Thursday, January 19, 2012

M3 Suspension Bits, Part 1

The M3 is the top dog in the E90 pack, but since the 335 and the M3 share the same floorpan, there's nothing to prevent a 335i owner from applying the M3 suspension goodies.  Most of these parts are available in the aftermarket, but the original-equipment M parts are nice because they are readily available, they fit perfectly, and they're relatively affordable.  Plus, the sway bars are hollow.  More on that later.

Our Mach V car came equipped with the OEM Sport Package springs and shocks, and we like the ride height and feel of those a lot, so for now we have left them alone.  We focused instead on the suspension bushings and the sway bars.

For the front of the car, the lower control arms can be swapped out to M3 bits.  These have two benefits.  First, they incorporate stiffer bushings, reducing slop and squirm when they are loaded.  The second benefit is that the arms are slightly different lengths from the 335i pieces, giving about 0.5 degrees extra camber, and perhaps a bit more caster. 

While we had the front suspension apart, we also installed the M3 front sway (or "anti-roll") bar.  The M3 version is the same diameter as the stock 335i bar, with the exception of one six-inch section in the middle of the bar.  The 335i version has a neck-down section that's much smaller in diameter.  The M3 bar has no such neck-down.  Along with the M3 sway bar, we swapped in M3 sway bar bushings, which are a more solid bearing style bushing, instead of the softer 335i bushing.

The rear control arms in the 335i can also be replaced with M3 units, but we opted to leave our stock parts in for now.  We did choose to replace the rear sway bar with the M3 version.  Since the rear subframe has to be dropped down to replace the sway bar, we also took the opportunity to replace the very soft 335i rear subframe bushings with the firmer M3 units.

After observing the similar front sway bar diameters between 335i and M3, the huge size difference in the rear bars was a bit of a shock!  The stock 335i RWD rear sway bar is 13mm thick.  The M3 bar is 20mm thick!  Now, that's not quite a directly comparison, because the 335i unit is a solid bar, while the M3 unit is hollow.  But in any case, the M3 bar is FAR stiffer.

While we're talking about hollow versus solid sway bars -- the stiffness of a sway bar in terms of resistance to twisting it proportional to the 4th power of the diameter.  That means the stiffness goes up very fast as the diameter gets larger.  It also means that the very outer part of the bar is the most important in terms of the overall stiffness.  The result is that a hollow bar is very close to a solid bar in terms of stiffness, since the center of the bar does very little of the work.  Of course, hollow bars are much lighter than equivalent solid bars.

The rear sway bar install/subframe bushing replacement is a pretty involved job, taking the good part of a day even for our experienced mechanic.  Besides being time-consuming, it takes some special tools to press the bushings in and out of the subframe.  It's not something we would recommend tackling on jack stands at your home unless you're a pretty experienced mechanic.

So after a good long day on the lift, it was time to set the car down on the ground and see how everything worked.  We'll give our subjective opinion in our next blog post.

M3 suspension bits -- buy them here.