4.0, 42RLE, NP241OR High Speed Vibrations After Re-Gear to 4.88 or Deeper
- Thread starter Mr. Bills
- Start date
If you turn your tire twice then your driveline turns almost 5 times if you have 4:88's
If you turn your tire twice then your driveline turns over 5 times if you have 5:18s 5:38's
If you have 307s it only turns a tiny over 3 see the pattern?
so say if you were 307 and you change to 4:88's your driveling is almost going twice as fast. Now consider that on the freeway.
Have you determined it is the front drive shaft causing the vibes? I'm actually going to be doing this. I talked to a tech from Tom Woods recently and I will be sending my Adams drive shaft to them to diagnose. I can remove the front drive shaft and the vibes disappear.
Dave is going to look into it for me, so we'll see what he says. But I know he's been trouble shooting @psrivats vibration issue after his re-gear to 4.88 for weeks now, and he isolated it to the front driveshaft, but is unable to fix it supposedly (maybe @psrivats will chime in with what he said exactly), so it seems in most of these instances it's related to the front driveline, which seems odd, because you'd think it would be the rear of anything.
No, but Dave is going to do it for me and see where it's coming from. He thinks it's coming from the front and from what I felt, it feels like that as well. With @psrivats, his is doing it from the front as well, which Dave confirmed.
Don't you find it odd that it's happening to the front driveshaft on most of these TJs and not the rear?
I think about it this way, since the front drive shaft is longer, it is more sensitive to changes. Kind of acts like a tuning fork. More tube length maybe? Either way it sucks that some of us are having these issues with no resolution in sight except to slow down the front drive shaft. I've been fighting it for over 2 years now.
Xposure
TJ Enthusiast
How is the drive shaft rotating faster? It’s fixed to engine-trans rpm and none of that changed. Isn’t it only the gears and axles that are spinning faster to turn the tires the same mph? I would think it’s either the gears themselves or the axles that are causing vibration. You engine rpm has probably changed some for a specific mph but that is minimal so I’d say the driveshaft is probably fine if it was fine before, your drivetrain from engine to driveshaft is still spinning just like stock. Or maybe I’m just misunderstanding what y’all are saying.
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Very curious for what Dave finds, it makes me nervous for sure on getting the deeper gears that are needed for the 42RLE
Weirdly I would have thought the opposite, not enough to argue about it. but I am really really hoping you are right since everything under my Jeep is brand new except my front driveline. Which seems to be in good shape, but if you guys get new front drivelines and if fixes your issues. I will definitely get Tom to build me a new one. I mean I probably will anyway, just later than sooner. Too many other fish to fry and mine isn't that bad. Also in my case I had to add the Ford flange so the driveline would bolt to the 8.8 so I thought that might be some of my issue too.
The driveshaft is bolted to the pinion gear it now has to turn more times to the one time it gets around the ring gear if that makes better sense.
Your driveline is bolted to the pinion and the tire is bolted to the ring, now your pinion has to turn more to get the tire to a full revolution.
I had 4.88 Revolution gears put in my old black Rubicon and it didn't have a single vibration even up to 90 mph on the highway. I drove that thing on so many long highway drives at 70+ and not a single vibration.
So it's not happening to all of them, but it it happening to quite a few.
Good way of explaining it!
Xposure
TJ Enthusiast
I think y’all may be thinking about it backwards. Everything is direct connected from engine through trans and driveshaft....it’s the differential that is now spinner faster given the same driveshaft speed. Everything up to your gears is the same as before/stock...the gears and then axle and wheels is where the new rotation speed comes into play. The driveshaft speed is based on the engine rpm and transmission gearing, then the differential multiplies that driveshaft speed.
I'll throw you a curve ball then, When I re-geared, I did the rear first. Stock drive shafts, no other changes except the rear re-gear and a new rear axle bearing because the seal was leaking a little. After doing the front a few weeks later, that is when the vibes started. removed the front drive shaft, vibes went away. I convinced myself that the setup in the front went south, so I had another shop re-redo the gears with a different brand. This netted no change. That is where I dove deep down the rabbit hole, replacing drive shafts, SYE, TC rebuild, adjustable control arms, etc, etc. After all this, the vibes are better, but still not acceptable. Now get this, At 60mph, the harmonic vibes almost go away, and are hardly noticeable. Explain that one! To add, the vibes do not change whether I'm under load, deceleration, coasting, does not matter, so it is a rotational issue.
Nope. The driveshaft rotates at gear ratio * tire rpm. At a given speed, tire rpm is constant for a given tire size. Lets say a 35" tire rotates at X rpm at 70 mph. Then with 4.10's, the driveshaft rotates at 4.10x. With 5.38's, the driveshaft rotates at 5.38x.
5.38x > 4.10x so the driveshaft is rotating faster. If you don't believe me, I'll just have to tell you to take my word for it.
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Xposure
TJ Enthusiast
Think of it all in order...say your driving on the interstate in a 1:1 transmission gear.....your going say 2700rpms and the trans input shaft is turning based on the engine rpm (it’s a fixed connection)...then the trans multiplies it or in this 1:1 example it’s the same, and it’s connected directly to the driveshaft....the driveshaft can’t change it’s directly connected to the trans....then the driveshaft feeds the differential and the differential multiplies it with a new higher speed of say 4.88 vs old 3.07 and then also sends the higher rotation speed to the axles/wheels
If the driveshaft was always fine before it should still be fine....only the gears and downstream of the gears changed their rotation speed. I’d look at the gears, the bearings, the axles, axle bearings, wheel bearings.....Id really consider all the bearings since they are designed for stock rotating speed and are now turning much faster than stock.
If the driveshaft was always fine before it should still be fine....only the gears and downstream of the gears changed their rotation speed. I’d look at the gears, the bearings, the axles, axle bearings, wheel bearings.....Id really consider all the bearings since they are designed for stock rotating speed and are now turning much faster than stock.
This discussion about driveshaft RPM also answers one of the question that was originally posed in this thread. Why do vibrations seem to be more prevalent with the 42RLE when compared with other transmissions? Well, the 42RLE needs deeper differential gears to make up for the .69 OD ratio it has. For example, a 5 speed with 35's should have 4.88's, while a 42RLE with 35's should have 5.38's. This means that the driveshaft is spinning about 10% faster if you have a 42RLE (5.38 / 4.88 == 1.102).
So, if you are going 70 mph with your 5.38's (42RLE), the driveshaft is spinning at the same rate as if you were going 77 mph with 4.88's (5 speed). Because the driveshaft is spinning faster, vibrations will be more prevalent with a 42RLE. This increase in driveshaft speed only has to do with the differential gear selection, not the transmission.
So, if you are going 70 mph with your 5.38's (42RLE), the driveshaft is spinning at the same rate as if you were going 77 mph with 4.88's (5 speed). Because the driveshaft is spinning faster, vibrations will be more prevalent with a 42RLE. This increase in driveshaft speed only has to do with the differential gear selection, not the transmission.
The transmission input speed would not be the same with different rear gears at the same speed. A 4.88 gear provides LESS wheel speed than a 3.07 gear, not more. When you regear, your engine speed increases for a given vehicle speed.
Great point! Of course how does the NP241OR factor into this? I only mention that since it was included in the title.
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