Quote:
Originally Posted by kdw215
First Post:
I've read a lot on here and on e90 about understeer... having come from a long line of BMW's I can say this is normal, what is odd is the concensous on here that the answer is a 'big rear bar'
E30, E36, E46 all have the same 'issue' as the current e92, e82, and all respond to the same solution. (being all rear wheel drive, front wheel drive responds differently)
The issue is 'roll induced understeer' the front slips first as it has less grip compared to the front. (put a bar which is too big on the front - and you can make it oversteer. Why? because the front has MORE grip)
Basically it's comes down to the roll ratio and the springs. If you look at the KW2 club sport you will see the answer. A quick calculation reveals a front to rear spring ratio of approx 63.5%
M3 KW Clubsport 63.5%
Front: 508 lb/in
Rear: 800 lb/in
KW Clubsport (non M3) 70%
Front: 400 lb/in
Rear: 570 lb/in
(this is most likely higher due to the difference in sway bars between the two)
Bilstein PSS10 62%
Front: 385 lb/in
Rear: 615 lb/in
Incidently this ratio is pretty close to all the Group A e30, e36 race cars which are between 59% and 61%.
The e30 crowd have known this for years, so it's well proven as an approach.
I would 'solve' the e82 understeer issue with a similar F to R spring ratio.
Stock the 135i is approx 34% Given 120f and 350r (not accounting for the progressive nature of the springs)
I notice the BMW Performance springs are estimated to be 190f 505r
(thats a 37% ratio, so not much different but a slight nod in the right direction) so we still have the same built in 'safety undeersteer'
make those front springs around 300-320lbs. And i would bet it would be a dream... (and note no sway bar changes) so rear traction should be unaffected which it can be by installing too big rear bar.
Really sway bars should be used to fine balance an already correctly set-up spring and damper combination, not the other way round.
Obviously this is just scraping the surface of chassis dynamics, and is a point of view based on expirence with BMW's in particular. and IMHO
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You don't need super wide front tires to reduce understeer. The key is to get the proper spring rates front and rear.
On E90post you will find lots of threads on suspension set up. Lots of good reading there! If you still have questions after doing some reading...PM or email me.
Harold