[tetsuo111]

My 328i xDrive is a daily driver and also tracked as often as I'm able. I'm preparing to drop my rear subframe to install Al RSBs and delrin differential bushings. I planned to further upgrade the rear suspension in the future with M3 control links, but since this is such a large job, I decided to do all rear suspension upgrades at once. The final piece of the puzzle is the rear sway bar. To be very clear, I don't feel excessive body roll since replacing the OEM suspension with TC Kline. In fact, I'm inclined to leave the OEM rear sway bar alone.

It was interesting to learn the M and non-Ms share the identical front sway bar size of 26.5 mm. All the 3-series, from 325i to M3 have this size front bar, even though part numbers are different between the M and non-Ms.

The rear is different - the M3 rear sway bar is 22.5 mm while the non-Ms share an identical 13 mm sway bar. Can someone share any insight on the different suspensions? This will help me to decide whether or not to include a new rear sway bar in my project.

I can understand non-Ms having a softer rear bar to make the car safer to drive (more understeer) for the average driver. But considering the front bars are identical, the difference in size between the rear bars is a mystery to me. Especially since bar stiffness is an exponential function of diameter. The massive difference in size strikes me as impossible for both suspensions to be balanced.

What am I missing here?

[CALWATERBOY]

Originally Posted by tetsuo111

What am I missing here?

Limited slip differential.

Drive M3 - you'll immediately see....feel....want....

M3 rear sway NOT recommended w/o LSD.

[tetsuo111]

I knew the LSD matters for suspension balance. I didn't realize just how much. Thanks for the insight.

[Manxman1950]

Originally Posted by tetsuo111

My 328i xDrive is a daily driver and also tracked as often as I'm able. I'm preparing to drop my rear subframe to install Al RSBs and delrin differential bushings. I planned to further upgrade the rear suspension in the future with M3 control links, but since this is such a large job, I decided to do all rear suspension upgrades at once. The final piece of the puzzle is the rear sway bar. To be very clear, I don't feel excessive body roll since replacing the OEM suspension with TC Kline. In fact, I'm inclined to leave the OEM rear sway bar alone.

It was interesting to learn the M and non-Ms share the identical front sway bar size of 26.5 mm. All the 3-series, from 325i to M3 have this size front bar, even though part numbers are different between the M and non-Ms.

The rear is different - the M3 rear sway bar is 22.5 mm while the non-Ms share an identical 13 mm sway bar. Can someone share any insight on the different suspensions? This will help me to decide whether or not to include a new rear sway bar in my project.

I can understand non-Ms having a softer rear bar to make the car safer to drive (more understeer) for the average driver. But considering the front bars are identical, the difference in size between the rear bars is a mystery to me. Especially since bar stiffness is an exponential function of diameter. The massive difference in size strikes me as impossible for both suspensions to be balanced.

What am I missing here?

The front bars are not the same - if you look closely, the centre section of the non-M bar is a thinner cross section. In fact even M3 bars vary - the E93 being the stiffest.

[feuer]

Originally Posted by tetsuo111

I can understand non-Ms having a softer rear bar to make the car safer to drive (more understeer) for the average driver.

That is the rule of thumb on fwd, on rwd is the opposite, softer would induce oversteer on rwd. The size depends from the whole suspension setup, mostly the spring rates and alignment, no so much LSD or not.

[tetsuo111]

I think it's a good idea to leave things as-is. If a LSD is in my future, then I'll re-visit the sway bars at that time.

Thank you for the replies.

[ajsalida]

Originally Posted by feuer

That is the rule of thumb on fwd, on rwd is the opposite, softer would induce oversteer on rwd. The size depends from the whole suspension setup, mostly the spring rates and alignment, no so much LSD or not.

I'm not sure I understand this comment about softer bar inducing more oversteer on RWD. I agree with the second part about spring rates and so forth, the M3 has quite a lot of different stuff going on than non-M3, higher springs rates, wider track, fatter tires, and then of course that big V8 sitting in front

Since I have installed the rear M3 bar on an XI, maybe I should chime in. I did this before installing a larger front bar, but after Koni Yellows and Eibach pro kit springs. Got a huge amount of additional oversteer. Very obvious and noticeable. Car immediately wanted to rotate around outside front tire on turn-in. Was kind of cool for a day or two then got annoying. Was obvious you'd not want this much rear roll stiffness differential F/R under most conditions (unless you were drifting).

Then I added the UUC front bar and it is nice and neutral, very very slight and controllable bias to rear stepping out, but nowhere near as scary bad as with only the big rear bar. The weird thing about BMW's and front bars is you'd think adding a big front bar would cause more understeer. But it doesn't, because of the McP front geometry, and esp on lowered cars, the bigger front bar makes the tire stay at more of an optimal angle to the road under cornering forces. So this makes the front tires stick better (more like it should). Without this the tire tread will lean out onto the outside edge away from the road and you lose traction long before you should. This is counter-intuitive but many people find the cars handle a lot better and more neutral with just the bigger front bar. Bonus is it is WAY easier to install than a rear bar. In my case the M3 bar was so far over the top F/R roll stiffness wise I had to balance things out. Probably should have tried just the front bar first.

Anyway once I added the UUC bar in front the handling balanced out nicely. I will say if I had it to do over again I'd probably go with the UUC rear bar instead of M3, as UUC rear is adjustable and M3 is not. The reason I went with UUC front is they are (or were) the only people making a bigger bar for XI.

I do not have an LSD but I am running much softer springs than OP, if the 400/600 is the spring rate on the TCK's he's got. Quite a lot of M3 track type folks running similar rates find too much rear bar is a problem. They are lifting the inside rear wheel in a corner, whole other problem. Just too much total rear roll stiffness (springs + bar) I guess.

But the springs I have now are relatively soft (Eibach fronts ZSP rears) so the bigger bars do not go over the top in terms of total roll stiffness. Hope this helps.

[feuer]

Originally Posted by ajsalida

I'm not sure I understand this comment about softer bar inducing more oversteer on RWD. I agree with the second part about spring rates and so forth, the M3 has quite a lot of different stuff going on than non-M3, higher springs rates, wider track, fatter tires, and then of course that big V8 sitting in front

Since I have installed the rear M3 bar on an XI, maybe I should chime in. I did this before installing a larger front bar, but after Koni Yellows and Eibach pro kit springs. Got a huge amount of additional oversteer. Very obvious and noticeable. Car immediately wanted to rotate around outside front tire on turn-in. Was kind of cool for a day or two then got annoying. Was obvious you'd not want this much rear roll stiffness differential F/R under most conditions (unless you were drifting).

Then I added the UUC front bar and it is nice and neutral, very very slight and controllable bias to rear stepping out, but nowhere near as scary bad as with only the big rear bar. The weird thing about BMW's and front bars is you'd think adding a big front bar would cause more understeer. But it doesn't, because of the McP front geometry, and esp on lowered cars, the bigger front bar makes the tire stay at more of an optimal angle to the road under cornering forces. So this makes the front tires stick better (more like it should). Without this the tire tread will lean out onto the outside edge away from the road and you lose traction long before you should. This is counter-intuitive but many people find the cars handle a lot better and more neutral with just the bigger front bar. Bonus is it is WAY easier to install than a rear bar. In my case the M3 bar was so far over the top F/R roll stiffness wise I had to balance things out. Probably should have tried just the front bar first.

Anyway once I added the UUC bar in front the handling balanced out nicely. I will say if I had it to do over again I'd probably go with the UUC rear bar instead of M3, as UUC rear is adjustable and M3 is not. The reason I went with UUC front is they are (or were) the only people making a bigger bar for XI.

I do not have an LSD but I am running much softer springs than OP, if the 400/600 is the spring rate on the TCK's he's got. Quite a lot of M3 track type folks running similar rates find too much rear bar is a problem. They are lifting the inside rear wheel in a corner, whole other problem. Just too much total rear roll stiffness (springs + bar) I guess.

But the springs I have now are relatively soft (Eibach fronts ZSP rears) so the bigger bars do not go over the top in terms of total roll stiffness. Hope this helps.

As you said:very slight and controllable bias to rear stepping out. That is after you installed stiffer UUC on the front. I was saying that if you go softer on the rear then already is, or softer in comparison to the "stock" front you will get oversteer as the front tires would stick better. I hope this clarify things a bit. Softer rear relative to stock rear and stock front will give you oversteer and this is only to RWD. This is why everyone is upgrading the front bar on RWD and the rear bar on FWD.

[The HACK]

Originally Posted by feuer

As you said:very slight and controllable bias to rear stepping out. That is after you installed stiffer UUC on the front. I was saying that if you go softer on the rear then already is, or softer in comparison to the "stock" front you will get oversteer as the front tires would stick better. I hope this clarify things a bit. Softer rear relative to stock rear and stock front will give you oversteer and this is only to RWD. This is why everyone is upgrading the front bar on RWD and the rear bar on FWD.

It's the opposite. And RWD and FWD has little to no effect. What type of suspension has a bigger determination.

ajsalida has it right. Stiffer rear bars on a multi-link rear will cause the rear to lose grip quicker. Stiffer front bars on a McPherson Strut CAN have the opposite effect because the bar prevents the suspension from compressing past the roll center.

The reason why some FWD cars with rear bars actually reduce oversteer, is because some popular FWD cars come with torsion beam rear suspension. When swaybar is applied, it effectively increases the rear spring rate, which is the same effect as adding bars on McPherson strut...It prevents the suspension from compressing past roll center.

In an ideal world, where front AND rear suspensions are the magical double A arm type, or multi-link push-rod (ala tube frame race cars or open wheels), an anti-roll bar introduced to prevent fully independent suspension movement will actually DECREASE grip to the end the bar is added to. A great example is the mighty NSX. The original version of the NSX came with equal sized swaybars front and back, on double wishbones all around. The end result was one of the most neutral chassis on the market (since the car is moderately powered and mid-engined with, if I recall, a 40/60 weight mix). The OTHER end result, was it has a propensity to swap ends because people don't know what to do when oversteer is initiated. So in subsequent releases they put thicker and thicker front bars, to a point where the car is heavily biased toward understeer rather than a neutral chassis, where you'd have to be truly stoopid to get the car to oversteer (it still will). On a car like the NSX, tuning the suspension was relatively straightforward. You can follow the text book and the results will mostly be predictable.

On a BMW? Not so much. Thicker bars up front may reduce or promote understeer. There's no real way to predict it. Thicker bars in the rear though, will most likely REDUCE understeer in that it actively takes away rear grip to promote OVERSTEER (again, think of understeer and oversteer as two ends of the car on a teeter totter) because it reduces ability for the two parts of the rear axle from moving independently in the middle of a turn.

But if you put a fully independent, multi-linked rear suspension on a FWD, and you increase the roll stiffness on that same FWD? You will promote OVERSTEER, not reduce it.

But, why in the world would you want to reduce understeer (and therefore increase oversteer) on a FWD?

[feuer]

Originally Posted by The HACK

It's the opposite. And RWD and FWD has little to no effect. What type of suspension has a bigger determination.

ajsalida has it right. Stiffer rear bars on a multi-link rear will cause the rear to lose grip quicker. Stiffer front bars on a McPherson Strut CAN have the opposite effect because the bar prevents the suspension from compressing past the roll center.

The reason why some FWD cars with rear bars actually reduce oversteer, is because some popular FWD cars come with torsion beam rear suspension. When swaybar is applied, it effectively increases the rear spring rate, which is the same effect as adding bars on McPherson strut...It prevents the suspension from compressing past roll center.

In an ideal world, where front AND rear suspensions are the magical double A arm type, or multi-link push-rod (ala tube frame race cars or open wheels), an anti-roll bar introduced to prevent fully independent suspension movement will actually DECREASE grip to the end the bar is added to. A great example is the mighty NSX. The original version of the NSX came with equal sized swaybars front and back, on double wishbones all around. The end result was one of the most neutral chassis on the market (since the car is moderately powered and mid-engined with, if I recall, a 40/60 weight mix). The OTHER end result, was it has a propensity to swap ends because people don't know what to do when oversteer is initiated. So in subsequent releases they put thicker and thicker front bars, to a point where the car is heavily biased toward understeer rather than a neutral chassis, where you'd have to be truly stoopid to get the car to oversteer (it still will). On a car like the NSX, tuning the suspension was relatively straightforward. You can follow the text book and the results will mostly be predictable.

On a BMW? Not so much. Thicker bars up front may reduce or promote understeer. There's no real way to predict it. Thicker bars in the rear though, will most likely REDUCE understeer in that it actively takes away rear grip to promote OVERSTEER (again, think of understeer and oversteer as two ends of the car on a teeter totter) because it reduces ability for the two parts of the rear axle from moving independently in the middle of a turn.

But if you put a fully independent, multi-linked rear suspension on a FWD, and you increase the roll stiffness on that same FWD? You will promote OVERSTEER, not reduce it.

But, why in the world would you want to reduce understeer (and therefore increase oversteer) on a FWD?

I had w201 16v with rear suspension very similar to e90, then I had an e36 and how e90 and to reduce understeer (most rwd are setup like that "because people don't know what to do when oversteer is initiated" as you said) I was advised by shops that professionally build race cars to go with larger front swaybar and leave the rear alone.
Additionally, I had quite few VW, from old with rear torsion bean and no rear swaybar what so ever to newer with 5 link rear line the Mark V cars, 2006 and up GTI and to reduce the understeer I was adviced to install larger rear swaybar.
So, sarcasm aside, I guess I have been miss informed and how the cars behaved after the changes was imaginary.

[Cloud9blue]

Originally Posted by feuer

I had w201 16v with rear suspension very similar to e90, then I had an e36 and how e90 and to reduce understeer (most rwd are setup like that "because people don't know what to do when oversteer is initiated" as you said) I was advised by shops that professionally build race cars to go with larger front swaybar and leave the rear alone.
Additionally, I had quite few VW, from old with rear torsion bean and no rear swaybar what so ever to newer with 5 link rear line the Mark V cars, 2006 and up GTI and to reduce the understeer I was adviced to install larger rear swaybar.
So, sarcasm aside, I guess I have been miss informed and how the cars behaved after the changes was imaginary.

There are too many other variables in play with all the different types of cars that you are trying to compare (spring rates, suspension types, etc.)...

As mentioned above, whether a thicker bar will induce or reduce grips all depends on the suspension camber change upon compression and the effective spring rates of that particular axle relative to the other axle.

For our chassis, at least on the RWD models, a thicker rear bar is not necessary at all, even with LSD in my opinion. X-drive models might be slightly different due to the additional load on the front tires due to the AWD system powering the front and the additional weight at the front axle.

[tetsuo111]

Resurrecting an old post.

I pulled the trigger on a Performance Gearing clutch LSD for the rear diff. Installing a rear sway requires dropping the rear subframe, so it would be nice to batch the installs together for convenience and economics.

My understanding is that UUC is the sole fabricator of front sway bars for xDrive. They make a matching rear bar, too. The rear is adjustable. The sizes are 25.5 and 19.5mm. For comparison, OE bars are 26.5 and 13mm front and rear. The M3 bars are 26.5 and 22.5.

Wondering if anyone has direct first hand experience using upgraded sways, especially in the rear, together with a proper, non-open differential on an E9x chassis? My primary use is for the track. At this point I'm leaning towards installing the bigger sway bars, mostly because I don't see a downside. The new bars would still be softer than stock M3 bars, and I expect the new PG LSD would perform at least as well as the BMW M3 OE unit.

Assumptions remain unchanged from my OP. My suspension is already significantly modified (see details in my signature and garage). Body roll is minor now, but a little added firmness may be welcome. Springs are 400 / 600 front and rear on TCK coilovers.

Thanks.

[feuer]

Originally Posted by tetsuo111

I pulled the trigger on a Performance Gearing clutch LSD for the rear diff. Installing a rear sway requires dropping the rear subframe, so it would be nice to batch the installs together for convenience and economics.

You aren't dropping the subframe for LSD install and need to only lower it, not completely drop it for swaybar install.
Here are some good articles about swaybars.
http://hubpages.com/autos/Anti-Roll-...r-for-your-car
http://www.houseofthud.com/cartech/swaybars.htm
http://www.stealthtdi.com/SwayBars.html
Pretty much all say FWD upgrade rear swaybar and RWD upgrade front swaybar.
Since your are AWD but with rear bias I would say upgrade just the front something that most of us here have done already.
Also from experience I don't see any direct relation between swaybars and LSD.
Vehicle weight and spring rates yes but LSD no.
I have been modifying MB's in the past and most had LSD from factory and some big name tuners never offer rear swaybar upgrades, at least not that I know of.
Additionally in the past I have been modifying BMW's as well.
I remember from (all non M) two e21 we had in the family as well as e36 came with LSD as factory option and the sway bars were the same with non LSD cars.
You have done extensive moding and no offense, I love naturally aspirated I6's, but is 328ix, heavy and AT, AWD too.
Why LSD? I can't justify it, honestly! I track my 335i with open diff.

I had to edit:
Your signature says stick and M3 dct padals? Is it MT or AT?

[tetsuo111]

Thanks feuer. You seem to have extensive practical experience. This is the kind of input I find helpful.

Those articles are better than many I've read. And that's the challenge, I've read so many articles my head spins. With information overload, I think I'll just try the new sways and gain experience. I'm probably over-complicating the entire subject and /or just not getting important parts of the explanation.

Interesting observation about your experience with LSDs and sways. Dinan and others recommend having an LSD as a prerequisite for increasing rear sway bar size.

I ordered the LSD for track use. On very tight, low speed corners I can feel the chassis struggle to grip and put power to the ground. The cost is lower exit speed, and lower terminal speed on the following straight.

Why 328ix wagon? I'm limited to one car at the moment that needs to fulfill very different needs. Basic transportation, AWD winter mountain vehicle transportation, and summer track car. It is a heavy car, no different from all modern BMWs. I made significant progress lightening the car. Stock the car is listed > 3800 lbs. It weighs around 3550 lbs now, and 3400 lbs is within reach. I looked for a 6MT for months before giving up and going with the Steptronic. "Stick" in my signature refers to mods for the purpose of traction, grip, and balance.

[putnampunk]

So how did your setup turn out? I'm considering LSD on my 328xi and subframe bushings plus inner brake lines as well as a bigger rear bar (M3) while I'm in there.

I've typically increased front bar stiffness with RWD cars (M3 links and cabby bars) and left the rear alone (or unhooked the rear bar on my track E30 with stiffish rear coilover springs). But with a heavier AWD wagon, it may make sense to help rotation.