The 75 and ZT Owners Club Forums - Unequal Rear Suspension Settings?

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- - Unequal Rear Suspension Settings? (https://www.the75andztclub.co.uk/forum/showthread.php?t=327451)

Quote:

Originally Posted by Mike Noc (Post 2992421)

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.

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

Simon

Quote:

Originally Posted by Kearton (Post 2992251)

(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?

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)