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      04-28-2013, 11:29 AM   #1
FogCityM3
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Sway-bar for understeer reduction

I have the ZCP w/ EDC as a DD and I'm pretty satisfied with the suspension and really don't want the suspension any stiffer w/springs or coil overs (or vehicle lowered) due to poor roads and my track use is only ~3x per year. However, I was wondering if adding only stiffer front (no rear) sway-bar/anti-roll bar (e.g. Dinan, RD Sport, turner etc) would help dial out the understeer significantly enough without any further mods. Thx in advance.

Last edited by FogCityM3; 04-28-2013 at 12:15 PM.
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      04-28-2013, 11:52 AM   #2
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No, increasing rear sway bar stiffness will reduce understeer. A less expensive option is to modify tire pressures. Higher in the front and lower in the rear will reduce understeer. Of course you don't want to make changes with pressure significant enough to cause poor wear patters. The stiffness of the tires sizes/types generally run on cars makes it a bit more challenging to get an effect here meaning you might have to run from 30-50 psi to get the desired effect.
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      04-28-2013, 12:06 PM   #3
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See the discussion in the suspension and track sections of the board. I have my car setup for F-stock tire class autocross by adding the Dinan front bar and pulling the alignment pins and maxing out the negative camber up front. The Dinan bar has really woken the car up in transients, and while I see a small increase in steady state understeer usually mid-apex area, the steady state understeer increase is balanced out some by the reduction in body roll (i.e. more favorable camber up front) and since I'm running same size tires front and rear even though I'm limited to stock wheel widths. The front bar also resulted in a decent increase in the ability to put power down coming off corners and elements as it allows the rear to hook harder, earlier (i.e. keeps more weight on inside rear).

All-in-all, I'm pleased with the way the car is handling. I just got overall FTD at a BMW CCA autocross at the Performance Center last weekend (BFG Rivals are awesome tires btw), and at my last club autox, I PAX'd first overall out of 95 drivers. The car is just a blast to drive. On the Perf Center course, which is long and more like a time trial than an autocross as speeds reach near the top of 3rd, the car exhibited no understeer issues at all -- was very happy to have the Dinan bar on it.

Here are the two links I was thinking about:
http://www.m3post.com/forums/showthread.php?t=408290
http://www.m3post.com/forums/showthread.php?t=823336
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Last edited by CSBM5; 04-28-2013 at 12:10 PM. Reason: added links
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      04-28-2013, 12:24 PM   #4
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Thx, yeah I've tried the PSI rear/front differential and it definitely helps..fine running this on the road, but more worried about doing this on the track, particularly effects of lower PSI on rear and associated negative effects on sidewall strength and grip/stability.

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Originally Posted by swamp2 View Post
No, increasing rear sway bar stiffness will reduce understeer. A less expensive option is to modify tire pressures. Higher in the front and lower in the rear will reduce understeer. Of course you don't want to make changes with pressure significant enough to cause poor wear patters. The stiffness of the tires sizes/types generally run on cars makes it a bit more challenging to get an effect here meaning you might have to run from 30-50 psi to get the desired effect.
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      04-29-2013, 11:18 AM   #5
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If you haven't already done so I would have the alignment checked by someone who knows what they are doing...a garage who specialises or on a BMW KDS machine - had mine done for the first time recently as the front end sharpness has faded and the toe was out front and rear.
Ask for the maximum negative camber on the front within the standard specs - should be -1 degree 30 minutes IIRC. Running 36 psi front will help to sharpen the turn in - I run that along with 36 psi on the back but you might find 34psi rear more to your liking.
The best balance I've had was with 1/2 worn PS2s on the front and new PSSs on the rear. I've now got new PSSs on the front as well and they are settling in nicely but I think the peak dry performance comes when they are about half worn.
It probably doesn't need saying but trying to carry too much entry speed or getting hard on the throttle too early makes the car understeery.
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      04-29-2013, 11:30 AM   #6
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The front sway bar aids in initial turn-in making it crisper and reacting faster but it'll add to mid-corner understeer if you don't balance it out with other suspension modifications. The rear sway bar is what reduces understeer.
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      05-31-2013, 01:53 PM   #7
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here is a good read...

http://www.tirerack.com/tires/tirete...jsp?techid=58&
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      05-31-2013, 03:31 PM   #8
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Good coilovers are no stiffer than stock, or minimally stiffer, on low rebound/compression setups. Sways are really just there for fine tuning. You would be much better off getting a decent set of coilovers.

The comments suggesting higher psi in front are off. In general, higher pressure in your tires reduces grip (and vice versa of course). If you are having understeer (front loses grip first) then you should LOWER the front pressure rather than raise. The problem will then become outside edge wear from sidewall rolling. It is a game of compromises.
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      05-31-2013, 03:37 PM   #9
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Quote:
Originally Posted by Porschefile View Post
Good coilovers are no stiffer than stock, or minimally stiffer, on low rebound/compression setups. Sways are really just there for fine tuning. You would be much better off getting a decent set of coilovers.

The comments suggesting higher psi in front are off. In general, higher pressure in your tires reduces grip (and vice versa of course). If you are having understeer (front loses grip first) then you should LOWER the front pressure rather than raise. The problem will then become outside edge wear from sidewall rolling. It is a game of compromises.
It depends on what pressure you are starting with and what condition you are trying to control. Higher pressures yield increased sidewall spring rate, so in transient conditions, turn-in, slaloms, etc, increasing front pressure up to a point will reduce understeer, as will reducing rear pressures (which lowers rear sidewall spring rate creating more rear rotation).

It's a misnomer to assume that higher pressures always equal less grip, especially on a street car. The goal is to maximize the contact patch's capability at peak steady state load *and* during transient conditions (in most cases). In many cases, this means adding air to get more grip.
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      05-31-2013, 03:38 PM   #10
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Here are modifications I would recommend without spending a fortune or compromising ride quality.

1. increase front negative camber by pulling the alignment pins
2. get wider front tires
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      05-31-2013, 04:15 PM   #11
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Quote:
Originally Posted by CSBM5 View Post
See the discussion in the suspension and track sections of the board. I have my car setup for F-stock tire class autocross by adding the Dinan front bar and pulling the alignment pins and maxing out the negative camber up front. The Dinan bar has really woken the car up in transients, and while I see a small increase in steady state understeer usually mid-apex area, the steady state understeer increase is balanced out some by the reduction in body roll (i.e. more favorable camber up front) and since I'm running same size tires front and rear even though I'm limited to stock wheel widths. The front bar also resulted in a decent increase in the ability to put power down coming off corners and elements as it allows the rear to hook harder, earlier (i.e. keeps more weight on inside rear).

All-in-all, I'm pleased with the way the car is handling. I just got overall FTD at a BMW CCA autocross at the Performance Center last weekend (BFG Rivals are awesome tires btw), and at my last club autox, I PAX'd first overall out of 95 drivers. The car is just a blast to drive. On the Perf Center course, which is long and more like a time trial than an autocross as speeds reach near the top of 3rd, the car exhibited no understeer issues at all -- was very happy to have the Dinan bar on it.

Here are the two links I was thinking about:
http://www.m3post.com/forums/showthread.php?t=408290
http://www.m3post.com/forums/showthread.php?t=823336
Here's a question for ya. I am running f-stock, which as you mentioned limits wheel widths. I am running 18x10 square. Am I out of compliance with the fronts because stock are x9? Of course, stock rears are x10. The rules as far as I am concerned are vague. Is the applicable width the largest tire or the single tire?
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      05-31-2013, 05:07 PM   #12
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I assume you have the comp package? i.e. 9x19F, 10x19R

Yes, then you are not class legal for F-stock with 10x19 all around. You have to run the same widths (and diameter) as standard on the car.

For non-comp package M3s, one has the option of running 18s or 19s, but they both have to be 8.5F and 9.5R.

Here is an excerpt from the rules (it seems pretty clear): "Any type wheel may be used provided it complies with the following: it is the same width and diameter as standard and as installed (including wheel spacers if applicable) it does not have an offset more than ” (6.35mm) from a standard wheel for the car."
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      05-31-2013, 06:30 PM   #13
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Quote:
Originally Posted by CSBM5 View Post
It depends on what pressure you are starting with and what condition you are trying to control. Higher pressures yield increased sidewall spring rate, so in transient conditions, turn-in, slaloms, etc, increasing front pressure up to a point will reduce understeer, as will reducing rear pressures (which lowers rear sidewall spring rate creating more rear rotation).

It's a misnomer to assume that higher pressures always equal less grip, especially on a street car. The goal is to maximize the contact patch's capability at peak steady state load *and* during transient conditions (in most cases). In many cases, this means adding air to get more grip.
Stiffer doesn't necessarily equal more grip. I ran PS2, AD08, RS3, slicks, etc., and each tire got more grip with less pressure, up to the point that it rolled on the sidewall too much. Try PS2's at 40 cold and then 30 cold and report back

Oh, and on a side note, sways on stiff in front generally decrease front grip. I know E9X are weird and can get opposite results, probably partially due to the fact that the suspension rides on bump stops at high lat g loads.

Richard's advice is the best unless you want to swap suspension. 18x10 all round rocks.

Last edited by Porschefile; 05-31-2013 at 06:35 PM.
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      05-31-2013, 06:36 PM   #14
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Quote:
Originally Posted by Porschefile View Post
Stiffer doesn't necessarily equal more grip. I ran PS2, AD08, RS3, slicks, etc., and each tire got more grip with less pressure, up to the point that it rolled on the sidewall too much. Try PS2's at 40 cold and then 30 cold and report back
..and less stiff doesn't necessarily equal more grip either. However, stiffer sidewall spring rate does increase transient grip up to a point, so like I said in my post, it all depends on what pressures you are starting with as the point is to end up with the optimum contact patch, under load, for the conditions you are going to impose on it.
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      05-31-2013, 06:55 PM   #15
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Quote:
Originally Posted by Porschefile View Post
Oh, and on a side note, sways on stiff in front generally decrease front grip. I know E9X are weird and can get opposite results, probably partially due to the fact that the suspension rides on bump stops at high lat g loads.

Richard's advice is the best unless you want to swap suspension. 18x10 all round rocks.
Yup front-engined cars like ours tend to roll too much in the front, resulting in bottoming of the suspension which overloads the outer front tire and hence understeer. The Grand Am GS M3 runs a HUGE bar up front.

I used to prefer a staggered set up on the M3, but as my driving progressed I preferred to have the car looser (i.e. rotate more freely), that's why I use a square set up now. Another reason is tire size availability.
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      05-31-2013, 07:01 PM   #16
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Check out this bad boy from Turner.

http://www.turnermotorsport.com/p-17...y-bar-kit.aspx
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      05-31-2013, 07:40 PM   #17
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Thx for the advice. Assuming I want to keep ZCP wheels, what tire size would be optimal for the fronts to further reduce understeer?

Quote:
Originally Posted by Richard@M-World View Post
Here are modifications I would recommend without spending a fortune or compromising ride quality.

1. increase front negative camber by pulling the alignment pins
2. get wider front tires
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      05-31-2013, 08:03 PM   #18
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Quote:
Originally Posted by Richard@M-World View Post
Check out this bad boy from Turner.

http://www.turnermotorsport.com/p-17...y-bar-kit.aspx
Holy moly.

Wider rims will help more than just wider tires, OP. Wider tires on stock wheels will just balloon the tires, lowering contact patch.
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      06-01-2013, 12:07 AM   #19
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Quote:
Originally Posted by Richard@M-World
Check out this bad boy from Turner.

http://www.turnermotorsport.com/p-17...y-bar-kit.aspx
Wow! For those that can't decide what to buy, a supercharger or a front sway bar!
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      06-01-2013, 12:30 AM   #20
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Quote:
Originally Posted by mhabs View Post
Thx for the advice. Assuming I want to keep ZCP wheels, what tire size would be optimal for the fronts to further reduce understeer?
According to this thread you should be able to fit the OEM ZCP rear wheels/tires on your front axle.

http://www.m3post.com/forums/showthread.php?t=517713
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      06-01-2013, 07:37 AM   #21
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Thanks...actually was wondering if a different size tire on the same front zcp wheel would make any difference. Alternatively, would putting a wheel spacer on front zcp wheel and keep wheel and tire make any difference? thx in advance

Quote:
Originally Posted by Richard@M-World View Post
According to this thread you should be able to fit the OEM ZCP rear wheels/tires on your front axle.

http://www.m3post.com/forums/showthread.php?t=517713
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