Hello everybody, I've got a lot of help off this forum so far and I thought I would pick your brains once again. I have a very weird issue nobody can seem to figure out. This may be long but thank you in advance for taking time to help.
The front of my truck about 35 mph roughly wants to bounce up and down. Only if I'm cruising at 35 mph does it do it. I can accelerate "through" 35 mph and it has no issues. It's not a side to side shake at all. It's up and down. I will put a list of everything I have tried to address.
1. Replaced tie rods and drag link
2. Replaced steering shaft with Borgeson unit
3. Has new trail grapplers I put on last year that have been rebalanced just in case.
4. Tried wheel centering rings but that was a waste
5. King pins seem tight as can be
6. Hubs function properly and still bounces whether engaged or disengaged
Here is what I am thinking the issues can be but I need help narrowing it down or atleast suggestions on how to check.
1. Leaf springs are starting to arch backwards and I am not sure on condition of bushings on the springs themselves or the frame (not sure how to properly check)
2. King pins are bad even though there is no movement at all
3. I've heard the springs on the upper kingpins can go bad causing this issue
4. Something funny with pinion angle I am not seeing

Keep in mind this is a prominent up and down movement. Not a little bounce. It will jar the entire truck up and down like running over speed bumps almost. Quite annoying trying to cruise in 4th with this issue. I can even push the clutch in and the bounce still is there. Also, it seems as though some days it is "better" as in it is not as bad as it usually is. Not sure why there either. Any suggestions fellas? Any help appreciated !

 

Have your drive shafts checked. Both my 91 and 92 had this annoying "basket ball bounce" at a specific speed. Not a shimmy or a shack, but a bounce. The rear yoke spread on my 91 shifting the drive shaft of center. My 92 was out of balance.
When I get this shake that feels like the truck is twisting and it only happens in a gentle turn, my rear brakes were out of adjustment.
Hope this helps.

 

With an up/down bounce I see no reference to installing new shocks? Might no fix the problem...but might mitigate the severity of the problem a bit.

Could be the tires. Maybe the bands in the tires are coming loose and causing the tire(s) to balloon at a specific point. With that ballooning in the tire(s) it would cause the front end to start bouncing. Do you know someone you could swap front tires with for a few hours?

 

I really don't think that there is a real good way to tell if the king pins are bad, except replacing them. Also, I believe that a locked up, bad front axle u-joint could cause that problem. It sounds like it could be a variation of the infamous death wobble, good luck...Mark

 

ties out of round.... jack up one wheel, spin wheel, use a stationary block near the tread of tire and watch the gap between tread and block... if its not perfect there's your front end hop. wheels balanced good? lug nuts tight?

 

Could also be spring wrap bouncing against your governor. Try shifting into neutral and let the engine idle when it happens.

 

Very interesting phenomenon

I would initially think out of round tires. Balanced tires can still be out of round. do as stated above and confirm you don't have a shifted belt in the tire.

Secondly, you can jack up the truck where the suspension is hanging. You will see whether or not the spring eye bushings are bad as you put pressure on the tires while slowly lowering the truck down. to do this, you either have to jack it up at the bumper, or the frame behind the front spring rear mount. It's easiest to do it on the bumper if you have a tall enough jack.

IMO, you're going to have to replace the front springs if they're reverse arched. The truck will ride so much better after doing so.

 

With the weight of the cummins

These wear out fast https://www.dieseltruckresource.com/...d.php?t=302761

Not a ton of money

 

I agree with checking your driveshafts. Especially the rear one

I also agree with there is no physical way to check king pins unless they are completely shot. The spring takes up all the force a person can use to check them

 

Ok great suggestions guys but everything mentioned I've already tried. I swapped complete set of tires to another truck my buddy had and his didn't bounce but mine still did with his tires. I've rotated countless times. Rear driveshaft has already been balanced. Shocks are new bilsteins 5100s. I have also shifted into neutral and let engine idle and same thing there until you get below 35 ish mph. I'm completely lost. I've tried everything except tearing into the front end and I don't wanna do that unless I've narrowed something down. The front end is tight as can be. I'm kindve leaning towards springs but I don't wanna throw parts at it when I have no idea. I'll try checking those bushings tomorrow but I'd say my jack isn't high enough. Well crap. anymore ideas? Lol

 

And bobva that link doesn't seem to work?

 

Just thinking out loud as well but if something was out of round wouldn't it bounce at all speed and get worse the faster you drive? They may be wrong but in my head (that's often wrong) it makes sense lol. Couple more things I forgot to mention. I replaced both transmission mounts as well as the transfer case mounts... And thanks once again guys !

 

Fixed

 

From what I gather, you haven't said that you checked the front axle u-joints and there really is no way to check the king pins without taking them apart. My front u-joints seemed good on the truck, but were bad when pulled out, same with the king pins...Mark

 

Thank you bobva. For that kind of money I'll try that for the heck of it. Well my thinking behind the axle u joints is that if I lock the hubs the bounce is still there same as unlocked. They only spin when locked correct? I have not had my front driveshaft checked but it wouldn't be spinning unless in 4wd. Not trying to say you are wrong at all just trying to talk this out in my head cause I'm stumped lol