As many of you know, a few members and I have been doing quite a bit of research/measurements/calculations on the M3 suspension. Here is one finding which I thought should be explicitly expressed
for the make benefit of the M3 community.
Here is why lowering springs may ruin your M3's ride and handling:
(I will try to explain in the simpliest words possible)
The M3 front suspension has about 5.75" of total suspension travel
from fully extended (i.e. on a lift) to fully compressed (let's assume no bump stop for now). About half of the 5.75", or 2.875",
is dedicated to compression.
Now, let's take lowering spring brand A for example, which drops the car by 1"
in the front. So 2.875"-1"=1.875", which means that you will still have 1.875" of compression travel left
, which is ok.
Well, guess what, there is a OE bump stop on the front suspension which is about 2.375" long uncompressed
, and with a 1" drop, you would already be riding on it.
The bump stop behaves progressively (see sample figure below), meaning the rate increases with more compression. And when this bump stop compresses to about 1" (from 2.375"), its rate shoots up, adding significant stiffness to the total effective spring rate. At this point the bump stop is almost like a hard piece of rubber in comparison to the rest of the system.
Now, we were left with 1.875" of compression travel after the drop. Subtract another 1" due to the bump stop, and one is left with 0.875" of effective compression travel
. (yes, it is about the width of a finger
The short compression travel (in this type of set up) does a few things:
1) You will get a very bumpy ride, especially when you go over imperfections, as the suspension is constantly hitting the compression limit.
2) This will constantly overwhelm the OE shocks, which is not designed to operate in such a high variable effective spring rate all the time.
3) Unpredictable handling. More weight is transfered to the wheel that has a stiffer rate.
When a lot of weight is transferred to the front outer wheel when you are turning, you get understeer.
Hope this helps. This is not to say that all lowering springs will behave this way, but obviously some should be avoided if you don't want to sacrifice the ride and handling just for the look.