4.56 or 4.88 gears?

[freedom_in_4low]

Believe it or not I just created a drive shaft rpm calculator inspired by this post. It is a common question to consider when deciding gear ratios and determining some other things. We've always known how to do the math but I figured it would be a good thing to have as a resource for people on our website. I haven't read this whole thread so some of what I'm saying might be redundant to other posts but here's my take. You want to gear your Jeep so that your drive shaft rpm at 60 mph is around 2,500 rpm at 60 mph. A lower number like 2,300 will gear your jeep for faster driving with less low end torque. A higher number like 2,700 will gear your jeep "lower" meaning you will be geared better for low speed off-roading and low speed torque, you'll be able to crawl better but it will require higher engine rpms to cruise at highway speeds. I think 2,500 is the best all around target but you can gear higher or lower as you wish. Just remember that low speed crawling and interstate driving are on two opposite sides of a see-saw, gearing for crawling means you make some sacrifices when it comes to high speed driving. You can find the calculator embedded on the bottom of this page.

https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations

anybody using this calculator remember you need to use ACTUAL tire diameter, not the nominal size on the sidewall, to get an accurate result.

My favorite way of doing this is to use the revolutions/mile published by the manufacturer, because I'm usually doing this kind of thing at my desk and it's quicker than going out to the garage with a tape measure and trying to get an accurate measurement of the center of my hub.

Rolling Diameter = (5280*12)/(pi*revolutions per mile)

Most 35's are in the neighborhood of 602 revs/mile which comes out to an effective rolling diameter of 33.5. With 4.88's, the difference between using 35 and 33.5 is 2812 vs 2938 driveshaft rpm.

Or another way is to skip the tire diameter altogether...

Driveshaft speed = Revolutions/mile * axle ratio * (vehicle speed/60)

 

[freedom_in_4low]

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.
View attachment 311961

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

yes, 100% agree. The force increases with the square of the velocity, but linear to mass. F=m(v^2)/r

 

[mrblaine]

I like plug and play (watching install video now). I'd prefer not to cut into the factory wiring if I can avoid it. Thanks!

It is all about perspective and philosophy. I strongly dislike running a wire from the engine compartment to the speedo sensor, then another one back to the engine compartment simply to interrupt and alter a signal. I much prefer to mount the correction box behind the glove box, pick up power and ground there with 18" of wire for each and then another pair of signal wires that are 24" long that tap into the signal wires right by the PCM. We essentially hook up all four leads using less wire than one signal run from the engine compartment to the t-case.

 

[freedom_in_4low]

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.

don't take it back! It's still useful information. For a long time around here the prevailing wisdom has been that there's basically no such thing as too much gear. The truth is that there are bounds on both ends, and that having the perfect cruise RPM at 75mph shouldn't be the only consideration.

I've put together a list trying to shed some light on our common vibration issues and I can say out of 37 rigs, the 23 that had been regeared have driveshaft rpm between 3738 and 3443 at 60 mph, with the average being about 3050. Extend that out to common highway speeds in most of the country and the picture gets even worse. My local highways have speed limits as high as 80 and my 4.88 with 35s puts me close to 4000 when the vibration sets in...and with all the data I have, I see that I'm one of the lucky ones (excluding those that don't have the issue of course!). We have one example that picks up the vibration around 2700 but if we (and the engineers at Jeep Truck developing the Rubicon with 4.1 gears) had followed your advice, this vibration conversation wouldn't even be happening. Unfortunately though, as you've picked up, they stuck us with some oddballs when it comes to low overdrives and we're stuck choosing the lesser of two evils.

 

[Dante]

Not to throw a monkey wrench into the clockworks, but I have the 3spd automatic tranny and am running 33s on my '99 TJ. I need to inspect the pumpkin on my Dana 35 to see where it's geared. Any recommendations on ratios in that?

FWIW: I live near mountains, and will be going up some hills regularly...possibly with a tiny camp trailer. I don't plan on running 35s, but I may put a 4" lift on later (it's a 2.5" now). I understand I may be battling pinion angles if I go higher on the lift & may need other mods to accommodate that (SYE, adjustable control arms, etc.) I may just go with a 1" body lift instead

and, of course, I'm asking this before actually searching the forum for my specific tranny/diff/ratio (sorry)

 

[freedom_in_4low]

Not to throw a monkey wrench into the clockworks, but I have the 3spd automatic tranny and am running 33s on my '99 TJ. I need to inspect the pumpkin on my Dana 35 to see where it's geared. Any recommendations on ratios in that?

FWIW: I live near mountains, and will be going up some hills regularly...possibly with a tiny camp trailer. I don't plan on running 35s, but I may put a 4" lift on later (it's a 2.5" now). I understand I may be battling pinion angles if I go higher on the lift & may need other mods to accommodate that (SYE, adjustable control arms, etc.) I may just go with a 1" body lift instead

and, of course, I'm asking this before actually searching the forum for my specific tranny/diff/ratio (sorry)

In the context of the driveshaft speed/vibration conversation...you have what might be the best transmission available on a TJ. With 33's, you could run stock 3.73 gearing and have the same highway rpm that I do, with 600 less rpm at the driveshaft. Plus, if you don't already have 3.73s, you could buy some factory 3.73 axles and swap them in for way less money than paying somebody to regear yours.

If I had a 32RH and planned on running 33's, I would throw 3.73s in it (if it didn't have it already) and move on with my life.

Out of those 23 rigs that reported vibration, only one is a 32RH and I suspect there is something different going on.

 

[JMT]

Next question. If the oscillating vibes are due to imperfections in the balance that are apparent only over 3300 rpm’s (I’m ruling out those who get vibes at lower rpm’s as people who need to have their shaft re-balanced or adjust pinion angle), then why do they usually go away when the front shaft is removed or front hubs are added?

 

[freedom_in_4low]

Next question. If the oscillating vibes are due to imperfections in the balance that are apparent only over 3300 rpm’s (I’m ruling out those who get vibes at lower rpm’s as people who need to have their shaft re-balanced or adjust pinion angle), then why do they usually go away when the front shaft is removed or front hubs are added?

I think the additive nature of the vibration waveform is such that one shaft isn't enough to feel, but two is. The transfer case is feeling both of them.

If the blue is the rear shaft, and the green is the front shaft, the orange is the transfer case. Get rid of the orange and the green and you cut your amplitude in half.

 

[JMT]

I think the additive nature of the vibration waveform is such that one shaft isn't enough to feel, but two is. The transfer case is feeling both of them.

If the blue is the rear shaft, and the green is the front shaft, the orange is the transfer case. Get rid of the orange and the green and you cut your amplitude in half.
View attachment 312052

I can see from your wavelength chart why the vibes would be oscillating. When the two shafts wavelengths correspond you get the most amplitude. Nice chart.

Then we ought to be able to test that by removing the rear shaft and leaving just the front in, putting it in 4wd and getting up to normal vibration onset, right? If the vibration problem is solved, then it’s due to the additive effect.

Just saying test it both ways.

 

[freedom_in_4low]

Then we ought to be able to test that by removing the rear shaft and leaving just the front in, putting it in 4wd and getting up to normal vibration onset, right? If the vibration problem is solved, then it’s due to the additive effect.

Just saying test it both ways.

in theory that should work. One issue is that many, maybe most, of us are running a front pinion angle that we only get away with because the front shaft isn't powered at highway speed. I suspect most of us driving with the rear shaft removed probably get a pinion angle vibration when running the front shaft only. Mine is probably a good 2 degrees low, which would only get worse when the pinion sinks under power. I know I did.

 

[bitbyte]

It is all about perspective and philosophy. I strongly dislike running a wire from the engine compartment to the speedo sensor, then another one back to the engine compartment simply to interrupt and alter a signal. I much prefer to mount the correction box behind the glove box, pick up power and ground there with 18" of wire for each and then another pair of signal wires that are 24" long that tap into the signal wires right by the PCM. We essentially hook up all four leads using less wire than one signal run from the engine compartment to the t-case.

So then what product are you recommending?

 

[freedom_in_4low]

So then what product are you recommending?

I don't think he's recommending a product, I think he's describing his technique of locating the speedo-healer to require the least amount of work in creating his own custom harness.

Like you, I also like plug-n-play so I used the Blue Monkey setup that runs the wires back to the sensor. But if that didn't exist and I was going to DIY the harness, I would follow blaine's advice and intercept the wire near where I was going to mount the box.

 

[Shawn at Tom Wood's]

I think the additive nature of the vibration waveform is such that one shaft isn't enough to feel, but two is. The transfer case is feeling both of them.

If the blue is the rear shaft, and the green is the front shaft, the orange is the transfer case. Get rid of the orange and the green and you cut your amplitude in half.
View attachment 312052

This is so on point! I was actually talking with my dad (Tom) this morning about creating a graph like this. Where it really gets complicated is when you add in waves for the tires, engine, transmission, heck probably even the wind turbulence. And then I think you'd have to plot things out so show the the changes in wave timing and amplitude as speeds increase. Surely at certain points there is going to be synchronization/harmonics between several different sources. Everything that is moving is vibrating on some level. Change the gearing and you change the points at which things synchronize, but is that going to result in more or less vibration? Luck of the draw often the answer to that question.

@freedom_in_4low How do you know all of this and know how to do these calculations? What is your profession? In my earlier posts when I said something like "Someone smarter than me, you are welcome to figure this out" I must have been referring to you. As well as others on this forum, lots of smart people here.

 

[Dante]

In the context of the driveshaft speed/vibration conversation...you have what might be the best transmission available on a TJ. With 33's, you could run stock 3.73 gearing and have the same highway rpm that I do, with 600 less rpm at the driveshaft. Plus, if you don't already have 3.73s, you could buy some factory 3.73 axles and swap them in for way less money than paying somebody to regear yours.

If I had a 32RH and planned on running 33's, I would throw 3.73s in it (if it didn't have it already) and move on with my life.

Out of those 23 rigs that reported vibration, only one is a 32RH and I suspect there is something different going on.

Sorry to muddle the conversation but, I have a 3.73 ratio (just looked at the diff) with no vibrations, and i'm not messing with it

I was mainly just making sure I wasn't putting any extra stress on the gears back there with the bigger tires.

Thanks for the tip, much appreciated

 

[Shawn at Tom Wood's]

I'd like to take a moment to acknowledge how funny it is that the OP asked "4.56 or 4.88?" and now we are 5 pages deep discussing physics/mechanics. It makes me imagine a situation in which someone askes Neil deGrasse Tyson "What time is it?" to which he replies "To answer that question we first must ask ourselves a few questions about our understanding of time itself. . . " Poor guy OP just wanted someone to tell him "4.88". Maybe it is funny or maybe it is discouraging if you are someone who is here just looking for a straight answer and recommendation.

Side note to that, @freedom_in_4low, some of this info and much of this conversation probably should be moved over to your thread about vibrations. That is where the casual searcher is more likely to find the information they might find helpful.

 

[JMT]

Sorry to muddle the conversation but, I have a 3.73 ratio (just looked at the diff) with no vibrations, and i'm not messing with it

I was mainly just making sure I wasn't putting any extra stress on the gears back there with the bigger tires.

Thanks for the tip, much appreciated

The gears will be fine. It’s your engine that has to produce more rpm’s to turn the bigger tires.

 

[JMT]

I'd like to take a moment to acknowledge how funny it is that the OP asked "4.56 or 4.88?" and now we are 5 pages deep discussing physics/mechanics. It makes me imagine a situation in which someone askes Neil deGrasse Tyson "What time is it?" to which he replies "To answer that question we first must ask ourselves a few questions about our understanding of time itself. . . " Poor guy OP just wanted someone to tell him "4.88". Maybe it is funny or maybe it is discouraging if you are someone who is here just looking for a straight answer and recommendation.

Side note to that, @freedom_in_4low, some of this info and much of this conversation probably should be moved over to your thread about vibrations. That is where the casual searcher is more likely to find the information they might find helpful.

This derailing of a thread happens a lot, and some people are well-known for it!

 

[freedom_in_4low]

This is so on point! I was actually talking with my dad (Tom) this morning about creating a graph like this. Where it really gets complicated is when you add in waves for the tires, engine, transmission, heck probably even the wind turbulence. And then I think you'd have to plot things out so show the the changes in wave timing and amplitude as speeds increase. Surely at certain points there is going to be synchronization/harmonics between several different sources. Everything that is moving is vibrating on some level. Change the gearing and you change the points at which things synchronize, but is that going to result in more or less vibration? Luck of the draw often the answer to that question.

@freedom_in_4low How do you know all of this and know how to do these calculations? What is your profession? In my earlier posts when I said something like "Someone smarter than me, you are welcome to figure this out" I must have been referring to you. As well as others on this forum, lots of smart people here.

I'm a mechanical engineer. I work in commercial and industrial HVAC manufacturing and currently write equipment control software, but before that I did about 10 years of modeling and simulation... Using math to predict real world behavior of components and systems. The majority of that is in fluid mechanics and heat transfer but we do have a lot of rotating equipment like fans and compressors. Throw 2 dozen 30" variable speed fans into a cabinet made of sheet metal and you can set up all sorts of vibration that worst case can end up causing fatigue failures in piping and dump hundreds of pounds of refrigerant in a matter of seconds. I'm currently working on a project where I'm programming specific frequencies for a pump to skip over because vibration testing revealed a failure risk at those speeds. Considering a failure could spoil 25% of the refrigerated product in a supermarket within 6 hours is a pretty big motivator to make sure that doesn't happen.

 

[mrblaine]

So then what product are you recommending?

My preference is for any that use a potentiometer to adjust the speedometer. I despise the correction factor bullshit and refuse to use it if at all possible.

 

[bitbyte]

I don't think he's recommending a product, I think he's describing his technique of locating the speedo-healer to require the least amount of work in creating his own custom harness.

Like you, I also like plug-n-play so I used the Blue Monkey setup that runs the wires back to the sensor. But if that didn't exist and I was going to DIY the harness, I would follow blaine's advice and intercept the wire near where I was going to mount the box.

Got it.

To be honest I'm not sure why they didn't intercept the signal where the factory harness plugs into the speedo cluster instead of under the vehicle.