4.0, 42RLE, NP241OR High Speed Vibrations After Re-Gear to 4.88 or Deeper

[Chris]

@jeepndogs beat me to it with critical speed. I've been in the shadows, reading these and other threads. Everyone has mentioned driveshaft balance...no one has mentioned Driveshaft stiffness. My opinion is that the front driveshaft is exceeding critical speed and starting to deform (or whip). The longer the tube is, the earlier it will experience deformation...also smaller diameter and / or thinner wall stock. I wonder if Tom Woods or someone would make a custom DS with a larger diameter or heavier wall. This would change the response.

Someone should take some measurements of their set-up and plug them into this calculator...see what it spits out.

https://spicerparts.com/calculators/driveshaft-safe-operating-rpm-calculator

Which might explain why it's happening with the front driveshaft as oppose to the rear... since the front is so much longer.

I think you might be onto something here. I'm certain Tom Wood would make a driveshaft to whatever spec the customer wanted. Those guys build custom driveshafts all the time.

If mine still has the vibration when I get it back, I'll be happy to be the test dummy on this (if @bobthetj03 doesn't beat me to it) and order a new front driveshaft if we can determine what thickness the driveshaft needs to be.

 

[AndyG]

I'd wonder if the speed and the double cardan are likely the combined culprits.

 

[bobthetj03]

Which might explain why it's happening with the front driveshaft as oppose to the rear... since the front is so much longer.

I think you might be onto something here. I'm certain Tom Wood would make a driveshaft to whatever spec the customer wanted. Those guys build custom driveshafts all the time.

If mine still has the vibration when I get it back, I'll be happy to be the test dummy on this (if @bobthetj03 doesn't beat me to it) and order a new front driveshaft if we can determine what thickness the driveshaft needs to be.

I wont mind if Tom Woods wants to dissect my driveshaft. We need to determine what the tube wall thickness is. If he can't find anything wrong with the one I send him, I may just have him build me a new one with a thicker tube wall and see what we get.

 

[ac_]

I wont mind if Tom Woods wants to dissect my driveshaft. We need to determine what the tube wall thickness is. If he can't find anything wrong with the one I send him, I may just have him build me a new one with a thicker tube wall and see what we get.

I'm in too!

 

[Chris]

I wont mind if Tom Woods wants to dissect my driveshaft. We need to determine what the tube wall thickness is. If he can't find anything wrong with the one I send him, I may just have him build me a new one with a thicker tube wall and see what we get.

This is what I was thinking as well. It certainly wouldn’t hurt anything to have a new front driveshaft with a thicker tube wall.

The more I think about this though, the more it makes sense.

 

[bobthetj03]

I have one more camping trip at the end of this month, then I'll pull my DS and ship it to Tom. Stay tuned.

 

[bobthetj03]

Mr Bills thread and mine have kind of shared a lot of info, so I'll try to keep my results on my thread, fyi.

 

[Mike_H]

From my Adams front driveshaft. Don't know what any of this means, just plugged in what the calculator wanted. I don't really understand how this calculator works without angles, but I'm going to assume it just bases everything off of deflection.

View attachment 59432

At a certain speed, the driveshaft itself (the tube) starts to deform. Doesn't matter what the angle is. Could be a fixed tube with no joint whatever. In your case, the safe speed was just over 4000 RPM. The next step is to find a calculator, like grim jeeper, to find your driveshaft speeds. If it's exceeding that critical speed... Then your probably in need of a heavier driveshaft.

 

[Chris]

I have one more camping trip at the end of this month, then I'll pull my DS and ship it to Tom. Stay tuned.

I’ll be eagerly awaiting any news. Bring this conversation we are having up with him and mention making it thicker. I think there is a lot of merit here.

 

[bobthetj03]

I’ll be eagerly awaiting any news. Bring this conversation we are having up with him and mention making it thicker. I think there is a lot of merit here.

See what Dave has to say about it as well.

 

[Mike_H]

I was playing with grim jeeper...it doesn't list pinon or DS speed. Would be easy for me to make an Excel sheet to calcluate that based on road speed, tire size and drive ratio though. Maybe Chris can take the sheet, and make a web editable version?

 

[zebra12]

Does weight play a factor? It seems logical to me a light and stiff driveshaft would deform the least.

 

[JDS123]

I was playing with grim jeeper...it doesn't list pinon or DS speed. Would be easy for me to make an Excel sheet to calcluate that based on road speed, tire size and drive ratio though. Maybe Chris can take the sheet, and make a web editable version?

I calculated mine earlier today. I run around 3200 DS RPM at 70mph. At 80, I run 80/70 * 3200 == 3656 RPM. While that isn't close to the maximum operating speed, is it close to the 1/2 true critical speed. Around that speed / driveshaft rpm is where I start to get vibrations. Interesting. Since I'm getting a tummy tuck done along with my frame swap, I might ask my shop if they can order my new driveshafts with thicker tubing. Might be interesting to see what they have to say.

Does anyone know what Tom Wood typically makes his driveshafts out of?

Easy formula for driveshaft rpm:

(5280 * 12 * MPH * GearRatio)
————————————————————-
(pi * TireDiam * 60)

 

[Rob5589]

Won’t fit. But that is possibly the solution. Look at the clearance there is between the shaft and the trans pan at full compression on the drivers side and full droop on the passenger side. A larger shaft would hit the pan. I had a discussion with one guy that said an aluminum shaft with a carbonfiber wrap would probably be enough to stiffen the shaft to raise the defection rpm but the price was astronomical to build it to SEE if it helped.

What about smaller diameter but thicker walled??

 

[Rob5589]

I calculated mine earlier today. I run around 3200 DS RPM at 70mph. At 80, I run 80/70 * 3200 == 3656 RPM. While that isn't close to the maximum operating speed, is it close to the 1/2 true critical speed. Around that speed / driveshaft rpm is where I start to get vibrations. Interesting. Since I'm getting a tummy tuck done along with my frame swap, I might ask my shop if they can order my new driveshafts with thicker tubing. Might be interesting to see what they have to say.

Does anyone know what Tom Wood typically makes his driveshafts out of?

Your ds rpm will be your engine rpm / the final drive ratio. What gear were you in?

 

[JDS123]

Your ds rpm will be your engine rpm * the final drive ratio. What gear were you in?

Here is how I find driveshaft rpm using known values:

(5280 * 12 * MPH * GearRatio)
————————————————————-
(pi * TireDiam * 60)


Your method works as well and is probably easier. I calculated it using both ways at 80 MPH in 5th gear at engine 3000RPM. The results were within 50RPM (The difference probably comes down to reading the tachometer correctly and exact tire sizes).

 

[jeepndogs]

Thicker wall will make it stiffer but will also in the rotational mass of the shaft. That also effects the critical speed of the shaft. I do a lot of high HP drag car chassis work. Driveshaft diameter, length, and wall thickness plays a huge factor in the race cars. Denny’s driveline builds a lot of the race shafts for some of the big drag racing teams. He also built the front shaft that is currently in my Jeep. It is a slightly thicker wall and was also high speed balanced. They have the ability to balance at 10,000 RPM. This shaft produces the least amount of vibration but it was still there. His opinion was if he went any thicker in the wall it would have added too much rotational mass and the critical speed would actually be reduced.

 

[Rob5589]

Here is how I find driveshaft rpm using known values:

(5280 * 12 * MPH * GearRatio)
————————————————————-
(pi * TireDiam * 60)


Your method works as well and is probably easier. I calculated it using both ways at 80 MPH in 5th gear at engine 3000RPM. The results were within 50RPM (The difference probably comes down to reading the tachometer correctly and exact tire sizes).

You are probably correct. That is the way it was explained to me, so.... I did edit, division for multiplication. Disclaimer: I will never, ever claim to be a math wiz. Hell, I can barely help my 6 year old.

 

[JDS123]

That is the way it was explained to me. Disclaimer: I will never, ever claim to be a math wiz. Hell, I can barely help my 6 year old.

Your method is perfectly valid. Engine Rpm / transmission ratio = driveshaft speed. I'm calculating it from tire rpm, you're calculating it from engine RPM. Both are perfectly fine methods (and yours is probably more accurate considering how hard it is to know the exact diameter of a tire).

 

[Chris]

See what Dave has to say about it as well.

Yes, I intend to!

I was playing with grim jeeper...it doesn't list pinon or DS speed. Would be easy for me to make an Excel sheet to calcluate that based on road speed, tire size and drive ratio though. Maybe Chris can take the sheet, and make a web editable version?

Yes, I could do this.

Thicker wall will make it stiffer but will also in the rotational mass of the shaft. That also effects the critical speed of the shaft. I do a lot of high HP drag car chassis work. Driveshaft diameter, length, and wall thickness plays a huge factor in the race cars. Denny’s driveline builds a lot of the race shafts for some of the big drag racing teams. He also built the front shaft that is currently in my Jeep. It is a slightly thicker wall and was also high speed balanced. They have the ability to balance at 10,000 RPM. This shaft produces the least amount of vibration but it was still there. His opinion was if he went any thicker in the wall it would have added too much rotational mass and the critical speed would actually be reduced.

So you're saying that even after your thicker front driveshaft, you still have vibrations?