4.56 or 4.88 gears?

[mrblaine]

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.

A minor but vastly important clarification, is everything upstream of the ring gear will have less of a mechanical advantage. The reason we can run the lower gear ratios with very low failure rates is due to a concept we all inherently understand but fail to apply.

The reason that manual transmissions rarely blow up first gear when taking off from a stop is the mechanical advantage is in favor of the gear set in the transmission. As you pointed out, the lesser mechanical advantage is in the higher gears which means that if we made a habit of trying to take off from a stop in the higher gears, something is very likely going to break. The exact same principle applies to the lower axle gears, they have a better mechanical advantage to move the same load.

 

[mrblaine]

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.

The issue is maybe easier to understand if you come at it from the other direction. A 5% increase in torque delivery with little penalty at only 5% higher RPM is not a bad thing at all. In order for the 5% greater chance of causing vibes to be accurate, every re-geared TJ would have to suffer from them and that simply isn't the case. Statistically, the number of rigs who wind up with vibes is relatively low and what we wind up seeing is a very vocal minority due to just how painful they are to chase down and fix.

The cause of the vibes which manifest at higher RPM is not quite as simple just the driveshafts spinning faster. We have chassis stiffness and second and third order harmonics that come into it which are incredibly difficult to diagnose and repair.

A good example of that is I have a 2004 TJ Unlimited. It has the cyclical harmonic vibe at 75. Minor but it is there. It has been there since it was bone stock with the stock tires and stock 3.73 gearing. I regeared it to 4.88 when I put the JK take-off 32's on it and it still has the same harmonic vibe that starts to be noticed at 75.

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
View attachment 311902

Some of the smarter folks at the factory made provisions to hang a very large damper off the back of the 241 transfer case in case it was needed on some of the JK's. There are pics of it if you dig around online. This is one of them.

If you compare that to other 241 Jeep cases you will note that the extended bolt bosses are cast into the back case half. That isn't a happenstance provision. They are very aware they have a harmonic issue or at the very least we can make a highly educated guess that there is a reason a damper is hung there and it would be very unusual for it to be other than to counteract some sort of vibration issue.

We also need to understand that while a lot of the vibration issues are resolved with a hub kit, we aren't actually solving the vibes with a hub kit, we are returning the rig back to civilized handling and steering. I can almost always get the vibes to acceptable levels by adjusting out front caster and raising the driveshaft to improve pinion angle. The end result is typically crap steering at speed with a vague on center feel, poor return to center and more wandering requiring constant correction than I'm willing to tolerate.

The hub kit is a shotgun blast to "fix" the problem when the reality is we could cut and turn the C's, get some caster back, stop the vague on center feel and wandering. It is unfortunate that we don't have an easy way to cut and turn the C's but very fortunate we can get back what we like and it only cost money and some time to install a hub kit.

The better part of that is not a single time has a hub kit not solved the problem for us.

You are correct in that there is no pat answer to predict what will happen and worse, no pat answer on how to solve it. What we do know is not everyone is going to get vibes if they regear and not everyone won't get vibes if they regear.

 

[mrblaine]

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.

Yes, no, maybe. I have a stocker that has them, now what? And it is one with the longer rear driveshaft which runs at less operating angle and is not a double cardan shaft.

 

[mrblaine]

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?

Because it isn't purely a balance and shaft speed issue. There are many more factors in play that cause the harmonic than just shaft speed. If it was, folks like Shawn would eventually build a product with much higher shaft speeds even if we had to put up with larger diameter drivelines out of different materials so we could achieve higher critical shaft speeds.

 

[mrblaine]

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.

The chart is leaving out the 1st, 2nd, and 3rd order harmonic relative to the engine firing sequence and what happens when those waves get in time with that sequence. They also don't take into account chassis stiffness, which t-case skid you are running and how the resonant frequencies of those come into play.

 

[mrblaine]

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.

We test the front only after we have run the rear and can't dial out shaft vibes due to imbalance as a way to check for imbalance. We always start with the rear installed first, dial out the rear vibes and then install and run the front shaft. It doesn't take long after you starting dialing out caster to know if you have a problem child on your hands that is not going to behave. Then you have a choice, you want vibes and good steering or lessened vibes and crap steering?

We generally accept that the rear pinion is forgiving on the low side and the front is forgiving on the high side. I have worked on one TJ Unlimited on 37's geared to 5.38's that had the front pinion about 6 degrees low and the rear was about 6 degrees high. In my world, that thing should vibe like crazy but it was butter smooth all the way up to 80 mph. All I know is anytime I think I know something, I run across something like that which shows me I know very little.

 

[mrblaine]

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.

That answer may be as easy as which connectors they could get their hands on and how easy it is for the average person to unplug and connect two connectors at the t-case versus pulling the dash apart.

 

[mrblaine]

The pcm uses the speed signal for other things, and then communicates it to the cluster digitally, so there's not really a good way to intercept and change it there. It's gotta be between the sensor and the pcm.

They are both digital signals. If it can be corrected before the PCM, it can be corrected after the PCM.

 

[mrblaine]

Fair enough. Then at the PCM instead of at the transfer case. Same principle.

PCM connector plugs are large and expensive as well as being attached to a very bulky harness. Much easier to do it elsewhere.

 

[mrblaine]

Exactly. That's what both of the mentioned solutions do. Sounds like Blaine prefers to turn a knob until the speedometer reads correctly, the speedo healer does some math in between to adjust it by a percentage.

I don't know what the signal format is; I always figured it was some sort of frequency or pulse width modulation but if a simple potentiometer works then it would have to be a scaled voltage. Unless the potentiometer solutions Blaine mentions are not literally just a potentiometer but an adjustment knob that shifts the modulation.

No knob. They have all been a small slotted plastic post recessed into the outer case that you turn with a small flat blade screwdriver.

 

[machoheadgames]

No knob. They have all been a small slotted plastic post recessed into the outer case that you turn with a small flat blade screwdriver.

Which ones have you used that have the screw? I am personally not opposed to splicing wires whatsoever. I recommended speedohealer for the other guy (while mentioning the option of splicing in a screw style) because it seemed more up his alley. I still have the speedo gear but wouldn’t mind knowing a decent screw style calibrator to keep in mind if I change my setup enough to need one.

 

[mrblaine]

Which ones have you used that have the screw? I am personally not opposed to splicing wires whatsoever. I recommended speedohealer for the other guy (while mentioning the option of splicing in a screw style) because it seemed more up his alley. I still have the speedo gear but wouldn’t mind knowing a decent screw style calibrator to keep in mind if I change my setup enough to need one.

The only one left in "production" that I am aware of is the Jet Accuspeed. They aren't on top of the shortage yet and sadly, are a bit confused. They have it listed for the auto only (IIRC) even after I called their engineering dept. and explained that the t-case output shaft neither knows nor cares what transmission it is connected to.

 

[machoheadgames]

The only one left in "production" that I am aware of is the Jet Accuspeed. They aren't on top of the shortage yet and sadly, are a bit confused. They have it listed for the auto only (IIRC) even after I called their engineering dept. and explained that the t-case output shaft neither knows nor cares what transmission it is connected to.

Gotcha. Is that 4 wires? Power, ground, speedo in and out?

 

[mrblaine]

Gotcha. Is that 4 wires? Power, ground, speedo in and out?

Yes, red, black, purple, green.

 

[Pacoson71]

Fun story about my regear to 5.38.

Break -in time was to be done on my 2 hr ride home from the shop.
If I remember, you need to stay below 45 MPH and let it cool after the first hr for a heat cycle.

I was pulling over every so often to let cars pass me, took more like 3+ hrs to get home!!!

45 MPH on the speedo was more like 28MPH before I put in the healer.

 

[Pacoson71]

It really did seem that I was going slow but at the time I didn't figure it out.

I laugh now.

 

[Jerry Bransford]

Fun story about my regear to 5.38.

Break -in time was to be done on my 2 hr ride home from the shop.
If I remember, you need to stay below 45 MPH and let it cool after the first hr for a heat cycle.

I was pulling over every so often to let cars pass me, took more like 3+ hrs to get home!!!

Same here, I took all back roads to avoid the freeway for 50 miles and it was a 3-4 hour trip.

 

[freedom_in_4low]

Ha. I didn't want to mess with that so I pulled my axle and took it to 50 miles to the shop in the back of my mini cooper.

On my next regear I used a closer shop and had 45-60mph speed limits the whole way home, so I stuck to 45 even though the guu told me he'd already heat cycled it.

 

[Pacoson71]

Ha. I didn't want to mess with that so I pulled my axle and took it to 50 miles to the shop in the back of my mini cooper.

View attachment 312278

On my next regear I used a closer shop and had 45-60mph speed limits the whole way home, so I stuck to 45 even though the guu told me he'd already heat cycled it.

Where's the other axle?

 

[freedom_in_4low]

Where's the other axle?

The other axle was a hp30 that I had geared before installing it, so I had taken it to the shop in the back of the TJ before. I swapped the front axle while the rear was getting regeared.