Unequal Rear Suspension Settings?
 

(ZT-T)
I recently thought that my left-hand rear spring might have broken as the wheel seemed to be sitting closer to the wing than on the other side. The springs aren't that old, but one of them had had to be replaced after only 10,000 miles (a presumed manufacturing flaw), so I thought that the other side might now have gone.

I drove it to my garage cautiously to be checked. I was pleased when they reported that both springs were fine, and in fact in very good condition. The culprit had been low tyre pressures - way lower than I'd expected. (I hadn't used it much since Xmas, so the monthly checks had been missed.)

I asked about the apparent difference in wing clearance between left and right sides, which I didn't think could have been caused by low tyre pressures alone. They said that BMWs - with which they were more familiar - have their rear suspensions set slightly lower on one side to offset the weight of the driver, in order to improve handling. Since MGR used a variation of the BMW rear suspension, the garage thought that they might have done that too.

Has anyone else heard of this and/or know whether that is the case?

I'd check your shock absorbers if I were you.

Simon

And the condition of your upper arms!

and the spring thickness and lengths. I think the garage are playing silly billies !!

Thanks all for the suggestions.

The shock absorbers were replaced about 15-20,000 miles ago, along with the upper suspension arms. The garage said that both these and the springs were in good condition.

Ride height is controlled primarlly by the springs, so i don't think it can be due to the shock absorbers. I didn't ask the garage whether they had checked the spring lengths, so that might be a possibility.

I think my next step will be to recheck the clearances with the car on level ground. My drive has a bit of a slope.

Has anyone heard of those differences being the norm on BMWs?

Have you ever fitted shock absorbers yourself Kearton?

Simon

Not on a car.
If you need to do, it I think it will be easier on the rear than the front as the springs are housed separately, so there is no need to remove them.

Springs should be replaced in pairs to ensure equal performance. Shocks do not set ride height.

I see that you are a professional engineer Ken so can you help me with this please?

Whenever I fit a shock absorber I have to raise the car body very high, beyond spring-determined height, in order to insert them. When the body is subsequently lowered the spring partially compresses but so does the shock absorber. A point of equilibrium appears to be reached. Surely this would depend upon the ability of both the spring and the pressurised shock absorber to compress, wouldn't it?

If I removed my shock absorbers completely, would you say that the car's body height would be identical to that with the spring/shock absorber combination fitted?

I'd be grateful if you could explain where I am going wrong with this.

Simon

I would think the answer is obvious but as the clubs technical advisor why don't you make it Simple Simon and remove your shock absorber and try it then you can let us all know

Another point is that if the shocks are not equal length when fully extended , then this will give a different ride height. I had this problem when i bought new shocks. The seller listed them as suitable for rover 75. They were in fact the mgzt rear shocks,which were shorter,as are the springs. Apparently , the last R75 used the mg set up on them, ( contempory models ) so ,if the shocks or springs are not the same each side , it will. Unless they were all renewed in pairs of the same,then you will have ride height differences. Even heavy duty springs (which i have fitted on the rear) sit much higher. When shorter shocks are fitted ,or shorter or stronger springs fitted, then they are already compressed lower than the originals. Simon has made a reply request,which i am interested in too .... ;)

Try a static damping test: Stand beside one corner of your car, push it down hard & then let go. When the spring pushes the body back up it overshoots the normal (rest) position then drops back to it. This is correct damping.

Explanation
When a car is sitting in its normal position (rest), the suspension must be partly compressed. (Typically by 1/3 of its travel as I understand it.) This is so that the body can move both down and up relative to rest when it goes over bumps.

A) If the damper were at its longest position when the car was at rest, when depressed it would go down, rebound to the rest position, then hit the top of the damper and break it.

B) If the damper were at its shortest position when the car was at rest, when you pushed down it would not move as it would be acting as a solid rod, giving a very uncomfortable ride.

An Aside
Drag racing cars often have rigid or very stiff dampers so that they don't compress when starting off so that the weight on the wheels, and hence the grip, is better.

I hope that helps.

It’s as simple as….
The springs are there to carry the weight of the vehicle and provide up and down movement of the suspension whilst keeping the vehicle level and to carry the weight of the vehicle. The ride height is regulated by the spring hence why you get ‘lowering’ springs. The damper is there to control and ‘dampen’ the operation of the Spring and in doing so keep the wheel on the road. The dampers are not weight carrying.
You can get lowered or shorter dampers but these aren’t to do with the height of the vehicle but the height of the spring, a lowered spring needs a shorter damper to control it effectively.

Yes agreed. But everything mentioned in the past posts will depend upon having the correct shocks and springs fitted , and that they are all functioning equally and correctly. We wont know this until this status has been confirmed from the original poster. Variants in springs and shocks will bring different ride qualities and properties ,but most importantly is that they are equal each side .Thanks sworks for the post. ... ;)

Yes, thanks Mark and Kearton.

This is the interesting bit.
I understand what you're saying Mark but if that was absolutely true wouldn't it be possible, with the car resting on its wheels, to undo the damper's fixings and just slide it out? But that's not possible.

As Kearton has said, the damper is partially compressed. It contains gas under pressure so doesn't that provide some support to the body, albeit much less than the spring?

Simon

As mark stated Simon the weight of the car is held by the springs . The dampers are slightly compressed in location to allow for dampening effect.
View it another way . Springs hold the weight and absorb impacts.
The 'shock absorbers' and in fact just dampers to dampen the springs rebound.
All I / we can do is explain it to you , we can't make you understand it.

Sent from my 2201116SG using Tapatalk

That's exactly what I did Simon, with the car on the ground unscrewed the bolt retaining the shock absorber to the hub and removed it. I had a suspension knock and wanted to eliminate the shock absorber so took it out of the equation.

Simples!

The only way shock absorbers can affect ride height (assuming the correct ones are fitted) is if they start to break up and debris blocks the internal channels the oil passes through. I've had this happen on my car where the NSR of the car got lower and lower as I drove because the shock absorber bound up until all the suspension movement was effectively used up. Left the car for half an hour and normal ride height was gradually resumed.





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As I said before it's quite Simple Simon try it on you car and let us know the results then we will have the evidence that you are usually so keen on

Hi Simon, that is not the case. I'm sorry if my explanation wasn't clear.

At rest, the spring will be partly compressed and thus under load.
The damper too will be partly compressed, but will not contain any pressure.

A damper only contains pressure when it is moving. Its job is to slow down movement. It does this by restricting/slowing the flow of a fluid between its two chambers. Once that flow has been completed, there is no pressure. If the flow is blocked for any reason, the damper will behave like a solid rod.

IHTH

Thank you Kearton. That's helpful.

Simon

Thanks Mike. It's interesting to know that a shock absorber internal fault resulted in lower ride height, albeit temporary.

Simon

Had something similar on my old ZT - problems was caused by a 75 shocker being fitted as opposed to a ZT at some stage so the heights were unequal. :shrug:

Springs carry all the weight of the vehicle that is not considered 'unsprung weight'.
A static vehicle, sitting 'neutral' on the 4 wheels, will find it's natural 'balance' and sprung weight 'level' - this is how the designer intended and all the different spring combinations that MGR specified take into account the variations of weight (engines/gearbox, body style...etc.) of differing models. For all these variables the damper/hydraulic strut will have it's physical dimensions = so that the damper internal piston is sitting somewhere about mid-stroke of the damper barrel (and probably not a great deal of dimensional difference between struts of all the models); depending on model, springs will vary a bit more in terms of free length and 'strength' (usually a common O/D with slightly differing wire diameter, number of coils and stiffness of material).
With any given vehicle in motion moving over undulating road surface(s), internally the strut may move similar distances whether the spring is highly compressed (shorter) or lightly compressed (longer) than the neutral position but the speed at which the piston moves will vary with how the oil medium (viscosity of which could also be a model variable) squishes to and fro 'through' the piston - this is dampening the wilder oscillations of the bouncing springs - themselves a function of vehicle weight, speed, road surface and a little bit of direction change (sway/balance).

I finally got a chance to check the ride heights on level ground after the garage corrected the tyre pressures. All pretty even. I thought the values might be of interest / use to others.
I think my original visual check was affected by the low tyre pressures, but also because I've realised that my drive slopes left to right, as well as more obviously rear to front. This must cause the RHS to rise a bit relative to the left when parked.

Ride Height Measurements (MG ZT-T CDTi)
(Road to highest point of the wheel arch, in line with the wheel centre. cms)

Rear L(67.5) R(67.5)

Front L(68.5) R(69)