[93formula]

i got a guy whos bent 2 drive shafts.

the car is a ls1 f body. he is the original owner, road races but doesnt drag race. the car is pretty much stock.

the first drive shaft bent with original suspension. at speeds of about 220-230 km/h. well some serious vibrations were met.

then it was replaced by a shop with a steel drive shaft for temporary use. tested it, ran fine for a bit at speeds of 220 km/h. then a short while later the vibrations came back.

went to the shop again, it was bent.

the only differenced between the two times was the second time he had some suspension mods done like new konis, relocation brackets, eibach pro kit, aluminum lcas.

now what the hell would cause two drive shafts to bend at speeds of over 220km/h???

[pknowles]

the critical speed is actually the torsional or lateral resonant frequency. This is a factor of the stiffness and inertia of the shaft.

It can be, but it doesn't have to be. I used to work in the engineering department of Kop-flex in Baltimore which makes basically drive shafts for Turbo-machinary. The natural frequency of the shaft is important as you don't want to operate there for a long time or under high load, but critical speed by definition is the lateral critical speed. Imagine two people (one on each end) swinging a jump rope, above the lateral critical speed your drive shaft will start to take shape like that jump rope (IMG:http://www.frrax.com/rrforum/style_emoticons/default/blink.gif) . A lot of times we designed shafts that would operate above the first lateral harmonic and sometimes above the second harmonic for their entire life as long as they are below the lateral critical speed.

All of that is for shafts that operate in a narrow RMP band, as long as you slowly ramp through the harmonics your fine. For automotive driveshafts which operate over a wide range of speeds a company may define the critical speed as the first harmonic, so you don't operate there for a long time. You could have a harmonic of the drive shaft at 100 mph, as long as you don't stay there you will be fine.

With my experience working with the products at work I sometimes wonder how an engine in a car can rev from 800 to 6000 rpm without going through any resonances.

As long as you have enough damping you can go through a harmoic.

All that crap aside, look at the engine and trans mounts like others have said; I killed a driveshaft when I broke the motor mounts in my Mustang. Drove around for a week with both motor mounts broken and didn't notice till I changed the oil. Well I was wondering what the thump under hood was when I popped the clutch, turned out to be the motor hitting the hood!