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]

I agree with the Doctor that the tires are the most likely culprit.

You can get short term flat spotting just from sitting overnight. Usually drives out in about 10 miles or less as the tire carcass warms. Have a good tire shop check balance and the Radial First Harmonic which is abbreviated R1H within the tire (or tyre (IMG:style_emoticons/default/biggrin.gif) ) industry. If this "bounce" is occuring after many miles of driving it wouldn't be flat spotting but as Doc mentions, could be a broken belt.

Here is the US, Hunter is the major supplier of tire balance equipment. You want a similar version - whatever is used in the UK.

A diligent tire tech will be able to determine if it is the tires, a broken belt, or balance using equipment like the Hunter equipment noted below.

https://www.hunter.com/wheel-balancers/road...ASAAEgIUu_D_BwE

There is also another possibility.

There is a suspension phenomena called freeway hop. Occurs quite often on California freeways due to the way they are built. I see you're from London so less likely but it could still be occurring.

Freeway hop is an interaction between road input frequency, the natural frequency of the engine/transmission mount scheme, and the suspension natural frequency.

When all of these frequencies are very close to each other you will get a vertical resonance that occurs at very specific speeds that is very unpleasant and feels like a "bounce". It usually can be driven out of by either speeding up or slowing down which changes the frequency alignment and it will stop. 50 - 60 mph is a very typical speed that freeway hop occurs at though out the automotive industry.

Check to make sure your engine and transmission mounts are good and that all fasteners are tight.

If everything above is OK (wheels, tires, mounts) then it may a freeway hop type phenomena. Does it occur on all roads or only one particular motorway? There are a number of production vehicles that are sold today in the market that are sensitive to freeway hop. Unfortunately there isn't an easy fix beyond semi-active suspension technology that many luxury manufacturers are now using.