[tomthecomic]

I was at the track today and broke a few things. I was at about 107 6200 rpm and had a bang. Looked under the car and the drive shaft was twisted and bent in half, now it is in the trunk. As I walked up to the time slip person he handed me the tab to the rearend yoke. What broke first?

[Rambo_The_Dog]

Originally Posted by tomthecomic

I was at the track today and broke a few things. I was at about 107 6200 rpm and had a bang. Looked under the car and the drive shaft was twisted and bent in half, now it is in the trunk. As I walked up to the time slip person he handed me the tab to the rearend yoke. What broke first?

I'd think the yoke busted and the driveline bounced off the pavement @ 107 6200RPM and that's what mangled it. Let's see some pictures!

[Crosley]

uhhhhhhhhhh, the U-joint.

what do I win?

[k-star]

You better pull the cover on the rear and look at the gears close...


Keith

[tomthecomic]

Believe it or not the the u is not broken. Could it have been the rpm? I ran out of gear and held the rpm for a few seconds. I only made 3 passes.

[tomthecomic]

This is the pic I don't know if you will get it. http://www.dallasclassicchevy.com/ph...0966482e97b564


If you do not and have time it is at dallasclassicchevy.com chat room under events ennis. Their are also some pics of nice cars.

[Deuce]

Stole the photo ... and posted it here ...

With the U-joint not broken ... I would guess it was the yoke that broke ...
and the damage to the driveshaft happened when it got loose ...

I would buy a aluminum driveshaft ( you have to get a replacement anyway )



FYI ... a taller set of tires on the rear would help with the RPM situation. You need to Redline @ the end of the strip ... about between the first timing light and the middle light.

[tomthecomic]

Thanks for posting the pic, I could not figure out how to do it. I was going to get a taller tier today but I think I will have to put that off. I have 26 but considering 27 or 28. The car did a 13.023 with the motor shut off at the end.

[novajohnb]

What is (was) the length of the drive shaft? (see critical speed below)

Was it stock? What's the OD (diameter) and metal thickness? (see critical speed below)

Were all the balance weights attached? Were there any dents in the drive shaft? (Missing weights or dents can affect the balance enough to cause vibration and failure.)


What was the engagement of trans yoke into the trans? (If you have less than 3/4 inch of play left, sometimes the yoke can bottom out if the rear squats. This could bend the shaft of break the tailshaft- once this happens the games over. If the yoke didn't have enough engagement, especially at suspension full droop if the rear rises under power, the drive shaft will vibrate at high speed. This vibration could kill the shaft.)

I suspect you may have found out the meaning of "critical speed." This is when harmonics bend the shaft like a snake until it snaps.

A lesson I learned a few years ago at the drag strip is a stock GM "G-Body" drive shaft has a critical speed of about 5,400 RPM. Not good on my 10 second Nova that goes through the lites at 6,800 RPM. What critical speed means is that each shaft with particular dimensions will NEVER be able to go over a certain speed without self- destructing.

Take a look at this calculator:
http://www.wallaceracing.com/driveshaftspeed.htm

If you are in fact exceeding this critical speed on a stocker, the best solution is to go aftermarket drive shaft with a larger OD. A shaft with the right dimensions can go 10,000+ RPM. A local truck drive line shop should be able to build what you need, as long as you provide the correct dimensions.

[novajohnb]

By the way, depending on your combination, taller tires may slow you down. You should aim to go through the traps about 500-1,000 RPM higher than peak power.

Check the rear yoke carefully, that there isn't any runout when you spin it, and that the u-joint locating "t its" are not damaged. Also check that the tranny tail shaft bushing did not work loose or spin in its bore and that the tail shaft is not cracked or bent.

[jimfulco]

A seemingly inconsequential dent in a driveshaft can be a major weak spot at high rpm.