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      02-12-2008, 09:46 AM   #1
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Upgrade Rear Sway Bar w/ Stock ZSP?

any thoughts in upgrading the rear sway bar to control dreaded body roll during cornering w/ stock ZSP suspension? I have searched but nothing really concrete about advantages/disadvantages in just upgrading the bar itself? I know our ZSP suspension (if you want to maintain stock ride) is good in itself, but the car still rolls like a beer truck around corners! In particular at high speeds

So for those who may have only thought about upgrading the sways, can you chime in? Just playing around with this thought, it may be pointless but better ask instead of speculating.

Gracias
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      02-12-2008, 10:45 AM   #2
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      02-12-2008, 07:02 PM   #3
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      02-12-2008, 11:26 PM   #4
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Mild option: M-sport 14mm. Full bore: H&R. Adjustable: UUC.
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      02-12-2008, 11:45 PM   #5
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Quote:
Originally Posted by fast_lap28 View Post
any thoughts in upgrading the rear sway bar to control dreaded body roll during cornering w/ stock ZSP suspension? I have searched but nothing really concrete about advantages/disadvantages in just upgrading the bar itself? I know our ZSP suspension (if you want to maintain stock ride) is good in itself, but the car still rolls like a beer truck around corners! In particular at high speeds

So for those who may have only thought about upgrading the sways, can you chime in? Just playing around with this thought, it may be pointless but better ask instead of speculating.

Gracias
Anyone who uses beer truck describing body roll will like the H&R or UUC sway bars. This is no brainer you don't need to say more

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      02-12-2008, 11:57 PM   #6
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I will be the contrarian here.

I installed a rear HR bar on the stock suspension and recently PAID to have it removed once I installed my coilovers. If you are driving on the street and are nowhere near the limit, the rear sway may make sense. It flattens the car a bit, dials out the understeer and makes the car "feel" sharper.

But with the stock open rear diff, it makes you wheelspin like no tomorrow. After corning, I literally could not get on power until almost in a straightline.

It's a bandaid and one that you will pay for later, imo. But for street-only use, it's acceptable.
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      02-13-2008, 12:14 AM   #7
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Quote:
Originally Posted by leftcoastman View Post
I will be the contrarian here.

I installed a rear HR bar on the stock suspension and recently PAID to have it removed once I installed my coilovers. If you are driving on the street and are nowhere near the limit, the rear sway may make sense. It flattens the car a bit, dials out the understeer and makes the car "feel" sharper.

But with the stock open rear diff, it makes you wheelspin like no tomorrow. After corning, I literally could not get on power until almost in a straightline.

It's a bandaid and one that you will pay for later, imo. But for street-only use, it's acceptable.
Have you considered upgrading the front sway bars to improve the balance?
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      02-13-2008, 12:19 AM   #8
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Quote:
Originally Posted by Ray View Post
Have you considered upgrading the front sway bars to improve the balance?
Yes. The rears went from 13mm to 20mm. That is a huge difference.

The fronts, with aftermarket, would go from 27mm to 27.5mm. That isn't a big difference, even with stiffness increasing to the 4th power of diameter.

Even if you did both front and rear, it'd drastically alter the balance towards the rear. Regardless, the rear thick sway with open diff = massive wheelspin on corners.
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      02-13-2008, 01:10 AM   #9
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Quote:
Originally Posted by leftcoastman View Post
Yes. The rears went from 13mm to 20mm. That is a huge difference.

The fronts, with aftermarket, would go from 27mm to 27.5mm. That isn't a big difference, even with stiffness increasing to the 4th power of diameter.

Even if you did both front and rear, it'd drastically alter the balance towards the rear. Regardless, the rear thick sway with open diff = massive wheelspin on corners.
http://www.e90post.com/forums/newrep...reply&t=113596

I listen to what you said in the past for your setup and that you actually went backwards. You really messed with you front to rear ride height and most likely have to much dampening in the rear. Front to rear ride height will mess with traction more than a sway bar.

Lets be clear. If you run stiffer rear spring or sway bar the effect is the same. You will have slightly more inside wheel spin with open diff. If you had a LSD you would not(also would also increase rear roll over open diff). The stock rear sway bar size was specified for a reason..it wasn’t for performance. Get rid of the RFT and open diff and it is a new ball game.

BTW, you got the stiffness thing wrong and you don’t understand torsional loading. It much easier if you pull out a calculator and do the math for this. You can use percentages to get a scale.

H&R Front: 531,441
H&R Rear: 160,000
Total Stiffness: 691,441

Stock Front: 493,155
Stock Rear: 28,561
Total Stiffness: 521,716

Orb

Last edited by Orb; 02-13-2008 at 09:28 AM.
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      02-13-2008, 10:21 AM   #10
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Orb's point is that although the stiffness formula is usually framed in terms of a %, derived from the ratio of difference in diameter to the 4th power, one needs to consider the value that is being increased. 10% of 100 = 10, whereas 10% of 10 is only = 1. Because the rear bar is so much smaller than the front, an equal % increase in each results in much more absolute stiffness increase in the front.
Switching to H&Rs results in the following change:

% increase in stiffness = (27/26.5)^4 = 1.07763468413, i.e., a 7.8% increase in front bar stiffness (if we believe the measurements, and if the materials have the same tension resistance).

For the rear, % increase = (20/13) ^4 = 5.60204474631 i.e., a 460% increase.

So just looking at the %s, it looks like rear stiffness has increased much more than the front change, a % ratio of roughly 460/7.8 or about 60:1. But if you look at the change in actual stiffness, it is more even, because the front bar was so much bigger to begin with (10% of a big bar is a lot more than 10% of a little bar). The change ratio of actual stiffness is roughly 13/4, or about 3:1.

Note that this formula does not work for hollow bars like the Hotchkis, nor adjustable bars. Note further that there are a lot of factors that enter into the end result. You really need a sophisticated modeling program to sort the effects of the variables out, and then a ton of road time to fine tune the art. This is why some folks are hesitant to mess with suspensions piecemeal.

My bottom line: if you want a simple moderate increase in roll stiffness and reduction of understeer, opt for a M-sport rear sway. If you want a major increase in roll stiffness, opt for the H&R set. But be wary that you have significantly changed the balance that BMW engineered, and you may need to adjust other components (e.g., get adjustable coilovers). As noted above, there are traction issues (inside wheel will lift off ground more with increased roll stiffness), and having a LSD puts another complex variable into the mix.

No-nos in my book include: lowering w/ stiffer springs (and doing nothing else), or making a big change in one or the other sway (small relative increase in rear being the desireable change).
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      02-13-2008, 08:12 PM   #11
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I'm not sure where the antagonism came from, but it appears to have clouded your reasoning.

Let me rephrase for you:

At first, I installed a rear sway only (stock suspension). How does that change ride height?

I ran open rear diff and RACE rubber with the thicker rear sway. It spun like crap. It spun even worse when I was running RFT. Either way, the rear sway made it spin.

The rear sway is not a panacea. If it was, I wouldn't have paid $400 to get it removed. Tire temps at the track indicated that it was a bandaid that wasn't effective in the longrun.

In my simple mind (sorry Orb, I'm not a genius like you) and my empirical testing on track with a pyrometer and video analysis, the rear swaybar may increase my entry speed, but kills my exit speed. Even if I were to upgrade the front sway, the balance of the vehicle is changed far too dramatically and results in a twitchy feeling at the limit.

Again, if you're only driving on the street - it'll make you feel faster. But at the limit on the track, you're likely to make a trip to the weeds.

Here's a tip: Kill the arrogance in your posts.

Quote:
Originally Posted by Orb View Post
http://www.e90post.com/forums/newrep...reply&t=113596

I listen to what you said in the past for your setup and that you actually went backwards. You really messed with you front to rear ride height and most likely have to much dampening in the rear. Front to rear ride height will mess with traction more than a sway bar.

Lets be clear. If you run stiffer rear spring or sway bar the effect is the same. You will have slightly more inside wheel spin with open diff. If you had a LSD you would not(also would also increase rear roll over open diff). The stock rear sway bar size was specified for a reason..it wasnít for performance. Get rid of the RFT and open diff and it is a new ball game.

BTW, you got the stiffness thing wrong and you donít understand torsional loading. It much easier if you pull out a calculator and do the math for this. You can use percentages to get a scale.

H&R Front: 531,441
H&R Rear: 160,000
Total Stiffness: 691,441

Stock Front: 493,155
Stock Rear: 28,561
Total Stiffness: 521,716

Orb
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      02-13-2008, 10:42 PM   #12
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I love these conversations Actually I really do since they do stimulate conversation and bring up some interesting points of debate. If only the election process could be these friendly...

I would agree that tuning a suspension is a difficult task when changing all variables. If I was to do it again, I would buy the Dinan complete suspension kit consisting of struts, shocks, lowering springs, front and rear sways, and camber plates. It is a complete "system" and takes the guess work out of the equation. And for most, will prove to be the best set up under most circumstances. I wish this was out a year ago. Check out the Dinan bar diameters. A bit more agressive than the BMW M Sport offering.

Keep up the good debate
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      02-14-2008, 12:21 AM   #13
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Quote:
Originally Posted by leftcoastman View Post
I'm not sure where the antagonism came from, but it appears to have clouded your reasoning.

Let me rephrase for you:

At first, I installed a rear sway only (stock suspension). How does that change ride height?

I ran open rear diff and RACE rubber with the thicker rear sway. It spun like crap. It spun even worse when I was running RFT. Either way, the rear sway made it spin.

The rear sway is not a panacea. If it was, I wouldn't have paid $400 to get it removed. Tire temps at the track indicated that it was a bandaid that wasn't effective in the longrun.

In my simple mind (sorry Orb, I'm not a genius like you) and my empirical testing on track with a pyrometer and video analysis, the rear swaybar may increase my entry speed, but kills my exit speed. Even if I were to upgrade the front sway, the balance of the vehicle is changed far too dramatically and results in a twitchy feeling at the limit.

Again, if you're only driving on the street - it'll make you feel faster. But at the limit on the track, you're likely to make a trip to the weeds.

Here's a tip: Kill the arrogance in your posts.
I mixed you up with someone else so please accept my apology.

In any case it doesn’t look like an over steer situation. You are just struggling with mechanical grip at the edge and the rear sway bar will indeed reduce grip to the wheel with the least amount of traction of course. If you put it in terms what the car feel like before you spin the rear wheels then you can determine the balance to some degree so other influences don’t dominate like drive traction and soft dampening then you might have a some what different opinion? Isolating a behavior is paramount to making any improvement even when it is limited and then we can only change what we can afford.

I’m sure you wouldn’t say no to a LSD and the balance would have been a lot different at the edge with stiff sway bar just based on the load is the opposite. That is why I say an LSD does just as much for handling dynamics than straight line traction.

Orb

Last edited by Orb; 02-14-2008 at 01:13 AM.
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      02-14-2008, 01:03 AM   #14
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I love these conversations
I'm actually quite honored to have had an opportunity to correspond with these two gentlemen off-post. Both bring a great deal of value to this forum.
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      02-14-2008, 01:08 AM   #15
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Quote:
Originally Posted by sg335 View Post
I love these conversations Actually I really do since they do stimulate conversation and bring up some interesting points of debate. If only the election process could be these friendly...

I would agree that tuning a suspension is a difficult task when changing all variables. If I was to do it again, I would buy the Dinan complete suspension kit consisting of struts, shocks, lowering springs, front and rear sways, and camber plates. It is a complete "system" and takes the guess work out of the equation. And for most, will prove to be the best set up under most circumstances. I wish this was out a year ago. Check out the Dinan bar diameters. A bit more agressive than the BMW M Sport offering.

Keep up the good debate
BTW, those Dinan bars are Eibach repainted...LOL.

Orb
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      02-14-2008, 01:10 AM   #16
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I agree that the LSD is a very important tuning tool and makes a significant difference in corning exit speed and reduction in inner wheel spin in a turn. The difference a Quaife made in my other car was night and day at a road course and made a two second difference at some tracks. When I upgraded sway bars after the Quaife install, there was little noticable difference with inside wheel spin as the car would still loose some grip (much less) but would be much more controllable. And they sways kept the car flatter with PSS9's and were good for an additional .5 seconds.

You need a Quaife!
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      02-14-2008, 01:38 AM   #17
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Orb...the eibach rear bar is 15mm, but it's only listed under 325i/328i/330i sedan on eibach's website, do you think it'll work for E92 335i?
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      02-14-2008, 01:48 AM   #18
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No worries. The biggest issue I had with the fat rear sway was that the limit became so much sharper.

In decreasing radius turns, with the upgraded sway, I had to be extremely careful with throttle steering. If I lifted too hard, I'd be facing the front end of the guy behind me.

For my driving style, that doesn't make for confidence, which for me doesn't make for speed.

For the short period of time that I drove with the Quaife and the thicker rear sway, I didn't get to take it on track. In my hooligan moments, I did find that getting the rear out was much easier than it is now (with Quaife, stock sway and coilovers). Granted, the coilovers have a bunch of toe-in dialed in the back, so power oversteer isn't as prevalent.




Quote:
Originally Posted by Orb View Post
I mixed you up with someone else so please accept my apology.

In any case it doesnít look like an over steer situation. You are just struggling with mechanical grip at the edge and the rear sway bar will indeed reduce grip to the wheel with the least amount of traction of course. If you put it in terms what the car feel like before you spin the rear wheels then you can determine the balance to some degree so other influences donít dominate like drive traction and soft dampening then you might have a some what different opinion? Isolating a behavior is paramount to making any improvement even when it is limited and then we can only change what we can afford.

Iím sure you wouldnít say no to a LSD and the balance would have been a lot different at the edge with stiff sway bar just based on the load is the opposite. That is why I say an LSD does just as much for handling dynamics than straight line traction.

Orb
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      02-14-2008, 01:50 AM   #19
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SG - are you talking about another car or your 335i? Specifically the comment about shaving off 0.5 seconds.

Quote:
Originally Posted by sg335 View Post
I agree that the LSD is a very important tuning tool and makes a significant difference in corning exit speed and reduction in inner wheel spin in a turn. The difference a Quaife made in my other car was night and day at a road course and made a two second difference at some tracks. When I upgraded sway bars after the Quaife install, there was little noticable difference with inside wheel spin as the car would still loose some grip (much less) but would be much more controllable. And they sways kept the car flatter with PSS9's and were good for an additional .5 seconds.

You need a Quaife!
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      02-14-2008, 04:59 AM   #20
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Dinan shows the name part# for both e90 330i and e92 335i, so the eibach sway bars should fit 335i too. Anyone care to comment on how the eibach stack up to h&r on paper?
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      02-14-2008, 06:19 AM   #21
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I don't understand why the average person would want such a dramatic change in the sway bar dia. for the rear considering the front doesn't get much larger. It's downright dangerous as the endogenous under steer turns into aggressive oversteer.
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      02-14-2008, 11:52 AM   #22
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Quote:
Originally Posted by leftcoastman View Post
I will be the contrarian here.

I installed a rear HR bar on the stock suspension and recently PAID to have it removed once I installed my coilovers. If you are driving on the street and are nowhere near the limit, the rear sway may make sense. It flattens the car a bit, dials out the understeer and makes the car "feel" sharper.

But with the stock open rear diff, it makes you wheelspin like no tomorrow. After corning, I literally could not get on power until almost in a straightline.

It's a bandaid and one that you will pay for later, imo. But for street-only use, it's acceptable.
good thought and point here....

I def don't want to wait to get back on the thorttle during hard cornering! all I want is to reduce the body roll with stock since I haven't had time to go w/ coilovers yet. the car is DD but I'm hoping to hit some driving schools later this year. I also don't want to upset the balance by throwing in the bar and it upsetting the rest of the suspension....that's why I wasn't sure if this is even worth doing figuring you still have to drop the sub frame to get it installed?
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