Different sized sway bars for 128/135?

[fishbulb]

My guess is that they will have different sized sways. Any one know for sure?

The reason I ask is because I just read a post about the Alpina B3 (tuned 335i coupe) where they use a smaller (ie softer) rear bar on the car to help turn in - and my guess help with traction as well:

"They then work there magic on the suspension, including reducing the rear anti roll bar diameter, to increase turn in precision."

Could be all BS - but I remember taking of the big rear sway on my turbo miata going back to the factory Tooth Pick sized one. Made the car MUCH easier to drive hard without worriing about the rear coming around.

Of course - i had already messed with what the factory did, so who knows?!? Just more food for thought as we wait....

[atr_hugo]

Yes they will - for the reasons you stated. There is a company that makes sway bars, ADDCO, that used to distribute a handy little wallet card that described how to adjust various suspension bits to impact understeer/oversteer. Tighten the front and loosen the rear to reduce understeer.

The notion is to increase the traction on the end that looses traction first or reduce the traction on the end that's still gripping. You diddle with tire pressures to do that at an autocross. (There were many nasty arguments regarding this many years ago - thinking 1996-97 time frame - when BF Goodrich confused the autocrossing world with a special tire that had different sidewall stiffness for the inner and outer edges - it inverted the usual tire pressure mantra - the conflaguration could be seen over the entire automotive end of the internet - heck it may have even spilled over into the porn side of the 'net ; -)

[atr_hugo]

Well - I misremembered (so sue me ; -). It was 'Quickor' that issued the wallet guide to handling. Here's the scoop:

Adjustments--------------To Increase Understeer------To 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 neagtive----------------------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

[atr_hugo]

So for the 135i - the weight balance is neutral, the section height favors understeer (more in rear than front), we don't know spring rates, sway bar sizes, tire pressure recommendations, or camber specifications yet.

[mikeo]

Past BMW front suspension designs, i.e., McP struts, were considerably "camber challenged" and actually had no front camber adjustment other than so-called "crash bolts" or elliptical bushings (or aftermarket adjustable camber plates of course). It appears the 1-Series still employs this type of front suspension, not the more sophisticated double A-arm design. Understeer can be greatly eliminated using the table above, but most importantly requires massive static negative camber (which is really not practical for street use). I run front -3.5 deg camber, left and right, for autocross events along with most all of the tweaks outlined in the table.

[atr_hugo]

There should be three variables to adjust in front (caster, camber and toe) and two in rear (camber and toe) and of course with MacPherson struts you're lucky to get toe. ; -) Crash plates it is!!

[ducati]

Increasing the rate of the swaybar will increase grip on the opposite end of the car. As posted above (in the revised post), decreasing the rate of the rear sway will INCREASE understeer and reduce rear roll stiffness. This should not increase turn in; "turn in" would feel slower. The post was wrong... or misinterpreted the reasoning.

Reducing the rear sway can indeed have POSITIVE results, but it will never increase turn in "precision" :smile:

I suspect the MSport suspensions will be the same between the two, but yes, I bet the stock suspension on the 128i will feature bars with smaller diameter/softer rate.

[mikeo]

Originally Posted by ducati

Increasing the rate of the swaybar will increase grip on the opposite end of the car. As posted above (in the revised post), decreasing the rate of the rear sway will INCREASE understeer and reduce rear roll stiffness. This should not increase turn in; "turn in" would feel slower. The post was wrong... or misinterpreted the reasoning...

While this is generally true, there are exceptions and the E36 and E46 happen to respond well to massive front sway bar setups and thereby counter-intuitively reduce understeer. Tomorrow's lesson will explain why.

[321]

along the lines of.... stiffer bar in front=more traction in front=less understeer?

[ducati]

Stiffer bar in front generally decreases traction at that end of the car, improving rear grip. There are a lot of factors involved, though, and I'm interested to hear mikeo's description of the forces at play.

[mikeo]

OK, here's my attempt at an explanation why E36 and E46 cars need a massive front bar. The dynamics of the MacPherson strut suspension are such that as body roll increases positive camber increases at a faster rate than body roll, causing the tire footprint to rollover from perpendicular thereby reducing grip and causing understeer. A massive front bar significantly reduces body roll allowing the tire to remain perpendicular (along with large static negative camber settings) to the pavement and generate maximum grip. I'm not going to get into spring rates but higher rate springs can also be used, up to a point, to reduce roll but most racers find the big front bar to be the best compromise. The big front bar is also why you see pics of BMWs cornering with the inside front tire off the ground, or "tri-cycling". Very common, but not ideal, a compromise.

[ducati]

I see the tricycle right above this post

Very interesting. I don't have any experience running a BMW on the track, so that's all new to me.

[mikeo]

And my front bar is not "massive" because I drive the car on the street. Still, corner workers regularly report that I tri-cycle.

[fishbulb]

great thread - anyone have good links for BMW suspension set ups?

[aesthetect]

Originally Posted by mikeo

OK, here's my attempt at an explanation why E36 and E46 cars need a massive front bar. The dynamics of the MacPherson strut suspension are such that as body roll increases positive camber increases at a faster rate than body roll, causing the tire footprint to rollover from perpendicular thereby reducing grip and causing understeer. A massive front bar significantly reduces body roll allowing the tire to remain perpendicular (along with large static negative camber settings) to the pavement and generate maximum grip. I'm not going to get into spring rates but higher rate springs can also be used, up to a point, to reduce roll but most racers find the big front bar to be the best compromise. The big front bar is also why you see pics of BMWs cornering with the inside front tire off the ground, or "tri-cycling". Very common, but not ideal, a compromise.

whoa.. counterintuitive indeed, i had no idea.. im going to have to read that again tomorrow.

[mikeo]

Originally Posted by aesthetect

whoa.. counterintuitive indeed, i had no idea.. im going to have to read that again tomorrow.

Yes, and I've about reached my level of incompetency, so if you want further info on this subject I suggest the link below and ask the experts.

http://forums.bimmerforums.com/forum...play.php?f=128

[imported_Bimmer]

Originally Posted by mikeo

OK, here's my attempt at an explanation why E36 and E46 cars need a massive front bar. The dynamics of the MacPherson strut suspension are such that as body roll increases positive camber increases at a faster rate than body roll, causing the tire footprint to rollover from perpendicular thereby reducing grip and causing understeer. A massive front bar significantly reduces body roll allowing the tire to remain perpendicular (along with large static negative camber settings) to the pavement and generate maximum grip. I'm not going to get into spring rates but higher rate springs can also be used, up to a point, to reduce roll but most racers find the big front bar to be the best compromise. The big front bar is also why you see pics of BMWs cornering with the inside front tire off the ground, or "tri-cycling". Very common, but not ideal, a compromise.

GOOD DEAL! your a genius, or im just tired and plain oll retarded, great explanation though!

[mikeo]

Genius, No. Frustrated engineer, maybe.

[ersatzS2]

Originally Posted by mikeo

The dynamics of the MacPherson strut suspension are such that as body roll increases positive camber increases at a faster rate than body roll, causing the tire footprint to rollover from perpendicular thereby reducing grip and causing understeer. A massive front bar significantly reduces body roll allowing the tire to remain perpendicular

That makes perfect sense. Sounds like if I don't change the springs, this calls for really extreme camber settings on a tight track. Based on all the marketing hype and general smugness of the bimmer community, and in my naiveté of things bimmer, I assumed bmws came with double wishbone front suspensions. I was really disillusioned when I took a look at the 335 at the dealership and saw the McStruts.


Anyhow, thanks for the substantive comments. Interesting. I'm really looking forward to the April trackday at Summit Point.

PS: just to add a clarifying comment on what seem to be lots of contradictory statements: in GENERAL, (once the car is set up) you soften the end that is sliding (lower pressures, softer springs, softer rebound rates, softer bars) and stiffen the end of the car with too much grip. (Gratuitous bonus comment: reverse everything for racing karts!)

[mikeo]

Yes, BIG static negative camber is the order of the day. I ran -3.7 for autocross/track and have heard of people doing more than that.