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      01-31-2012, 06:13 PM   #284
mdputnam's Avatar

Drives: 135i & M235i Convertibles
Join Date: Dec 2011
Location: SoCal

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It seems that over the years that this thread has existed, people keep asking, "Why does a stiffer front roll bar reduce understeer on a 135i?" From what I've just read in this thread no one has given a clear explanation. Maybe this will help:

Understeer happens while you are cornering and the horizontal force on the front tires exceeds the "grip" (counter acting frictional force) the tire can provide, while the back tires still maintain their grip. At the most basic level, to get maximum grip from a tire you need to maximize the area of the contact patch (the part of the rubber tire that is touching the asphalt) and maintain an even distribution of load throughout the contact patch. The best way of doing this is to keep the rubber tire tread flat and parallel to the asphalt. That is easier said than done because there are many things that keep this from happening, like tire side wall flex, tread squirm, tire inflation, road imperfections, and... (drum roll please) suspension geometry. For reasons we won't go into here keeping the alloy wheel at 0 or slightly negative camber (the top tilted in compared to the bottom) will help to keep the tread of the outside tire flat and parallel to the asphalt with an even load distribution during cornering. Of the two front tires that is the one with the highest load during cornering.

Earl S Mac Pherson and His Strut
The front of the 135i has a type of suspension that is called the Mac Pherson Strut. From a purely performance viewpoint this is unfortunate as Mac Pherson Struts have a couple of geometry drawbacks (see this site ). One draw back is when a Mac Pherson strut is compressed towards its extreme, the top of the alloy wheel tilts towards the outside relative to the bottom (positive camber). This is bad because that reduces the contact patch and increases the load on the outside edge of the contact patch. Remember, big area, even load = maximum grip.

A picture is worth a thousand words. Check out this amimation, see how as you begin to compress the Mac Pherson strut, it slightly increases the alloy wheels negative camber (the top of the wheel moves in relative to the bottom of the wheel) which is good for maintaining the rubber tread of the outside tire flat and parallel to the asphalt during cornering. But, as it is compressed further to its extreme, the top of the wheel starts to tilt out (towards positive camber) the size of the contact patch is reduced, the load distribution is uneven, and the outside front tire looses its grip (understeer). The big stiff M3 roll bar works, because it prevents the Mac Pherson strut from swinging into an highly compressed position that reduces negative camber and in its extreme ultimately creates a positive camber.

I have never carefully examined and measured the 135i suspension. Suspension design, as in life, is way more complicated than the simple fables we tell here. There are thousands of things that an engineer must consider when designing a suspension. We only lightly touched on one of them. So, after reading this I wouldn't recommend updating your resume and sending it off to Munich.
Extra Credit
For 20 points, can anyone explain why adding a stiffer roll bar to a double wishbone front suspension (like the RX-8 has) increases understeer?

Last edited by mdputnam; 02-01-2012 at 12:11 PM.. Reason: Grammer
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