[Tambohamilton]

My E91 330D has the most hilariously spindly rear ARB that I've ever seen, but the front is pretty meaty. I see lots of folk saying that putting even bigger front bars in (and often leaving the rear as it is) improves front end grip, but that goes against what I thought I knew.

Please can somebody explain why the big front bar is magic on the E9x? Thanks.

TL;DR
A massive front sway bar of course helps the car stay flatter in turns, which has 3 effects:
1. Faster steering response - feels way more responsive to inputs.
2. Helps maintain negative camber; prevents the outside front suspension from compressing too far, and reduces the body roll which would otherwise increase (make less negative) camber.
3. Applies a lot more vertical load to the outside front wheel, at the expense of removing it from the inside front wheel...gives less front grip overall (probably).
So, the front bar isn't magical on the E9x, but it does improve the feel of some things. Read on for much more in depth discussion.

[Blown95ImpalaSS]

Big front bar does not improve absolute grip. It does improve steering feel and makes the car respond more immediately to driver inputs/seem more agile to a certain point. A truly monster front bar will make the car push or understeer at the limit.

A big rear bar does the same thing and feels great on the street. Lots of people on this platform suggest leaving rear bar stock to avoid unwanted behavior from our e-diff (open diff) and excess wheel spin on corner exit. True LSD upgrade is suggested to make best use of bigger rear bar.

Related:
https://www.e90post.com/forums/showthread.php?t=1613871

[Tambohamilton]

Thanks. Basically confirmed what I thought; add more rear bar and put up with one-tyre-fire for some extra front end grip, or replace bars in pairs (upgrade to m3) to keep some balance and improve turn-in feel...at the expense of some e-diff action.

[Silnismo]

Worth noting the front bar upgrade is a much easier install oppose the rears since you have to drop the rear subframe to get the rear bar in and out.

I upgraded my front sway bar to a eibach on my E90 and its one my favorite modifications.

[Tambohamilton]

Originally Posted by Silnismo

Worth noting the front bar upgrade is a much easier install oppose the rears since you have to drop the rear subframe to get the rear bar in and out.

I upgraded my front sway bar to a eibach on my E90 and its one my favorite modifications.

Yeah, I guess that's why most people only upgrade the front bar, and the resulting difference in response is good enough. I'll need to do the rear subframe bushes on mine pretty soon anyhow, so I think I'll put a rear bar in it at the same time.

[rothwem]

The rear bar is smaller because the multi-link in the rear has more geometric anti-roll built into it. The struts at the front have a lower roll center, thus a larger roll couple, and a larger bar is needed for the same effect. The rear suspension also does a better job maintaining camber as the car rolls, so it has more traction than the front for equivalent roll angles and doesn't need as stiff of a bar to maintain contact patch.

True, most road cars are tuned for a bit of understeer, but a perfectly neutral or even slightly oversteering E90 would still have a less stiff sway bar in the rear.

[TheMidnightNarwhal]

Hahaha OP I looked at my rear sway bar for curiosity and you're right it definitely looks noodle sized

[bbnks2]

Originally Posted by rothwem

The rear bar is smaller because the multi-link in the rear has more geometric anti-roll built into it. The struts at the front have a lower roll center, thus a larger roll couple, and a larger bar is needed for the same effect. The rear suspension also does a better job maintaining camber as the car rolls, so it has more traction than the front for equivalent roll angles and doesn't need as stiff of a bar to maintain contact patch.

True, most road cars are tuned for a bit of understeer, but a perfectly neutral or even slightly oversteering E90 would stiff have less stiff sway bar in the rear.

Right, but most people don't understand this and they also don't understand that it's only true to a certain extent. Throwing a huge front bar on these cars does not make it handle better.They already under-steer like crazy in stock form. The rear "noodle" bar is part of the problem. Move to a more balanced sway bar setup (like the E92 M3) and the car will be way more balanced yet still slightly under-steer oriented. The stock staggered tire setup is the next hurdle.

A modest rear bar upgrade will not pick up tires off the ground. The "e-diff" thing is also blown out of proportion. Put too big of a rear bar on the car and you will have issues though. Being RWD, rear sways can be detrimental to putting down power on the inside wheel. I have yet to see ANY 135i setup in such a way that it is lifting rear tires though. My own car has an E92 M3 rear sway, 18k rear springs, and relatively soft 6k front springs and an E92 front sway. The car still doesn't pick up rear tires despite the front of my car being set up significantly softer than most others and the rear being set up significantly stiffer. The car feels fantastic at autocross though. No plowing around courses eating up front tires...

[Tambohamilton]

Originally Posted by rothwem

The rear bar is smaller because the multi-link in the rear has more geometric anti-roll built into it. The struts at the front have a lower roll center, thus a larger roll couple, and a larger bar is needed for the same effect. The rear suspension also does a better job maintaining camber as the car rolls, so it has more traction than the front for equivalent roll angles and doesn't need as stiff of a bar to maintain contact patch.

True, most road cars are tuned for a bit of understeer, but a perfectly neutral or even slightly oversteering E90 would still have a less stiff sway bar in the rear.

Nice one. I hadn't considered antiroll from the geometry at all. Makes sense, though I can't really envisage where the roll centres really are.

Originally Posted by bbnks2

Right, but most people don't understand this and they also don't understand that it's only true to a certain extent. Throwing a huge front bar on these cars does not make it handle better.They already under-steer like crazy in stock form. The rear "noodle" bar is part of the problem. Move to a more balanced sway bar setup (like the E92 M3) and the car will be way more balanced yet still slightly under-steer oriented. The stock staggered tire setup is the next hurdle.

A modest rear bar upgrade will not pick up tires off the ground. The "e-diff" thing is also blown out of proportion. Put too big of a rear bar on the car and you will have issues though. Being RWD, rear sways can be detrimental to putting down power on the inside wheel. I have yet to see ANY 135i setup in such a way that it is lifting rear tires though. My own car has an E92 M3 rear sway, 18k rear springs, and relatively soft 6k front springs and an E92 front sway. The car still doesn't pick up rear tires despite the front of my car being set up significantly softer than most others and the rear being set up significantly stiffer. The car feels fantastic at autocross though. No plowing around courses eating up front tires...

See that's what I thought, but there's the vast majority of folk on the forums throwing front bar and spring at their cars. Glad to hear you've bucked the trend and it's working for you. When I build up the funds I think I'll do similar, though probably won't get as far as squaring up the tyres.

[Silnismo]

Originally Posted by bbnks2

Right, but most people don't understand this and they also don't understand that it's only true to a certain extent. Throwing a huge front bar on these cars does not make it handle better.They already under-steer like crazy in stock form. The rear "noodle" bar is part of the problem. Move to a more balanced sway bar setup (like the E92 M3) and the car will be way more balanced yet still slightly under-steer oriented. The stock staggered tire setup is the next hurdle.

A modest rear bar upgrade will not pick up tires off the ground. The "e-diff" thing is also blown out of proportion. Put too big of a rear bar on the car and you will have issues though. Being RWD, rear sways can be detrimental to putting down power on the inside wheel. I have yet to see ANY 135i setup in such a way that it is lifting rear tires though. My own car has an E92 M3 rear sway, 18k rear springs, and relatively soft 6k front springs and an E92 front sway. The car still doesn't pick up rear tires despite the front of my car being set up significantly softer than most others and the rear being set up significantly stiffer. The car feels fantastic at autocross though. No plowing around courses eating up front tires...

Reducing body roll on a 3000 plus pound car will increase the handling I guarantee you that, no matter how you slice it.

Simple as that, that's all the bar does.

[Tambohamilton]

Originally Posted by Silnismo

Reducing body roll on a 3000 plus pound car will increase the handling I guarantee you that, no matter how you slice it.

Simple as that, that's all the bar does.

It's really not that simple. Let's say we just threw a massive (like 30mm) rear bar on it, and left the front one stock. There would be less body roll, but the handling would not be improved...or increased.

[stevesingo]

Originally Posted by bbnks2

Right, but most people don't understand this and they also don't understand that it's only true to a certain extent. Throwing a huge front bar on these cars does not make it handle better.They already under-steer like crazy in stock form. The rear "noodle" bar is part of the problem. Move to a more balanced sway bar setup (like the E92 M3) and the car will be way more balanced yet still slightly under-steer oriented. The stock staggered tire setup is the next hurdle.

A modest rear bar upgrade will not pick up tires off the ground. The "e-diff" thing is also blown out of proportion. Put too big of a rear bar on the car and you will have issues though. Being RWD, rear sways can be detrimental to putting down power on the inside wheel. I have yet to see ANY 135i setup in such a way that it is lifting rear tires though. My own car has an E92 M3 rear sway, 18k rear springs, and relatively soft 6k front springs and an E92 front sway. The car still doesn't pick up rear tires despite the front of my car being set up significantly softer than most others and the rear being set up significantly stiffer. The car feels fantastic at autocross though. No plowing around courses eating up front tires...

Is it not the case that even as stock, the rear springs are much stiffer than front? This is down to motion ratio. The rear springs act on the suspension much further inboard from the movement path of the wheel than the fronts. Front MR is 0.96 Rear MR is 0.563.

I would argue that the E90 does not "understeer like crazy". I find them quite well balanced and the balance can be throttle adjusted.

I also find that the handling balance is more oversteer bias when driven in the wet and this is due to front/rear roll stiffness. A certain degree of roll is required to generate grip as increased vertical load on the tyre increases the grip-up to a point where the grip/vertical load ratio becomes non-linear. Grip in turn generates roll-to a point where excess roll causes the unloading of the inside tyres to the extent that the grip/vertical load ratio becomes non-linear. If there is less grip at the tyre contact patch in the first place, you don't get roll and don't get the increased loading of the outer contact patch.

In normal dry conditions, the front/rear ratio of roll resistance is fixed to give stable balance. In the wet, the greater inherent roll resistance of the rear geometry (roll centre vs mass distribution vs CofG) means the there is less rear roll in comparison to the front and the car becomes a little more over steer biased.

It is a compromise which is unavoidable due to mass distribution of a saloon and the compromise (cost) of the front suspension geometry be MacPherson Strut.

[bbnks2]

Originally Posted by Silnismo

Reducing body roll on a 3000 plus pound car will increase the handling I guarantee you that, no matter how you slice it.

Simple as that, that's all the bar does.

No. Absolutely false.

Unless you have some other definition of what "handling" is than I do of course. I would 100% rather have a car that has a lot of body roll than one that has a ton of under-steer. Please understand the difference in how I am interpreting "handling.".

Go to an autocross and watch a street Miata whip around a course. They have massive body roll. Not a single person will complain a Miata doesn't handle well. Body roll affects transitional responsiveness and how you toss the cars weight around. Body roll does not necessarily mean the car is handling poorly. Do rally cars handle poorly because they have a ton of chassis movement? If you are talking about camber loss due to body roll... that is a single factor out of the MANY that dictate how a car handles. Dialing in 2.0*+ of static camber is more than enough to offset camber loss running street tires.

[bbnks2]

Originally Posted by stevesingo

Is it not the case that even as stock, the rear springs are much stiffer than front? This is down to motion ratio. The rear springs act on the suspension much further inboard from the movement path of the wheel than the fronts. Front MR is 0.96 Rear MR is 0.563.

I would argue that the E90 does not "understeer like crazy". I find them quite well balanced and the balance can be throttle adjusted.

I also find that the handling balance is more oversteer bias when driven in the wet and this is due to front/rear roll stiffness. A certain degree of roll is required to generate grip as increased vertical load on the tyre increases the grip-up to a point where the grip/vertical load ratio becomes non-linear. Grip in turn generates roll-to a point where excess roll causes the unloading of the inside tyres to the extent that the grip/vertical load ratio becomes non-linear. If there is less grip at the tyre contact patch in the first place, you don't get roll and don't get the increased loading of the outer contact patch.

In normal dry conditions, the front/rear ratio of roll resistance is fixed to give stable balance. In the wet, the greater inherent roll resistance of the rear geometry (roll centre vs mass distribution vs CofG) means the there is less rear roll in comparison to the front and the car becomes a little more over steer biased.

It is a compromise which is unavoidable due to mass distribution of a saloon and the compromise (cost) of the front suspension geometry be MacPherson Strut.

You might be the only person I have ever seen state an E8x/E9x doesn't under-steer lol. Could you be mistaking torque induced over-steer when laying down power for actual chassis balance mid-corner?

Yes, in stock form the car is quite well balanced once you run a more square wheel setup, but, it is most certainly under-steer biased still. A bigger front sway bar does NOT help that. The car will understeer more every time. I've been there and done that.

Your comments about roll couple seem to be based on sound logic, but, you're making a lot of assumptions that aren't true when applied to this car. Or, at least they don't impact handling to the extend you infer based upon the concepts you are presenting. Yes, rear geometry maintains a better camber curve. That doesn't mean the car will still be balanced if you put a big ass front sway bar on the car without also adding more rear roll stiffness.

The rear of the car is not set up stiffer than the front. Again, in the wet you might be mistaking power over-steer for chassis balance... Touch the gas and sure the rear kicks out but toss the car into a steady state turn in a parking lot in the rain and the front tires will slide out... That's under-steer. One of the biggest issues I had with the 135i power was that the factory throttle mapping is calling for 80% load at as little as 20-30% pedal angle. The car needs linear throttle mapping.

I understand motion ratio and the front/rear geometry differences. Hence why I run 6k/18k (maintaining the stock 1:3 ratio) and the more neutral balanced M3 sway bar setup (M3 front bar is still stiffer than the rear bar). I was comparing MY setup to that of OTHER modified cars which usually go for something more like 10k/12k and huge front bars. The comparison I was drawing was to provide context to my comment that a mild rear sway bar upgrade will not magically render the rear wheels useless. Again, everything is relative so if you put too big a rear bar into the car then yeah you might be limiting the ability of your RWD car to lay down power on corner exit but a modest bar will not have that outcome even with the stock diff.

I run a 135i so their are minute differences but the corner weights and geometry is for all intents and purposes of this thread the same. I've done all sorts of math and geometry extrapolations on paper. I've ran countless different suspension setups. That experience makes me pretty confident in what I am saying at this point.

With that being said everyone has a different driving style. Everyone has a different opinion on what a well handling car feels like. If you can explain a little more as to why you think the stock 15mm bar is neutral balanced with the front 26mm bar then please share. I am not the kind of person that won't admit they were wrong but I will argue my point until I hear something logical presented lol.

[Tambohamilton]

Originally Posted by stevesingo

Is it not the case that even as stock, the rear springs are much stiffer than front? This is down to motion ratio. The rear springs act on the suspension much further inboard from the movement path of the wheel than the fronts. Front MR is 0.96 Rear MR is 0.563.

Correct me if I'm wrong, but don't you need to multiply by the square of the motion ratio to calculate the wheel rate? This would make the factory 3:1 (R:F) spring stiffness pretty balanced.

Originally Posted by stevesingo

I would argue that the E90 does not "understeer like crazy". I find them quite well balanced and the balance can be throttle adjusted.

You're a better driver than me then! And maybe your car has much less turbo lag than mine...

Originally Posted by stevesingo

I also find that the handling balance is more oversteer bias when driven in the wet and this is due to front/rear roll stiffness. A certain degree of roll is required to generate grip as increased vertical load on the tyre increases the grip-up to a point where the grip/vertical load ratio becomes non-linear. Grip in turn generates roll-to a point where excess roll causes the unloading of the inside tyres to the extent that the grip/vertical load ratio becomes non-linear. If there is less grip at the tyre contact patch in the first place, you don't get roll and don't get the increased loading of the outer contact patch.

Wight transfer is increased by body roll, but it happens regardless. So, due to the tyre curves, it's generally better for the car to be as flat as possible in turns, and the roll stiffness to be pretty equal F-R.

Originally Posted by stevesingo

In normal dry conditions, the front/rear ratio of roll resistance is fixed to give stable balance. In the wet, the greater inherent roll resistance of the rear geometry (roll centre vs mass distribution vs CofG) means the there is less rear roll in comparison to the front and the car becomes a little more over steer biased.

I don't follow this at all. Are you saying that the rear of the car rolls less than the front in the wet? I don't think water softens the chassis that much...

Thanks for the good discussion everyone

[Silnismo]

Originally Posted by bbnks2

No. Absolutely false.

Unless you have some other definition of what "handling" is than I do of course. I would 100% rather have a car that has a lot of body roll than one that has a ton of under-steer. Please understand the difference in how I am interpreting "handling.".

Go to an autocross and watch a street Miata whip around a course. They have massive body roll. Not a single person will complain a Miata doesn't handle well. Body roll affects transitional responsiveness and how you toss the cars weight around. Body roll does not necessarily mean the car is handling poorly. Do rally cars handle poorly because they have a ton of chassis movement? If you are talking about camber loss due to body roll... that is a single factor out of the MANY that dictate how a car handles. Dialing in 2.0*+ of static camber is more than enough to offset camber loss running street tires.

You are comparing apples to oranges my friend, a miata that weighs 2000 pounds should have way less body roll then a 3000 pound 4 door bimmer so very little is needed to address the Miata's body roll oppose to a e90 which is way heavier.

If all the variables stay the same (stock vs stock) and you upgrade just front bar, you will reduce body roll in the front which will allow you to take a corner way better then stock.

[bbnks2]

Originally Posted by Silnismo

You are comparing apples to oranges my friend, a miata that weighs 2000 pounds should have way less body roll then a 3000 pound 4 door bimmer so very little is needed to address the Miata's body roll oppose to a e90 which is way heavier.

If all the variables stay the same (stock vs stock) and you upgrade just front bar, you will reduce body roll in the front which will allow you to take a corner way better then stock.

That is not how that works but ok.

[gjm120]

A little sidetrack..

Will a rear ARB from a M3 Convertible make a mess of my handling? Bar is 23.6mm, my car is an E82 128i.
Is there a front upgrade that will balance out handling with the M3 cabrio bar?

[Tambohamilton]

Originally Posted by Silnismo

You are comparing apples to oranges my friend, a miata that weighs 2000 pounds should have way less body roll then a 3000 pound 4 door bimmer so very little is needed to address the Miata's body roll oppose to a e90 which is way heavier.

If all the variables stay the same (stock vs stock) and you upgrade just front bar, you will reduce body roll in the front which will allow you to take a corner way better then stock.

But the heavier car should also have proportionately stiffer suspension, so why should it have more body roll?

Also, how would a larger front bar only reduce body roll at the front? Sure, the chassis will flex, but the rear will also roll less. The difference is that the front tyres will be working harder because they're the ones sorting out the body roll, so they will have less traction available for cornering (see above response regarding tyre curves; lateral load vs vertical load).

[Silnismo]

Originally Posted by Tambohamilton

But the heavier car should also have proportionately stiffer suspension, so why should it have more body roll?

Also, how would a larger front bar only reduce body roll at the front? Sure, the chassis will flex, but the rear will also roll less. The difference is that the front tyres will be working harder because they're the ones sorting out the body roll, so they will have less traction available for cornering (see above response regarding tyre curves; lateral load vs vertical load).

Less body roll means improved weight distribution amongst the front 2 tires, and because your are cornering flatter less weight is being transferred to the outside tire.

[Biginboca]

Originally Posted by Silnismo

Less body roll means improved weight distribution amongst the front 2 tires, and because your are cornering flatter less weight is being transferred to the outside tire.

I disagree, and generally wonder where you are getting these strange ideas. Can you find any articles to back up these theories? I would like to see the logic behind your arguments.

The stiffer bar in your example makes the inside tire want to lift and transfers more weight to the outside tire. The exact opposite of what you believe. In other words, your car will now stay flatter through the turn... as you simultaneously lose traction at a lower threshold.

The only real benefit I can see of the stiffer front bar on a stock suspension is controlling the loss of camber when our Mac Strut front suspension compresses. And you aren’t even taking about that single benefit.

Generally a stiffer bar will reduce grip on the end (front or back) where it’s added. Like a stiffer rear bar is typically added on front wheel drive cars to make them over steer cause they lose rear grip with a stiff rear bar.

A larger front bar is used on our cars to take away front grip and reduce under steer. You might perceive that as “improved handling” but depending on many other variables you might actually make for a slower lap time due to ultimately reducing overall traction.

But there’s a much bigger picture going on here which is spring rates. There are track cars with High spring rates that don’t even run a swaybar on one or both ends. Spring rates, dampers, and the rest of your suspension settings (camber, toe, etc) work in synergy and need to be dialed in correct relationships before you even look at sway bars which are the last piece of the puzzle.

Thinking you can just throw on a stiffer front sway bar on a stock suspension and automatically “improve handling” is naive.

[Tambohamilton]

Originally Posted by Biginboca

I disagree, and generally wonder where you are getting these strange ideas. Can you find any articles to back up these theories? I would like to see the logic behind your arguments.

The stiffer bar in your example makes the inside tire want to lift and transfers more weight to the outside tire. The exact opposite of what you believe. In other words, your car will now stay flatter through the turn... as you simultaneously lose traction at a lower threshold.

The only real benefit I can see of the stiffer front bar on a stock suspension is controlling the loss of camber when our Mac Strut front suspension compresses. And you aren’t even taking about that single benefit.

Generally a stiffer bar will reduce grip on the end (front or back) where it’s added. Like a stiffer rear bar is typically added on front wheel drive cars to make them over steer cause they lose rear grip with a stiff rear bar.

A larger front bar is used on our cars to take away front grip and reduce under steer. You might perceive that as “improved handling” but depending on many other variables you might actually make for a slower lap time due to ultimately reducing overall traction.

But there’s a much bigger picture going on here which is spring rates. There are track cars with High spring rates that don’t even run a swaybar on one or both ends. Spring rates, dampers, and the rest of your suspension settings (camber, toe, etc) work in synergy and need to be dialed in correct relationships before you even look at sway bars which are the last piece of the puzzle.

Thinking you can just throw on a stiffer front sway bar on a stock suspension and automatically “improve handling” is naive.

You're both right, to a point.

Yes body roll increases weight transfer since the cog of the car shifts slightly with the body roll...unless the roll axis is exactly aligned with the cog.

However, if one axle has vastly higher roll stiffness than the other, it's outside wheel will see far higher vertical load than the outside wheel on the other axle, and vice-versa for the inside wheels. The result of which is that the outside tyre can generate a bit more grip (tyre curves aren't linear), and the inside tyre generates a lot less grip (net; less grip) on the axle with high roll stiffness.

So, to get the best performance (highest grip), we need to balance the roll stiffness at both ends. Or we can use roll stiffness to remove grip from one end to balance it with the other, which gives neutral handling.