too large rear sway = understeer?

[avus]

i've heard in the past that adding a rear sway that is too big will cause understeer. is this true?
what would be considered too big for the 1series?

reason i ask is that i'm comparing the H&R and Eibach ones against the stock rear sway (which i think is 12mm)

H&R
Rear bar diameter is 20mm

Eibach
Rear bar diameter is 15mm

[Red135]

I would like to know this also. I asked this question previously and it wasn't answered. Maybe you will have better luck.

[mikeo]

Eaxctly the opposite. Large rear swaybars tend to cause oversteer (or less understeer if not too large or stiff).

[Red135]

The front bars in the two sets are similar. Considering the cast difference in the rear bar sizes, which would be the better way to go?

[atr_hugo]

Do a search - there are lots of opinions and a couple of decent answers out there - there are also a couple of suspension engineers around here.

A larger rear bar will reduce understeer - but it has other consequences besides. There is an informed school of thought that says work on the end that has the problem. The problem seems to stem from 'skinny'/narrow tires in the front.

Work with someone who has done some suspension tuning (and you have good recommendations for their work).

[Red135]

I refer to my post above. I have searched already and posted the question already. I guess it is too much to ask for.

[mikeo]

First point to make on both the H&R and Eibach is that both are non adjustable, so what you get is what you get. I prefer adjustable bars if I'm going to dump the OE setup.

IIRC, the OE rear bar is 15mm, so the Eibach does not represent a change unless they use a super-stiff material (which I doubt). So that said, if you want a non-adjustable set of bars I'd go with the H&R setup.

Although Hugo is correct that larger front tires may be all you need to bring the minor understeer under control, the 135 does tend to have excessive body roll in low-speed, max grip turns. Reducing that roll will help keep the MacPherson strut front from rolling over into positive camber, which you don't want, and should help understeer in that instance. The siffer rear H&R bar helps with the understeer as well.

[SCA1]

On my Cooper S a larger rear bar helped alot with the understeer, making the car much closer to neutral than it was with the stock rear bar.

On the 135 maybe a slightly wider front tire (225?) coupled with a larger/stiffer rear bar may work together to significantly reduce or even eliminate the understeer. The only way to tell for sure is to give it a go and see what happens. It would also be great to have an adjsutable rear bar, but I'm not aware of one in production at this point.

[jeremyc74]

I've got a question about the sway bar as well, and this looks like a fine place to ask it instead of creating a new thread.

I've never had a RWD car that needed understeer correction, and I don't honestly understand why changing the rear sway bar helps the issue. Is it actually making the front bite harder, or just reducing grip in the rear?

I'm all for putting wider tires on the front, but I'd also like to reduce the body roll some.

TIA

[Johnnyr135]

I am getting the H&R front and rear sways installed Saturday I will let you know what I think.

[atr_hugo]

There are a handful of things you can control at both ends of the car related to suspension. There are a whole bunch of things you can't (like most of the suspension geometry outside of toe and camber). The pieces you can fiddle with easily are, springs, shocks, and anti-roll bars (and tires, tire pressure, and ride height).

Quickor Engineering used to have a handy wallet card describing how to deal with understeer/oversteer. I'll recreate their table below:

Adjustment / Increase Understeer / Increase Oversteer
Front Tire Pressure / Lower/ Higher
Rear Tire Pressure / Higher / Lower
Front Tire Section / Smaller / Larger
Rear Tire Section / Larger / Smaller
Front Wheel Camber / More positive / More Negative
Rear Wheel Camber / More negative / More positive
Front Springs / Stiffer / Softer
Rear Springs / Softer / Stiffer
Front anti-sway bar / Thicker / Thinner
Rear Anti-sway bar / Thinner / Thicker
Weight distribution / More foreward / More rearward

So stiffening the rear bar (making it thicker) will reduce roll but so will stiffer springs. They both should reduce understeer.

And we haven't even scrathced the surface of shocks.

[atr_hugo]

I know that doesn't answer the fundamentals of your question Jeremy, but it'd take a refresher course in physics (or suspension dynamics) to lay it all out and your best bet may be a serious sitdown with one of Carroll Smith's books, like "Tune to Win".

[Sherifftruman]

Something else to think of on bars and Mikeo kind of touched on this. The outer diameter is not the be all end all measurement. Some bars are solid and some are hollow, with hollow bars having various wall thicknesses and various metal alloys.

What you really need to know is how stiff the bar is compared to say, the stock bar. That way you know what you are doing. Otherwise, you might have 2 bars that are different thickness and the thinner one is actually stiffer.

[mikeo]

Sherifftruman is right, more info on the stiffness of the Eibach rear bar vs. the OE bar is needed and the manufacturer or dealer should be questioned.

[adc]

Go with an adjustable setup for both front and rear. That way you can increase general roll stiffness both front and rear, which will make the car continue to work as one piece - and you will also be able to increase the rear stiffness a little more than you did the front stiffness, to help with the understeer.

What I was hoping is to hear is from someone who installs the Performance Catalog Suspension, together with a set of say UUC sways, wider tires up front (with either staggered or square setup) and a little extra negative front camber (via alignment pin removal).

[jeremyc74]

Originally Posted by atr_hugo

your best bet may be a serious sitdown with one of Carroll Smith's books, like "Tune to Win".

Funny...I was just thinking the same thing :thumbup:

[Rye]

Besides increased oversteer, if going too stiff, you can get wheel-hopping while taking a corner or even lifting of the inner wheel.

[professor]

very interesting, but are there any adjustable sway bar options out there? racing dynamics or acs perhaps?

[WAY]

I am going to leave the rear bar untouched eventhough my track day on Friday showed a very understeery car! With more pressure up front, it helped a lot. I am investigating stiffer suspension and increasing the camber on the front. I don't like stiffer rear bar as it will suffer when you drive your car quick on b-roads (ie, bumps will cause less contact patch on the rear with stiffer rear sway bar).

[TazioNYC]

Originally Posted by Johnnyr

I am getting the H&R front and rear sways installed Saturday I will let you know what I think.

What did you think?

[Johnnyr135]

I had the front bar installed today and it is great turn in is much sharper and the body roll is much less. I am very surprised the car did not come from the factory like this because I do not see a trade off for this improved performance. The ride is not more harsh or twitchy, the car takes hard corners much flatter and more composed I cannot wait till I get the rear bar installed. The shop I took it to was to busy to get the rear bar installed I am really thinking about tackling the rear bar myself tomorrow because I am so excited because the front made such a big difference. I cannot recommend getting the H&R bars enough.

[Orb]

Originally Posted by WAY

I am going to leave the rear bar untouched eventhough my track day on Friday showed a very understeery car! With more pressure up front, it helped a lot. I am investigating stiffer suspension and increasing the camber on the front. I don't like stiffer rear bar as it will suffer when you drive your car quick on b-roads (ie, bumps will cause less contact patch on the rear with stiffer rear sway bar).

Bad idea....I have a working load transfer sheet based on Claude Rouelle (magic number) for this car now. I see about sharing more details later.

Go measure the rear motion ratio and look at what happens when you increase spring rate. The math as follows:

Rear wheel spring rate: 0.58^2 x (coil spring rate)
Front wheel spring rate: (cos 14)^2 x (coil spring rate)

The number will show that every 50 lb/in you increase in the front you need to increase the rear 130 lb/in to keep thing even. Since the stock car has 130 lb/in spring and the rear is 350 lb/in and under steers fairly bad when pushed. If you want to run a 285 lb/in spring on the front then your going to need a 560 lb/in on the rear. If you go less spring rate on the rear than this the car will under steer. You got Clubsport at 400 lb/in for the front and the rear are 570 lb/in in the rear. You’re going to need a 1000+ lb/in spring in the rear to keep it the same as stock……Side bar…car can’t handle a spring greater than 635 lb/in with out major load transfer problem due to rear bushing…..we have a problem.

The stock 135i “magic number” load transfer calculations are 17% over what neutral steer car for a starting point. H&R roll bar will put he car back within 3% of the magic number with stock springs. Once thing is very clear H&R did their homework but the bar don’t scale well when spring rate increase and heads towards under steer. BTW, the M3 has neutral steer and it matter because it shares most of the same geometry and weight distribution.

The problem with your car is load transfer and this is the first and last thing when tuning. It will be difficult if not impossible to fix the suspensions with this car without change to the M3 rear sub frame bushing. The bushings just deflect way too much with stiffer suspension and result in unpredictable handle and some very nasty transient load condition. Getting it right means you should not need more that 2 degrees on negative camber in the front so you can win the camber game but your options are limited. The calucated roll numbers support this.

The common track folklore of non stagger high profile tires and big front bar will send you 180 degrees in the wrong direction. There is very little room in tuning for maximizing the speed of this car on the track.

Orb