D50 bucks and hops when turning in 4x4, but I'm running it as a FWD, what gives?

I'm my quest to get rid of all the vibrations in my rig I have been eliminating problems one by one. Continuing the effort I took off the back half of the rear drive shaft to eliminate that from the equation and have driven the last few days as a FWD. (Which is both not that different and pretty weird all at the same time, just depends on how much skinny pedal you give her!)

Problem: my front end hops around and bucks under load when turning. But only under load, it will coast wheel lock to wheel lock without complaint.
And it acts up with little steering input, less than half of a turn it's wobbling quite a bit.
But since the rear isn't connected and it's a totally open front differential, why does this happen???

Thank you!
Joshua

Could be the axle shaft ujoints. Or wear in the hubs.

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Or both...

Check the U-joints, but also check for lateral slack in the axle stub at the knuckle end. They are supposed to be supported by a needle bearing. Get a crow bar and try prying up and down on the U-joint to see where the slack is.

Don't have one or both hubs that are not unlocking , thus still engaged and binding because of a bad U-joint.

It sounds as though the axle is in 4 wheel drive, on dry pavement. You might want to check to be absolutely sure it's not.

gandalf said:

It sounds as though the axle is in 4 wheel drive, on dry pavement.

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This is exactly what is going on. He didn't mention putting the rear driveshaft back in. It's pretty normal sounding to me. There's a reason why you shouldn't put drive power to your front axle on dry pavement. This isn't quite like a front wheel drive car. It's engineered to drive from the rear axle and only use the front when needed. I think that you should be able to tell if you have a rear driveshaft vibration or not. I think that it's time to start using the rear axle to drive the truck again.

U joints are not constant velocity joints they need to operated in pairs to cancel out the elliptical rotation. When turning the ujoint are at a angle and the wheel is wanting to speed up and slow down twice per revolution.

Even without watching the video, you explained it perfectly to me.

hce said:

U joints are not constant velocity joints they need to operated in pairs to cancel out the elliptical rotation. When turning the ujoint are at a angle and the wheel is wanting to speed up and slow down twice per revolution.

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Wow, even I didn't know that. Thanks!

Just to be clear, I think everyone here is pointing you toward your axle ujoints not the ones in the front driveshaft. That is my vote from where its coming from as well. Hell my 2017 F250 does that when I leave the front hubs locked. I don't know why they don't move to cv joints on the front axle. I ran a 3/4 ton chevy plow truck with them for years and they held up fine without all the popping and jumping in turns.

hce said:

U joints are not constant velocity joints they need to operated in pairs to cancel out the elliptical rotation. When turning the ujoint are at a angle and the wheel is wanting to speed up and slow down twice per revolution.

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Thank you for posting!
Wow I've never understood how that worked until now! I've always wondered why matching the pumpkin angle to the transmission was so important. Since I don't lift my vehiculars I've never had to worry about it though.

And that certainly sheds light on why my front end acts all funny under power. But why does it turn fine when coasting? Maybe the test would be to coast in gear while turning rather than out of gear the way I tested it.

I've always thought the front end hopped and bucked on pavement in 4x4 because it was binding with the rear and the energy had to go somewhere. But really it's just the nature of u-joints on a steer axle. Crazy.

I'm kind of shocked that this is the standard design for the better of like 70 years. But hey when you can do something cheap and dirty AND strong as all get-out, why change it?!?

Edit: my outer ujoints are new within a couple years and get greased, the inner was fine when I changed the outers. I jacked up both wheels individually and the bearings are good, put her in 2nd gear and there is no wobble or issue with the driveline or noise at all. So I think the start and end of the issue is just the nature of ujoints on a steer axle.

Problem solved! I love it when the answer is physics and not chasing down old dying parts.

Try your king pins IV seen them have a bunch of slop and act this way bit if a getting man front hubs and ujoints

Oledirtypearl86 said:

Try your king pins IV seen them have a bunch of slop and act this way bit if a getting man front hubs and ujoints

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D50 has ball joints but those are tight also. I replaced them 4-5 years ago now but I checked them with the wheels up in the air.

The_Josh_Bear said:

I've always thought the front end hopped and bucked on pavement in 4x4 because it was binding with the rear and the energy had to go somewhere. But really it's just the nature of u-joints on a steer axle. Crazy.

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It also does the same thing if you leave the transfer case in 2WD but lock the hubs.

The_Josh_Bear said:

And that certainly sheds light on why my front end acts all funny under power. But why does it turn fine when coasting? Maybe the test would be to coast in gear while turning rather than out of gear the way I tested it

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When not in gear the output on the transmission is not being forced to run at a constant rpm from the engine. The transmission output can know speed up and slow down as it is driven by the front end, although this could still produce some vibration and weird sounds from the driveline banging back and forth as slack is taken out one way then the other.