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      03-10-2010, 02:20 AM   #1
rzm3's Avatar

Drives: E92 M3
Join Date: Aug 2007
Location: USA

iTrader: (17)

Arrow Why (Aggressive) Lowering Springs May Ruin Your M3, Part II

This is a continuation of this thread...

Actual Bump Stop Test Data

By chance I was able to put an OE front bump stop into a force/displacement machine and measure its behavior, which is shown in red.

As I mentioned in the previous thread, the OE bump stop behaves progressively, meaning its stiffness increases with more compression. And when this bump stop compresses to about 1" in length (1.875" mark on the graph), its rate shoots up, adding significant stiffness to the total wheel rate. At this point the bump stop is almost like a hard piece of rubber in comparison to the rest of the system.

And the data shows just that...

So What Does This All Mean?

This means that in stock form, M3's front suspension has roughly 1.875"-2" of compression travel. In this range, compression force rises linearly, meaning the wheel stiffness is consistent (stiffness is slope of force/displacement graph). Consistent stiffness = predictable weight transfer = predictable/neutral handling.

Now, take a lowering spring that drops the front suspension more than 1". Because of the drop, the suspension now compresses from the right of the 1.0" orange line, and is only left with <~0.875" (aka very little) of linear, predictable compression travel.

Those with an aggressive drop will also easily run into the highly non-linear, unpredictable stiffness range of the suspension travel (right side of the second orange line). This of course leads to more unpredictable handling vs. OE. Moreover, because the front axle can now become very stiff at relatively short travel, more understeer will be experienced during cornering.

Next step is to get actual spring data.

This is meant to be an objective, fact-based discussion. Comments to my analysis are more than welcome.
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