[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???

[firehawkclone]

I would start to check all the motor and tran's mount's! Is there a poly trans mount on the car? I would also check the output shaft on the tran's too!

FYI the steel shafts are really bad from the get go, that's one reason why GM went to a alum shaft.

[z28barnett]

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???

Drive shafts have what is know as a "Critical Speed".

At this "Critical Speed", the imbalances in the drive shaft cause it to flex slightly, this flexing causes more imbalance and more flex. This cycle can continue until the drive shaft fails. If you are lucky it won't pole vault the car end over end.

The LT1 steel drive shaft had problems with the 3.42 gearing, the drive shaft was at "critical speed" near the top speed of the the car and even had some vibration at around 80mph. I remember some magzine testing crew that lost a drive shaft at 150mph. They were lucky and nothing really bad happened.

The critical speed is related to the stiffness and weight of the drive shaft. You will want a larger diameter aluminum or CF shaft.

If the bending is only happening at speed and is not related to suspension problems binding the drive shaft some how. Then your guy has almost lost the drive shaft twice. If you want to run that speed safely you should run out and buy a better drive shaft.

One other thought, have you checked your transmission rubber mount? If it has split it could be letting things flop around and causing failures due to that.

http://www.gmpartsdirect.com/results.cfm?s...umber=SS-Camaro

At the bottom of the above page they list the LT1 & Ls1 aluminum drive shafts.

Z28

[trackbird]

http://www.lingenfelter.com/Merchant2/merc...tegory_Code=P08

I'm using one of these and highly recommend it.

[93formula]

thanks for the replies guys.

the fist one he bent was a stock ls1 drive shaft. was replaced by a 3 inch steel one. when he took the steel one back to the shop they told him he over spun it.

as for bushings there all stock. the vibrations only started at over 200 kms so i don't think the tranny mount has a problem. ive had a split one and it shook all the time.

ill mention upgrading the drive shaft.

[35th_Anniversary...]

the critical speed is actually the torsional or lateral resonant frequency. This is a factor of the stiffness and inertia of the shaft.
stresses and deflections are magnified when you are running at or near the resonant frequency. We do a lot of torsional analysis work here at work as we make products that are similar to engines with rotating & reciprocating parts. A torsional failure would have broken the shaft at a trademark 45 degree angle. IMO if it was a resonance problem it must be a lateral resonant frequency it was running at. Of course I can't say whether or not this is the actual failure mode.

If you ever see your mirrors vibrating when a bass note plays or at a certain engine RPM the mirror is vibrating in resonance. blow across a half empty coke bottle and yu are hearing a 1/4 wave resonance. Pluck a guitar string and it is vibrating in resonance.

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. (Maybe when I get bored I will do one) The torsional resonant frequency can't be too high with all the inertia in the system, counterweights, pistons, connecting rods. And when you have a low torsional resonant frequency you will have higher modes of vibration as well, so even if you are below 800 RPM on your first mode you still have to contend with yur 2nd, 3rd, 4th, etc. Plus in an engine you also have to worry about additional resonances from half orders.

[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!

[CamaroSS]

Hello everyone,

I appreciate the input on my driveshaft issue. We did find the apparent cause of the problem and it was related to a misaligned yoke on the original aluminum shaft. We are assuming the misalignment caused the shaft to become unbalanced from previous high speed excusions.

After the replacement steel shaft bent I sent both shafts to another driveline shop and had them spin the aluminum shaft again to see if it was repairable. They found that it was bent and also found that the yoke was slightly out of alignment. The first shop claimed the shaft was beyond repair and that is why the steel shaft was fabricated. The aluminum shaft has been repaired and will be installed tomorrow and hopefully will solve the problem. We did find out that the steel shaft was not specifically designed to run over 5900 rpm (shaft speed) as they were unaware of the vehicle it was going in. I did not look after ordering or installing the steel shaft but it was 3" OD and I am unsure of the wall thickness.


Thanks again for the responses.