67 Rear axle bounce

[oldman3]

Ok guys I'm going to try this again. This time I'm asking the question in the correct forum.

I posted a question in the 67-72 message board in May 2011, about this problem. That thread number is 463544 if anyone wants to read it. I got back several answers and tried a couple things, but still have the same problem. So her goes.

I redid a friends 67 pickup a couple years ago. We installed a drop kit from CPP. Kit #6370BDK 3" front/5" rear. We did a 5" coil spring drop #RCS603-5 in the rear with relocators for shocks, new shocks, c noth, etc. The pickup is a 67 longbed stepside. It was orginally a 1/2 ton heavy duty long bed. We kept the longbed and added a gas tank at the rear of the frame. What is happening now is when you run down the road at about 70 mph, the rear axle bounces like a rubber ball. It's not real bad, but you can see it if you watch from another vehicle. Since that original post I've removed the 5" coil spring (thinking it couldn't handle the weight) and installed 4" drop heavy duty coil springs #RCS603-HD from CPP. Changed the angle of the shocks, changed tires front to rear (same size) and it has made NO difference.

I've got about 25 pics of the old setup and changes made. I've got pinion angle, trans angle, two piece drive shaft angle, trac bar angle, shock angles, etc. When I first posted this problem I was not aware of the Suspension forum, I now check and read it daily. I have a couple items I think maybe the problem, but I want to check here and see what others think.

I'll stop running off at the mouth and post some pics with info....Jim

[oldman3]

Pic 1 is ride height with 5" drop coil.
Pic 2 is original bracket from CPP for bottom of shock
Pic 3 is new bracket I made to move bottom of shock forward for better angle.

[oldman3]

Pic 1 old shock angle with 5" coil spring
Pic 2 an other view

[oldman3]

Pic 1 view of new shock angle looking up.
Pic 2 another view
Pic 3 new shock angle ( front to rear of truck) 75 degrees
Pic 4 new shock angle ( left to right side of truck) 80 degrees
Pic 5 the new 4" HD col spring (coils are larger and 1 more that the 5" coil)

[oldman3]

Pic 1 View showing whole setup of rear suspension
Pic 2 from axle to carrier bearing
Pic 3 from carrier bearing to trans
Pic 4 Hard to see this, but shaft from axle to bearing is at 0 degrees (level) and shaft from bearing to trans is at 5 degrees up. Two piece drive shat is in phase.
Pic 5 Trac bar is on a 3 degree up from frame to axle mount.

[oldman3]

Pinion angle is 3 degree up and the trans angle is 5 degree down.
Drive shaft is 66" overall, with 30" from trans to center and 36" from center to rear axle.

Open for questions and hopefully some answers....Jim

[BMERDOC]

My two guesses would be tire related. The first one would be a busted belt in one of the tires but you've already rotated the tires with no change. The second would be that the wheels aren't hub centric and the wheels are not centered causing an oval rotation instead of circular rotation causing the bounce. Those things have been my top two experiences with a situation like this. Good luck and if I think of anything else Ill chime in again.

[oldman3]

Originally Posted by BMERDOC

My two guesses would be tire related. The first one would be a busted belt in one of the tires but you've already rotated the tires with no change. The second would be that the wheels aren't hub centric and the wheels are not centered causing an oval rotation instead of circular rotation causing the bounce. Those things have been my top two experiences with a situation like this. Good luck and if I think of anything else Ill chime in again.

Thanks Bmerdoc, tires were one of my first guesses, but it seem like they are OK. I even jacked it up, put a stationary tool near the side wall and rotated the tire, doesn't show anything,but it wasn't doing 70 mph either....Jim

[lolife99]

My first guess would be rear shocks or tires.
Can you install some "known" tires that ride good?

I had some Toxic Shocks on a truck years ago and they sucked.
I switched shocks and I couldn't believe how much better it drove.

I really like the lower shock mount brackets you built!

[LONGHAIR]

It pretty much has to be a tire or at least a balance situation. Have you tried spin balancing those tires?
Or maybe even truing? It could be that the tires are not "round", even though they balance ok.
Truing is kind of a specialty thing though, it may take some effort to find someplace that can do it.

[oldman3]

Originally Posted by lolife99

My first guess would be rear shocks or tires.
Can you install some "known" tires that ride good?

I had some Toxic Shocks on a truck years ago and they sucked.
I switched shocks and I couldn't believe how much better it drove.

I really like the lower shock mount brackets you built!

Keith Thanks for the response. I've tired to find someone with good tires/wheels to install, but nothing yet. Well check with a couple more friends. Shocks were one of my first thoughts. What would you recommend in shock?, brand, single or double tube, which is best. I think the current closed and open length is OK. Anything you suggest I can check with the owner. Thanks on the bracket I was happy with how much better they made the angle of the shocks....Jim

[lolife99]

I would check with some of the board members that run Bilsteins.
I have no experience with them, but hear they are great.
I ususally just run KYB's or Belltech drop shocks.

But I would change wheels and tires, to know for sure.

[oldman3]

Originally Posted by lolife99

I would check with some of the board members that run Bilsteins.
I have no experience with them, but hear they are great.
I ususally just run KYB's or Belltech drop shocks.

But I would change wheels and tires, to know for sure.

Will start checking on the Bilsteins, or KYB's. Will try the tires/wheels first, thanks...Jim

[BMERDOC]

Do you have a trust worthy shop around that can check the tires? A road force balancer would be a plus. How new are those tires? Were they balanced? Did they sit around for a while before they were mounted and balanced?

[oldman3]

Originally Posted by BMERDOC

Do you have a trust worthy shop around that can check the tires? A road force balancer would be a plus. How new are those tires? Were they balanced? Did they sit around for a while before they were mounted and balanced?

Right after getting the new wheels, he purchased tires, had them mounted and balanced by a local shop with a good record....Jim

[broey]

X2 on the road force balance test.

[broey]

if its only at high speed I would think its a rotational balance problem. you could rotate the tires front to back if they are the same and see what happens. or pull some wheels off another truck that you know is good.

[oldman3]

Originally Posted by broey

if its only at high speed I would think its a rotational balance problem. you could rotate the tires front to back if they are the same and see what happens. or pull some wheels off another truck that you know is good.

broey, had already done this. Tires and wheels on the front were as smooth as silk, changed to the rear they bounce, and the rear to the front were smooth.

[broey]

one more thing, based on the pics I see, I dont see any balance weights on the wheels, should be some sticky weights just in from the rear lip of the wheels. do you see any?

[oldman3]

Originally Posted by broey

one more thing, based on the pics I see, I dont see any balance weights on the wheels, should be some sticky weights just in from the rear lip of the wheels. do you see any?

Will have to check on the weights, can't say for sure. I do know that were high speed balanced when purchased....Jim

[broey]

dang, well. if you rotated them and it still happens its probably not the wheels...

[ulakovic22]

I'm just tossing stuff out there, but I would think since it only happens at highway speeds whatever the issue, its small. Shocks not stiff enough, springs too stiff or bent axle. I see it has a 5 lug on it, but the truck is a 67.....were the flanges drilled, axle swap or is the rear end out of a 70+ truck or something else?
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[oldman3]

Other info I forgot to post
#6569RACK 5 lug rear axles, complete kit (new drums etc) from CPP.
#6372SBK Heavy Duty sway bar kit (front and rear) from CPP.
The 3/5" drop kit we ordered came with the short adjustable trac bar not the long one.
If I find anything else I forgot, I will post it....Jim

[oldman3]

Ulakovic22, sorry, I forgot to mention that we did an axle swap, we got new 5 lug axles from CPP, along with new bearings, etc. Again one of my thought was a bent axle, but the bounce is on both sides, not one. I wouldn't think both new axles would be bad, but I've been wrong before. If axles were bad I would think in would have shown up when I did the stationary marker and spin test (see post 10). Thanks for the input and keep throwing things out there, maybe something will finally show up....Jim

[Slow Build]

Have you tried rotating the rear u-joint connection 180 degrees? Somewhere in the back of my mind I think I remember someone doing this and it helped.

Also it must be an optical illusion but it looks like your rear driveshaft is lower at the center bearing than at the rear end. I've never had a two piece drive shaft so I don't know what kind of considerations you would have for angles at the center bearing but since the angles between the tranny and the center bearing are basically fixed I would think the angle from the rear of the bearing and the rear driveshaft might be important.

I'm just throwing it out there. Don't know if it's relative or not

Good luck