Rear sway bar delete?

[DustenT]

In the search for an M Coupe rear sway bar I ran into a guy that told me to just remove the rear sway bar. Apparently some of the M Coupe guys do this on their track cars and it handles really nice.

First off, has anyone heard of this? What do you think this would do? I'd think it would make the rear-end less stable.

Thoughts?

[Rhys H]

I disagree with what that guy said, it really does not make sense to me. By definition its a hardened steel bar connected to the frame and both lower control arms to prevent excessive body roll. I question if this guy knows what he is talking about

[pdxmotorhead]

I've found some cars respond to removing the bar if they are running stiff springs. I disconnect the rear bar on wet tracks as well.

Dave

[Rhys H]

I find that quite interesting. What would the handling characteristic of your car be on wet ground with the rear sway bar installed opposed to without the rear sway bar installed?

[DustenT]

Originally Posted by pdxmotorhead

I've found some cars respond to removing the bar if they are running stiff springs. I disconnect the rear bar on wet tracks as well.

Dave

These are the guys exact words:

Go with H&R 28mm front bar and delete the rear bar. It's a sharp setup for our rear semi-trailing arm suspension. You can compensate with higher rear springs rates but it's not necessary. I run Koni DAs with TCK 500lb springs in the front and 600lb in the rear. Not harsh at all.

I need to find out what the spring rate is for my HR sports, but I'm sure it's not 600#.

[96cali]

If you need a rear Mcoupe sway I have some emails from June I can pass on to you. I did not buy, not sure if he still has it. On a similar note, if you sell you mtech sways give me a shout per favore.

[tastade]

Looking around on the internet, a stiffer rear sway bar increases oversteer and a stiffer front sway bar increases understeer. So if you removed the rear sway bar, you should get more understeer. Is that what you want? I imagine that more understeer in wet conditions is exactly what you would want (as the wet will likely make the car oversteer).

[DustenT]

Originally Posted by tastade

Looking around on the internet, a stiffer rear sway bar increases oversteer and a stiffer front sway bar increases understeer. So if you removed the rear sway bar, you should get more understeer. Is that what you want? I imagine that more understeer in wet conditions is exactly what you would want (as the wet will likely make the car oversteer).

So the stiff 28mm front bar and no rear bar must even everything out? Damn, I might have to give this a shot. It only takes 10 minutes to remove the rear bar, or disconnect it and zip tie it up out of the way. I'll have to wait until Autox season starts again tho.

Anyone else willing to give this a shot?

[AlaskaBlue]

That is my general plan. I will swap the KW springs for some higher rates like mentioned and give it a shot. I have a problem with getting a rear wheel to come up in the air and the no rear bar would alleviate that problem.

[DustenT]

Ok, I need to share this because I've never gotten such a detailed response to a PM on bf.c.

Original post:

Originally Posted by dustent

I have 26mm's up front now, will I notice a huge increase with the 28mm's?

Remove the rear bar? Serious? How does that feel? I have HR sports and Bilstein sports but looking to move up to a more "race" setup, mostly for AutoX now but hope to move up to the big tracks soon.

Dusten.

Response:

Originally Posted by stician

Oh so you have an aftermarket setup. I don't think 28 will feel different than 26 but removing the rear will give you a lot of rear end grip. It hooks up. Try detaching the rear bars with your current set and go find your favorite curve. I'm dead serious. I didn't figure this out so I'm not smart. But TC Kline is and they setup my car. I also have their rollbar which I love.

I haven't autox much. Not trying to sound snooty but I prefer track days. I like the seat time and fluidity. Plus you see awesome cars at events.

Here's something I found and saved...

The low down on semi-trailing arm geometry

1. The geometry of the design imparts a change in toe-in/ou through the suspension travel. From ride height under compression, toe-in increases, and the reverse as the suspension goes into full droop. The means the rear of the car "steers". Turn a corner hard, and the inside rear is dropping, going to tow out, while the outside toes-in under compression. This is not a neutral condition, therefore is not desirable, as it is effected by bumps. Hitting a bump in a corner can creat unpredictable rear end "steering" effect, depending on which side the bump is on. Not a good thing.

2. The geometry of the system has a strong tendancy to "Squat" under acceleration loads. This is overcome somewhat by placing the arms slightly downward at the hub under static load, but this is a bandaide solution. However, those that have lowered their cars have negated this in the M coupe and roadster, as the trailing arm is no no longer properly oriented to produce the "Anti-squat" geometry BMW deisnged into the car. Stiffer springs mask some of this, but not all of it.

3. Opposite to squat, the STA system tends to lift on the rear of the car under hard braking. Also not desirable. Stiffer springs actually make this worse. This is one reason the front springs in most kits are so much stiffer than stock. If the front isn't allowed to dive, the rear is somewhat restrained as well.

4. STA geometry causes the rear wheels to move in an arc, not straight up-and-down, as the suspension moves. This imparts a small change in wheelbase, as well as making it more difficult to connect devices, such as sway bars and springs. If you've lowered your car, look at how screwed up the sway bar link angles are now!

The best performing cars to use Semi-Trailing Arm suspensions include the 240Z, BMW E30 M3, and now the Z3. It imparts a handling dynamic that can be quite fun to drive, but is no longer competitive, as it can be a ltitle unpredictable. Not a good characteristic when driving at the limit. The only way to really make it work well is to severely restrict suspension movement and travel, in order to minimize its negative effects. This is how club racers make the E30 M3 really fly. This is also why the M coupe responds so well to stiffer and lower springs and stiffer dampers.

The STA system is very strong, is compact and reasonably lightweight. It is also a fairly inexpensive independant rear suspension design, compared to others in use today. Finally, it seems to work best in short wheelbase cars. This is another reason it is used in the Z3 chassis.

It is not an ideal system, but it does work fairly well. It also produces a very active handling character which is what BMW advertises it provides. In the end, a four or five link design, with neutral geometry, would make the M coupe a faster and more easily driven car. It's just not worth the time and effort required to make the required modifications.

Discuss!

[aceyx]

I like this idea. The front must be linked to get proper initiation on a turn. The rear has to get traction to provide power--if they're linked, and the body is rolling, this is going to "lift" the inside tire, losing traction.

Not something I would do for street, but trackdays I'd give it a shot.

[G.T.]

Originally Posted by AlaskaBlue

That is my general plan. I will swap the KW springs for some higher rates like mentioned and give it a shot. I have a problem with getting a rear wheel to come up in the air and the no rear bar would alleviate that problem.

Good call. The "hot" set-up on the Mk1 & Mk2 VW GTIs I raced with - mine included - involved no bar and high spring rates in front, BIG rear bar and not-quite-as-high spring rates in the rear.

The higher spring rate compensates for the missing bar and allows you to put the power down with both wheels. You also save a bit of weight by removing it.

Not to say that this only works for FWD cars. It isn't uncommon to see RWD cars with no bar and stiff springs in the rear + bar and slightly less-stiffer springs to match up front.

[andy]

The thing to remember is that the effective weight of the spring the car experiences changes in ratio with the actual wheel weight of the car. Gustave's site had a good writeup on that concept:

http://www.e30m3performance.com/tech...e/eff_rate.htm

So, on my car with 550f and 700r springs, my actual spring rate is really approx 486 front, 314 rear...actually less stiff in the rear than in front. With the setup I'm using now, I'm still getting some steady-state understeer which isn't so bad at Laguna but kills me at Sears Point.

The best thing to do is experiment and see what you can find out. With the lighter springs I used with the M42 my car would rotate nicely on the power, but I may need a little more bar in the rear to get it working like that myself, given the new setup.

-A-

[andy]

Oh, and IME, FWD cars typically go for pretty stiff in front and really stiff in back to get them to rotate better - but again that's wheel ratio, not necessarily the actual spring rate. The other cars I raced at the SJGP would hit the railroad tracks and get a good two feet of air under the rear wheels...hilarious.

[G.T.]

Originally Posted by andy

The other cars I raced at the SJGP would hit the railroad tracks and get a good two feet of air under the rear wheels...hilarious.

That's why my tire budget was manageable...I only used three wheels at a time!

Funny thing is that when a RWD car lifts the front tire when cornering hard, I think it looks cool. But when a FWD car makes like a bad doggie, I think it looks kinda cartoonish.