Dana 44 with 5.89s

Shawn at Tom Wood's said:
From my perspective, yes the shaft will be spinning too fast. Over 4,000 drive shaft RPM, you can find a drive shaft RPM calculator at the bottom of this page. Here's where running low (numerically high) gears at modern highway speeds can become a problem.

  1. The industry standard balancing machine only goes to 3,400 RPM. That means that anything above that can't be tested in most drive shaft shops. You enter into uncharted territory and there might be vibrations that show up at those speeds.
  2. Centrifugal force of any minor imbalances increases more dramatically at higher RPMs. Every time the speed of the drive shaft doubles the force of an imbalance quadruples. Think exponential but not exactly. More on that here https://wranglertjforum.com/threads/what-gears-do-i-want.51422/post-1168187. Any minor imperfections in the shaft or in the yokes or bearings the shaft attaches to are much more likely to generate vibrations at those 3,500+ RPMs. It's not a guarantee but its much more likely to occur when people are running 5.XX gears.
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The theory is fine, but I question how relevant it is when 5.38s and 35's are popular already (and have been for years) and are pushing 3800-4000 rpm at highway speed (70-75) with no issues? There are a lot of 5.38/35 folks out there....they don't fit in the guidelines either, but it all still works fine. 5.89 isn't that far away from the 5.38s many folks already have.

The unfortunate problem for Jeep owners is they have the transmissions they have, and nobody wants to sacrifice performance for driveshaft speeds. The 42RLE sucks when it's undergeared, and the only way to keep it in your guidelines for driveshaft rpm is to be undergeared. Not trying to be rude or say driveshaft rpm doesn't matter....just seems like the real world success maybe could possibly outweigh the theoretical problems.

The only "vibe" issues I know of being attributed to the faster driveline speeds are the pulsing harmonic vibes, not the unbalanced driveshaft type where it feels like you're driving on the wake-up bumps. The unbalanced shaft vibes don't seem to be that much of a problem - at least, it seems to consumers that if you can balance it to 3400, it's good enough to keep going. The harmonic vibes on the other hand seem to happen somewhat regardless of the circumstances. I had harmonic vibes with 3.55 gears going 80 mph, and my driveshaft speeds were acceptable due to the 3.55 ratio.
 
Shawn at Tom Wood's said:
From my perspective, yes the shaft will be spinning too fast. Over 4,000 drive shaft RPM, you can find a drive shaft RPM calculator at the bottom of this page. Here's where running low (numerically high) gears at modern highway speeds can become a problem.

  1. The industry standard balancing machine only goes to 3,400 RPM. That means that anything above that can't be tested in most drive shaft shops. You enter into uncharted territory and there might be vibrations that show up at those speeds.
  2. Centrifugal force of any minor imbalances increases more dramatically at higher RPMs. Every time the speed of the drive shaft doubles the force of an imbalance quadruples. Think exponential but not exactly. More on that here https://wranglertjforum.com/threads/what-gears-do-i-want.51422/post-1168187. Any minor imperfections in the shaft or in the yokes or bearings the shaft attaches to are much more likely to generate vibrations at those 3,500+ RPMs. It's not a guarantee but its much more likely to occur when people are running 5.XX gears.
Click to expand...

That’s all very interesting. Basically with 35s and the 5.89s you’d be limited to like 60mph to stay under that 3400 rpm limit.

Out of curiosity I put my current combo in and it’s right inline with my findings. Right when my DS RPMs get over 3400 there’s a noticeable increase in the vibrations you feel.

I may still run the risk as any decent trip my Jeep is trailered and I normally only drive it on short trips around town.
 
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machoheadgames said:
The theory is fine, but I question how relevant it is when 5.38s and 35's are popular already (and have been for years) and are pushing 3800-4000 rpm at highway speed (70-75) with no issues? There are a lot of 5.38/35 folks out there....they don't fit in the guidelines either, but it all still works fine. 5.89 isn't that far away from the 5.38s many folks already have.

The unfortunate problem for Jeep owners is they have the transmissions they have, and nobody wants to sacrifice performance for driveshaft speeds. The 42RLE sucks when it's undergeared, and the only way to keep it in your guidelines for driveshaft rpm is to be undergeared. Not trying to be rude or say driveshaft rpm doesn't matter....just seems like the real world success maybe could possibly outweigh the theoretical problems.

The only "vibe" issues I know of being attributed to the faster driveline speeds are the pulsing harmonic vibes, not the unbalanced driveshaft type where it feels like you're driving on the wake-up bumps. The unbalanced shaft vibes don't seem to be that much of a problem - at least, it seems to consumers that if you can balance it to 3400, it's good enough to keep going. The harmonic vibes on the other hand seem to happen somewhat regardless of the circumstances. I had harmonic vibes with 3.55 gears going 80 mph, and my driveshaft speeds were acceptable due to the 3.55 ratio.
Click to expand...

It's not just theory, it often manifests as actual vibration problems. I only explain the theory to back up my opinion as to why running too low of gears can sometimes cause problems. I'm explaining the reason behind the very real world results we often see when people over-gear. "Sometimes" is really the key word here. I know that sometimes people run 5.38 with no vibrations but "no vibrations" is highly subjective. Everything that is moving is vibrating, it is just a matter of intensity and how perceptive the driver is to those vibrations. I always say that predicting vibrations is like predicting the weather, a chance of rain doesn't mean it's 100% going to rain, it means there is a chance. Same thing here, there is a chance that the higher drive shaft RPMs will result in a noticeable vibration. My goal is mainly to get people to understand that there is a potential trade-off when running those gears, that making changes to one thing affects something else. Some people definitely have issues when running those gears. Of course I hear about it more often than the average person because we are who people contact when people are trying to fix those vibrations. It is not accurate though to say that these issues are entirely theoretical and that there are "no issues". I think you just haven't talked to as many people with those issues as I have.

There are many causes of vibrations, again anything that is moving is vibrating. So that fact that you had high speed vibrations with 3.55 gears does not disprove the fact that centrifugal force increases at higher speeds, it just means you had a different experience and had vibrations that came in at lower drive shaft RPMs. I bet if you went 100 miles an hour they would have been even worse though. Your vibrations may have been drive shaft related, they may have been from something else, they may have been from the drive shaft and several other things all contributing to the vibration.

There are no black and white truths here, it is all a spectrum. Vibrations are partly due to random chance and how a multitude of factors all stack up. Also, vibrations are subjective. That's why i very intentionally used words like "might" and "more likely", because I want to be clear that we're talking about possibilities and probabilities, and that the higher the speed the higher the chances you have of running into an issue.
 
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Shawn at Tom Wood's said:
It's not just theory, it often manifests as actual vibration problems. I only explain the theory to back up my opinion as to why running to to low of gears can sometimes cause problems. I'm explaining the reason behind the very real world results we often see when people over-gear. "Sometimes" is really the key word here. I know that sometimes people run 5.38 with no vibrations but "no vibrations" is highly subjective. Everything that is moving is vibrating, it is just a matter of intensity and how perceptive the driver is to those vibrations. I always say that predicting vibrations is like predicting the weather, a chance of rain doesn't mean it's 100% going to rain, it means there is a chance. Same thing here, there is a chance that the higher drive shaft RPMs will result in a noticeable vibration. My goal is mainly to get people to understand that there is a potential trade-off when running those gears, that making changes to one thing affects something else. Some people definitely have issues when running those gears. Of course I hear about it more often than the average person because we are who people contact when people are trying to fix those vibrations. It is not accurate though to say that these issues are entirely theoretical and that there are "no issues". I think you just haven't talked to as many people with those issues as I have.

There are many causes of vibrations, again anything that is moving is vibrating. So that fact that you had high speed vibrations with 3.55 gears does not disprove the fact that centrifugal force increases at higher speeds, it just means you had a different experience and had vibrations that came in at lower drive shaft RPMs. I bet if you went 100 miles an hour they would have been even worse though. Your vibrations may have been drive shaft related, they may have been from something else, they may have been from the drive shaft and several other things all contributing to the vibration.

There are no black and white truths here, it is all a spectrum. Vibrations are partly due to random chance and how a multitude of factors all stack up. Also, vibrations are subjective. That's why i very intentionally used words like "might" and "more likely", because I want to be clear that we're talking about possibilities and probabilities, and that the higher the speed the higher the chances you have of running into an issue.
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Thank you for weighing in here, this info is very helpful!

I wonder how likely it is for an LJ’s rear driveshaft to suffer vibes with 5.89s.

How much does driveshaft length play a role into perceived vibrations?
 
PNW_LJ said:
How much does driveshaft length play a role into perceived vibrations?
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I think that there are two ways in which length can play a role in vibrations. One is the "critical speed" of the drive shaft but in the context of a TJ or LJ this is pretty much a non issue. It is in things like pickup trucks where this starts to become a significant concern. The other way in which length might contribute is simply that a longer shaft is heavier. More mass means more potential for vibrations. Though that extra 10 inches of tube in an LJ shaft doesn't increase the weight of the shaft by much, only 2-3 lbs, roughly a 10% increase in the net weight of the shaft.
 
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Shawn at Tom Wood's said:
I think that there are two ways in which length can play a role in vibrations. One is the "critical speed" of the drive shaft but in the context of a TJ or LJ this is pretty much a non issue. It is in things like pickup trucks where this starts to become a significant concern. The other way in which length might contribute is simply that a longer shaft is heavier. More mass means more potential for vibrations. Though that extra 10 inches of tube in an LJ shaft doesn't increase the weight of the shaft by much, only 2-3 lbs, roughly a 10% increase in the net weight of the shaft.
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Okay, thank you for that.

So compared to a TJ, an LJ is less susceptible to vibrations due to drivetrain lift (whether suspension lift or tummy tuck), but it is slightly more susceptible to vibrations at very low gear ratios. Interesting.
 
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