Hello there and welcome to the BMW Car Club of America.

If you are a BMW CCA member, please log in and introduce yourself in our Member Introductions section.

What is the impact of tire diameter on vehicle performance?

Discussion in 'Autocross' started by Brian A, Jun 19, 2009.

    • Member

    Brian A

    Post Count: 657
    Likes Received:7
    I am becoming frustrated by the lack of tire options in the stock 195/65R14 or 205/60R14 size for my 1991 318i. For autocross, I wish to upgrade to "Extremely High" Performance Summer tires.

    I am thinking of buying a set of 16" rims which has a huge number of tires available. The correct size would be 205/50-16, but there are even more tires available in 205/55-16 (including the exalted Bridgestone Potenza RE-11s).

    205/50-16 typically make about 865 revolutions per mile and 205/55-16 typically make about 835 revolutions per mile. This is a 4% reduction. Is this significant? Would I notice it in my performance?
    • Member

    MGarrison

    Post Count: 2,781
    Likes Received:142
    The smaller overall diameter (od) you go, the more you increase your accelerative capabilty. For auto-x, you might be choice constrained depending on whatever class you run. If you are in stock class, you might have to stick with whatever size is required. If you throw that consideration out the window, then I'd say run 225/50-14 Hoosier A6's. You'll have tires that stick, you'll get out of the box quicker, and have a slightly lower center of gravity for better handling. You'll probably also be in a class where the tire choice would make you hopelessly uncompetitive because the other cars are more heavily modified.

    16" tires, you won't get going quite as quick, but you shouldn't have to shift as much - if you find you're always on the rev limiter and being forced to up & downshift, you might find you make up the time difference from that by less shifting due to a 15" or 16" larger od tire/wheel combo. I doubt from the seat of your pants you could feel any difference in acceleration between the two different 16" wheel sizes. Don't forget, larger wheels mean more unsprung weight. Whatever you opt for, you'll want to be staying in the torque band for the bulk of your runs.

    This should at least allow some theoretical comparisons for sizes, which I think would be the same percentage difference in rpms for any given gear.

    http://www.1010tires.com/tiresizecalculator.asp
    • Member

    Brian A

    Post Count: 657
    Likes Received:7
    Thanks for the help in sorting this out.

    Yeah, I am trying to stay within a particular car performance classification, but the Golden Gate Chapter classification system is a points system so I could move to R comps or change overall tire circumference without any sort of disqualification if I acknowledge the points they add to my car.

    At this point, I do not want to get into R comps. I just want to stay in what we call "B" class, which I still qualify for if I use summer tires. I just want to get the grippiest "summer" tires I can get. That is why I was wondering about the effect of a slightly larger diameter since there are really nice tires available in the 205/55R16 size but not teh 205/50R16 size.

    It is interesting that you suggest 225/50-14. Does the 225 width actually fit into an E30 without spacers/rolling/flares? I had thought that 205 was the widest I could go.

    That diameter calculator is very useful. Thanks for including the link. I hadn't seen it before. And, yeah, I like my 13.8 lb bottlecaps. I wish I could keep them, but there aren't many tires available for them anymore.
    • Member

    az3579

    Post Count: 3,270
    Likes Received:3
    Have you considered 15" wheels/tires? I've seen a plethora of available tires for this size.
    I would say 205-50-15 or 215-50-15's (more choices for 205 though).
    • Member

    MGarrison

    Post Count: 2,781
    Likes Received:142
    Oh, so NOW you want to get picky?!? ;)

    I actually haven't tried a 225/50 on a 14" rim on mine, but - I'm thinkin' about it! 225/50-15's are what I usually run for a street tire, & 225/45's for track. When I first ran 225/50's (before there were any 45 series in 15" dia. sizes), they caught the inside top inner lip of the fender, so I had to roll the fenders. It _may_ be possible to roll fenders without completely cracking the paint off the fender, but it would be time consuming - I suspect you'd have to do it very, very slowly on a hot day, and warming the paint and fender, I suppose with a heat gun. And still no guarantee you could do it without cracking the paint. Flares are only necessary for something wider than a 225.

    I didn't take that kind of time, nor did the condition of my paint warrant extra care. I have run 215/55-14's on 14x7 rims, those rubbed the plastic fender liner in the wheel well on turning. I think on that particular set of rims, clearance to the strut was fine, as it also has been w/ 15" rims. You might need a spacer w/ a stock rim, I forget the offset on these 14" ones I have. My car is also lowered, but that's probably a moot point for comparison; sits lower, less roll, but rubs. Stock suspension sits higher, but rolls more, and probably still rubs.

    There's a ton of ultra-hi-perf summer tires in 205/50-15, including your RE11, why not that? Same width, less unsprung weight and slightly less acceleration loss due to dia. increase. And, no guarantees from me, but probably no rubbing.

    The Tire Rack is showing a 15x7 TRMotorsport C1 wheel with slightly less offset than the Kosei, which should give you a nominally wider track-width, and they're $16 less expensive, and 1/2lb less weight than the Kosei. Sounds like a reasonable deal to me - Kosei's are fine too, but need a hub-centering ring, which are a bit of a nuisance when wheel-swapping. Whenever you buy wheels, considering buying an extra 1 or 2. If the wheel gets discontinued, finding replacements can be a hassle, and although some wheels are available for years - many are not.
    • Member

    mattm

    Post Count: 204
    Likes Received:5
    I run the 225 45 15 on my 1996 E36 318i. No clearance problems that I am aware of. Kosei wheels (15x7) with the backspace machined to the proper depth by The Tirerack.
    • Member

    MGarrison

    Post Count: 2,781
    Likes Received:142
    E30 though, not E36 -
    • Member

    mattm

    Post Count: 204
    Likes Received:5
    Yes, I know, but I don't think the tire fitments were not all that different. I do have a set of used Hoosier A6s for sale that could be used for a driver's school and fitment test. I suppose I should actually post it somewhere on the list. Pics available. $100 for all four plus shipping. I am in Connecticut
    • Member

    MGarrison

    Post Count: 2,781
    Likes Received:142
    Tire sizes that might work on either car, maybe not - clearance for fender rubbing is a different matter, and wheel-dependent. An E36 has different suspension geometry, 15" is a stock dia. for E36, and +1 for an E30; E36's require high-offset wheels, E30's use medium-offset wheels. Width increase over stock width for an E30 is from 185 or 195 to 225, 30-40mm; an E36, it's going from a stock 205, so 20 mm. Yeah, the 20mm difference between a 205 that clears without rubbing and 225 isn't much on a ruler, but enough that clearance without rubbing is far less likely for a non-M E30, whereas 225 is no problem for an E36. Rubbing clearance might likely start to become an issue for an E36 in a similar fashion for widths over stock in the same 30-40mm amount, at least in a stock dia. wheel, without spacers, I would guess, although I'm no E36 expert.

    I think you should sell your tires to Botond :D

    http://www.gruppeb.com/BMWwheels/fitments/

    As an aside (and what thread of this length doesn't deserve to be taken off topic by now), this -> (http://www.gruppeb.com/Mole-Killer/ ) was funny - very funny.
    • Member

    mattm

    Post Count: 204
    Likes Received:5
    Stock offset for my E36 is 47mm. Mine are set at 41mm. A click under 1/4"
    • Member

    Brian A

    Post Count: 657
    Likes Received:7
    I'll consider anything. Compared to 16" (or 17") the selection for 15" wheels is tiny. I targeted 16" just because of the tire selection.

    Not too picky: I just want options that ACTUALLY FIT MY CAR!!!!!!


    At this point I am open to anything. I got so thoroughly trounced at autocross last weekend, I am in complete denial that it could have anything to do with ME. Autocross is just like golf: its 80% equipment. The reason why I can't hit a golf ball straight either has nothing to do with me and just that I don't have this year's latest $700 driver.

    I am okay with 15"; its just that if I am going to buy something different than my bottle caps (I have three sets of bottlecaps BTW), I would buy the size with the highest number of tires available. That is clearly 16" or 17". The idea of 17" wheels makes me feel puce, so I settled on 16".

    I was scrounging used wheels at Double 02 Salvage today and learned that they sell new Rota RB 15" wheels with 25mm offset for $550 for a set of 4. Ever heard of them? Popular with the 2002 crowd apparently.

    Long live the bottlecap.

    Do they come with wheels? (...:D)
    • Member
    • Technical Service Advisor

    mooseheadm5

    Post Count: 1,880
    Likes Received:15
    E36 tires are too tall for an E30.
    • Member

    MGarrison

    Post Count: 2,781
    Likes Received:142
    lol

    Yes, but it is sounding like you actually want to be somewhere within the zone of competitive - you have little enough oomph as it is, why bleed off more of whatever you do have for a bigger tire variety when you have an adequate selection of ult-hi-summer-perf tires offering the performance you want in the width you want in a smaller 15" dia.?

    If they weigh less than the 13lbs of the TRmotorsports wheels and are less than their $99/whl price, then you probably have a find. If they weigh substantially less, they might be worth the extra $50. I like the 'minilite' look of the Rotas, I think that's a great looking wheel for E30's.
    • Member

    Brian A

    Post Count: 657
    Likes Received:7
    15x7 Rota RBs weigh 14.4 lbs. I can understand how there would be substantial suspension performance differences with higher unsprung mass, but I have trouble believing that the associated rotating mass affects acceleration much. This seems like urban myth.

    I am warming to 15" rims. I would still like to figure out if I could squeeze 225 width tires into my wheel wells without modications (rolling fenders). Even though the 318i is a lot better/faster car than most people think, it is not at the top of the performace food chain and I don't want to go over the top modifying it. It is a lot of fun as-is.
    • Member

    MGarrison

    Post Count: 2,781
    Likes Received:142
    I didn't mention that or suggest it - would only be an effect in the overall hp-to-vehicle-weight sense, which isn't going to be much when you're talking a difference of 1-1.5lbs across each of 4 wheels. Obviously suspension dynamics are a different story.

    Somebody somewhere out there has had to stick 225's on a stock-suspended car; perhaps a bimmerforums search if no one else has a definitive answer about clearance.
    • Member
    • Technical Service Advisor

    mooseheadm5

    Post Count: 1,880
    Likes Received:15
    Rotational inertia does make a difference. Not so much when going from a 15" to a slightly lighter or heavier 15" wheel, but definitely when buying cheap heavy 16" wheels in place of 14" wheels. It shows up as a horsepower difference on an inertial dyno and it is not a myth. It affects acceleration and braking. The extra wheel weight affects suspension and steering. I would go with 15" wheels of any reasonable weight and be done with it.
    I don't believe that you can run 225s without rolling your fenders.
    • Member

    CRKrieger

    Post Count: 1,616
    Likes Received:20
    When you consider that autox is a game of hundredths - sometimes thousandths - of a second, no small effect is negligible. To be the most competitive, I would find the best 14" tires you can. Then, quit obsessing over equipment details (Yes; it's just like golf.) and get a season or two of competition under your belt. Learn how to adjust your tire pressures during the day. Learn how to 'read' the course and adjust your driving to get faster each and every time around a course - or know why you didn't. Have fun and make your goal simply to get better every time.
    Now you're going the other direction. You can be compulsive about wringing every last millisecond out of your car or you can enjoy it. But somehow, I don't think you can do both. :cool:
    • Member

    Brian A

    Post Count: 657
    Likes Received:7
    A tire / source recommendation is very welcome.

    Yeah, lots of driver left to fix, but I am not that bad. Attended Evolution Phase 1 Autocross school last Saturday too.


    I am just "assessing" rather than "obsessing". There are new "Extreme" summer tires on the market now that generate almost R-comp levels of grip. It is now a factor. Cars with larger diameter wheels are using them. I recently replaced my tires with some pretty awful ones. A major error on my part (fortunately they were only $59 each). It's a serious noticeable difference.

    I need to decide whether to do something or not. It is looking to me like "something" means dropping $900 for 15" or 16" rims and a set of excellent summer tires. It would be nice to have competitive grip. (If I remove my Dinan chip I drop enough points that I can upgrade to Extreme Sooper stick-um summer tires and stay in the same class.)
    • Member

    az3579

    Post Count: 3,270
    Likes Received:3
    I just ordered a set of tires for my 15" Kosei wheels. I've been hearing excellent reviews from people about the Kumho Ecsta XS wheels (treadwear 180). They're relatively cheap ($88/piece in the size I ordered 205/50ZR-15 91W) and I believe they're aimed at autocrossing but are still street legal. The person I talked to at Tire Rack even recommended it over what I was originally looking for (Yokohama s.Drive), saying that the XS is better suited for autocross than anything because that is what it was designed for. The s.Drive is a summer tire, but mainly designed for a good life while still providing good grip. The XS tires are still good for Street Touring class, which I am trying to stay in. See note 1 below.

    I find the 15" wheel to be good for what I use it for and think you should consider it. It may not be a looker, but the Kosei K1 wheels available on Tire Rack for I think $115 a piece if I remember correctly. They're a good set of lightweight wheels and while not being my favorite (others love the design), they make a good companion for auto-x or track days. I believe they are 15x7.5 in size with an offset around 38? If you order from Tire Rack, they come with all hardware required to install, including the hub centric rings.




    Note 1 - besides, it looks badarse. :D
    [IMG]
    • Member

    CSBM5

    Post Count: 340
    Likes Received:3
    Uh, no. I wouldn't use the adjective "almost" here. They can't compete in any way with a Hoosier A6/Kumho V710/BFG R1. They are definitely sweet street tires (I run the Star Specs on my M5 for street driving), but nothing like a pure autocross R-comp tire.

Share This Page