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

[freedom_in_4low]

I absolutely have not proven your point. The driveshaft does not need to be perfectly aligned across. People with ford 8.8’s in TJ’s don’t have vibrations, people without adjustable track bars at all don’t have vibes. Riddle me this, if your driveshaft can handle 15 degrees (or more) downward, why can’t it handle 1 degree side to side?

the DC joint handles 15 degrees downward, but the pinion is pointed straight with the driveshaft and often vibrates if off by a degree. There's no way to set your pinion in line with the driveshaft with 1 degree side-to-side unless you're ok with having a 1 degree thrust angle.

 

[jodomcfrodo]

the DC joint handles 15 degrees downward, but the pinion is pointed straight with the driveshaft and often vibrates if off by a degree. There's no way to set your pinion in line with the driveshaft with 1 degree side-to-side unless you're ok with having a 1 degree thrust angle.

When it comes to track bar angles and moving the axle back and forth, we are talking about much less than one degree. There is some play to be had in pinion angles. I’m saying nothing has to be perfect as evidenced by 8.8’s and people with lifts and no adjustable track bars. Claiming that the difference in track bar mounts can cause vibrations when driving on the highway makes no sense. Your suspension is at basically ride height while doing that anyways, and the difference in side to side angle is extremely small.

 

[freedom_in_4low]

When it comes to track bar angles and moving the axle back and forth, we are talking about much less than one degree. There is some play to be had in pinion angles. I’m saying nothing has to be perfect as evidenced by 8.8’s and people with lifts and no adjustable track bars. Claiming that the difference in track bar mounts can cause vibrations when driving on the highway makes no sense. Your suspension is at basically ride height while doing that anyways, and the difference in side to side angle is extremely small.

I was merely pointing out the fallacy in your logic with respect to the angle comparison. I have a 44 so I've never been interested in the 8.8 but even if that seemed like the right option in other ways, the off-center pinion would be enough to guide me in a different direction.

I'm not sure who is advocating large track bar angles in the rear end; the most common recommendation I see around here is to use the relocation brackets that most lift kits come with, which returns the axle pretty close to centered. Mine was off by 1/4" at 3" of lift and a stock bar with the relocation bracket which is about a 1.2° angle and though I was able to tune out the vibrations with enough transfer case drop, it was EXTREMELY sensitive to pinion angle (usually took 3-5 rounds of adjustments every time I changed something) and I suspected the lateral component was related. I put on an adjustable bar when I did my SYE so it's centered now and I've been able to hit a vibration-free pinion angle on the first try ever since.

 

[Bill M.]

I absolutely have not proven your point. The driveshaft does not need to be perfectly aligned across. People with ford 8.8’s in TJ’s don’t have vibrations, people without adjustable track bars at all don’t have vibes. Riddle me this, if your driveshaft can handle 15 degrees (or more) downward, why can’t it handle 1 degree side to side?

I am talking u-joint phasing which is where the vibrations come from. You are talking driveshaft angle which if to great will cause u-joint to bind. Two different things.

 

[jodomcfrodo]

I am talking u-joint phasing which is where the vibrations come from. You are talking driveshaft angle which if to great will cause u-joint to bind. Two different things.

Ok I'm open to hear what you have to say then since maybe I'm mis-interpreting it. How could having a different track bar mount change vibrations? I could only see there being a problem if the lateral angle is fairly large.

 

[mrblaine]

the DC joint handles 15 degrees downward, but the pinion is pointed straight with the driveshaft and often vibrates if off by a degree. There's no way to set your pinion in line with the driveshaft with 1 degree side-to-side unless you're ok with having a 1 degree thrust angle.

The driveshaft doesn't know up from down.

 

[mrblaine]

I am talking u-joint phasing which is where the vibrations come from. You are talking driveshaft angle which if to great will cause u-joint to bind. Two different things.

One glaring flaw in your theory is that we still find and deal with the harmonic vibe with the mid arm and no trackbar that moves the axle side to side under travel.

 

[Bill M.]

When it comes to phasing small angle differences can make for vibration problems. DC joints take care of their own phasing internally. Regular u-joints when used in tandem and set at the same angle cancel phasing issues. But with a DC and u-joint combo the there is no second u-joint to cancel the speed change of an out of alignment single u-joint.

Thus it becomes critical that the pinion face is square to the DC joint in all planes.

 

[Bill M.]

One glaring flaw in your theory is that we still find and deal with the harmonic vibe with the mid arm and no trackbar that moves the axle side to side under travel.

Sounds like the mid arm has some pinion angle changes through travel.

 

[jodomcfrodo]

When it comes to phasing small angle differences can make for vibration problems. DC joints take care of their own phasing internally. Regular u-joints when used in tandem and set at the same angle cancel phasing issues. But with a DC and u-joint combo the there is no second u-joint to cancel the speed change of an out of alignment single u-joint.

Thus it becomes critical that the pinion face is square to the DC joint in all planes.

Where I'm a little confused is with the trackbar mount. If you are cruising at ride height down the highway, how could the trackbar angle matter?

 

[bobthetj03]

Why would a drive shaft that is perfectly happy all of a sudden not be happy when the only change made is a change in gearing?

 

[JMT]

The driveshaft doesn't know up from down.

Exactly, just like the earth doesn't know up from down. Of course, then Australia wouldn't be "the land down under."

 

[Steel City 06]

Has anyone ever tried a true CV driveshaft, like one using a Rzeppa joint? If a Rzeppa were installed at both ends, the driveshaft could run any angle with no change in motion (except at binding)

 

[jodomcfrodo]

Why would a drive shaft that is perfectly happy all of a sudden not be happy when the only change made is a change in gearing?

The driveshaft increases in speed when you regear. Regears often just exacerbate issues that were around beforehand. If you regeared from a diff ratio of 1 to 2, your driveshaft moves twice as fast at the same speed. I think you regeared from 3.73 to 4.56, so your driveshafts now spin 1.22 as fast as they used to. 65 mph with 4.56 is 80 with 3.73 when it comes to driveshaft speeds.

 

[freedom_in_4low]

The driveshaft doesn't know up from down.

I wouldn't either if someone was spinning me around at 3800 rpm!

I recall about 10 years ago I high-sided my motorcycle around 60mph and my best guess at my rate of spin is ~120rpm. I could perceive that my field of vision was alternating quickly between mostly green and mostly blue but it was happening far too quickly to process either my position or fortunately, the injuries I was sustaining.

 

[bobthetj03]

The driveshaft increases in speed when you regear. Regears often just exacerbate issues that were around beforehand. If you regeared from a diff ratio of 1 to 2, your driveshaft moves twice as fast at the same speed. I think you regeared from 3.73 to 4.56, so your driveshafts now spin 1.22 as fast as they used to. 65 mph with 4.56 is 80 with 3.73 when it comes to driveshaft speeds.

So with that being known, let's find a solution to keep a drive shaft happy at 1.22 times faster rotation. I'll buy pizza-n-beer!

 

[Bill M.]

Where I'm a little confused is with the trackbar mount. If you are cruising at ride height down the highway, how could the trackbar angle matter?

Good question, it would not matter if your TJ/LJ sets at the same static height as it does at freeway speeds but often dynamic and static height differ. Thus with a high acute angle of a trackbar those height changes could effect driveline alignment.

Many of the followers of this thread have committed they have harmonic vibrations. That leads one to believe several small vibrations are adding up to the vibration they are committing on.

 

[Bill M.]

So with that being known, let's find a solution to keep a drive shaft happy at 1.22 times faster rotation. I'll buy pizza-n-beer!

Give me your credit card and will be back with the pizza-n-beer.

 

[jodomcfrodo]

Good question, it would not matter if your TJ/LJ sets at the same static height as it does at freeway speeds but often dynamic and static height differ. Thus with a high acute angle of a trackbar those height changes could effect driveline alignment.

Many of the followers of this thread have committed they have harmonic vibrations. That leads one to believe several small vibrations are adding up to the vibration they are committing on.

How much a lateral difference do you think there is between the trackbar mount I run and the one you are recommending due to the squatting that happens on the freeway? Why do vibes not appear like crazy when you load the rear down with weight and really squat the suspension?

 

[Bill M.]

How much a lateral difference do you think there is between the trackbar mount I run and the one you are recommending due to the squatting that happens on the freeway? Why do vibes not appear like crazy when you load the rear down with weight and really squat the suspension?

I am not recommending any trackbar or trackbar location. I am only recommending that lateral as well as vertical alignment of the driveline system be considered when chasing vibration problems.
When it comes to your personal vehicle I don't have enough information to make any comment to your questions.