If your points total has you just barely in an upgraded class and you don't find an answer off the chapter website, perhaps query your local auto-x chair.

    All that aside - if you're just giving auto-x a shot for fun, then, have fun. Far more important to concentrate on your driving for the day, walking the course, memorizing it, visualizing, thinking about looking up and ahead, through the course, where to get your eyes next, etc.

    Ok, got curious and looked - from: http://www.boston-bmwcca.org/Autocross/autocross-rules.aspx, for "Non-BMW's" (Mini's are treated as BMW's for classification purposes, but perhaps the same rationale applies for the BMW classing system where questions might arise)

    "Anything available on the exact model and year of your car as standard or optional equipment, as installed on the factory assembly line"

    "Any wheel of diameter and width identical to a stock or factory optional wheel, offset within +/- 0.25 inch, is considered stock."

    The question would be whether the same applies to BMW/Mini.

    That doesn't exactly answer whether you have to stay apples-to-apples for tire-change points calculations in the sense of, say, Granny Smith to, uh, hormone-enhanced Granny Smith, or, Golden Delicious to hormone-enhanced Granny Smith. Obviously, the most conservative approach is assessing points for all aspects of the tire change, assuming you've only installed wider/lower aspect-ratio tires on the same o.e. rims, then that's 4.26 points. Looks like a '05 Mini Cooper S starts with 102 base points, leaving only 1.9 points before being bumped out of class D into Class C... so, yeah, seems like it might be worth seeking some clarification from your auto-x committee or chair. Another question perhaps, the .pdf isn't totally clear, at least to me, for the "sports seats" points assessment - racing seats would obviously be aftermarket seats, but is the intent specifically for aftermarket seats, or are you supposed to assess points because your model came with sports seats from the factory, or are original seats, even though described as sport seats by the maker, considered stock, and therefore not subject to points assessment? I don't know if your car came with self-described "sports seats", but those extra points, if they apply, could bump you up a class, making the tire points question academic. Seems to me they would be meaning aftermarket, non-factory sports or racing seats, but lacking any further specifics, ya don't know, and 5 pts. here means being in one class or another.

    Interpreting auto-x rules... I remember getting in arguments over the rules of Monopoly!
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    keetkb

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    Thanks again for helping to sort this out and all the information. I'm not on the edge of a class change now (in "C" class with mods of CAI, RSB, and tires) but I'm considering more mods in the future as I learn to drive better. I'll definitely have to look into the OEM/options concept for tires, seats, and such that you brought up.
    Cheers!

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