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

[Mike_H]

I just had another thought...Is it possible for the T-case damper and the Crank Damper to get Out of Phase and set up a vibration? Probably not...

If it were me, I think the first thing I would do is make DAMN sure the harmonic balancer on the engine is in good shape. They can wear out...I might actually call these guys.

http://www.atiracing.com/products/dampers/

They already make a tunable harmonic balancer. No need to reinvent the wheel.

Then, I'd start looking at the T-case damper. Make sure its in good shape. As far as how to "fix" the issue...Still not sure if adding weight is the right answer. It might be a matter of allowing more movement, so you need more rubber.

Something that has been bugging me. You guys are finding that you're getting the vibs after a regear. When you regear, you're increasing engine speed, right? Then you come back and say turning off OD makes the problem go away...Which probably increases RPM yet again. How come these Vibs don't happen at stock RPM? That is slow...and in my mind, it should be present, if its present at the low RPM after a re-gear. I realize this isn't making much sense...I'll need to edit later and try to clean up the language.

It must be entirely related to driveshaft RPM if that is the case.


Another Theory. Does anyone know what the OD switch actually does? Does it put the transmission in Third Gear, or does it just prevent the Torque converter from Locking out. If its preventing the Converter from locking out, THAT is the real answer. You've now added a giant viscous coupler into the system and allowed for the vibrations to be absorbed by ATF, essentially.

 

[psrivats]

@Ranger_b0b it has to do with the increased torque the engine generates in OD when the rpms drop.

https://en.wikipedia.org/wiki/Overdrive_(mechanics)?wprov=sfla1

If the engine crankshaft has issues, it would be felt before regear. Like wise in the phasing. It would be good to check the health of those two but I suspect you'll find that they are fine.

Now take a case where you have regeared, and think about when OD would engage, the torque at that rpm range and what would happen vibration wise. The stock damper is designed to work for a set of design params and now we have changed everything with the regear.

 

[bobthetj03]

Still trying to wrap my head around this. Ok, say my jeep has 3.73 gears and I'm running on 30's. My engine RPM at 65mph in 5th gear is 2500. No vibes. So, I re-gear to 4.56's and change to 33's. Engine RPM at 65mph in 5th gear is 2500, yet now I have vibes. They say the drive shafts are spinning 30% faster, but I can't comprehend that. Splain Lucy?

 

[glwood]

Still trying to wrap my head around this. Ok, say my jeep has 3.73 gears and I'm running on 30's. My engine RPM at 65mph in 5th gear is 2500. No vibes. So, I re-gear to 4.56's and change to 33's. Engine RPM at 65mph in 5th gear is 2500, yet now I have vibes. They say the drive shafts are spinning 30% faster, but I can't comprehend that. Splain Lucy?

Without any suspension changes in the mix?

 

[bobthetj03]

Without any suspension changes in the mix?

In this scenario, no changes to the suspension. Yeah, I know 33's won't do well on a stock TJ, but that is not the question I'm trying to ask.

 

[psrivats]

Still trying to wrap my head around this. Ok, say my jeep has 3.73 gears and I'm running on 30's. My engine RPM at 65mph in 5th gear is 2500. No vibes. So, I re-gear to 4.56's and change to 33's. Engine RPM at 65mph in 5th gear is 2500, yet now I have vibes. They say the drive shafts are spinning 30% faster, but I can't comprehend that. Splain Lucy?

Below is the right formula for the calculations from what I understand. the 63360 in the denominator is used to get the vehicle speed in mph (ie it's a scaling factor). Transmission gear ratio relates engine revolutions and driveshaft revolution. For direct drive gear (1:1), the engine rpm and driveshaft rpm is the same. The differential gear ratio relates how many driveshaft revolutions there are for each axle revolution. The transfercase gear ratio acts to multiply the transmission gear ratio. Hope this makes sense. @Ranger_b0b or @jjvw or @Chris can correct me if I am wrong.

If you run the math, you'll see what happens with the scenario you specify.

A bit busy at work now, but I will reply in a little while showing different scenarios. Easy to build a table in microsoft excel. I will show a nice table.

 

[Tigerman]

If I may venture an opinion on the matter @psrivats ...

First, it is an awesome thread and it is very cool that you did the research and put it all together. I learnt a lot. It so happens that a friend of mine ran into this problem with his Jeep a while back, when he lifted, upgraded the tires and re-geared the Jeep. To get rid of the non-harmonic vibrations we installed a double cardan driveshaft and an SYE kit of course with adjustable control arms. That took care of the normal vibrations but the true nature of the vibrations was revealed passed 65 mph. Cyclic, low rumble, coming and going. We figured it was harmonic and something was generating a frequency which was hitting the natural resonant frequency of the entire structure.

We fixed it a little unorthodox. What we did is we offset the driveshaft slightly from its perfectly aligned position using thin washers and introduced a hardly noticeable vibration in the driveline from the start. This vibration threw off the frequency and stopped the harmonic vibration, although not entirely but it started occurring less vividly and at much higher speeds.

I think a better solution would be to try to change the natural resonant frequency of the entire structure by making it higher. The NRF for a structure is given by the following formula, if I am not mistaken: =(1/2π)*√(K/M), where K is the stiffness of the structure and M is the material. We cannot change the material of the structure but we can change its stiffness. I propose adding an additional cross member which can be adjusted for tension between the frame rails, close to the skid plate. That, theoretically, should stiffen the structure and increase the NRF.

Cheers

 

[bobthetj03]

If I may venture an opinion on the matter @psrivats ...

First, it is an awesome thread and it is very cool that you did the research and put it all together. I learnt a lot. It so happens that a friend of mine ran into this problem with his Jeep a while back, when he lifted, upgraded the tires and re-geared the Jeep. To get rid of the non-harmonic vibrations we installed a double cardan driveshaft and an SYE kit of course with adjustable control arms. That took care of the normal vibrations but the true nature of the vibrations was revealed passed 65 mph. Cyclic, low rumble, coming and going. We figured it was harmonic and something was generating a frequency which was hitting the natural resonant frequency of the entire structure.

We fixed it a little unorthodox. What we did is we offset the driveshaft slightly from its perfectly aligned position using thin washers and introduced a hardly noticeable vibration in the driveline from the start. This vibration threw off the frequency and stopped the harmonic vibration, although not entirely but it started occurring less vividly and at much higher speeds.

I think a better solution would be to try to change the natural resonant frequency of the entire structure by making it higher. The NRF for a structure is given by the following formula, if I am not mistaken: =(1/2)*√(K/M), where K is the stiffness of the structure and M is the material. We cannot change the material of the structure but we can change its stiffness. I propose adding an additional cross member which can be adjusted for tension between the frame rails, close to the skid plate. That, theoretically, should stiffen the structure and increase the NRF.

Cheers

Can you go into a little more detail on what you did? Did you do this to both drive shafts or just the front?

 

[psrivats]

If I may venture an opinion on the matter @psrivats ...

First, it is an awesome thread and it is very cool that you did the research and put it all together. I learnt a lot. It so happens that a friend of mine ran into this problem with his Jeep a while back, when he lifted, upgraded the tires and re-geared the Jeep. To get rid of the non-harmonic vibrations we installed a double cardan driveshaft and an SYE kit of course with adjustable control arms. That took care of the normal vibrations but the true nature of the vibrations was revealed passed 65 mph. Cyclic, low rumble, coming and going. We figured it was harmonic and something was generating a frequency which was hitting the natural resonant frequency of the entire structure.

We fixed it a little unorthodox. What we did is we offset the driveshaft slightly from its perfectly aligned position using thin washers and introduced a hardly noticeable vibration in the driveline from the start. This vibration threw off the frequency and stopped the harmonic vibration, although not entirely but it started occurring less vividly and at much higher speeds.

I think a better solution would be to try to change the natural resonant frequency of the entire structure by making it higher. The NRF for a structure is given by the following formula, if I am not mistaken: =(1/2)*√(K/M), where K is the stiffness of the structure and M is the material. We cannot change the material of the structure but we can change its stiffness. I propose adding an additional cross member which can be adjusted for tension between the frame rails, close to the skid plate. That, theoretically, should stiffen the structure and increase the NRF.

Cheers

Interesting solution. You had the driveshaft intentionally slightly offset? That frequency related to the sqrt(stiffness/mass) is why Jim Frens was saying once could potentially change the skid and curb this problem ..

 

[Tigerman]

Interesting solution. You had the driveshaft intentionally slightly offset? That frequency inversely related to the sqrt(mass) is why Jim Frens was saying once could potentially change the skid and curb this problem ..

Yes, we did intentionally offset the driveshaft.

 

[Mike_H]

@Ranger_b0b it has to do with the increased torque the engine generates in OD when the rpms drop.

Except engines don't generally develop more torque as RPM decreases. Usually, they develop less...That is why you need to downshift to get up hills and whatnot.

 

[Tigerman]

Can you go into a little more detail on what you did? Did you do this to both drive shafts or just the front?

Did this only to the rear driveshaft. Basically we installed thin washers under two of the feet where the drivesaft attached to the yoke.

 

[psrivats]

Except engines don't generally develop more torque as RPM decreases. Usually, they develop less...That is why you need to downshift to get up hills and whatnot.

This is from another forum. Hope this clarifies why there is additional torque and vibrations when pushing that button.

“Overdrive” technically refers to the existence (and use of) a gear that turns the transmission’s output shaft faster than its input shaft is spinning, so that the engine can turn more slowly to maintain a particular speed. Put another way, non-overdrive transmissions tend to have a top gear with a 1:1 ratio — for every one revolution of the input shaft, the output shaft turns one revolution, as well. Overdrive transmissions, however, typically have a high gear around 0.7:1, meaning that a little less than 3/4ths of a turn of the input shaft still spins the output shaft one full turn.

In an automatic transmission, “overdrive” gear is just another gear, and the transmission will only shift into it when that’s warranted, based on vehicle speed, engine speed, load, throttle position and pressure and rate of change, and other factors.

But if your automatic transmission gear selector has an “Overdrive” (or just “OD”) setting, you should generally use that for all normal driving. Towing might be an example of when you might want to not leave the shifter in OD.

There is another aspect of automatic transmissions worth covering, and which is often inaccurately considered “overdrive” — a lock-up torque converter.

A torque converter is sort of like a clutch — it’s a fluid coupling between the engine’s crankshaft and the transmission’s input shaft. In the old days, torque converters always “slipped”, so they were inefficient — they couldn’t achieve a 1:1 ratio. That was bad for fuel economy. In the late-1970s/early-1980s, manufacturers came out with “lock-up” torque converters that would physically connect the input side of the torque converter (the housing) to the output side (the stator) under certain situations — basically just light throttle, like cruising on the highway. Often, it’s this torque converter “lock-up” feature that is controlled by a button on a shift lever. And, normally, it, too, can and should be left engaged (so that it automatically locks-up when conditions are right). Again, towing would be a time that torque converter lock-up would not be ideal, because torque converters are relatively fragile, so it can make sense to disable the lock-up function.

But modern transmissions and their controllers are adept at choosing the right gear and the proper lock-up status under any driving condition, so it should be better to just leave it in Overdrive and let the car figure out what’s best to do when.

 

[Mike_H]

Still trying to wrap my head around this. Ok, say my jeep has 3.73 gears and I'm running on 30's. My engine RPM at 65mph in 5th gear is 2500. No vibes. So, I re-gear to 4.56's and change to 33's. Engine RPM at 65mph in 5th gear is 2500, yet now I have vibes. They say the drive shafts are spinning 30% faster, but I can't comprehend that. Splain Lucy?

Ok..what you're asking about is angular acceleration, and its the reason Dynamics is so dang hard! Think about a record player. Now imagine a dot about 1/2" from the center and another dot 1/2 from the outer edge. When you turn the record player on, they will both spin at the same RPM, right? They have to. HOWEVER, the distance each will travel is very different. If the inner dot is at 1/2" from center, it will travel 3.1415 (pi) inches per revolution. If the outer dot is at 5 inches from center (for easy math) it will travel 31.415" in that same revolution. Its angular acceleration is much higher.

In your scenario, you've changed from 3.73 gears to 4.56. For each rotation of the tire, the driveshaft will rotate that number of times...

a 30" tire at 60 mph is 1 mile per minute. There are 5280 feet in a mile so that is 63360 inches. Your 30" tire has a circumference of 94.25" so it takes 672.25 rotations to travel a mile or its turning at 672.25 RPM. Now, your driveshaft is spinning at 2507.5 RPM (because of the 3.73:1 Ratio).

Lets do the math for the 33 inch tire and 4.56 gears...

63360\33(pi) = 611 RPM (tire)
611* 4.56 = 2787 Driveshaft RPM.

Your driveshaft is really only going 10% faster...Not 30.

 

[Mike_H]

From what I have read, when a transmission is in overdrive, the torque multiplying relationship between the engine and the drive wheels is reversed. I'll post some references shortly.

Please do. That doesn't make sense to me...so I'd like to learn.

 

[psrivats]

Please do. That doesn't make sense to me...so I'd like to learn.

I just edited my post above .. hope that clarified things.

 

[Mike_H]

I just edited my post above .. hope that clarified things.

It doesn't. Lock-up torque converters don't multiply engine torque. They simply constrain the transmission to the engine and, in essence, make it more like a manual. Did you see my above post about the OD button and one theory I have about why it might cure the vibrations?

 

[psrivats]

It doesn't. Lock-up torque converters don't multiply engine torque. They simply constrain the transmission to the engine and, in essence, make it more like a manual.

Let me look up at how overdrive is implemeted in the 42RLE. Will get back to you on the torque question.

Did you see my above post about the OD button and one theory I have about why it might cure the vibrations?

This bit?

Another Theory. Does anyone know what the OD switch actually does? Does it put the transmission in Third Gear, or does it just prevent the Torque converter from Locking out. If its preventing the Converter from locking out, THAT is the real answer. You've now added a giant viscous coupler into the system and allowed for the vibrations to be absorbed by ATF, essentially.

That makes a lot of sense, honestly. The OD button does not put the trans in a lower gear from what I know.

 

[jodomcfrodo]

I think @Ranger_b0b 's question has more to do with engine torque at certain rpms. I think the answer to his question is this:

Engines don't operate at their maximum torque all the time. You would have to be full throttle all the time for this to be the case. When you go into overdrive, the engine has to increase its torque output to maintain the same speed as it has less of a mechanical advantage now due to the gearing change. So the engine could be operating at 50% capacity in 3rd gear and 75% capacity in 4th gear. This is why you have to increase throttle input when in a higher gear to maintain the same speed. So maximum engine torque decreases as RPM falls, but this does not mean that engine torque decreases in the real world as engines typically don't operate at maximum torque (for a certain RPM).

Little hard to explain, but I think that is why engine torque technically increases in overdrive even though RPM falls.

 

[psrivats]

I think @Ranger_b0b 's question has more to do with engine torque at certain rpms. I think the answer to his question is this:

Engines don't operate at their maximum torque all the time. You would have to be full throttle all the time for this to be the case. When you go into overdrive, the engine has to increase its torque output to maintain the same speed as it has less of a mechanical advantage now due to the gearing change. So the engine could be operating at 50% capacity in 3rd gear and 75% capacity in 4th gear. This is why you have to increase throttle input when in a higher gear to maintain the same speed. So maximum engine torque decreases as RPM falls, but this does not mean that engine torque decreases in the real world as engines typically don't operate at maximum torque (for a certain RPM).

Little hard to explain, but I think that is why engine torque technically increases in overdrive even though RPM falls.

This is how I understand it as well. Thanks for writing it so clearly. The added torque is what creates the vibrations as well.