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.

[dr914@autoatlanta.com]

maybe a knot in one of the tires, and or a broken steel band? Also an out of round tire can cause it. MANY MANY times when we take these 914s our of our display area after sitting for several years they bounce up and down as they travel down the road. The tires have become flat spotted from sitting

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.

[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.

[Superhawk996]

I keep re-reading your OP. You say you had the suspension checked.

What was checked?

The symptoms you describe also sound like it could also be too high of a rear spring rate and/or not enough damping. Especially if this is happening at low speeds (i.e. 30 mph).

Read what is posted above, clarify what's occurring and we'll eventually get you on the right path.

[SirAndy]

Is the car lowered a lot? It could be riding on the bump stops ...
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[dan10101]

I never assume because one person told you that something is fine, that they are correct.

What you are describing is a shock not functioning. You can check pretty easy by pressing down hard on the 4 fenders and seeing if it bounces more than once.

Since the shocks were just worked on i suspect a shock mount or defective shock.

Without a more definite description of the bouncing it's hard go guess beyond that.

A separated or flat spotted tire can do that but it would be constant with some speeds being more noticeable that others.

What about a broken spring or torsion bar? Is one corner sagging?

Is it only when you are on the brakes or does it continue when you just coast. A warped rotor can cause a vibration under braking.

[IronHillRestorations]

Is the brake pedal pulsating? I think it’s probably front struts though