M3 vs Eibach swaybars

[Ski Killset]

The Eibach is 28mm also, and cheaper than the M3 bar. Why are
So many people going with the M3?

[John_01]

The E93 M3 bar is not constant 28mm diameter. It has a slimmer section in the centre. It is also hollow and therefore lighter. One advantage of the M3 bar is the rubber bushings don't need lubrication.

[128Convertibleguy]

Pretty modest differences in all regards. No doubt some people like the idea that the M3 bar is "BMW".

Like much here, matter of taste. The differences between these two are small in comparison with the diffeence between either and OE.

[Ski Killset]

So if the Eibach is solid, it would be considerably stiffer. I wonder if then you would need the rear bar upgraded to balance.

[new2the3]

I'm pretty happy with my E93 M3 bar.

[1GrayBulldog]

Originally Posted by Ski Killset

So if the Eibach is solid, it would be considerably stiffer. I wonder if then you would need the rear bar upgraded to balance.

Given the fact that Eibach's Pro-Plus Kit comes with an upgraded rear bar as well as the front 28mm bar upgrade, I'd say it's it's at least something to consider.

[128Convertibleguy]

Originally Posted by Ski Killset

So if the Eibach is solid, it would be considerably stiffer. I wonder if then you would need the rear bar upgraded to balance.

Depends on how you define "considerably". The inside metal on a sway makes much less of a contribution to stiffness than the outside metal. Boring a hole that's 1/2 of the diameter in a swaybar only reduces the stiffness to 94% of the solid bar.

As far as needing to balance the solid bar with the rear, the front and the rear bar do different things in different ways, so a simple comparison of stiffness increase says little. But increasing the size of an average rear bar by 1mm (say from 15 to 16) increases the stiffness by 30%. The added metal is on the outside. Now that's "considerably stiffer".

[1GrayBulldog]

Originally Posted by 128Convertibleguy

Depends on how you define "considerably". The inside metal on a sway makes much less of a contribution to stiffness than the outside metal. Boring a hole that's 1/2 of the diameter in a swaybar only reduces the stiffness to 94% of the solid bar.

As far as needing to balance the solid bar with the rear, the front and the rear bar do different things in different ways, so a simple comparison of stiffness increase says little. But increasing the size of an average rear bar by 1mm (say from 15 to 16) increases the stiffness by 30%. The added metal is on the outside. Now that's "considerably stiffer".

TIL. Thanks for the info!

[Kgolf31]

Mis-information throughout this thread.

A M3 Swaybar is COMPLETELY different than an Eibach 28mm bar. The M3 is hollow, and also thinner in the mid-section where it is MOST IMPORTANT. The Eibach bar is 28mm throughout.


As well, DO NOT UPGRADE THE REAR BAR. I absolutely hate when people recommend throwing a rear bar on a BMW. It makes no sense, all you're doing is making it harder to put power down, and thus making the rear end twitchy.

If anything, remove the bar.

[Ski Killset]

Right I understand hollow vs solid vs 1mm larger diameter and all that. The weight difference is negligible and seeing that it is near centerline of the hubs it probably wouldn't make a huge difference anyways.

So slightly stiffer and slightly less expensive, I will probably go with the Eibach bar.

I am adding a Quaiffe and a 3.46 final drive, I think I will likely need a rear bar after that.

Thanks

[Black Prinz]

Originally Posted by Kgolf31

Mis-information throughout this thread.

As well, DO NOT UPGRADE THE REAR BAR. I absolutely hate when people recommend throwing a rear bar on a BMW. It makes no sense, all you're doing is making it harder to put power down, and thus making the rear end twitchy.

If anything, remove the bar.

Curious, as I understand it, the base 128 has no rear bar but upgrading to the sport option adds one & the Eibach kit also includes a rear bar. Just want to understand your reasoning.

[Kgolf31]

Originally Posted by Ski Killset

Right I understand hollow vs solid vs 1mm larger diameter and all that. The weight difference is negligible and seeing that it is near centerline of the hubs it probably wouldn't make a huge difference anyways.

So slightly stiffer and slightly less expensive, I will probably go with the Eibach bar.

I am adding a Quaiffe and a 3.46 final drive, I think I will likely need a rear bar after that.

Thanks

You won't.

Originally Posted by Black Prinz

Curious, as I understand it, the base 128 has no rear bar but upgrading to the sport option adds one & the Eibach kit also includes a rear bar. Just want to understand your reasoning.

Because when you add a rear bar, you stiffen up the wheels and make weight transfer to the front of the car.

When you do a front bar, you're essentially transferring weight from the front wheels to the rear, this is where you increase traction.

Now, if you add a rear bar, you're essentially transferring weight front the rear to the front...which will now result in less grip

[128Convertibleguy]

Originally Posted by Kgolf31

When you do a front bar, you're essentially transferring weight from the front wheels to the rear, this is where you increase traction.

Now, if you add a rear bar, you're essentially transferring weight front the rear to the front...which will now result in less grip

Disagree. Bars transfer weight at the end they're on side to side, not front to rear. In a very obvious manner, they push down on one side, and up on the other. That has implications for front/rear balance, but the mechanism is not weight transfer front/rear, it's more complicated.

Sway bars are _really_ complicated. Among other things, they work differently on different cars. Beware of any web site discussing them that does not cover that point. Beware rules of thumb that may not apply to your situation.

If you want to get a feel for the complexity, read through this thread, between guys who somewhat know what they're doing. Ignore the fact that you don't understand half of it, I don't claim to be able to follow it, either. I do know the issues they're talking about are real, and important.

http://www.eng-tips.com/viewthread.cfm?qid=312246

If you want to begin to understand sway bars with respect to your car, get about 4 for each end, and rent a track for a day. That will be about 16 runs, and about 16 changes. You must either be, or hire, a good test driver. From the late, great, Carroll Smith.

"If all this makes you think we're going to be too busy at the track to drink beer and chase girls, you've got it right."

[Kgolf31]

Originally Posted by 128Convertibleguy

Disagree. Bars transfer weight at the end they're on side to side, not front to rear. In a very obvious manner, they push down on one side, and up on the other. That has implications for front/rear balance, but the mechanism is not weight transfer front/rear, it's more complicated.

Sway bars are _really_ complicated. Among other things, they work differently on different cars. Beware of any web site discussing them that does not cover that point.

A Tire can only take 100% of grip.

When you force a tire to accept 120% of grip, that 20% of grip needs to go somewhere...it goes to the rear.

There is a reason why BMWs only need a front bar. I'm probably not explaining it good enough, but do your research and you'll see that it is a bad idea.

When TCKline tells me to never upgrade a rear bar, when I initially wanted to buy one from him when I tracked my Z4M, tells you something.

[128Convertibleguy]

Originally Posted by Kgolf31

A Tire can only take 100% of grip.

When you force a tire to accept 120% of grip, that 20% of grip needs to go somewhere...it goes to the rear.

There is a reason why BMWs only need a front bar. I'm probably not explaining it good enough, but do your research and you'll see that it is a bad idea.

When TCKline tells me to never upgrade a rear bar, when I initially wanted to buy one from him when I tracked my Z4M, tells you something.

I'm not arguing with your conclusion, just your theoretical explanations.

I think I agree with your conclusion (with sway bars, I'm never that confident). Here's my explanation why.

On a BMW the front sway serves principally to limit roll. That causes the front MacPherson strut suspension, which has an awful camber curve, to grip better. That probably outweighs the fact that the bar also transfers weight to the outside front tire. So, it (probably) causes less understeer. That also could be stated as more oversteer.

The camber curve at the back is much better, and people also tend to run high static negative camber. Unlike the front, it's easy to dial it in. So, at the rear, the weight transfer effect dominates, and a larger rear bar reduces grip at the rear.

The combination of a larger front bar and a larger rear bar will (probably) create too much oversteer.

Disclaimer. Sway bars are really complicated. People mess around with them without full understanding. You shouldn't blindly trust much of what you'll read on the net about bars. That includes this post.

[Ski Killset]

I've been racing high horsepower rwd light cars with unequal a arm front and solid axle rears for years. So the wholr strut/irs thing is weird.

I will start with a front bar and the M3 bits and probably camber plates and kws. This car handles like a pig stock on runflats!

[Kgolf31]

Originally Posted by 128Convertibleguy

I'm not arguing with your conclusion, just your theoretical explanations.

I think I agree with your conclusion (with sway bars, I'm never that confident). Here's my explanation why.

On a BMW the front sway serves principally to limit roll. That causes the front MacPherson strut suspension, which has an awful camber curve, to grip better. That probably outweighs the fact that the bar also transfers weight to the outside front tire. So, it (probably) causes less understeer. That also could be stated as more oversteer.

The camber curve at the back is much better, and people also tend to run high static negative camber. Unlike the front, it's easy to dial it in. So, at the rear, the weight transfer effect dominates, and a larger rear bar reduces grip at the rear.

The combination of a larger front bar and a larger rear bar will (probably) create too much oversteer.

Disclaimer. Sway bars are really complicated. People mess around with them without full understanding. You shouldn't blindly trust much of what you'll read on the net about bars. That includes this post.

Explained it a lot better than me.

A ton of variables goes into a car and turning, but yes, a front bar can help maintain camber under load.

[Black Prinz]

Appreciate the insight you and 128Convertibleguy have provided.

Originally Posted by Kgolf31

When TCKline tells me to never upgrade a rear bar, when I initially wanted to buy one from him when I tracked my Z4M, tells you something.

Am I correct in interpreting the above as stating there is no need to upgrade from the Z4M's 22.5mm bar as opposed to to upgrading from Z4 18mm(stock)/19mm(sport) to 22.5mm?

As opposed to you advocating:

Originally Posted by Kgolf31

If anything, remove the bar.

Disclaimer I have a stock "base" 128i and am looking at the Eibach kit; now I understand why the rear bar is so puny but would I not be potentially jeopardizing myself and others by upgrading the front and doing nothing to the rear? Eibach is only a 2mm increase from sport.

[Kgolf31]

Originally Posted by Black Prinz

Appreciate the insight you and 128Convertibleguy have provided.



Am I correct in interpreting the above as stating there is no need to upgrade from the Z4M's 22.5mm bar as opposed to to upgrading from Z4 18mm(stock)/19mm(sport) to 22.5mm?

As opposed to you advocating:



Disclaimer I have a stock "base" 128i and am looking at the Eibach kit; now I understand why the rear bar is so puny but would I not be potentially jeopardizing myself and others by upgrading the front and doing nothing to the rear? Eibach is only a 2mm increase from sport.

If we're talking about the Z4, no...there is no need to touch the rear bar. You can get the rear to rotate around fairly easy. Upgrading the rear bar will make the rear end MORE UNSTABLE. It is reverse thinking from what people would imagine.

As far as the 128i, I'm still undecided on what to do. I'm personally running the M Sport sway bar (OEM for me) and don't know if I should keep it, or remove it. Removing it would help keep the rear end more stable, but keeping it "might" help with rotation. I've only done 3 events in the car, and I'm finally getting the car to rotate on the stock bar, so I don't think an upgraded bar is needed.

Like I said prior, I'm running a 28mm Eibach up front and stock in the rear. The car is setup fairly well, and I don't think anything is needed in the rear. Removing the bar is fairly common to help put power down...if 135i owners can listen from someone, they would get that bar out ASAP.

[John_01]

Originally Posted by Kgolf31

Because when you add a rear bar, you stiffen up the wheels and make weight transfer to the front of the car.

When you do a front bar, you're essentially transferring weight from the front wheels to the rear, this is where you increase traction.

Now, if you add a rear bar, you're essentially transferring weight front the rear to the front...which will now result in less grip

I don't agree with this reasoning, although I don't dispute the outcomes as you explained them. I am posting this because I think you got the background info about weight transfer slightly backwards.

Adding a firmer front sway bar increases the weight transfer on the front axle. More weight transfer means the load balance between left and right wheels changes more quickly in response to steering input. The initial effect of the weight transfer isn't necessarily the best thing for the grip at the front of the car. It places the greatest loading on the outside front wheel, so the outcome will depend on the ability of the tire to handle it. It will depend on all aspects of the tire's dynamics, eg. size, camber, pressure, temperature etc. If the car has lots of static front camber and big wide front tires it can work well, but in other situations it causes understeer.

Adding a rear sway bar does the opposite. It increases weight transfer at the rear axle. This typically reduces the amount of rear cornering grip and increases the front corning grip. Sudden weight transfer on the rear axle during cornering (due to firm damping or sway bars) will destabilise the car and make it more prone to oversteer.

The decision to change sway bars may depend on the spring rates used. On a stock car normally the largest part of roll stiffness comes from the front sway bar. Increasing the front bar size will make the overall car dynamics firmer so the car will have less body roll in corners. In contrast the rear sway bar is light relative to the springs used. The rear sway bar is only a small component of the roll stiffness at the rear of the car. For people who install coil-over suspension they will often increase the spring rates by a factor of 2 (double) or more. It may not be intentional, but its going to have a bigger effect on the increasing the roll stiffness at the rear of the car than at the front of the car. Because of increasing the rear weight transfer, it is likely the shift the car's handling balance to become more neutral, ie. less understeer.

[Kgolf31]

Originally Posted by John_01

I don't agree with this reasoning, although I don't dispute the outcomes as you explained them. I am posting this because I think you got the background info about weight transfer slightly backwards.

Adding a firmer front sway bar increases the weight transfer on the front axle. More weight transfer means the load balance between left and right wheels changes more quickly in response to steering input. The initial effect of the weight transfer isn't necessarily the best thing for the grip at the front of the car. It places the greatest loading on the outside front wheel, so the outcome will depend on the ability of the tire to handle it. It will depend on all aspects of the tire's dynamics, eg. size, camber, pressure, temperature etc. If the car has lots of static front camber and big wide front tires it can work well, but in other situations it causes understeer.

Adding a rear sway bar does the opposite. It increases weight transfer at the rear axle. This typically reduces the amount of rear cornering grip and increases the front corning grip. Sudden weight transfer on the rear axle during cornering (due to firm damping or sway bars) will destabilise the car and make it more prone to oversteer.

The decision to change sway bars may depend on the spring rates used. On a stock car normally the largest part of roll stiffness comes from the front sway bar. Increasing the front bar size will make the overall car dynamics firmer so the car will have less body roll in corners. In contrast the rear sway bar is light relative to the springs used. The rear sway bar is only a small component of the roll stiffness at the rear of the car. For people who install coil-over suspension they will often increase the spring rates by a factor of 2 (double) or more. It may not be intentional, but its going to have a bigger effect on the increasing the roll stiffness at the rear of the car than at the front of the car. Because of increasing the rear weight transfer, it is likely the shift the car's handling balance to become more neutral, ie. less understeer.

In bold, that is in a perfect world.

Typically you overload the front tires, so adding a rear bar only increases understeer.

BMW are understeer pigs, it is their natural habitat....a rear bar offers no gain for front grip.


When you lift a wheel in a BMW (which is fairly common) you give the other front wheel 100% load. However, we all know there was other grip available from that lifted tire...so it goes to the inner rear tire.

What do you think happens when a FWD car lifts a rear wheel?

RWD Cars (especially a BMW) you put a big front bar, a FWD car (like a Mini) you add a big rear bar.

Spend money on a rear bar if you want, but all you're doing is compromising rear grip, where you need it the most.

[Suds]

From running auto-x in the One, the E-93 front sway allowed me to be much more competive with my colleagues without increasing side wall tire wear. Downside my cowl creaks from the added stiffness, but it is worth it.

Also, the rear is more settled when doing quick maneuvers on the highway.