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      11-01-2014, 01:42 AM   #1
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question about e92 sway bars

I currently have an e92 335i and the current suspension mods are kw v1 coilovers, michelin pss tires, and front strut tower brace. I love how my car handles and it has stiffened up alot compared to my stock sport suspension, but it still feels a tad bit floaty and could use a bit less body roll in the front and the back (not that it isn't good enough now but i want more!) and yes i have read about the m3 rear bushings but my question is in regard to sway bars. So I keep reading that stiffening the front and rear swaybars will actually decrease the grip limit but give the illusion of better handling on the street by decreasing body roll. My question is, if this is true and the stiffer bars do reduce the grip limit, why does the m3 which we all know handles better than our non-m have them? Is it all part of a bmw illusion or is there some part of the m3 suspension that is able to make use of the stiffer bars to increase grip? very confused over here lol
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      11-01-2014, 06:16 AM   #2
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Excellent question.

Sway bars *can* reduce the grip limit if you overdo it. If you apply just the right amount of anti-roll you can improve overall grip slightly by better maintaining tire geometry to the road. Beyond that point where you start to lift too much pressure off the inside tires you will then approach diminishing returns. That's why chassis tuning is so critical, and why the right way to go about this is to change sway bars last (which, quite smartly, is what you've done!)

Obviously, the most apparent perceived improvement with stiffer sways is the transient response to steering input. The car feels "lighter" as it reacts much more quickly... but as you noted, you can lose steady-state effectiveness if you haphazardly add lateral stiffness.

The other thing to keep in mind is to make sur your front to rear stiffness is properly tuned. This can be used to dial in or out understeer and oversteer, but so car relative tire widths and camber. Lots to think about and attribute changes from, which is why it's best to isolate variables and change one thing at a time, and work methodically from the simplest most isolated changes (alignment, tires, spring rates, damper rates, then sways).

The M3 also has the benefit of a wider track width (distance laterally between the tire centers). This factors into the overall geometry of the suspension and makes the ideally tuned anti-roll stiffness typically higher than a non-M.
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      11-01-2014, 01:51 PM   #3
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Quote:
Originally Posted by ashmostro View Post
Excellent question.

Sway bars *can* reduce the grip limit if you overdo it. If you apply just the right amount of anti-roll you can improve overall grip slightly by better maintaining tire geometry to the road. Beyond that point where you start to lift too much pressure off the inside tires you will then approach diminishing returns. That's why chassis tuning is so critical, and why the right way to go about this is to change sway bars last (which, quite smartly, is what you've done!)

Obviously, the most apparent perceived improvement with stiffer sways is the transient response to steering input. The car feels "lighter" as it reacts much more quickly... but as you noted, you can lose steady-state effectiveness if you haphazardly add lateral stiffness.

The other thing to keep in mind is to make sur your front to rear stiffness is properly tuned. This can be used to dial in or out understeer and oversteer, but so car relative tire widths and camber. Lots to think about and attribute changes from, which is why it's best to isolate variables and change one thing at a time, and work methodically from the simplest most isolated changes (alignment, tires, spring rates, damper rates, then sways).

The M3 also has the benefit of a wider track width (distance laterally between the tire centers). This factors into the overall geometry of the suspension and makes the ideally tuned anti-roll stiffness typically higher than a non-M.
thanks for the response, what would you recommend diameter wise front and rear? there aren't any tracks near where i live so i doubt it will see much use there but I do drive the canyons pretty aggressively on a regular basis. The main reason i am considering sways is to hopefully feel more planted on the street.
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      11-01-2014, 02:27 PM   #4
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To be honest I'd do the bushings and front+rear m3 arms first... Is that something you are willing to consider? This will get rid of the float and vagueness in both ends of the car.
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      11-01-2014, 02:40 PM   #5
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Quote:
Originally Posted by ashmostro View Post
To be honest I'd do the bushings and front+rear m3 arms first... Is that something you are willing to consider? This will get rid of the float and vagueness in both ends of the car.
^This

I'd suggest spherical bearings throughout too but daily driving might be too harsh. Great on the track tho and probably thru canyons as well.

Sent from Eagles Canyon Raceway where it was 32F this morning making for an interesting first lap
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      11-01-2014, 03:08 PM   #6
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Haha on what tires bud?
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      11-01-2014, 03:36 PM   #7
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Quote:
Originally Posted by ashmostro View Post
To be honest I'd do the bushings and front+rear m3 arms first... Is that something you are willing to consider? This will get rid of the float and vagueness in both ends of the car.
any idea on the cost of those parts? labor is like 75-80 an hour at the shop i go to im not mechanical enough to tackle that job myself haha
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      11-01-2014, 04:32 PM   #8
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I would budget $1700 for parts and labor if you buy TRW instead of OEM labeled arms (TRW is the manufacturer of both) + subframe bushings.

That seems like a lot in comparison to the raw cost of sways, but consider that you also have to drop the rear subframe to install rear sways, so practical cost of just doing front and rear sways is $1000-1300 depending on what kit you get. Used parts will help but the labor on the rear is the priciest part.

Front sways are easy enough to do in an hour by any competent tech.

I'd say if you just did the bushings and arms, you'd notice say a 90% improvement in stability and clarity. With just sways you'd really only be gaining a fraction of that, say 20%. I know this is very subjective so don't take it too literally, rather just consider how strongly we are encouraging the "smart path" based on trial and error.
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      11-01-2014, 10:19 PM   #9
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Haha on what tires bud?
Toyo R1Rs. Yeah. It was, um, different. Wheeeeeee...
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      11-01-2014, 10:21 PM   #10
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My last DE of the year is next Friday... I probably will not run my r1rs and stick with the PSSs for the same reason, lol
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      11-01-2014, 10:25 PM   #11
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My last DE of the year is next Friday... I probably will not run my r1rs and stick with the PSSs for the same reason, lol
Good thinking.

Maybe I should check the weather report next time. Nahhhhh, that'd be planning ahead n stuff.
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      11-01-2014, 10:27 PM   #12
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Haha sounds about right!
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      11-02-2014, 01:15 PM   #13
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Quote:
Originally Posted by ashmostro View Post
I would budget $1700 for parts and labor if you buy TRW instead of OEM labeled arms (TRW is the manufacturer of both) + subframe bushings.

That seems like a lot in comparison to the raw cost of sways, but consider that you also have to drop the rear subframe to install rear sways, so practical cost of just doing front and rear sways is $1000-1300 depending on what kit you get. Used parts will help but the labor on the rear is the priciest part.

Front sways are easy enough to do in an hour by any competent tech.

I'd say if you just did the bushings and arms, you'd notice say a 90% improvement in stability and clarity. With just sways you'd really only be gaining a fraction of that, say 20%. I know this is very subjective so don't take it too literally, rather just consider how strongly we are encouraging the "smart path" based on trial and error.
If I were to just do the front m3 sway bar would it throw off the balance of the car?
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      11-02-2014, 10:23 PM   #14
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Not if you don't go too stiff. The coupe bar works well. So does the bmw performance bar but I'm not sure if you can buy that on its own... That would be ideal with a stock rear bar.
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      11-02-2014, 11:27 PM   #15
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Remember the M3 has wider track, wheels and tires. Much better bushings throughout. And most importantly, it has a mechanical LSD.

Anti roll bar will reduce the overall traction available in most cases. However, if the reduction in roll induced camber change can be had with a larger bar, you will actually pick up some traction. This is especially true on the front of the BMW, starting in the E36 days. Adding a rear bar with out LSD, you will be taking more traction away from the rear than adding.
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      11-02-2014, 11:37 PM   #16
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Quote:
Originally Posted by HP Autosport View Post
Remember the M3 has wider track, wheels and tires. Much better bushings throughout. And most importantly, it has a mechanical LSD.

Anti roll bar will reduce the overall traction available in most cases. However, if the reduction in roll induced camber change can be had with a larger bar, you will actually pick up some traction. This is especially true on the front of the BMW, starting in the E36 days. Adding a rear bar with out LSD, you will be taking more traction away from the rear than adding.
So doing a front bar that is not too dramatic will have a positive effect? If so which would you recommend?
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      11-03-2014, 03:11 AM   #17
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Quote:
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So doing a front bar that is not too dramatic will have a positive effect? If so which would you recommend?
This is a helpful web page for your question: E82, E90, E92, E93 Front Sway Bar Upgrade Comparisons

I suggest the 26.5mm e92 M3 front bar to pair with the stock rear sway.
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      11-03-2014, 12:16 PM   #18
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Quote:
Originally Posted by uberbeast21 View Post
So doing a front bar that is not too dramatic will have a positive effect? If so which would you recommend?
e93 front bar with stock rear bar.

It looks like your concerns are multiple. The "floaty" feeling has a lot to do with your dampers so I recommend you address that first. Subframe bushings will be the other.
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      11-03-2014, 05:51 PM   #19
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Turner motor sports also recommends the front e93 m3 bar as a modest improvement keeping everything else stock.
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      11-03-2014, 05:56 PM   #20
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Based on those data points it seems the smaller e92 bar isn't enough of an increase to warrant the change. I do happen to be running an e93 bar up front with a Hotchkis rear and really like the balance... but I'm not running stock spring rates so all is not equal.
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      11-03-2014, 10:41 PM   #21
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Quote:
Originally Posted by ashmostro View Post
Based on those data points it seems the smaller e92 bar isn't enough of an increase to warrant the change. I do happen to be running an e93 bar up front with a Hotchkis rear and really like the balance... but I'm not running stock spring rates so all is not equal.
thanks for the help im going to do the e93 m front with trw m control arms up front as well do you think this will leave the car out of balance or is it just going to help turn in and body roll?
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      11-04-2014, 11:41 AM   #22
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I would also vouch for the e93 m3 front sway bar which I upgraded from the Bmw performance from sway. Sure it introduced a tad more understeer, but I can easily counter it with some proper trail braking techniques. Turn in was noticeably sharper with the m3 bar. Plus I feel like like the car is easier to control at its limit this way, given how much torque a FBO n54 makes. I didn't even changing the rear sway by the way.
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