What gears do I want?

Diggin up old thread... Sorry
So after having the 5.13 gears awhile what's your opinion of the performance now?
Did you ever get the driveline vibes some have had with going to 5.13 gears because of driveshaft rpm's?
I'm about to make a decision on which gear ratio I will go with (also considering 4.88). My Jeep is a 2006 LJ Rubicon with the dreaded 42rle Auto Tranny, 33 inch tires. Illinois is pretty flat.

Hello, I did not get any vibrations with the 5.13. I did get them when I lifted the Jeep, which was more to do with pinion angle. I solved that with a SYE and an Adam's CV driveshaft.

I loved how the Jeep behaved with 5.13s and 33s. I went for 35s soon after.
 
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Realize that there is no magic line between 4.88 and 5.13 where vibes appear. Some get them with 4.56 and some don’t get them at all with 5.38. Obviously the deeper you go the faster the shaft is spinning and more chance for vibes. Both 4.88 and 5.13 are deep enough that you’re in vibe risk territory. Personally I’d choose what makes the Jeep drive the best over delaying vibes a few mph.

Exactly. If one is going to fret about vibes, they may as well stay stock because there is no way to know. And if harmonic vibes are the ones people are worried about, I had those on stock gears with 31’s when I’d go 80-85. Double cardan and SYE on 2.5” lift. Personally I think the double cardan shaft is prone to harmonic vibes and the deep gears just exacerbate it.

I also think a lot of people don’t realize what harmonic vibes are and so the ones that say they don’t have any vibes may just not realize they have them. Basically just sounds like a bass hitting periodically and at changing rhythms. I notice it much more with the hardtop on than not.
 
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Hello, I did not get any vibrations with the 5.13. I did get them when I lifted the Jeep, which was more to do with pinion angle. I solved that with a SYE and an Adam's CV driveshaft.

I loved how the Jeep behaved with 5.13s and 33s. I went for 35s soon after.

I have no plans on ever going to 35's and what I have been reading is that the Rubicon with 42rle, 33's and 5.13's are more prone to the harmonic vibes. All because of driveshaft revolutions and maybe something to do with the 241 transfer case output shaft to the front driveshaft angle. I want to do the 5.13 gears because I'd rather be a little overgeared than under. From my research the 4.88 gear is ok but the optimal gear ratio is a little lower than that. This vibe thing has me jumping back and forth between 5.13 and 4.88. I lived with harmonic vibes on my previous 03 TJ Rubicon and didnt like it at all. I am sure the LJ would be very happy and a pleasure to drive with 5.13 gears.
 
I have no plans on ever going to 35's and what I have been reading is that the Rubicon with 42rle, 33's and 5.13's are more prone to the harmonic vibes. All because of driveshaft revolutions and maybe something to do with the 241 transfer case output shaft to the front driveshaft angle. I want to do the 5.13 gears because I'd rather be a little overgeared than under. From my research the 4.88 gear is ok but the optimal gear ratio is a little lower than that. This vibe thing has me jumping back and forth between 5.13 and 4.88. I lived with harmonic vibes on my previous 03 TJ Rubicon and didnt like it at all. I am sure the LJ would be very happy and a pleasure to drive with 5.13 gears.
The harmonic vibe is known to occasionally happen even with 4.88. I'd go with 5.13.
 
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When I purchased my '06 LJ Rubicon with 42RLE transmission it was equipped with 315/75R16 tires ("metric 35's") and 4.88 gears. No vibrations, but it was a dog in OD.

I experienced vibrations when I re-geared to 5.38 using the same "metric 35" tires. I replaced the front drive shaft with a unit from Tom Wood and played with pinion angles, which helped, but I didn't have the patience to continue playing so being the "old skool" jeeper I am, I converted to Yukon manual hubs. Vibrations gone.

Unless things have changed since I moved to Mexico, the deepest one can go with a non-Rubicon is still 5.13. That would be a good ratio for 33" tires and the 42RLE. With a Rubicon one can go to 5.38, which is about 5% deeper than 5.13 and what I would suggest for 35" tires and the 42RLE. Either ratio will be a improvement over 4.88 gears - even if one has to install hubs to cure any resulting vibrations.

Its only money.
 
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I'm going to copy and paste a post from a previous thread. There are lots of factors to consider when re-gearing, the drive shaft is only one of them. But as far as the drive shaft is concerned when you get into those 3,500+ drive shaft rpms the force of any slight imbalances become more and more pronounced. Use the calculator on the bottom of this page https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations?_pos=1&_sid=d6ec76a03&_ss=r to calculate your drive shaft RPM at certain speeds, then read the info below.

Okay, I'm back with more math that I may or may not fully understand. I knew that the increase in force (vibrations) couldn't be linear but couldn't figure out how or why. It is not exponential, it is quadratic. I think. When I was first doing the force calculations I was using an overly complicated calculator that also require me to convert units of measure before inputting the values I wanted. I found a simpler calculator which I'll link below. I re-calculated the numbers, based on 1 ounce spinning at 3" radius. The values here are almost arbitrary though, I actually meant to do 3" diameter but entered it as radius but the principal should be the same. I did calculations for speeds in 200 rpm increments from 0 to 4,000. I then created a line graph in excel. Here's what that looks like.
force chart.jpg



You can see that the increase in force and therefor the increase in vibrations due to minor imbalances or loose parts is not linear. My understanding after reading a bunch is that when speed doubles centripetal force quadruples. You can see in the graph above that at 1,000 rpm the force is around 25. Double the speed to 2,000 and the force is about 100. Double again and to 4,000 rpm and the force is almost 400.

Drive shaft rpm is not the only thing to consider when choosing gears but I think this sheds some light on why many people develop high speed vibrations once their drive shaft rpms get up above 3,000 or so. The faster you are going the more a little increase in rpm is going to have an effect on increased forces that cause vibrations.
The amount of force created from an imbalance going from 3,600 to 4,000 is the about the same as going from 1,000 to 2,000. In essence at a certain point 400 rpm can have the same effect at introducing a vibration as a 1,000 rpm increase earlier did earlier on.

Centripetal Force Calculator

Check this centripetal force calculator if you want to estimate the centripetal force acting upon a body in circular motion.
favicon-04452dcdb0.png
www.omnicalculator.com
 
I'm going to copy and paste a post from a previous thread. There are lots of factors to consider when re-gearing, the drive shaft is only one of them. But as far as the drive shaft is concerned when you get into those 3,500+ drive shaft rpms the force of any slight imbalances become more and more pronounced. Use the calculator on the bottom of this page https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations?_pos=1&_sid=d6ec76a03&_ss=r to calculate your drive shaft RPM at certain speeds, then read the info below.

Okay, I'm back with more math that I may or may not fully understand. I knew that the increase in force (vibrations) couldn't be linear but couldn't figure out how or why. It is not exponential, it is quadratic. I think. When I was first doing the force calculations I was using an overly complicated calculator that also require me to convert units of measure before inputting the values I wanted. I found a simpler calculator which I'll link below. I re-calculated the numbers, based on 1 ounce spinning at 3" radius. The values here are almost arbitrary though, I actually meant to do 3" diameter but entered it as radius but the principal should be the same. I did calculations for speeds in 200 rpm increments from 0 to 4,000. I then created a line graph in excel. Here's what that looks like.
force chart.jpg



You can see that the increase in force and therefor the increase in vibrations due to minor imbalances or loose parts is not linear. My understanding after reading a bunch is that when speed doubles centripetal force quadruples. You can see in the graph above that at 1,000 rpm the force is around 25. Double the speed to 2,000 and the force is about 100. Double again and to 4,000 rpm and the force is almost 400.

Drive shaft rpm is not the only thing to consider when choosing gears but I think this sheds some light on why many people develop high speed vibrations once their drive shaft rpms get up above 3,000 or so. The faster you are going the more a little increase in rpm is going to have an effect on increased forces that cause vibrations.
The amount of force created from an imbalance going from 3,600 to 4,000 is the about the same as going from 1,000 to 2,000. In essence at a certain point 400 rpm can have the same effect at introducing a vibration as a 1,000 rpm increase earlier did earlier on.

Centripetal Force Calculator

Check this centripetal force calculator if you want to estimate the centripetal force acting upon a body in circular motion.
View attachment 349584 www.omnicalculator.com

Thank you for this!
 
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I'm going to copy and paste a post from a previous thread. There are lots of factors to consider when re-gearing, the drive shaft is only one of them. But as far as the drive shaft is concerned when you get into those 3,500+ drive shaft rpms the force of any slight imbalances become more and more pronounced. Use the calculator on the bottom of this page https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations?_pos=1&_sid=d6ec76a03&_ss=r to calculate your drive shaft RPM at certain speeds, then read the info below.

Okay, I'm back with more math that I may or may not fully understand. I knew that the increase in force (vibrations) couldn't be linear but couldn't figure out how or why. It is not exponential, it is quadratic. I think. When I was first doing the force calculations I was using an overly complicated calculator that also require me to convert units of measure before inputting the values I wanted. I found a simpler calculator which I'll link below. I re-calculated the numbers, based on 1 ounce spinning at 3" radius. The values here are almost arbitrary though, I actually meant to do 3" diameter but entered it as radius but the principal should be the same. I did calculations for speeds in 200 rpm increments from 0 to 4,000. I then created a line graph in excel. Here's what that looks like.
force chart.jpg



You can see that the increase in force and therefor the increase in vibrations due to minor imbalances or loose parts is not linear. My understanding after reading a bunch is that when speed doubles centripetal force quadruples. You can see in the graph above that at 1,000 rpm the force is around 25. Double the speed to 2,000 and the force is about 100. Double again and to 4,000 rpm and the force is almost 400.

Drive shaft rpm is not the only thing to consider when choosing gears but I think this sheds some light on why many people develop high speed vibrations once their drive shaft rpms get up above 3,000 or so. The faster you are going the more a little increase in rpm is going to have an effect on increased forces that cause vibrations.
The amount of force created from an imbalance going from 3,600 to 4,000 is the about the same as going from 1,000 to 2,000. In essence at a certain point 400 rpm can have the same effect at introducing a vibration as a 1,000 rpm increase earlier did earlier on.
Great

Centripetal Force Calculator

Check this centripetal force calculator if you want to estimate the centripetal force acting upon a body in circular motion.
View attachment 349584 www.omnicalculator.com

Thanks Shawn, Great info!
 
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I'll put it this way. When I was considering gear ratios and reading up on the topic I had the following concerns, and this is what I know now after living with my decisions based on the input from the good folks in this forum.

1. 5.13s being too deep causing for high revs all the time - This is not the case. even when I was on 33s, the Jeep drove so much better and it never feels like it is winding up disproportionately to the speed I'm driving. I'd even consider 5.38 since you're not limited to 5.13 with the Dana 30 up front.
2. Vibrations was always going to be a factor for me - I ran with a t-case drop for a while with a little bit of a vibrations. This was while I was waiting on the SYE and Double Cardin shaft. Now, the belly pan is back up, pinion angle adjusted and the Jeep rides like a stock vehicle. For you, this is not an issue since your Rubi already has the SYE.

In your shoes, I'd go with 5.38 (what I read was, with the 42RLE, deeper the better when it comes to gearing) and maybe consider a better quality driveshaft from Adam's or Tom Wood's IF vibrations become a factor.
 
1. 5.13s being too deep causing for high revs all the time - This is not the case. even when I was on 33s, the Jeep drove so much better and it never feels like it is winding up disproportionately to the speed I'm driving. I'd even consider 5.38 since you're not limited to 5.13 with the Dana 30 up front.
Great comments but 5.38 is only available up front with a Dana 44, the standard Dana 30 is limited to 5.13.
 
The most important is not to a mistake like mine and stick with 4.56 gears. These will limit to 31 inch tires, because anything beyond will be tolerable, but underpowered. I have 32s now with 4.56, and it is kinda OK, but feels just wrong
 
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And one more thing to remember that I have learned:
Regearing with a goal of returning to stock is not a good idea. The stock gearing sucked right out of the factory, as it was a compromise between different gearboxes having same gearing.
TJ Rubicon came with 6 speed and 4.10, when in reality 4.10 is slightly too high for 6 speed. This is why when the Israeli military did their research for their TJ, they ended up with 32 inch tires (in reality 31.5) and 4.10 gears.
Same TJR came with 4 speed auto and 4.10, and is significantly under geared. Stock TJR with auto should have 4.56 gears, which would be ideal for him.
I am replacing my 32 inch to 31 inch, and this would be the ideal ratio for 4.0, 42RLE 4 speed, 4.56 gears and 31 inch (30.5 in reality).
 
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Thanks for all your knowledge fellas! I am really leaning for the 5.13 gear. I talked to a Jeep shop in St Louis today and was asking if they had ever done a Jeep like mine. He said he would highly advise goin anything over 4.56 with 33's. I immediately ended the conversation and told him thanks for his time. I am mainly worried about the vibes after installing deep gears but want the Jeep to be happy. I will deal with any vibes later if they arise. Again my Jeep is a 2006 LJ Rubicon with the 42rle auto and 33" tires.
 
Thanks for all your knowledge fellas! I am really leaning for the 5.13 gear. I talked to a Jeep shop in St Louis today and was asking if they had ever done a Jeep like mine. He said he would highly advise goin anything over 4.56 with 33's. I immediately ended the conversation and told him thanks for his time. I am mainly worried about the vibes after installing deep gears but want the Jeep to be happy. I will deal with any vibes later if they arise. Again my Jeep is a 2006 LJ Rubicon with the 42rle auto and 33" tires.

Since you have a Rubi I would go 5.38 for 33’s.
 
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Since you have a Rubi I would go 5.38 for 33’s.

I wouldn't.

I would choose 5.13 for an '06 LJ Rubicon with 42RLE and 33" tires unless it is primarily an around town/trail vehicle that won't see many Interstates or if one lives at elevation and frequently pulls grades. One must also consider the 4:1 low range, which combined with 5.38 gears will be a stump puller and great for technical obstacles but for average trails, fire roads, etc. will have the engine at unnecessarily high rpms all day. Its also too low a ratio for deep sand or when some speed is required.

I have 5.38 gears in my '06 Rubicon LJ with 35" tires and like that combination in high range, but in low range the 4:1 ratio was too often too low, which is why I installed a Rubicrawler to get a 2.72:1 middle ratio. That helped tremendously.

I can imagine how 5.38 gears would be with 33s and that 4:1 ratio. I wouldn't like it most of the time.
 
I wouldn't.

I would choose 5.13 for an '06 LJ Rubicon with 42RLE and 33" tires unless it is primarily an around town/trail vehicle that won't see many Interstates or if one lives at elevation and frequently pulls grades. One must also consider the 4:1 low range, which combined with 5.38 gears will be a stump puller and great for technical obstacles but for average trails, fire roads, etc. will have the engine at unnecessarily high rpms all day. Its also too low a ratio for deep sand or when some speed is required.

I have 5.38 gears in my '06 Rubicon LJ with 35" tires and like that combination in high range, but in low range the 4:1 ratio was too often too low, which is why I installed a Rubicrawler to get a 2.72:1 middle ratio. That helped tremendously.

I can imagine how 5.38 gears would be with 33s and that 4:1 ratio. I wouldn't like it most of the time.

If 4LO is annoying with 5.38, 5.13 will hardly be better. Only by a tiny bit. I’d rather the lower gears for the street. The RC is always an option for him also later anyways. Granted it’s expensive but so is everything else like the gears and possibly hubs.
 
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