Harmonic vibrations in Jeeps: A new theory (please read!)

[Chris]

Regardless of any other factors, I mentioned this to you when we chatted. This stuff is a real pain in the ass for shops. You can spend countless hours that you aren't sure will be paid for and even if they are paid for, it isn't comfortable to charge someone for that much time with sometimes no change for the better and often times change for the worse. I don't blame any shop that throws in the towel or lags on a project like this because there is little about it that doesn't suck.

We are shooting blind with stuff like this with only experimental solutions based in experiences that gives us places to just start to make a difference. That and it is very hard to disrupt a tight schedule of other rigs that have known solutions to be applied to them in order to get them out the door.

Not picking sides, but I can easily understand the issue.

I have the exact same vibration as Sri does Blaine. We drove our Jeeps back to back on the same day, and the only difference at all is that mine comes on at about 60 mph, whereas his comes on at about 40 mph. Of course mine is geared to 5.38 with 35s, and his is geared to 4.88 with 32s. Either way, both of our issues didn't present themselves until after the re-gear.

The shop who is installing my Savvy mid-arm and outboard shocks on Monday, told me that they are very familiar with this vibration (he even described it in detail as a harmonic and cyclical vibration) and he said every time they see it, it has to due with the driveshaft spinning at a faster speed from the re-gear, and needing to be balanced a particular way.

He told me he is very confident that their go-to driveshaft shop here in Dallas, Oregon can fix it, as he's fixed all the others. I wish I had gotten more details, but he really seemed to know what was going on, so I figured I'll let them check it out.

Obviously I remain very skeptical about this, since I know that Sri had his driveshaft balanced after the re-gear, and they said it was just fine.

Either way, I told my shop to take both the driveshafts out (front and rear) and send them to their guy, just to see if he finds something that someone else hasn't. I suppose it never hurts to have a second opinion.

Anyways, we'll see. I'll report back with what (if anything) they find.

 

[mrblaine]

I have the exact same vibration as Sri does Blaine. We drove our Jeeps back to back on the same day, and the only difference at all is that mine comes on at about 60 mph, whereas his comes on at about 40 mph. Of course mine is geared to 5.38 with 35s, and his is geared to 4.88 with 32s. Either way, both of our issues didn't present themselves until after the re-gear.

The shop who is installing my Savvy mid-arm and outboard shocks on Monday, told me that they are very familiar with this vibration (he even described it in detail as a harmonic and cyclical vibration) and he said every time they see it, it has to due with the driveshaft spinning at a faster speed from the re-gear, and needing to be balanced a particular way.

He told me he is very confident that their go-to driveshaft shop here in Dallas, Oregon can fix it, as he's fixed all the others. I wish I had gotten more details, but he really seemed to know what was going on, so I figured I'll let them check it out.

Obviously I remain very skeptical about this, since I know that Sri had his driveshaft balanced after the re-gear, and they said it was just fine.

Either way, I told my shop to take both the driveshafts out (front and rear) and send them to their guy, just to see if he finds something that someone else hasn't. I suppose it never hurts to have a second opinion.

Anyways, we'll see. I'll report back with what (if anything) they find.

I'm aware. We have also played the high speed driveshaft balancing game and like most things, sometimes it works, sometimes it doesn't. Hopefully it does and the issue goes away.

I've dealt with a few, more than a few and sometimes it is just easier to quit throwing good money after bad and do a hub conversion in the front. Sadly and oddly, not once has that not fixed the problem.

If I could fix it with a high speed driveshaft balance, nothing would make me happier.


The main reasons I don't believe that is the answer or ever will be 100% of the time is due to the RPM change. 4.10's to 5.38's is 31% faster driveshaft RPM. The Drive ratio of 1-1 versus the OD ratio of .69 is 31% slower between OD and Drive. If the entire issue was RPM related, we would see the vibes in Drive with the stock set up and go away in OD. Or, you would see them go away in OD since you are now spinning the shaft at stock speeds once you re-gear.

It has been my experience that the harmonic is there in both drive and OD practically regardless of anything major based on just ground speed. I'm actually not convinced that we don't have a gear set balance issue at times.

I've also played the game of pull the front shaft, dial in the back, pull the back shaft dial in the front and then put them both back in and the vibes come back.

 

[Chris]

I'm aware. We have also played the high speed driveshaft balancing game and like most things, sometimes it works, sometimes it doesn't. Hopefully it does and the issue goes away.

I've dealt with a few, more than a few and sometimes it is just easier to quit throwing good money after bad and do a hub conversion in the front. Sadly and oddly, not once has that not fixed the problem.

If I could fix it with a high speed driveshaft balance, nothing would make me happier.


The main reasons I don't believe that is the answer or ever will be 100% of the time is due to the RPM change. 4.10's to 5.38's is 31% faster driveshaft RPM. The Drive ratio of 1-1 versus the OD ratio of .69 is 31% slower between OD and Drive. If the entire issue was RPM related, we would see the vibes in Drive with the stock set up and go away in OD. Or, you would see them go away in OD since you are now spinning the shaft at stock speeds once you re-gear.

It has been my experience that the harmonic is there in both drive and OD practically regardless of anything major based on just ground speed. I'm actually not convinced that we don't have a gear set balance issue at times.

I've also played the game of pull the front shaft, dial in the back, pull the back shaft dial in the front and then put them both back in and the vibes come back.

The odd thing about all of this is that some Jeeps have the vibration after re-gear, and others don't. That's something that really puzzles me.

For instance, on my last TJ, I re-geared it to 4.10 and put 33s on it. No vibration whatsoever, and I even took it up to 90 mph. Yet my new TJ with 5.38 and 35s has it really bad. I can't explain that, but it almost seems like it could be a gear set balance issue.

Do you think that it's worth exploring the harmonic balancer on the driveshaft at all? For instance, a heavier harmonic balancer.

Dave had mentioned that sometimes he was able to get rid of it with different skid plates, notable the Nth Degree or the Savvy skid. I have the Rokmen skid on there as we speak, but I ordered the Savvy one the other day. While I doubt that will do anything, I'm going to try it just to rule it out (not to mention I like the design of the Savvy skid better).

In the end though, you're right. Rather than throw endless amounts of money on it, if I can't fix it, I'll ultimately just get manual locking front hubs.

 

[tomtaylz]

I ordered the Savvy one the other day.

:yikes:

 

[psrivats]

@mrblaine, I completely understand your point, sometimes shops are reluctant to work on it not wanting to rack up a giant bill to the customer for labor. But that's not quite the case here. We discussed that prior to me leaving the Jeep there. I'm perfectly ok if the proverbial towel is thrown after trying everything they can, but the trying part is yet to happen.

I did eventually speak with Dave and he said he's going to work on it and give my Jeep back one way or the other soon. I'll update on findings when I have one. I have a good local mechanic who has agreed to take on the project if I don't have success at the other place.

@Chris did you try driving your Jeep without the front driveshaft yet? Or just going to have your shop try their trick first?

 

[psrivats]

I'm aware. We have also played the high speed driveshaft balancing game and like most things, sometimes it works, sometimes it doesn't. Hopefully it does and the issue goes away.

I've dealt with a few, more than a few and sometimes it is just easier to quit throwing good money after bad and do a hub conversion in the front. Sadly and oddly, not once has that not fixed the problem.

If I could fix it with a high speed driveshaft balance, nothing would make me happier.


The main reasons I don't believe that is the answer or ever will be 100% of the time is due to the RPM change. 4.10's to 5.38's is 31% faster driveshaft RPM. The Drive ratio of 1-1 versus the OD ratio of .69 is 31% slower between OD and Drive. If the entire issue was RPM related, we would see the vibes in Drive with the stock set up and go away in OD. Or, you would see them go away in OD since you are now spinning the shaft at stock speeds once you re-gear.

It has been my experience that the harmonic is there in both drive and OD practically regardless of anything major based on just ground speed. I'm actually not convinced that we don't have a gear set balance issue at times.

I've also played the game of pull the front shaft, dial in the back, pull the back shaft dial in the front and then put them both back in and the vibes come back.

It's very interesting to me that a front locking hub conversion fixes the problem for most problem Jeeps. I still don't completely understand why, and I still don't understand why there is a damper in the rear and not the front. I keep thinking about it and hopefully one day I'll understand what's going on.

I'm well aware that there are fully stock TJs with this problem as well. That points to design/implementation choices from the factory being marginal and we really reveal the problem with our mods (lifts, SYE, regear ..). What I want to know is WHY the problem exists (i.e. the physics of it) and then figure out what can be done to fix it in a logical manner.

We have clues - Jim Frens' comments, the damper and it's inclusions in specific Jeeps, the 55Hz frequency the damper is supposed to address, Dave/Blaine's practical experiences and anecdotes in trying to address the issue in various Jeeps - but the whole picture is still not quite clear. Someday it'll become clear hopefully.

 

[Chris]

@Chris did you try driving your Jeep without the front driveshaft yet? Or just going to have your shop try their trick first?

Not yet. I plan on doing that once they get done installing the mid-arm, outboard shocks, and RubiCrawler. If they aren't able to solve it, I'll start diagnosing even more. However, I'm going to give them the benefit of the doubt first, since these guys do seem to know their stuff.

 

[psrivats]

Not yet. I plan on doing that once they get done installing the mid-arm, outboard shocks, and RubiCrawler. If they aren't able to solve it, I'll start diagnosing even more. However, I'm going to give them the benefit of the doubt first, since these guys do seem to know their stuff.

That's fair. Keep us posted please. I hope whatever trick they attempt fixes it.

 

[bobthetj03]

Then there is me, which screws up the whole 42rle theory.

 

[psrivats]

Then there is me, which screws up the whole 42rle theory.

It's always, always good to have a hard case to test your theories against you have no idea how many times I've read your thread trying to see if I missed something. I still may have!

 

[bobthetj03]

I've been dealing with it for so long, I've forgotten stuff I've already done and may have repeated stuff, more than once. I'm still not willing to throw in the towel, but I'm getting close.

 

[jodomcfrodo]

The main reasons I don't believe that is the answer or ever will be 100% of the time is due to the RPM change. 4.10's to 5.38's is 31% faster driveshaft RPM. The Drive ratio of 1-1 versus the OD ratio of .69 is 31% slower between OD and Drive. If the entire issue was RPM related, we would see the vibes in Drive with the stock set up and go away in OD. Or, you would see them go away in OD since you are now spinning the shaft at stock speeds once you re-gear.

You have to remember that the transmission plays no part in driveshaft speed. If you are going 70mph in 3rd gear, your driveshaft is spinning at the same RPM at 70mph in 4th gear. The tires are spinning at a set RPM and the driveshaft is mechanically locked to the tires through the diff so the driveshaft RPM can't change from gear to gear. The engine RPM is what makes up for the change in final drive ratio.

I do agree with you that a manual hub conversion is probably the best way to fix this. I've been eyeing one myself, but I just can't convince myself to put that amount of money into a dana 30 (though, I am buying your brakes in a few weeks so I guess I'm a bit of a hypocrite).

I still don't understand how @psrivats could have vibration at 40mph. That makes me think there must be something else besides driveshaft speed playing into the vibrations. 40mph is shockingly low for vibrations.

 

[bobthetj03]

I have them at 55mph.

 

[jodomcfrodo]

I have them at 55mph.

I used to have horrendous vibrations right at 56mph. It was so bad you couldn't look in the mirror. Turned out the slip joint of my stock front driveshaft was shot. You could push on the driveshaft and move it up and down nearly 1/4".

I think the biggest issue when it comes to vibrations is that we can't readily drive others Jeeps. Your vibration could be totally different than the one I had and I wouldn't be able to tell over a forum.

 

[psrivats]

You have to remember that the transmission plays no part in driveshaft speed. If you are going 70mph in 3rd gear, your driveshaft is spinning at the same RPM at 70mph in 4th gear. The tires are spinning at a set RPM and the driveshaft is mechanically locked to the tires through the diff so the driveshaft RPM can't change from gear to gear. The engine RPM is what makes up for the change in final drive ratio.

I do agree with you that a manual hub conversion is probably the best way to fix this. I've been eyeing one myself, but I just can't convince myself to put that amount of money into a dana 30 (though, I am buying your brakes in a few weeks so I guess I'm a bit of a hypocrite).

I still don't understand how @psrivats could have vibration at 40mph. That makes me think there must be something else besides driveshaft speed playing into the vibrations. 40mph is shockingly low for vibrations.

The 40mph is with the standalone skid crossmember. With stock skid they are at 55-60 mph. But they are very intense and really untolerable.

 

[bobthetj03]

I used to have horrendous vibrations right at 56mph. It was so bad you couldn't look in the mirror. Turned out the slip joint of my stock front driveshaft was shot. You could push on the driveshaft and move it up and down nearly 1/4".

I think the biggest issue when it comes to vibrations is that we can't readily drive others Jeeps. Your vibration could be totally different than the one I had and I wouldn't be able to tell over a forum.

Yes, it is hard to convey what is felt via a forum. A detailed explanation does help. When we say "harmonic, cyclic, sin wave" type of vibrations, one can generally come to a conclusion of what that vibrations feels, sounds like.

 

[jeepndogs]

I've also played the game of pull the front shaft, dial in the back, pull the back shaft dial in the front and then put them both back in and the vibes come back.[/QUOTE]

This exactly my Lj. I can pull one shaft and it’s perfectly fine, doesn’t matter what shaft, as long as only one is spinning no vibration. Once they are both in rotation forget it. After spending probably 2x what the hub conversion cost trying to avoid the hub conversion I sucked it up and did it. 10K miles later I can drive the Jeep anywhere I want and no vibrations.

 

[mrblaine]

You have to remember that the transmission plays no part in driveshaft speed. If you are going 70mph in 3rd gear, your driveshaft is spinning at the same RPM at 70mph in 4th gear. The tires are spinning at a set RPM and the driveshaft is mechanically locked to the tires through the diff so the driveshaft RPM can't change from gear to gear. The engine RPM is what makes up for the change in final drive ratio.

I do agree with you that a manual hub conversion is probably the best way to fix this. I've been eyeing one myself, but I just can't convince myself to put that amount of money into a dana 30 (though, I am buying your brakes in a few weeks so I guess I'm a bit of a hypocrite).

I still don't understand how @psrivats could have vibration at 40mph. That makes me think there must be something else besides driveshaft speed playing into the vibrations. 40mph is shockingly low for vibrations.

No, I understand that ground speed and driveshaft speed are related but the vibes change whether it is in drive or OD on some. It would be nice to find a better OD ratio for these transmissions.

Vibes at 40 is usually indicative of a pinion angle issue.

 

[Mike_H]

Sri, that single crossmember test they did is suspect, in my mind. They removed a whole bunch of material from a stock skid, hoping to improve vibration. It is much less stiff in that cut configuration than stock, so it's not surprising to me that your vibs got worse. You did get a result tho... Less stiff skid, vibs come on earlier. If they were to complete that test properly and bolt in a nice frame crossmember, that was very stiff and properly supported the transmission, something like the Savvy support, or the gen-right one, then you've got something to test.

 

[jodomcfrodo]

No, I understand that ground speed and driveshaft speed are related but the vibes change whether it is in drive or OD on some. It would be nice to find a better OD ratio for these transmissions.

Vibes at 40 is usually indicative of a pinion angle issue.

If I'm honest, I've been having a hard time following the theories behind the OD versus non-OD vibes. All that seems to change in my mind is engine RPM. Very interesting problem...