Drive Shaft Vibrations

Shawn at Tom Wood's

Shawn at Tom Wood's

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Great video. Curious about critical speed. I suspect (maybe improperly, because I haven't done the math) that some factory front driveshafts in our TJ's hit critical speed with 5.38s in the axle. Any merit? Could wall thickness variation cause some to have dynamic vibration while others don't?
 
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Mike_H said:
Great video. Curious about critical speed. I suspect (maybe improperly, because I haven't done the math) that some factory front driveshafts in our TJ's hit critical speed with 5.38s in the axle. Any merit? Could wall thickness variation cause some to have dynamic vibration while others don't?
Click to expand...
Great question. One that I think about often actually. For the most part we use the critical speed calculator found here https://spicerparts.com/calculators/critical-speed-rpm-calculator However, several months ago a trainer from Spicer was here giving us a presentation, among the things he talked about was critical speed. I asked him about how different materials affect critical speed and he did not know the answer. The critical speed calculator on their website does not specify. I do know however that Spicer only uses CREW tubing. We only use DOM tubing. I don’t know if that affects the results of the critical speed or not but I would assume that it has some effect, as would the material and grade of steel. I think that there are too many variables to say that the critical speed is always X without knowing and calculating all the other factors. Also, you are going to find a lot of conflicting information such as this https://pstds.com/critical-speed-chart and this http://www.wallaceracing.com/driveshaftspeed.php. The calculator on the Wallace Racing website has more robust options for data input which makes me think more thought and consideration has gone into it.

However, even according to Spicer's critical speed calculator the stock front shaft on a TJ is good for about 4,300 rpm. Even with 35" tires and 5.38 gears you'd need to go about 85 mph before you hit the critical speed of the front shaft. Upgrade to an aftermarket shaft with 2" .120" wall tube and you get closer to 100 mph before hitting critical speed. In a stock shaft I think the probably loose parts and the low standard factory balance is more likely to cause problems at those speeds than the critical speed is.

For the most part though, I use Spicer's critical speed calculator as a guideline but admittedly am not afraid to push the limits at times when necessary. After all, Spicer also recommends that a universal joint's maximum operating angle at 3,000 rpm is 5.8 degrees. That's a rule that gets broken in virtually every lifted vehicle.
 
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Great information, especially for me as I’ll be purchasing a new DS from you in the coming months when new axles and SYE go on.

The write up has a suggestion if 2500-2800 RPM, and every recommendation I see on here has your calculator showing well over 3k rpm’s for the DS at 70, 75mph. Are we just a essentially asking for trouble here? Is 3400-3800 rpm DS that out of bounds?

Much appreciated.
 
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cpwolf said:
Great information, especially for me as I’ll be purchasing a new DS from you in the coming months when new axles and SYE go on.

The write up has a suggestion if 2500-2800 RPM, and every recommendation I see on here has your calculator showing well over 3k rpm’s for the DS at 70, 75mph. Are we just a essentially asking for trouble here? Is 3400-3800 rpm DS that out of bounds?

Much appreciated.
Click to expand...
I can only cast my vote as the drive shaft guy. My vote is to keep the drive shaft rpm close to that 2,500-2,800 range at 60mph and under 3,500 at max cruising speed. I know there are some threads on here about ring and pinion gearing where everyone says to gear low, 5.xx gears even for 33 or 35 inch tires. I understand that this is for purposes of achieving ideal engine RPM which makes sense. But it does come at a cost, it makes the shaft spin very fast at highway speeds. High drive shaft RPMs mean a higher chance of high speed vibrations. Also, the more RPMs you have on your drive shaft the faster things will wear out. That being said, most TJ guys seem to think that this trade off is worth it given the benefits of running those low (high number) ring and pinion gears.
 
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Shawn at Tom Wood's said:
I can only cast my vote as the drive shaft guy. My vote is to keep the drive shaft rpm close to that 2,500-2,800 range at 60mph and under 3,500 at max cruising speed. I know there are some threads on here about ring and pinion gearing where everyone says to gear low, 5.xx gears even for 33 or 35 inch tires. I understand that this is for purposes of achieving ideal engine RPM which makes sense. But it does come at a cost, it makes the shaft spin very fast at highway speeds. High drive shaft RPMs mean a higher chance of high speed vibrations. Also, the more RPMs you have on your drive shaft the faster things will wear out. That being said, most TJ guys seem to think that this trade off is worth it given the benefits of running those low (high number) ring and pinion gears.
Click to expand...

From reading on here, lots of the folks driving these are severely risk averse folks (Joking, they don't seem to be) so just one more risk to lift it. Guess I'll need U Joints in bulk, ugh, ha, although going to 4.88 might not be as bad.
 
cpwolf said:
From reading on here, lots of the folks driving these are severely risk averse folks (Joking, they don't seem to be) so just one more risk to lift it. Guess I'll need U Joints in bulk, ugh, ha, although going to 4.88 might not be as bad.
Click to expand...

with the 4.88's and my 35s at 80, I'm turning just under 4000 RPM at the DS. Mine drives well...for now. Rock solid Rear View mirror picture. Once I put the vib demon 42RLE in...who knows what's going to happen, LOL
 
Mike_H said:
Great video. Curious about critical speed. I suspect (maybe improperly, because I haven't done the math) that some factory front driveshafts in our TJ's hit critical speed with 5.38s in the axle. Any merit? Could wall thickness variation cause some to have dynamic vibration while others don't?
Click to expand...
Not much need to worry about critical speed in a TJ. Worst case are the SE front shafts that are 1.75 inch diameter and as Shawn has pointed out are good to 4300 rpm.

Wall thickness becomes an issue when you see a lot of the "crawler" shafts that are made from .134-.250 wall tubing just from its sheer weight and how true the tubing may or may not be.
Shawn at Tom Wood's said:
Great question. One that I think about often actually. For the most part we use the critical speed calculator found here https://spicerparts.com/calculators/critical-speed-rpm-calculator However, several months ago a trainer from Spicer was here giving us a presentation, among the things he talked about was critical speed. I asked him about how different materials affect critical speed and he did not know the answer. The critical speed calculator on their website does not specify. I do know however that Spicer only uses CREW tubing. We only use DOM tubing. I don’t know if that affects the results of the critical speed or not but I would assume that it has some effect, as would the material and grade of steel. I think that there are too many variables to say that the critical speed is always X without knowing and calculating all the other factors. Also, you are going to find a lot of conflicting information such as this https://pstds.com/critical-speed-chart and this http://www.wallaceracing.com/driveshaftspeed.php. The calculator on the Wallace Racing website has more robust options for data input which makes me think more thought and consideration has gone into it.

However, even according to Spicer's critical speed calculator the stock front shaft on a TJ is good for about 4,300 rpm. Even with 35" tires and 5.38 gears you'd need to go about 85 mph before you hit the critical speed of the front shaft. Upgrade to an aftermarket shaft with 2" .120" wall tube and you get closer to 100 mph before hitting critical speed. In a stock shaft I think the probably loose parts and the low standard factory balance is more likely to cause problems at those speeds than the critical speed is.

For the most part though, I use Spicer's critical speed calculator as a guideline but admittedly am not afraid to push the limits at times when necessary. After all, Spicer also recommends that a universal joint's maximum operating angle at 3,000 rpm is 5.8 degrees. That's a rule that gets broken in virtually every lifted vehicle.
Click to expand...

Had my first true critical speed failure a couple years ago. Customer asked me to retube a old Chevy pickup shaft no big deal. Until it came back in pieces with the tubing unzipped. He neglected to tell me he was street racing and was doing 140mph when it let go.

I am not sure that materials make as big of a difference in critical speed as they do in how well it balances. A lot of the Spicer tubing of late is not as straight and round as it used to be and that sure makes it a lot more work to turn out true well balanced shafts. I have also been told by Spicer in the past that my standards might be to stringent even though I pretty much just follow what is published in the back of the Neapco catalog.
 
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I just replaced my front drive shaft with a new one from Shawn. It completely eliminated the annoying “thrumming” that had been getting progressively worse over the last year or so. I previously had the stock drive shaft rebalanced and that didn’t solve the problem. So I rolled the dice and bought the new one. I should have done it a long time ago. Mileage was 110k but it has also been flat towed about 35k. The rear shaft is also Tom Woods and has about 50k total miles on it.

Another thing….. For a long time I have occasionally had problems with bucking when engaging the clutch in either first or reverse. It seemed to come and go in intensity but often I had to slip the clutch judiciously to prevent it. I dropped the skid plate to make replacing the front drive shaft easier, and I figured I might as well replace the transfer case mount while I was in there to see if that would make any difference. I replaced the mount with a new Mopar part even though the old one looked perfect. And guess what? The bucking is completely gone! Who would have thunk?
 
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Chasing driveshaft vibrations has become a hated hobby of mine. At least I've learned from this thread that it's likely all dynamic. I don't recall how many times I've had my driveshafts balanced, but I've never quite been able to eliminate that harmonic drone at somewhere between 70 and 75 mph. I'm running 35's with 4.88 gears and 42RLE, so RPM at 75 mph is about 3,500. Any suggestions are welcome.
 
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