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      09-20-2011, 09:51 PM   #1
jnm
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Wheel sizes for street vs track (19 vs 18), why?

So the 1M comes on 19" wheels, but I notice that most people getting another set of wheels for the track go for 18" wheels.

Bear with my neophyte question here (not much experience tracking my own car, a big reason why I am/was hoping to get a 1M) but why is that?

I mean the streets (at least around the Bay Area where I live) are rather bumpy compared to any track so wouldn't you want smaller wheels with a taller tires on the street to improve the ride, while on the track which is smooth wouldn't you want a larger wheel with not as tall tire in order to have less sidewall flex and less slip angle (i.e. more direct steering feel)? Or is it just a question of more/better tire choice in 18" than in 19"?
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      09-20-2011, 09:53 PM   #2
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      09-20-2011, 10:08 PM   #3
Mahlzeit
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18s if chosen wisely make up a lighter wheel/tire package than same width 19s. Reducing unsprung weight which helps acceleration, braking and suspension control. Stiff sidewalled 18s can still offer sharp turn in.
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      09-20-2011, 10:25 PM   #4
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Quote:
Originally Posted by Mahlzeit View Post
18s if chosen wisely make up a lighter wheel/tire package than same width 19s. Reducing unsprung weight which helps acceleration, braking and suspension control. Stiff sidewalled 18s can still offer sharp turn in.
+ 1

They are also a little cheaper to burn up on the track and at least for now there is also a larger selection in the 18" size.

Finally, if you go off the track, the extra sidewall rubber can save some rim/suspension damage.

There is a difference between "show" and "go."

If you look at real race cars, they all have sidewalls, but in street cars the sidewalls are dissapearing. Unsprung weight is important when you are looking for an edge.



Look how much more sidewall even a Formula car has over cars with 19s and 20s.


Last edited by nachob; 09-20-2011 at 10:35 PM.
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      09-20-2011, 10:38 PM   #5
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Read through this wiki: http://en.wikipedia.org/wiki/Unsprung_mass. It will give you a great overview in the benefits of lighter/smaller wheels, besides that of costs (which is true as well).

Here's another link that can explain things a little better: http://victorylibrary.com/brit/M-sprung-c.htm

Basically, it plays a large role on your suspension while in a track scenario. This should be the main reason that people are doing this, in addition to just the cost benefits.

To the comment on unsprung weight: it's not unsprung weight that helps with acceleration. I believe what your talking about is rotational inertia.

Last edited by v1k0d3n; 09-21-2011 at 02:11 AM.
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      09-21-2011, 12:56 AM   #6
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Quote:
Originally Posted by v1k0d3n View Post
....It does NOT help with acceleration.
True if the overall diameter is maintained with the new configuration.

However, during alot of discussion about aftermarket LSD options for my M3, I learned that going from a 40 profile to a 30 or swapping out your 19' with 18's and keeping the same sidewalk height will yield the same effect of going to a 4.1diff on a 6MT M3. Result, 6% increase in torque output and better low end acceleration.
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      09-21-2011, 01:21 AM   #7
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I know already about all the benefits of lower unsprung weight, but that would apply for both street and track, you never have too little unsprung weight. Is there that much weight difference between a 19" and an 18" wheel? (I wouldn't have thought so). If so, wouldn't you want to run 17" or even 16" for the track then? Or wouldn't you have the same effect by getting lighter 19" wheels?

Part of what I'm trying to figure out here is why does BMW M go to 19" wheels for the 1M or the M3 competition package? Looks only? I thought "M treatment" was more about go than show...
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      09-21-2011, 01:42 AM   #8
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Quote:
Originally Posted by v1k0d3n View Post
It does NOT help with acceleration.
You are 100% wrong. Decreasing rotating mass will always increase your acceleration and decrease your braking distances.
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      09-21-2011, 01:43 AM   #9
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Metak - wow, I would've never expected that big of a difference. That's huge! That said, I need to stop posting when it's so late. I notice my misspellings/typos and general errors are plentiful on the iPad when it's this late. It's not the unsprung weight that results in increased accelleration, but it's the rotational inertia. Should have been more clear. I went on a tangent entirely before I even realized it.

I brought the unsprung issue up because that can really impact cornering and traction stability on a track. Less wheel/tire weight has less impact to suspension and thus retains better traction.

jnm - it could be partially for looks. Does anyone know what the smallest wheel would work on the 1M, yet still allow for proper brake caliper clearance?
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      09-21-2011, 02:12 AM   #10
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Quote:
Originally Posted by simple_simon View Post
You are 100% wrong. Decreasing rotating mass will always increase your acceleration and decrease your braking distances.
Corrected ^
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      09-21-2011, 02:17 AM   #11
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Besides inertia consideration, unsprung weight only matters when there are bumps to absorb, right? Given a perfectly smooth road there should be no difference between a heavy wheel and a light wheel, right? (I'm assuming here that lighter unsprung weight means that you can use less damping and therefore more compliant shock absorption and therefore less bouncing of the wheel over the bumps and therefore more often in contact with the ground and therefore more grip). (please someone tell me if there's a flaw in my reasoning here).

Now given the assumption (true at least in the Bay Area) that the street is (much) more bumpy than the track wouldn't you want lighter wheels even more on the street than on the track? (on top of a higher (therefore more flexible) sidewall also helping swallow the smaller bumps).

Just trying to make sense of the fact that the 'performance model'/competition package of cars seems to now mean 19" wheels rather than 18" only for people that actually track the car to swap them for 18", why not just sell the performance cars with 18" wheels and just be done with it?

Not like BMW (or other car makers) actually make sense in their decisions, if that was the case they'd make a lot more 1Ms for the US for example.
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      09-21-2011, 08:43 AM   #12
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^^^ You're forgetting the whole "form over function" dynamic.
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