Quote:
Originally Posted by cyniclaus
Doyle, I assume you must be familiar with this thread...
I wonder what your opinion is of the discussion therein where it is mentioned that BMW seems to have engineered the car for a rear spring rate approximately triple that of the front and that moving to a rate that is double (or even less) contributes to understeer? |
I am indeed familiar with that thread. Like many things on the internet, take it with a grain of salt (you are supposed to be a cynic, right?
). There is certainly good info in there, but there are alot of opinions/half-truths/mis-construed conclusions.
The stock frequency bias is great for a smooth, street car, but seriosuly compromises some driving dynamics.
If you look at all the top e90 platforms, they all run variations of the bias where the rear spring is 1.5 to 2 times higher than the rear (as opposed to 3 times). KW M3 clubsport (and therefore M3 GTS/CRT, Alpina B3 GT3), GT4/S2000, HPA, certain unnamed Grand Am teams, all use a similar frequency bias.
Step 1 is to understand the "unchangeables". This is the suspension geometry/design and to some degree the weight. With a MacStrut design up front, controlling the front roll is the key to a proper handling BMW. In the past some have decided that one wheel with tons of grip is better than 2 wheels with little grip. This is the "3 wheel motion" that both Orb and Harold mention. So, instead of a heavy bar up front, you use stiffer springs to control the roll and maintain contact patches with both wheels.
For the xi's, this is even more important. Here are the reasons why a stiffer front spring than even the RWD cars is needed:
-the front wheels must steer and turn at the same time
-due to the open front diff, if you pop one wheel in the air, all the power gets directed to the wheel with no traction (yes, I know DTC brakes that wheel and tries to divert power to the other wheel, but the point is there is a delay and a dead weight loss of power application)
-The front travel is even shorter than the RWD struts, so combined with extra weight, extra roll the xi might hit the bumpstops more frequently
When all of this is taken into account (combined with the general thought that a stiff front end is prefer so the car can take set early), you end up with the front spring rates between 400 and 500 for a street/track setup.
Now, in the rear, since there is a multi-link setup and the car has a RWD bias, drivers are going to want a softer rear end. On a more practical matter, due to the design of the rear suspension, there is a very low motion ratio. This means that you'll have to run a higher spring rate, but not too high as to cause bind/stiction. Most reccomendations that I have seen (Harold, Orb, TCK, RRT, etc) feel that 700-800 is the sweet spot. Anything north of around 800 will need different bushings (Delrin) and really good shocks, so it should be reserved for serious track rats.
It is my belief that the xi's will need stiffer rear springs than the RWD cars, for two reasons:
-we send power to both front and rear, therefore, the need for the rear wheels to put down power is diminished
-with an AWD car, there is a strong tendency to power-on understeer. Since weight gets transferred to the rear, and the car is still recovering from a turn, the front end tends to get light (front inside wheel, in particular). To control that weight transfer, stiffer rear springs might be needed.
I plan on experimenting with stiffer rear springs (around 800-850) at some point. I also wonder if a rear LSD might help (if power is getting to the ground at the rear, less power will need to be directed to the front).
From there, we can choose the sway bars to balance out the chassis. We have a veritable cornucopia if rear bar choices, but a paltry 2 for front bar choices. A vendor seriously needs to hop on that. The UUC front bar was designed around the stock frequency bias, so it is uber-stiff (going back to the 3 wheel motion). With the proper springs selected, none of the rear bars out there are stiff enough to balance it. It is a moot point, anyways, as getting a matching rear bar would likely start diminishing mechanical grip. So, that is why either the e90/92 M3 bar or H&R e90/92 bar is usually suggested. The M3 bar is better with the lower end coilovers (H&R, KW V1/3). The H&R bar is better with proper spring rates.
There you go. Super long answer to a simple question!