Bouncing car, Car bounces when slowing down Options

[gaocs]

I have a really odd thing happing in my car that I hope someone can help with.

About two years ago I had new shocks fitted all round to my 1970 1.7 and since then the car has had an unusual symptom where it "bounces" up and down as you are going along... but it doesn't happen all the time. It always happens when I have been driving at say 50mph+ and then have to slow down, the car then continually bounces up and down as it's going along, especially at slow speeds. This always happens for example when I'm on a motorway and then the traffic slows right down and is then start-stop traffic. It feels like you are on a trampoline.

I always thought it was more of a "seat of the pants" feel, but today my son was driving behind me and he says you can visibly see the car bouncing up and down.

I've had the suspension checked and everything seems ok, so does anyone have any ideas as to what can be causing this? Could it be engine mounts or something else? The wheels are new as are the tyres so it's not a problem related to them not being round. The "bounce" is a few inches of movement up and down.

Thanks.

[Superhawk996]

One other obvious item comes to mind.

914 rear wheels are lug-centric. Meaning the rear hub has no wheel pilot and the the center rotation point of the wheel is established by the wheel lugs.

Depending on what brand of wheel you're using (OEM or aftermarket) as well as what type of wheel lugs you using. Wheel lugs have three common profiles.

Flat - usually used on heavy trucks. Definitely won't find on a 914.
Round
Conical.

The wheel lug pocket profile and the lug need to match. Also make sure you have sufficient lug engagement if you're using any sort of spacer.

If you are using wheel spacers, these are notorious for causing wheel balance problems. You measure the wheel / tire and it is OK . . . but the spacer was never measured. Disregard if you're not using wheel spacers.

Jack up rear of car. Ideally use a dial indicator to measure how much radial run-out the wheel/tire has. Rotate wheel by hand. Factory Spec. is 1.5mm maximum.

If you don't have a dial indicator you could used a fixed visual reference like a stack of cans to gauge how round the wheel/tire assembly is on the vehicle as it rotates relative to the stationary object.

Bottom line, ideal case is Zero radial run-out. If you have enough to see easily with the naked eye - you have way too much. Loosen lug nuts and try to let the wheel recenter by gradually tightening the lugs in an X pattern. Look again and see if it has improved.