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tires

Discussion in 'E30 (1984-1993)' started by ricco39, Jul 27, 2009.

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    • Member

    CRKrieger

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    You confuse purism with pragmatism. As a gearhead and enthusiast of 40 years, I have learned what works and what does not. I am not saying "Don't run non-stock wheels." I am saying "Don't run 17" wheels on that particular car."
    Have you ever learned about tire contact area? If not, you have some research to do before we go any further with this discussion.

    It's interesting that you mentioned rotational inertia instead of acceleration, and you're correct with that more precise term. Now, think about it: not only does the larger wheel/tire accelerate slower (which you know), it also brakes worse for the same reason. So you have a disadvantage at the beginning and at the end of every straightaway.
    I spent a decade autocrossing regularly (still do occasionally - look for last year's O'Fest results to see how many other 5ers I outran) and I have been an HPDE and car control (e.g. Street Survival) instructor since 1992. I think I know what I'm talking about.
    OK; if there are too few 15" choices, why jump up two inches in diameter? Do you think you'll find a better tire than my 16" Yoko Advan A038s?
    ... unless you autocross. I've won and lost events by hundredths of a second. A bigger wheel/tire combination only means I would have lost more.

    I also have direct experience with different size wheels and tires on the same car. I have run 14", 15", 15.35" (TRX 390 mm), and 16" wheels on 3.5 liter E28s. While there are quite a few guys who think like you ignoring advice like mine and trying the same thing on their E28s, there are none who later deny that the advice on heaviness in the wheels is correct. It is a tangible feeling of clumsiness of the car that goes away when they go back to a smaller, lighter wheel/tire, regardless of the sizes involved.

    z31maniac guest

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    1. Fair enough, but again for street use, it's not really a big deal. (From someone who has run stock weaves, 16 x 7 SSR Comps, and now 17 x 8 Work's on the exact same E30 with the same suspension/brake setup). Yes, there is a SLIGHT noticeable difference, but again, since I'm not racing mom's minivan from stoplight to stoplight, I'm really not that concerned with it.

    2. Go ahead educate me rather than being condescending.

    The braking thing is an interesting point, so I'll throw this out as a thought experiment. Let's say your stock E30, with proper technique (ie not just slamming the brakes before the tires are loaded up), is still capable of locking up the 205s, now the 245s afford you more contact area that will allow you to use more of your remaining braking capacity. Does that make up for the increased rotational inertia? I don't know, but it's an interesting thought anyway.

    3. Come down to OK for the next BMWCCA trackday and I'll let you drive my completed swap car and you can find a stock E30 to drive. If you lap faster in the stock E30, then I'll shut my mouth, otherwise we both know the twice the power/TQ, increased braking capacity and suspension on my swap car would annihilate a stock 318 around a road course.

    4. Back to the same thing with 15s, want to go wider than 215 in an OD that will fit on an E30? Not gonna happen without Rcomps, you could run a 225/50/16, but that is 1.1" taller if I remember correctly. A038s, yes there are better tires, those are old news.

    5. I like how you conviently disregarded STREET ONLY car, so that you could bring up an Auto-X reference.


    I'm all for a healthy debate and learning from those with more experience. But remember that just because someone doesn't have AS MUCH experience as you do, doesn't mean they don't have any. I see this attitude alot in the car scene from the older crowd and I'm still having trouble figuring out why.

    It was pleasantly absent when I was in the sportbike scene and doing trackdays on my R6.
    • Member

    CRKrieger

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    There is no increased contact area from tires with the same inflation pressure.

    Here's your enlightenment:

    Pounds per square inch.

    Easy to challenge someone who doesn't drive an E30 in the first place to make a 2600-mile round trip to meet your terms. You need to find, or set up, an objective test to overcome the theoretical weaknesses in your arguments. Otherwise, we gotta rely on simple physics and math which doesn't favor you.
    Hey, you're the one who brought up track events and miniscule differences. I only pointed out where it matters.
    I didn't say you didn't have any. I'm saying you didn't have this and that you haven't thought it all the way through. You have already decided what you are going to do and you are determined to bulldoze anyone whose opinion differs, regardless of the reason. When the math & physics doesn't work out for you, you play the 'age' card or the 'attitude' card or the 'it doesn't really matter' card. I see this attitude a lot from the younger crowd, but I don't wonder why. I've been there and done that. Until you take the time to learn what I'm getting across instead of arguing it reflexively, you will go your own way and ignore me. I already know that. But maybe, if I can get you to think about the entire picture instead of the small parts you're seeing now, you'll see it as I do:

    More is not always better.
    • Member

    az3579

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    Kids, please stop arguing. Seriously.
    • Member

    Brian A

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    Ha ha. Too true.

    Fast forward to August 4, 2039:

    z31maniac: You clowns had to put AIR in the tires before you tried to drive it! Don't you kids know anything??

    CRKrieger III: Back off! It's a 535i which doesn't even have a gauge to show if the air tank is full or not. So, duh. Why did Grandpa keep an extra wheel in the trunk, btw?

    The great thing about the BMW CCA website is that it is friendly enough to ask dumb questions. We all have them.

    z31maniac guest

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    We are discussing, not arguing, I doubt CR is upset, nor am I. I am geniunely interested in his opinion, but I can disagree with his opinion and still respect it.

    I apologize for assuming you drove in an E30, I figured that was your E30 in your avatar. So I apolgize for my wrong assumption.


    Although I am curious about the tire assertion, I just don't understand how there isn't increased contact area between two tires that are 40mm different in width. I'm not saying you are wrong, I'm just saying I don't understand.


    Exactly what mathmatical calculations and physics models are you using to conclude that a stock 318 would be faster around a road course than an E30 with sorted suspension, more powerful brakes and much wider tires, twice the power, 12% more weight and marginally different F/R weight distribution.

    Thrust curves? Power to weight ratios? Your basically claiming that I can't assert the swap car would be faster because I have no empirical evidence, yet you turn around and say the 318 would be faster without empirical evidence.


    I have thought about it, but other than a small increase in rotational inertia (and techincally drag from more/stickier rubber contact on the ground) I don't see the bigger wheel/tire combo being the hinderance that you are claiming. If it is, why does a certain well-known Nationally competitive DSP E30 run 18 x 10.5" with 285/30/18s. Or why RX-8s stuff 285s on 8" wide stock wheels.

    I'm not trying to not listen, but most of what you have said in the thread, is "I have the experience, I know, I'm right, you're mistaken." I have no problem being wrong, I just want it explained to me is all.
    • Member

    Brian A

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    I too am interested because I don't know much about this stuff.
    That, I can answer. The size of the contact area is related to weight and air pressure. Say a tire has to support 1,000 lbs. If the tire is at 10 psi, the contact area would be 1,000/10 = 100 square inches. A wide tire has a wide short contact patch and a narrow tire has a narrow long contact patch. Both contact patches are 100 sq in in area though.

    It'll be interesting to read your two's debate about this all tomorrow (plus all the other stuff above).

    Obviously, various tire / wheel configurations need different tire pressures (for example, to limit roll-under of the sidewall during autocross, my 195/65R14s needed 40 psi front and 37 psi back: I don't get a very big contact patch). Lower profile tires may not need such high pressures, which I suspect is beneficial, but I am stepping beyond my depth of understanding if I speculate further.

    Out of curiosity, I looked up Forumla 1 tire specs (http://www.f1-country.com/f1-engineer/tyres/tyres.html) and calculated that they are equivalent to F: 355/46R13 and R: 385/43R13. They have a remarkably high profile. They are run at a very low 16 psi ("1.1 bar"). Somehow they don't have roll-under problems.

    z31maniac guest

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    The F1 tires would just be down to incredibly stiff sidewall construction. It's so tedious/difficult that the engineers X-ray the wheels after tire mounting to ensure the wheel wasn't damaged.

    How's that for fragile!
    • Member

    eam3

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    I had 7x16 BBS RS with 205/50ZR16s on my E30 325is and they looked really good on it but it also looked to be as high as I would go to retain a proper look. I've seen regular E30s (non M3) on 17" wheels and they look kind of goofy and ricey. I would definitely stick with 15" or 16" on a non-M3 E30.
    • Member

    CRKrieger

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    True.
    No yer not. Don't bs me, whippersnapper! :mad:
    :( Damn kids. Can't tell the difference between an E30 and an E28 (which is what's in my avatar, along with me and my 12-y-o daughter). If you were really on the ball, you'd recognize that as Turn Six at Road America. ;)
    That puzzled me at one time as well. Then, one day, one of the best automotive engineers I know looked at me and said exactly the same four words I wrote to you. Then the light went on. Brian nailed it. For a given tire pressure, your contact patch does not change size with different profile tires; it only changes shape.

    Now, you have to figure out all the ramifications of changing that shape. Here's a hint: a tire grips best along the longest axis of the contact patch. Your turn. Analyze acceleration, braking, lateral acceleration, and net changes in handling between your wheels and tires and stock wheels with the same tires.
    Hold on there! You're changing six factors at once! That only leads to results that are inconclusive. Have you ever done experimental lab work? I spent 8 years in industrial R&D after almost the same in academic science labs. I can assure you that, to see a result and attribute it to a factor, you only change one thing at a time. We are talking about wheels and tires here, so that is the only thing we change to decide what works best. The fewer changes, the better. You're talking about diameter and section width at the same time, so I'll let you get away with that one 'double' change. But to get a reliable result, you need to assume all other factors are the same.

    So let's hypothetically put your big fat wheels and tires on that same 318. Or put those stock wheels and the same model tires on your smoke-breathin' E30. Which will work better, and at what task(s)?
    Where did I say that? I only said that the mathematics and physics works against your contention that bigger and wider tires are better on a 'street' E30.
    What does their competition run? Do they never lose to that competition? At a certain point, sheer power does become a factor in the equation, but any near-stock E30 is going to be power-limited, not traction-limited. Even the 325is isn't easily able to spin its tires under acceleration. There's little doubt that the 318 doesn't (absent serious clutch abuse in the lowest gears). A lot of what I've learned comes from driving low powered cars as fast as higher powered cars. That perspective is different from yours. Where I look to exploit handling and braking advantages for faster lap times, you simply say, "More power". That explains how I (driving a stock Audi 4KQ on R compounds) could drive faster in every corner than an Audi Sport Quattro (with 3 times the power, twice the tire width, and much better brakes) and still have a slower lap time. A well-trained chimpanzee can go fast in a straight line. It takes a driver with the ability to exploit well-thought-out equipment to make up for that. There will ALWAYS be somebody out there with a hypothetically faster car. My goal is to be able to keep up or outrun them with 'inferior' equipment.
    Let's start with comparing wheels and tires. It could be years before we get around to the final decision whether the 318 is actually faster - and where it might be.

    z31maniac guest

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    1. See kids, just a friendly debate with the occasional terse delivery.

    2. Hahahaha, whippersnapper!

    3. :eek: Always just took a glance and it looked like a Euro bumpered E30 with an Alpina style valance.

    4. "For a given tire pressure, your contact patch does not change size with different profile tires; it only changes shape."

    We weren't just talking about profile, but also width. If you look at my post I said a 245 vs a 205.

    5. I suppose the senility is setting in early.:D grant made a comment about the 318 keeping up in a tight auto-X with a swap car, to which I agreed, but pointed out the extra power/suspension/tire would annihilate it on a road course. I completely understand how to test variables, but again, that wasn't the discussion grant and I were having.

    The bigger wheels and tires can provide a sharper turn-in because of the lower profile, possibly "slower" as well because of the extra rotational inertia. The larger tire can also provide higher lateral loading, but will take more energy to accelerate/brake.

    So the question is, does the increase in cornering capability outweigh the acceleration disadvantage. On a 318, I don't have the data, but I would point to every competitive Auto-X that will take the weight penalty for the extra grip. I realize its anecdotal, but still a real world observation.

    6. I never said that bigger/wider wheels are better on a "street" E30, I said they are better for MY E30 given the desired use and modification level. What I said was that the difference is going to be miniscule on the street, which it is. I've run weaves with 195/60s, SSR comps with 225/45/16 and now my Work's with 215/40/17 (which are much wider than the 225/45/16). And the difference is barely noticeable. I suppose you might be losing 1-2% in fuel economy with larger wheels.

    7. Similar sizes. I try to follow it pretty closely even though my car will be completely outclassed in the Class I ended up in with this build. But that's fine, because I'm building the car for me to be fun, not for a certain class.

    Trust me, I know all about cornering skill, I come from sportbikes and track days. Nothing better than passing a liter bike in the turns or on the brakes while on your "little 600".

    8. Refer back to number #5.
    • Member

    CRKrieger

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    OK; 'section width'. Also, we're talking about total contact area spread out over four tires. So, for a given tire pressure, your total contact area does not change size with different section width tires.

    Do you get that yet? Brian had a pretty coherent explanation.

    And you continued your discussion with Grant using me as the antagonist instead. So who's senile here? :confused:
    Two is not 'every'. I have always been competitive and that has not always been my choice. Besides, you're only halfway there. An increase in potential lateral acceleration alone does not make a good handling car. What else are you missing?
    Which means one (or more) of three things:

    A. You haven't objectively tested it, or
    B. You haven't quantified it, or
    C. You haven't exceeded the handling limits in each case.

    More homework to do.

    z31maniac guest

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    Two? I only provided two examples to prove a point. You are grasping at straws on this one, I'm not talking about YOU the instructor being able to beat up on the local schulbs at you're local Auto-X (that's driving skill, not car/setup), I'm talking about once you get guys that ALL can drive.

    Every National level Auto-X (at least the one's that trophy so the top 1/3) uses the maximum size tires the class will allow, STX M3s on 285s, CSP Miatas on 275 Hoosiers, 'Vettes on 315/335, Evos on 295s, etc etc etc.


    Again, on the street, it does not have this huge effect you are talking about!!!! Are you suggesting people should be driving at 9/10ths on the street and approaching the handling limits of the car in a non-competiton environment? What would be the point of "testing" wheels/tire combos against each other that are driven on the street?

    The stance you are taking now is one purely to play devil's advocate, not actually debate, which is boring.
    • Member

    CRKrieger

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    Every one of which is able to overcome its tire traction with power. The E30 318 doesn't do that, so tire selection might be the widest available, but it isn't simply the biggest available. If you refuse to recognize the distinction and what it means, then we may as well quit.
    I am suggesting that, whenever a car is driven anywhere near its limits, be it on the street or on a track or on a skidpad, that the dynamic handling is affected by the tires. A competent driver knows what that handling limit characteristic is and must drive with it in mind. You are choosing to ignore that factor in your campaign to use the biggest baddest tires you can on anything, so yeah; I'm bored, too. Forget it.
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