4.56 or 4.88 gears?

[JMT]

Also, if you gear to high (low number like 3.73 or 4.10) your drive shaft as well as your engine and everything else upstream from the pinion is going to have less of a mechanical advantage. This means they will have to work harder to move your Jeep forward, this puts more strain on things and makes you more likely to break a drive shaft when off-roading.

To answer your question more directly though @CHAWK I personally think you will be better suited with 4.56 and think 4.88 is too low (high number) for your tire diameter. 4.88 is what I'd expect if someone is running 37" tires. The problem with recommendations is often that people assume that they know what your needs and goals are, they might assume you are building your jeep for off-roading only. It is nuanced, and what is best for you is not necessarily what is best for the next guy. Either that or people don't understand what they are recommending in the first place.

The calculator is helpful for determining driveshaft speeds, but the transmission and OD ratios matter for everyday practical driving.

I have the 6-speed and 456 gears which I set up for 33’s. Shaft rpm’s at 60 according to your calculator are 2787. I could pull the long gradual hills in ID and have power to spare. My mpg averaged 13. I went to 35’s and most of those hills I need to downshift to keep speed. My mpg reduced to 11.

According to the calculator and recommended 2500rpm I should be running 37’s. I can’t imagine the drag that would be.

Bottom line. There are other factors besides driveshaft speed that should be considered.

At what shaft rpm would you see inevitably causing vibes?

 

[Jerry Bransford]

To answer your question more directly though @CHAWK I personally think you will be better suited with 4.56 and think 4.88 is too low (high number) for your tire diameter.

While everyone here respects your opinions Shawn, it's his 42RLE automatic transmission that is causing us to recommend the lower 5.13 ratio and certainly no higher than 4.88 which I personally would not go with either. 4.56 would be a terrible choice even for 33" tires with the 42RLE transmission. The 42RLE has a very steep .69 Overdrive ratio that drops the engine rpms much lower than any other transmission in use in our Wranglers. The engine would be lugging with 4.56 on the highway and almost lugging with 4.88.

 

[Shawn at Tom Wood's]

Sounds like I should have read the whole thread before commenting. Or refrained entirely. I didn't read the part about the transmission and overdrive. I trust @Jerry Bransford's opinion here and it sounds like his tire diameter and gear ratio work out similarly to 33s with 5.13 and he of course knows better than I do whether or not the performance in his jeep is ideal with that setup.

@JMT, Yes I know that there drive shaft is not the only factor that should be considered. But drive shafts are of course the main factor I can speak on. Speaking more broadly we often see people gear 5.13 with 33" tires and then they tell us that they have a drive shaft vibration at 70 or 80 mph. When doing the math we realize that at those speeds the drive shaft is spinning way faster than we can balance it. Our Axiline balancing machine tops out at 3,300 rpm. With this proposed tire and gear combo that 3,300 drive shaft rpm is met at 63 mph. At 80 it is spinning close to 4,180 rpm, way faster than we or any shop can replicate in our balancing machines. I think there is a correlation between how fast the machine will go and how fast a drive shaft really should be pushed. The faster a shaft spins the more amplified any imbalances in the shaft or any vibrations caused by things like slight play in the transfer case or pinion bearings will become. Also, the faster things are spinning the more heat and friction they generate and the faster they will fail. Again, I'm mainly speaking from the viewpoint of the drive shaft. These are the types of things I mean when I say that the answers are nuanced and that there are trade offs when deciding things like gear ratio. Where we are, in Utah, we have 80 mph speed limits in some places which means people really drive 85 or 90. I don't want to see drive shafts spinning at 4,500+ rpm for long stretches. Of course I don't think anyone should drive that fast in a lifted Jeep but that doesn't mean they wont.

I was so excited to create and share a drive shaft rpm calculator that I didn't read the whole thread. I also just didn't want to read 3 pages of arguing before addressing the OP. But in light of the transmission info I will rescind my recommendation and say that my post is mainly to serve as tool for figuring drive shaft speeds with different gear ratios. And I guess chiming in on what the drive shaft wants, although the drive shaft's vote doesn't always get counted.

 

[Jerry Bransford]

Your driveshaft rpm calculator is destined for a lot of use Shawn. I'm afraid driveshaft rpm has not been much of a topic for many Jeep forums and it's clear that it should have been. Thanks for sharing it with us, I will be checking it out and using it!

 

[Shawn at Tom Wood's]

I think one thing we are all missing in our replies is qualifying what "Highway speeds" means. In some places it means 55 in others it is 80+. I don't think anyone has asked the OP what typical highway cruising speeds are for him. Does anyone know the math to figure shift points based on the information provided so far? This is not a rhetorical question, I'm really curious. I foresee the possible scenario in which a guy gears his jeep to where the transmission can't decide which gear it wants to be in for most of the guys daily commute.

 

[pagrey]

There was another thread recently about shift points. The 42RLE will shift about the same RPM, slightly lower due to the reduced load with a higher gear ratio. You can use Grimm Jeeper to figure out the shift point speeds if you know where it shifts now with your driving habits or simply convert with the old/new ratio. The transmission doesn't use speed so the conversion is a bit easier.

 

[Jamison C]

Sounds like I should have read the whole thread before commenting. Or refrained entirely. I didn't read the part about the transmission and overdrive. I trust @Jerry Bransford's opinion here and it sounds like his tire diameter and gear ratio work out similarly to 33s with 5.13 and he of course knows better than I do whether or not the performance in his jeep is ideal with that setup.

@JMT, Yes I know that there drive shaft is not the only factor that should be considered. But drive shafts are of course the main factor I can speak on. Speaking more broadly we often see people gear 5.13 with 33" tires and then they tell us that they have a drive shaft vibration at 70 or 80 mph. When doing the math we realize that at those speeds the drive shaft is spinning way faster than we can balance it. Our Axiline balancing machine tops out at 3,300 rpm. With this proposed tire and gear combo that 3,300 drive shaft rpm is met at 63 mph. At 80 it is spinning close to 4,180 rpm, way faster than we or any shop can replicate in our balancing machines. I think there is a correlation between how fast the machine will go and how fast a drive shaft really should be pushed. The faster a shaft spins the more amplified any imbalances in the shaft or any vibrations caused by things like slight play in the transfer case or pinion bearings will become. Also, the faster things are spinning the more heat and friction they generate and the faster they will fail. Again, I'm mainly speaking from the viewpoint of the drive shaft. These are the types of things I mean when I say that the answers are nuanced and that there are trade offs when deciding things like gear ratio. Where we are, in Utah, we have 80 mph speed limits in some places which means people really drive 85 or 90. I don't want to see drive shafts spinning at 4,500+ rpm for long stretches. Of course I don't think anyone should drive that fast in a lifted Jeep but that doesn't mean they wont.

I was so excited to create and share a drive shaft rpm calculator that I didn't read the whole thread. I also just didn't want to read 3 pages of arguing before addressing the OP. But in light of the transmission info I will rescind my recommendation and say that my post is mainly to serve as tool for figuring drive shaft speeds with different gear ratios. And I guess chiming in on what the drive shaft wants, although the drive shaft's vote doesn't always get counted.

Your driveshaft rpm calculator is destined for a lot of use Shawn. I'm afraid driveshaft rpm has not been much of a topic for many Jeep forums and it's clear that it should have been. Thanks for sharing it with us, I will be checking it out and using it!

I agree with Jerry. Your explanation of driveshafts, speeds, and balancing at certain speeds adds a lot more variables to consider when dealing with vibrations after a regear. Things I've never even thought of. Thanks for sharing your knowledge.

 

[Shawn at Tom Wood's]

I'm going to add to the conversation again. It has been expressed by others on this thread that the potential for vibration problems is insignificant because there is only a 5% difference between 4.88 and 5.13. I think this is paradoxical because if 5% is insignificant in terms of the possibility of vibrations 5% should also be insignificant in terms of the purported benefits of 5.13 over 4.88.

Also, I believe the likelihood of vibrations to be exponential as speed increases. Surely increasing the speed from 80 to 100 would have a greater likelihood of creating problems than increasing the speed from 40 to 50 would, even though they are both a 25% increase. Or you could frame it as miles per hour and not a percentage. I'd expect a greater chance of problems in the 80 to 100 scenario than I would in a 40 to 60 scenario, both increases of 20 mph. I feel like I know in my heart that vibrations as a result of centripetal/centrifugal force increase exponentially as speed increases but wanted to see if I could figure it out with math. I'm way too dumb to know how to actually do this math so I used an online calculator. I used 3" as the diameter, this is an educated guess as the majority of the diameter of the drive shaft in question would be 2" but the ends, the yokes, which account for around half the net weight of the shaft are closer to 3.5" diameter. So I averaged it and called it 3". I then ran the formula based on 1 ounce at 2,800 rpm and again at 2,940 rpm, 5% faster. The results were 45.72 Newtons of force at 2,800 rpm and 50.6 Newtons at 2,940. This is a 10.2% increase in centripetal force. So the increase in force, percentage wise, is more than double the increase in speed. Interestingly though if I halve the numbers and do the 5% speed increase calculations I still come up with about a 10.2% increase in force. This doesn't support my exponential increase theory. However, the force does appear to increase at a disproportionate ratio to rpm. I don't think it is as simple as 5% faster only means 5% more vibrations. Also, lets remember that the vibrations are always there, it is a matter of amplitude and intensity. The crossover point at which the intensity and amplitude become a problem is both highly subjective and highly dependent on a ton of other factors (luck). There are certainly lots of lengthy threads dedicated to "vibrations after re-gear". It obviously opens up a can of worms. And one person's anecdote of "I run X and Y" with no problems doesn't nullify another persons "I run X and Y and I have a horrible vibration".

I'll double down on saying that there is a ton of nuance with these things and there is not a one size fits all answer to a question like "what gear ratio should I be running?". Subjectivity plays a huge role too.

Someone smarter than me feel free to figure out the math pertaining to rates of increased speed and how they relate to rates of increased vibrations. Here's the force calculator I used.
http://hyperphysics.phy-astr.gsu.ed...Centripetal force = mass x velocity2 / radius

 

[bitbyte]

In reading all of this and seeing a comment or two on changing pinion angles to alleviate or at least partially mitigate the vibrations... I have begun to wonder if there's a correlation between the amount of lift and the likelihood you'll run into vibration issues? A higher lift is certainly going to give you worse driveline angles.

 

[Jamison C]

So here's something I just thought of too @Shawn at Tom Wood's . As Jerry and JMT mentioned we have to take final drive ratios from the transmission into account. The 42RLE has a final drive ratio of .69, which if I'm doing my math right would result in a 31% decrease in drive shaft rpms from the engine to the diffs, no?

According to your calculator, the tires I plan to get measure at 32.8" which will be paired with a desired ratio of 5.13. If the final drive ratio on the 42RLE was 1:1, the driveshaft would be at 3154 RPMs at 60MPH. However, if I decrease the final drive ratio by 31% to account for the .69 overdrive, this would drop my driveshaft RPMs to 2176.

Maybe my math is off, and someone who is less dumb than me can counter me, but does anyone see where I'm going with this? Yes? Bueller? Bueller?

 

[machoheadgames]

So here's something I just thought of too @Shawn at Tom Wood's . As Jerry and JMT mentioned we have to take final drive ratios from the transmission into account. The 42RLE has a final drive ratio of .69, which if I'm doing my math right would result in a 31% decrease in drive shaft rpms from the transmission to diffs, no?

According to your calculator, the tires I plan to get measure at 32.8" which will be paired with a desired ratio of 5.13. If the final drive ratio on the 42RLE was 1:1, the driveshaft would be at 3154 RPMs at 60MPH. However, if I decrease the final drive ratio by 31% to account for the .69 overdrive, this would drop my driveshaft RPMs to 2176.

Maybe my math is off, and someone who is less dum than me can counter me, but does anyone see where I'm going with this? Yes? Bueller? Bueller?
View attachment 311918

A transmission can’t decrease the driveshaft rpm. Driveshaft rpm is constant based on gear ratio and tire size. You are reducing the engine rpm to turn the driveshaft the same number of times when in OD. This is why with 5.13 the driveshaft has to spin fast and yet the engine doesn’t since the OD is so tall.

 

[Jamison C]

A transmission can’t decrease the driveshaft rpm. Driveshaft rpm is constant based on gear ratio and tire size. You are reducing the engine rpm to turn the driveshaft the same number of times when in OD. This is why with 5.13 the driveshaft has to spin fast and yet the engine doesn’t since the OD is so tall.

I feel so unbelievably dumb right now, because deep down I knew that.

 

[Shawn at Tom Wood's]

I feel so unbelievably dumb right now, because deep down I knew that.

I've spent the last two hours trying to wrap my mind around some of the concepts discussed in this thread. Somehow this has turned into some high level math and engineering. It certainly has my brain feeling like it needs a nap. Probably because of my "calculations" the convo has taken a turn into the weeds of engineering, physics, and math that I for one can't figure out and has me feeling dumb too. Which brings up another point, the educated and experienced engineers did a lot of calculations and work to design a jeep a certain way, I don't think that even they had all the "right" answers. Interestingly though if you run the numbers on gearing vs tire size for a stock tj (I think it is 30" tires and 4.10 gears?) and then for 33s and 4.56, it is almost exactly the same ratio as stock. Meaning it would get drive shaft, transmission, and motor speeds, relative to vehicle speed, right where the eggheads at Jeep put them. But loads of people like their jeep better geared lower and the entire aftermarket industry is there to "improve" stock designs. So who knows?!

 

[Jerry Bransford]

A couple things about larger tires together with the suspension lifts installed to clear them... larger tires have more rolling resistance and taller Jeeps have more wind resistance (and the Jeep usually weighs more too) so it actually takes more engine rpms to turn bigger tires on a lifted Jeep than it did for a stock Jeep with smaller tires to have the same performance.

 

[JMT]

Sounds like I should have read the whole thread before commenting. Or refrained entirely. I didn't read the part about the transmission and overdrive. I trust @Jerry Bransford's opinion here and it sounds like his tire diameter and gear ratio work out similarly to 33s with 5.13 and he of course knows better than I do whether or not the performance in his jeep is ideal with that setup.

@JMT, Yes I know that there drive shaft is not the only factor that should be considered. But drive shafts are of course the main factor I can speak on. Speaking more broadly we often see people gear 5.13 with 33" tires and then they tell us that they have a drive shaft vibration at 70 or 80 mph. When doing the math we realize that at those speeds the drive shaft is spinning way faster than we can balance it. Our Axiline balancing machine tops out at 3,300 rpm. With this proposed tire and gear combo that 3,300 drive shaft rpm is met at 63 mph. At 80 it is spinning close to 4,180 rpm, way faster than we or any shop can replicate in our balancing machines. I think there is a correlation between how fast the machine will go and how fast a drive shaft really should be pushed. The faster a shaft spins the more amplified any imbalances in the shaft or any vibrations caused by things like slight play in the transfer case or pinion bearings will become. Also, the faster things are spinning the more heat and friction they generate and the faster they will fail. Again, I'm mainly speaking from the viewpoint of the drive shaft. These are the types of things I mean when I say that the answers are nuanced and that there are trade offs when deciding things like gear ratio. Where we are, in Utah, we have 80 mph speed limits in some places which means people really drive 85 or 90. I don't want to see drive shafts spinning at 4,500+ rpm for long stretches. Of course I don't think anyone should drive that fast in a lifted Jeep but that doesn't mean they wont.

I was so excited to create and share a drive shaft rpm calculator that I didn't read the whole thread. I also just didn't want to read 3 pages of arguing before addressing the OP. But in light of the transmission info I will rescind my recommendation and say that my post is mainly to serve as tool for figuring drive shaft speeds with different gear ratios. And I guess chiming in on what the drive shaft wants, although the drive shaft's vote doesn't always get counted.

Shawn, I really appreciate your response. I hope my earlier post didn’t come across malicious. I didn’t intend that at all. I’ll never know near as much as you about driveshafts or any closely related areas. I love the perfection you and your family strive for and continually accomplish!

I think your driveshaft rpm calculator is going to help solve the most difficult problem on this forum: oscillating vibrations. This is what I was hoping to learn from your answer and I think the data generated by using your calculator may give insight into the underlying cause. Hubs usually solve it, but not always. There is still much more for us to learn. Thanks for sharing your expertise!

 

[JMT]

I've spent the last two hours trying to wrap my mind around some of the concepts discussed in this thread. Somehow this has turned into some high level math and engineering. It certainly has my brain feeling like it needs a nap. Probably because of my "calculations" the convo has taken a turn into the weeds of engineering, physics, and math that I for one can't figure out and has me feeling dumb too. Which brings up another point, the educated and experienced engineers did a lot of calculations and work to design a jeep a certain way, I don't think that even they had all the "right" answers. Interestingly though if you run the numbers on gearing vs tire size for a stock tj (I think it is 30" tires and 4.10 gears?) and then for 33s and 4.56, it is almost exactly the same ratio as stock. Meaning it would get drive shaft, transmission, and motor speeds, relative to vehicle speed, right where the eggheads at Jeep put them. But loads of people like their jeep better geared lower and the entire aftermarket industry is there to "improve" stock designs. So who knows?!

The Jeep engineers must have run into this problem bc they added the harmonic balancer to the output shaft. We eliminate that and then get problems. Does that mean if you guys designed something that fulfills that function the vibration problem would be solved?

Another thing: you could have two rigs with the same gear, transmission and tire size and both shafts pass the test on the machine at 3300rpm, and one get vibes and the other not at higher driveshaft rpm’s due to one shaft being better balanced. But who could know without a machine that is spinning them faster (I.e. a Vehicle set up to do so)

 

[sfexeter]

The more I read and try to understand this stuff, the less I seem to comprehend. I have a 2002 4.0 with 5-speed manual and am about to go to 35" tires with 4.88 gearing. Its been a mental roller coaster ride thinking that I made a good decision, then a terrible decision, back to a good..... You get the point. Hopefully I don't regret my choice since I'm dropping serious $ on the re-gearing!

Hats off to all of you who can truly wrap your minds around this stuff!

 

[L J]

The more I read and try to understand this stuff, the less I seem to comprehend. I have a 2002 4.0 with 5-speed manual and am about to go to 35" tires with 4.88 gearing. Its been a mental roller coaster ride thinking that I made a good decision, then a terrible decision, back to a good..... You get the point. Hopefully I don't regret my choice since I'm dropping serious $ on the re-gearing!

Hats off to all of you who can truly wrap your minds around this stuff!

Do you drive at 70ish mph on the interstate? Are you regularly driving up hills and mountains? If yes, I'd seriously consider 5.13s. I'd only go 4.88 if I was driving 80+ mph regularly, lived in the flatlands and seldomly encountered hills and mountains.

 

[Shawn at Tom Wood's]

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.

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.

Here's the force calculator I used this time.
https://www.omnicalculator.com/physics/centripetal-force

 

[Shawn at Tom Wood's]

Shawn, I really appreciate your response. I hope my earlier post didn’t come across malicious. I didn’t intend that at all. I’ll never know near as much as you about driveshafts or any closely related areas. I love the perfection you and your family strive for and continually accomplish!

I think your driveshaft rpm calculator is going to help solve the most difficult problem on this forum: oscillating vibrations. This is what I was hoping to learn from your answer and I think the data generated by using your calculator may give insight into the underlying cause. Hubs usually solve it, but not always. There is still much more for us to learn. Thanks for sharing your expertise!

Not at all! I hope my reply didn't come across as defensive. We're all just sharing our thoughts and trying to help each other and others understand these complicated issues. I appreciate your and everyone else's input as I can otherwise be too narrowly focused on the one thing I know, drive shafts. Even then I won't pretend to know everything about drive shafts, always learning new things.