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      10-18-2011, 12:26 PM   #1
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For track, why 18 inch wheel instead of 19?

Hi, guys, is there any good reason of choosing 18 instead of 19 wheels for the track day? I saw lots of you have 18 inch setup when you go to track.

The reason why I'm asking is that I want to install new BBK. But I am afraid that 18 is too small for 6 piston 380mm kit.

thx
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      10-18-2011, 12:41 PM   #2
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I run 19's at the track, but think the 18's are used because there is a larger selection, they're cheaper and don't think r-comps come in 19's.... other than that have yet to hear a compelling argument as to why 18's are "better" than 19's....am sure there's more to follow....
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      10-18-2011, 12:45 PM   #3
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As soon as you are on the right tires with the right suspension set up, wheel size will be minor.
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      10-18-2011, 12:49 PM   #4
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Quote:
Originally Posted by zpactfc View Post
Hi, guys, is there any good reason of choosing 18 instead of 19 wheels for the track day? I saw lots of you have 18 inch setup when you go to track.

The reason why I'm asking is that I want to install new BBK. But I am afraid that 18 is too small for 6 piston 380mm kit.

thx


Im using 18X10 Apex wheels all around with 275/35 tires, and StopTech brakes, 380mm rotor with ST-60 caliper in front, and it works fine, no spacers needed front or back.
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      10-18-2011, 01:29 PM   #5
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Larger diameter wheels have higher rotational moment of inertias and tend to decrease stopping times and reduce acceleration.

How much are they reduced? Depends on the wheel.
Is it a huge issue for a weekend racer? No.
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      10-18-2011, 01:48 PM   #6
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more initirtia, heavier rims = more unsprung weight, cheaper tires/rims.
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      10-18-2011, 02:12 PM   #7
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A lot more tire selection with 18" sizes. Also, less rotational mass along the outside diameter with a smaller rim.
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      10-18-2011, 03:14 PM   #8
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      10-18-2011, 03:24 PM   #9
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Why not run 15's then... Even smaller... An argument for infinite regress doesn't work...
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      10-18-2011, 03:28 PM   #10
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Quote:
Originally Posted by klammer View Post
Why not run 15's then... Even smaller... An argument for infinite regress doesn't work...
cause the smallest rim to fit on a M3 is 18in, that's why.
I know many non-M guys run 17s when they drag and it improves their times.

When you see race-spec cars, they don't generally run large rims

380mm 6 piston ST will fit APEX, but brembos do not.
you would need TE37s, TC-IIIs and such to fit 380MM Brembos with an 18 in rim
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      10-18-2011, 03:52 PM   #11
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F1 car runs 13s.

Part of it is having two wheels or equal overall diameter (with tires) and having different rims will result in the smaller rim being lighter due to just the rubber/nitrogen versus aluminum in the wheel.
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      10-18-2011, 04:37 PM   #12
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Fair enough. Just seems that these days the technology is making that moot, the BBS lightweight FI's are the lightest wheel out there and they're 19's, also thought that wheel width has more to do with weight, than size, but get that a bigger wheel has more weight further away, but just seems like we're getting to the point where metal is getting lighter than rubber and the whole wheel weight, not just rims, becomes important, but that's just me thinking out loud
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      10-18-2011, 04:41 PM   #13
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Quote:
Originally Posted by Superfly_M3 View Post
more initirtia, heavier rims = more unsprung weight, cheaper tires/rims.
I thought the 19s for this car were actually lighter??
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      10-18-2011, 04:49 PM   #14
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Quote:
Originally Posted by klammer View Post
Fair enough. Just seems that these days the technology is making that moot, the BBS lightweight FI's are the lightest wheel out there and they're 19's, also thought that wheel width has more to do with weight, than size, but get that a bigger wheel has more weight further away, but just seems like we're getting to the point where metal is getting lighter than rubber and the whole wheel weight, not just rims, becomes important, but that's just me thinking out loud
Total wheel weight and moment of inertia aren't completely tied together. Incoming maths!

A large portion of the wheel's mass is the rim itself, not the hub-mounting face and spokes, and that's the portion that's farthest away from the center of rotation. I = mass * dist_to_center^2 (in general for a thin-walled cylinder). For a two-piece wheel, a 19" rim will have a MoI of about 11.4% more than the 18" rim. The face/spoke center piece will also have a somewhat larger MoI, though maybe less pronounced. MoI is used to calculate the energy needed to maintain or change motion (e.g. go fast or stop going fast).

This also discounts the additional weight / MoI of the 18" tire combination you would need to run, but the overall mass of the added tire is very small and, at the radius at which you are adding material, does not span the entire width of the wheel providing for a relatively small MoI.
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      10-18-2011, 05:03 PM   #15
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Quote:
Originally Posted by CT View Post
Total wheel weight and moment of inertia aren't completely tied together. Incoming maths!

A large portion of the wheel's mass is the rim itself, not the hub-mounting face and spokes, and that's the portion that's farthest away from the center of rotation. I = mass * dist_to_center^2 (in general for a thin-walled cylinder). For a two-piece wheel, a 19" rim will have a MoI of about 11.4% more than the 18" rim. The face/spoke center piece will also have a somewhat larger MoI, though maybe less pronounced. MoI is used to calculate the energy needed to maintain or change motion (e.g. go fast or stop going fast).

This also discounts the additional weight / MoI of the 18" tire combination you would need to run, but the overall mass of the added tire is very small and, at the radius at which you are adding material, does not span the entire width of the wheel providing for a relatively small MoI.
where are you coming up with your numbers for your math???
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      10-18-2011, 05:11 PM   #16
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I understand the math, as I stated, just seems like technology is making things moot, like all other things. Wheels are heavier now, but seems to me that trend is changing that's all and soon we'll be bitching about what tires weigh. I get the argument about size of wheel, I'm just taking it a step further saying tire weight, which is furthest away, will matter more than wheel weight as wheel mfg gets better, stronger, lighter. Tire companies aren't as concerned with weight, more about other things.... Wonder what the differences in tire weights are between 18's and 19's and if that weight being furthest away to move becomes more of an issue than wheel weights. Just seems like people throw out equations w/out thinking about all the variables, and that it's a bit deeper than expected. Wonder if the few pound difference in tire weight further away could make more of a difference than a larger weight diff on the rim, that's all. Not sure if I'm wrong or right but just seems like a logical question to ask.
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      10-18-2011, 05:26 PM   #17
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Quote:
Originally Posted by klammer View Post
Fair enough. Just seems that these days the technology is making that moot, the BBS lightweight FI's are the lightest wheel out there and they're 19's, also thought that wheel width has more to do with weight, than size, but get that a bigger wheel has more weight further away, but just seems like we're getting to the point where metal is getting lighter than rubber and the whole wheel weight, not just rims, becomes important, but that's just me thinking out loud
Not everyone can run $8000 BBS FIs......

You aren't comparing apples to oranges here

I'm sure if BBS made FIs in 18s, they'd be lighter than their 19in counterpart
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      10-18-2011, 05:36 PM   #18
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18 is easier for square setup, no? I think 19" square setup could be a bit tough for finding the right clearance up front. I have square 275/35R18 on Apex Arc-8s. They're much lighter than my ZCP rims. It looks sexy and works well for track. (It took some getting used to, mind you!)

On the other hand, I've had lots of fun on the stock 19s with AD08s in staggered setup. Tires are more expensive for 19s, though.

With square, you can rotate tires. With staggered, you get less risk of oversteer.
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      10-18-2011, 05:51 PM   #19
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There is a sweet spot for rim size and too large of a rim can decrease performance and so can too large of a rim. Many race series have a limit for rim size so the fact that a particular race car runs a particular size rim may be because it is the maximum allowable size.

There are many factors at play here, unsprung weight, rotational mass and slip angle among others. These tend to interact with each other and as you change rim size to improve one parameter to gain performance you may actually be downgrading performance by making another area worse.

I can tell you what I was told by someone who has a lot of experience in this area. The coach for the Drivers Club at Lime Rock told me that the Club's M3 is fastest around the track with 18s and the suspension on the second stiffest setting. This is coming from someone whose job is driving an M3 around a race track.

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      10-18-2011, 05:53 PM   #20
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Quote:
Originally Posted by klammer View Post
I run 19's at the track, but think the 18's are used because there is a larger selection, they're cheaper and don't think r-comps come in 19's.... other than that have yet to hear a compelling argument as to why 18's are "better" than 19's....am sure there's more to follow....
18'' track wheels are usually lighter and so are most 18'' tires.Wide variety of track tires are also another reason to go 18.

OP,you do not need to go as big as 380mm.. No real advantage over the 360mm's other than looks.But a real disadvantage in added rotating mass.Car will stop and perform just as well on the 360mm setup with Motul 660 or Castrol SRF.

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      10-18-2011, 06:20 PM   #21
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Quote:
Originally Posted by J08M3 View Post
where are you coming up with your numbers for your math???
It's just 19^2 / 18^2 (mean rim diameter into I = 1/4*m*D^2). I can do the long-hand calc if you'd rather
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      10-18-2011, 06:30 PM   #22
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Quote:
Originally Posted by klammer View Post
I understand the math, as I stated, just seems like technology is making things moot, like all other things. Wheels are heavier now, but seems to me that trend is changing that's all and soon we'll be bitching about what tires weigh. I get the argument about size of wheel, I'm just taking it a step further saying tire weight, which is furthest away, will matter more than wheel weight as wheel mfg gets better, stronger, lighter. Tire companies aren't as concerned with weight, more about other things.... Wonder what the differences in tire weights are between 18's and 19's and if that weight being furthest away to move becomes more of an issue than wheel weights. Just seems like people throw out equations w/out thinking about all the variables, and that it's a bit deeper than expected. Wonder if the few pound difference in tire weight further away could make more of a difference than a larger weight diff on the rim, that's all. Not sure if I'm wrong or right but just seems like a logical question to ask.
I haven't done the math since FSAE 2005, but the 18 v. 19 debate (including tire data from the Milliken tire test consortium) almost always yields the 18" wheel as the clear performance winner. Effective brake area is always a concern when reducing wheel diameter, but at these sizes you're splitting hairs on the 'very conservative' end. Assuming the same brand is used, to eliminate the large impact of manufacturing method, slip angle, alignment torque, and the spring-mass-spring-damper results all net greater benefits from the 18" wheel and tire combination. Contrary to popular belief, stiffer is not always better - there's a very careful balance between corner independence, roll and ride stiffness, ride frequency and jounce v. rebound sensitivity.

Make no mistake, tire manufacturers are gaining ground in traction technology. It's just not readily apparent because there's a dearth of publicity on the matter and the layman is unable to really understand the gains. I-beam spokes? Marketable. Reorganization of ply direction or slight tweaks to the rubber compounds used? Impossible to explain.
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