The Bounce

[ampico-kid]

A good shock will dampen on the down stroke. A good shock should have full travel up and down. A good shock will resist as the needle valve is adjusted. Do not put a big wrench on the needle valve , you will seriously damage it. If it will not adjust with very little effort, do not force it. To test the shock in place, bolted to the frame, take off the shock arm clamping bolt, pull the shock arm off the shock shaft, and use a crescent wrench to turn the shaft. Better yet ask a knowledgeable club member to help you.
The OEM shock was referred to as an instrument for good reason.

I did check the two front shocks (easier to access), and they both felt good to me. Little resistance on the up stroke, and very definite strong resistance on the down stroke. Shock linkages all felt secure and tight as they should be. I didn't check the rears. It appears that the shocks on the car have been rebuilt or are original type replacements. They're nice and clean and leak free. I can check the rears, although my sense is that the bounce I'm feeling is primarily in the front of the car.....but I could be wrong about that.

 

[nkaminar]

You can adjust the shocks.

 

[rotorwrench]

I think I would check each drum/hub for rotational concentricity with a dial indicator. If it's been having this problem for an unknown period of time then it may have a hub/drum that is running off center due to manufacturing defects. If all run true and bearings are in good condition then it would likely be a balance problem in one or more of the rotating assemblies. The best way to balance is still the old Bear method of rotating each assembly separate from the others and checking the balance that way. It could have a drum/hub imbalance but that's not easy to find without a dynamic check of the whole system. Problems like this are not common but occasionally a defective part can show up. A lot of wear on the stub races of the rear axle housing can allow a lot of rear wheel movement so definitely check the rear wheel bearings for play in excess of norms.

 

[ampico-kid]

You can adjust the shocks.

Yes....that's true. I guess first I need to check all of them, only checked the front two. They seemed pretty stiff, or at least what I would have considered stiff. I'm going to have someone rock the car while I watch them and see how much movement and/or lost motion might be present. Then I might try adjusting them. Thanks for the pointer.

 

[Marshall V. Daut]

Although the front spring has been suggested numerous times as the possible culprit for bouncing, I haven't seen the spring hangers (shackles) mentioned. If the bushings inside the bottom spring leaf eyes and the perches are worn - as they usually are from age and lack of lubrication - the shackles will be loose inside the bushings, allowing the front spring and axle to rock to and fro. This will cause changes to the caster of the front axle as the car moves forward. The rocking motion could manifest itself as a bounce as the spring rapidly moves back and forth. This may only kick in at the same speeds cited by the poster because the physical forces acting on the front spring and axle finally overcome static resistance offered by these parts. Any movement here will most certainly be felt in shaking, shimmying, wandering and probably bouncing.
Additionally, if the four nuts on the backside of the shackles are loose, this will also allow the front spring to rock, creating unwanted gyrations and effects upon the front alignment. Alternately, if the nuts are too tight from over-aggressive assembly, the spring will not be able to do its job, meaning to flex up and down in response to road conditions. You should be able to use a long pry bar between the top of the front axle and the shackles to see if the spring can move up and down. If the spring ends cannot be lifted using a pry bar, either the nuts are too tight or the shackles are frozen inside the spring leaf ends and the perches. Usually just one side has seized, which also causes the car to lean to one side or the other. By the way, if the ends of the springs are resting on top of the axle, the spring is fatigued or broken and cannot perform its function. Replace or repair the front spring to achieve the necessary gap between the top of the axle and the spring leaf ends.
When checking the shackles, also ensure that the four large nuts securing the front spring to the frame are tight and that there is no looseness between the spring U-hangers and the front crossmember. There were different styles and lengths of the U-hangers and thread lengths, depending upon year and number of spring leaves (heavy-duty versus standard number of leaves). If the wrong ones have been installed and the spring is loose inside the front crossmember, there will be movement that will translate into unwanted gyrations. Ensure that the correct U-hangers are in place and are tightly secured.
Marshall

 

[Brentwood Bob]

If the shocks are new to you include a check to top off the fluid, or at least verify that they have oil . Marshall, as usual is spot on. Some people probably even put oil on the springs.

 

[Rob Doe]

ampico,

I have felt similar bounce like sensations in our coupe. It seems to come from the rear of the car rather than the front. I think post #23, by rotorwrench is the most likely source of our issues. His answer, if I understand correctly, is two part??? A hub and drum may not have been mated correctly, or the drum was incorrectly machined off center at some point when new brake linings were installed. The other is that the race on the end of the axle housing can become flattened on the bottom due to the load always bearing there. I have no clue what sensations that would cause. Others here might know.

I don't pretend to know the math equations involved, but I suspect that an out of balance situation would probably not diminish with increasing speeds of the rear wheel rotation. They probably grow exponentially???

 

[WHN]

The first thing that I would do is jack up the front of your car. Put jack stands under frame so that the front end is now hanging free.

Now you can check everything involved with your cars front end. Look at everything. Wheels, springs, shocks, everything. If all is OK.

Do the same thing to the rear end.

Good luck.

 

[rotorwrench]

An imbalance can feel relatively normal but cause sympathetic vibrations in other parts of the car during the changes in rpm. Each individual part will have a different frequency that will set off resonant vibration reactions.

Larger communities generally have a front end shop that can dynamically balance any one or all of the wheel assemblies on the car. Odd situations can lead to different methods to correct them at times. This would eliminate all questions about balance being a possibility.

I balanced a lot of rotor systems in my career as a helicopter mechanic. A person never stops learning about what causes vibration and ways to make them go away.

 

[Rob Doe]

ampico-kid, how did this turn out?

I recently added 4oz. glass beads to all four tires and I think, think, that I have noticed an increase in the number of times at low speeds that our coupe has a smooth bounce sensation for brief periods, usually after a stop at an intersection. I suspect the beads are setting in various positions and until the wheels spin fast enough to spread them and achieve their balance again, the sensation is there.

There are some places where the road actually has smooth waves or wash boards in the asphalt. Sometimes a close look on the next visit they can be seen. The sensation I'm speaking of is like an elliptical wheel would give.

The glass beads, if my observations are accurate, don't settle in until the upper 30's mph. and they become even more effective at the upper 40's and higher. It's visible in the movements of the lights and fenders. At 50 plus, its like the lights are locked in???

Comments please.

 

[Dr Foot]

Make sure that all tires are evenly mounted on rims.

 

[ampico-kid]

Thanks for checking back with this post. So far...no luck. As I mentioned early in this post I changed out all 4 wheels (at the same time) from my Coupe and got the same results, and that is...between about 15mph and 25-30mph on a flat level road, the car starts to bounce up and down enough that you will bounce on your seat. It's very cyclical and consistent. The faster you go, the faster the bounce. It may still be there at higher speeds, but maybe you just can't feel it as much. I jacked up the car and manually rotated the wheels, one at a time, to see if there might be something in the wheel bearings, but nothing shows up. I jacked up the rear and started the car and let the rear wheels (one at a time) spin and they appear good, I don't see any wobble. All of the wheel bearings feel secure with no wiggle. It's not the engine or flywheel, I can step on the clutch disengaging all of that and it's still there. Shocks all work well....disconnected each of them and checked them out. Seems to me it must be something between the transmission and the differential? But what? And how to check it? Have been thinking of doing a video to show it in action. If no solution soon, I'll do that and post a link to it here. Thanks again for checking in......Bob

 

[gdmn852]

Hello, check rear axle and hubs , if they don’t fit tight,will give a odd oscillation, on my 31 they treated the rear axle like the front apparently,backed off nut and axle wasn’t tight to hub . I checked axle to hub fit and tightened down nuts and new cotter pins . Felt somewhat like a loose wheel.

 

[nkaminar]

Every dynamic system has a critical speed or rpm. When that critical rpm is reached the osculations are the greatest. When above the critical rpm they diminish. An unbalance will make the osculations at the critical rpm worse. If perfectly balanced then there are no osculations at the critical rpm. Dampening will also diminish the osculations. Damping can come from the shocks or from springs that are long overdue for some grease.

The tires on the wheels can be balanced by using one of the front hubs. Back off the brake on one hub (count the clicks) and back off the bearing (count the number of cotter key positions). Mount the four wheels one at a time on this hub and see where the wheel stops. The bottom part of the wheel will be the heavy spot and lead weight needs to be added opposite that location until the wheel stops at random places.

Your lug nuts can also be bad which would make the wheel run out of true and cause a bounce. Check to see if there is any looseness on all 4 wheels by trying to shake the wheel by gripping it at the top and bottom. Correct any looseness. Check to see if the rear hubs are tight on the axles. Make sure the lug nuts don't bottom out on the studs before they are tight on the wheels.

Jumping up and down on the bumpers can test the shock systems. If the car bounces easily then you need to fix or adjust the shocks.

Make sure all 4 wheels are not out of round. Put a block of wood next to the outside diameter of the tire and rotate it. There should be no more than 1/16 inch out of round.

 

[ampico-kid]

Thanks for all of the useful (and back yard mechanic oriented) suggestions of things to check. Don't know if they are accurate, but the wheels presently have lead balancing weights on the wheels indicating at one time they were balanced. I can recheck though. I did find one thing out today. I used a pry bar to see if the front spring could be moved on the perch. As I was applying pressure on the passenger side I noticed that the spring perch was wobbling where it goes through the axle. It wasn't an extreme amount of movement, but it was clearly visible. The driver's side seems tight. Just wondering if that might be a contributing factor to the bounce?

Every dynamic system has a critical speed or rpm. When that critical rpm is reached the osculations are the greatest. When above the critical rpm they diminish. An unbalance will make the osculations at the critical rpm worse. If perfectly balanced then there are no osculations at the critical rpm. Dampening will also diminish the osculations. Damping can come from the shocks or from springs that are long overdue for some grease.

The tires on the wheels can be balanced by using one of the front hubs. Back off the brake on one hub (count the clicks) and back off the bearing (count the number of cotter key positions). Mount the four wheels one at a time on this hub and see where the wheel stops. The bottom part of the wheel will be the heavy spot and lead weight needs to be added opposite that location until the wheel stops at random places.

Your lug nuts can also be bad which would make the wheel run out of true and cause a bounce. Check to see if there is any looseness on all 4 wheels by trying to shake the wheel by gripping it at the top and bottom. Correct any looseness. Check to see if the rear hubs are tight on the axles. Make sure the lug nuts don't bottom out on the studs before they are tight on the wheels.

Jumping up and down on the bumpers can test the shock systems. If the car bounces easily then you need to fix or adjust the shocks.

Make sure all 4 wheels are not out of round. Put a block of wood next to the outside diameter of the tire and rotate it. There should be no more than 1/16 inch out of round.

 

[Ruth]

I used a pry bar to see if the front spring could be moved on the perch. As I was applying pressure on the passenger side I noticed that the spring perch was wobbling where it goes through the axle. It wasn't an extreme amount of movement, but it was clearly visible. The driver's side seems tight. Just wondering if that might be a contributing factor to the bounce?

Probably a good idea to tighten your spring perches. Being lose they will wallow out the holes in the axle and make thing worse.

 

[rotorwrench]

A tread separation event is common on older tire carcasses. You should roll each tire off the car to check for bad carcass. A good tire will roll straight with a good push. A bad tire will not roll straight and may flop over when it rolls onto the bad spot. Many times a person can see or feel the bulge on a bad tire. This will eliminate one more possibility.

 

[imacrazy]

I would say the tires have flat spots on them from sitting around a lot, or out of balance.
Just my thoughts.

Bill

 

[Ruth]

A tread separation event is common on older tire carcasses. You should roll each tire off the car to check for bad carcass. A good tire will roll straight with a good push. A bad tire will not roll straight and may flop over when it rolls onto the bad spot. Many times a person can see or feel the bulge on a bad tire. This will eliminate one more possibility.

I would say the tires have flat spots on them from sitting around a lot, or out of balance.
Just my thoughts.

Bill

Fellas, he said in an earlier post that he changed out all four wheels and tires from his other car with no love.

 

[ampico-kid]

Thanks for the input Bill, but if you read my explanation thoroughly you'll see that I replaced all four wheels with known good wheels from my 30 Coupe and still had the problem.

I would say the tires have flat spots on them from sitting around a lot, or out of balance.
Just my thoughts.

Bill