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Rumble and Shake in 4WD?


QuasarDecimari
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Awesome. Seriously thanks for all your help, this thread blew up to be bigger than I thought, but I’m hopeful that some unfortunate soul that gets the same issue as me can use all this to find the problem too.

 

 

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Aaaaand continuing on my list of ways to troubleshoot this BS:

I’m waiting to get ahold of that dial indicator, but in the meantime, I made YET another discovery (which will likely disappoint like the rest of mine).

I decided to push around on the driveshaft, checking for play, and while it was nice and solid, I went down to the differential pinion flange. All was good, no play, solid. Move up to the transfer case output flange, and I could wiggle it around a little. Maybe about 1mm or so of movement side to side, or up and down.

I can get ahold of the output shaft bearing for about $9, and it’s no difficult thing to do at this point, but what do you guys think?

Anyone want to shake theirs around a little and convince me it’s normal?

 

 

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Dont have my copy of the FSM on my phone but what does it say in regards to acceptable output flange play? should have some inspection guidelines

Edited by TowndawgR50
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Dont have my copy of the FSM on my phone but what does it say in regards to acceptable output flange play? should have some inspection guidelines

I’m assuming the “Mainshaft Front Bearing” specs is what I should look at.

7561d3229c6e345cae60aa91a76d9ee1.jpg

 

I’m just curious if I need to measure that with a feeler gauge with the flange off?

 

 

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Have you tried rotating your tires just to rule them out?

 

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They WERE rotated and balanced before winter, but now that you say that, I haven’t thought about the fact that there was no use for 4Hi on the roads since most everything was slow 4WD on the trails. I might go ahead and rotate them real quick.

 

 

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I’m assuming the “Mainshaft Front Bearing” specs is what I should look at.

 

Nope, mainshaft is the shaft between transmission and rear driveshaft. There's a separate "front drive shaft" with front and rear bearings. I'm not sure you're going to be able to get accurate play measurements unless it's off the truck and fully disassembled, unfortunately.

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Nope, mainshaft is the shaft between transmission and rear driveshaft. There's a separate "front drive shaft" with front and rear bearings. I'm not sure you're going to be able to get accurate play measurements unless it's off the truck and fully disassembled, unfortunately.

Ah, fantastic. I might raise the rear axle and unlock my front hubs, put everything in 4LO and just watch the front driveshaft spin, see if there’s any wobble at that point.

I’m thinking it couldn’t HURT to do that bearing, given how simple the removal and installation is, and how cheap the bearing is for me.

 

 

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Although that method might sound like a good idea, there are 2 issues I'd like to point out that may not help in your quest. 1) your front suspension will be completely unloaded (at full droop in fact), as apposed to having the normal "road force" load, therefore it may not replicate the problem as well, or at all. 2) even at idle speeds this could prove to be EXTREMELY dangerous, especially if it does start to replicate any heavy vibration or wobble. I've personally seen the aftermath of several failed attempts, similar to this scenario, once at a home garage (where the unfortunate guy got pinned under his running car while trying to diagnose a "strange noise", and 3 incidents at 3 different repair shops (2x's cars fell off a lift during "drivetrain diagnostic" and once the tech was using a floor jack, the car torqued when he tapped the gas pedal, came off the jack and crashed into the building). Luckily, in these particular instances, there were no life threatening injuries, but it does happen. There's a reason dyno shops use a 4 point tiedown system. Just food for thought.

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I wonder if it’s vibrating hard enough to catch it with the GoPro again..

 

Here’s the movement on mine. I’d definitely say it’s at least 1mm or maybe slightly more overall from side to side.

 

https://youtu.be/E2V4E1UP3t4

 

Edit: The angle makes it look like it’s not lateral movement but it is.

 

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Edited by onespiritbrain
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I wonder if it’s vibrating hard enough to catch it with the GoPro again..

 

Here’s the movement on mine. I’d definitely say it’s at least 1mm or maybe slightly more overall from side to side.

 

Huh, but all is well with your 4WD? No vibrations seeming to be influenced by that? That looks like about how much mine moves.

I let my buddy borrow my GoPro but I got it back today, so I’ll get it under there and try some different angles.

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I may have a new one for you guys.

I was looking at rotating my tires, and as I was checking out the treads, I remembered something. I bought my tires used, and I suppose you get what you pay for sometimes, but here's the thing. I looked up the original tread depth (mm) for my tires, which brand new tires are 18.5mm deep. I grabbed my digital caliper, and took some measurements. My front tires are sitting at 9.2mm deep (50% worn), but my rear tires are actually at 6.2mm deep (66% worn)...

That makes the rear tires about 33% shorter than my front tires.

If my tires were brand new, they'd be around 803.6mm in diameter. My front tires, therefor, are approximately 794.3mm. Converting that to circumference in inches, that's 101.85" traveled per revolution on the front tires.

Rear tires: 791.3mm, so 101.47" traveled per revolution.

That's a difference of 0.38" of travel from the front to rear. Of course, to figure out the revolution differences this would make in the differential gearing all the way through the rest of the drivetrain and transfercase, there'd need to be a lot more math done...

 

I guess the major question here is, with the difference between the two tires being within a third of each other, could this be causing binding along the drivetrain? I don't hear any grinding, the transfer case doesn't make any noticeable sounds, and the diff fluid shows no signs of shavings since my last fluid change...

 

Regardless, I have absolutely no idea why I let the tire shop get away with putting the much-more worn tires on the rear... The rear tires are going to wear much faster than the front.

Edited by QuasarDecimari
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A locked center diff and different sized tires will definitely cause a problem.

 

I don’t know when it becomes an issue though and if you’d see vibration from a variance in tread wear.

 

 

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A locked center diff and different sized tires will definitely cause a problem.

 

I don’t know when it becomes an issue though and if you’d see vibration from a variance in tread wear.

 

 

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What do you mean specifically by "locked center diff"? I have the original differentials for the pathy, LSD 4.363 gearing.

Differentials are made to withstand small variances in tire wear, of course, but it's unclear how far those variances can be.

I'll be throwing the rear tires on the front today, and if I have time, I'll crack down on the math behind the ratios for the gearing.

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What do you mean specifically by "locked center diff"? I have the original differentials for the pathy, LSD 4.363 gearing.

Differentials are made to withstand small variances in tire wear, of course, but it's unclear how far those variances can be.

I'll be throwing the rear tires on the front today, and if I have time, I'll crack down on the math behind the ratios for the gearing.

 

Locked center diff is usually more commonly heard when talking AWD vehicles that can lock driveshaft output to 50/50, in addition to variable front/rear output depending on traction conditions. Here, he just means when the transfer case is engaged in 4H/4L....output is locked to both driveshafts 1:1.

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I’m sorry, by center diff I was talking about the transfer case.

 

See if the vibration is there with the 6.2mm tires on front and back.

 

.........front.........

9.2—open—6.2

............|............

............|............

.....[TC lock].....

............|............

............|............

6.2—open—9.2

.........rear..........

 

That should work without issue.

 

 

 

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Edited by onespiritbrain
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Im sorry, by center diff I was talking about the transfer case.

 

See if the vibration is there with the 6.2mm tires on front and back.

 

.........front.........

9.2open6.2

............|............

............|............

.....[TC lock].....

............|............

............|............

6.2open9.2

.........rear..........

 

That should work without issue.

You know, since I'm going through college for mechanical engineering, I really should be trying to solve that sort of issue mathematically myself haha.

But hey, doesn't hurt to skip reinventing the wheel (no pun intended).

That makes perfect sense, though, since the spider gears will put up with the slight difference in revolutions between the two sides of the vehicle, but in the end, ratios should balance out as 1:1 for the front and back. Redoing my math from earlier (wrong mm conversion), if the front tires are traveling 101.89" per revolution, but the rears are traveling 101.52" per revolution, that puts the ratio at 0.9964:1 rear to front revolutions. For the ring gears, at 233mm (doing 28.82" per revolution), the ratio is the same, meaning the rear ring gear rotates at 28.72" per revolution, while the fronts are rotating 28.82". There's a 10th of an inch rotation difference between the front ring and the rear ring, meaning the driveshafts are also rotating at SLIGHTLY different speeds, and so on with the rest of the transfer case.

Alright guys, time to bust out the jack stands, impact, and torque wrench... the moment of truth awaits!

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I doubt it would make a difference. People run tires of the same size but different brands without issue. The PO had my wd21 like that. The size isn't consistent across manufacturers, for example even though they call something a 31 it may be 30.5 and another manufacture would be 31.5

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I doubt it would make a difference. People run tires of the same size but different brands without issue.

Aaaaaand back to square 1.

I DID swap the tires to even things out, aired them up and hit the road before class. On my way home tonight, I’ll kick it in 4Hi and see what happens.

 

 

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My dearest friends,

I would like to proudly announce to you that this incredibly annoying issue has finally come to a close. :happy:

In combination with the very worn differential bushings, the tire size differences in wear made all the difference in the end. Those bushings definitely needed replaced, and drastically reduced the front end vibrations, but the main cause of the issue actually boiled down to the tires themselves. Since I got these tires before winter, I never needed to use 4WD on paved roads, and when I used it, it was at low speeds on the trails. I never would have guessed that the issue actually resided in the ratios in which all of the drivetrain must maintain.

Without a nearly 1:1 front to rear rotation ratio for the tires and driveshafts, the transfer case struggles to output the same speeds with the tires under load and rotating different distances. This, in turn, likely caused the front differential to behave strangely, with the driveshaft and axle shafts not exactly cooperating at the same speeds, causing harmonics to shove the differential back and forth on the bushing mounts, wearing them faster and causing the terrifying vibrations.

With the tires properly rotated, as to maintain a proper ratio as close as possible between the front and rear differentials, the issue vanished. The spider gears in the differentials put up with the fractional differences in rotations between both sides of the front and rear, allowing the power to be distributed properly and everything to rotate evenly. This, in turn, also fixed the transfer case shifting issue. With the front tires matching the rear tires in speed, the transmission-like setup of the transfer case can sync up the speeds and properly engage and disengage on the fly, without damaging anything.

HUGE thanks to everyone that cracked down on this issue with me. As stupid simple as the conclusion was for this, I think it teaches me a lot about how important tire wear is, and possibly teaches us all how far the limits go for our vehicles and their gear tolerances. I'm just thankful that I limped it around in 2WD all winter instead of gritting my teeth and hauling it around while it was shaking, as something was likely to break.

I hope this thread sticks around, this should be very helpful for anyone out there with the same issues!

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Holy cow! This reads like one of those urban legends people throw around when trying to diagnose some unknown issue, but are running out of ideas. But in this case it makes the difference. Nice!

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