Re: some numbers for ya (archive)

While trying to remain univolved, your approximation of the rubber "web" stiffness sucks. The think has a wildly nonliear co-efficient of elasticity and the upper half is in compression therefore it's not even constant within the bush...also you assume that the entire moment of the wheel inertia in not shared between bushes and goes through one bush, you ignore the damping effect of the rubber of the tyre and the effect of the gas strut, otherwise I'm sure your results are adequate....it's pretty unlikely that a company such as BMW have made this mistake, repeatedly for the last 20 years, mind you I guess theie engineers work in metric so you never know :-)

Okay, how's this WimE? The front rotors and pads weigh 20 lbs, tire and wheel 35 lbs, spring 4 lbs, and strut ~40 lbs, Total =~ 100 lbs. Mass=W/g
The "ultimate driving machine" thrust bushings are a rubber web in which the attachment bolt 'floats' in the middle. The bolt attaches to the chassis and the rubber web suspends the thrust control arm. The stiffness of the rubber web is probably on the order of 1 to 2 klbs/inch, lets give it 1500 lbs/inch.
Now, who would design a suspension and put a resonant frequency in the operating region? Surely not BMW
Remember the formula? Natural frequency, f=sqrt(K/M)/2pi =~ 12.2 Hz, which is about 56 mph using 225/60-15 tires...
So you see i have done a little math and research into why these suspensions suck. Now i'm doing more research in hopes of actually solving the problem instead of replacing crappy bushings with more crappy bushings.
Search the archives under 3-,5-,7- series for front wheel shimmy and see how many folks are suffering from this DESIGN DEFECT.
Can you say RECALL?

Follow Ups:

Don't prompt him! It's his homework, not yours;-) THG 06:43 10/02/2000 (1)
- Yes, you're right...I'm calm now ;-) Paul Ash 08:28 10/02/2000 (0)

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