Quote:
Originally Posted by inTgr8r
In the case of the narrower subframe with more twisting forces,
would it not be more beneficial to use solid bushings to minimize forces on the chassis and impart them more directly to the wheels?
(subframe & chassis become unitized)
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The more solid the connection, the higher the peak forces.
Any kind of dampening reduces peak forces and dissipates the total amount of energy over a longer timespan.
So the softer the bushings, the lower the stress on the chassibolts and chassis itself (mainly the spotwelds). And the wider the chassisbolts/mounts, the more area it has to dissipate energy and the lower the lever forces.
I dont know if its a real problem with the z4 but with the e46 its a major problem. Although that has also to do with a design flaw. With the z4 the rear subframe sits in a full cage (the rear tension/pull rods enclose both the petrol tank and the subframe) and with the e46 the subframe sits in a C section (the tension/pullrods dont enclose the rear subframe, just the petrol tank).
But however you look at it, the subframe is much much stronger than the chassis so it can tear it up without any doubt.
It obviously also has to do with what kind of mileage you expect from your car. If you do 2k miles/year and you expect to keep the car for another 5 years it wont be as much of a problem (20k miles), but usually a car is designed to last 200k miles or so without major disasters.
you can also look at the front axle. The subframe from the fornt axle is bolted directly into the chassis, but again that subframe is twice the width if not more.