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MG Midget and Sprite Technical - quarter elliptic radius arms

On a quarter elliptic Sprite or midget should the radius arms
a) Float so that the rubber in the bushes at each end act for shock absorbance or should they, once that car has been lowered and sitting in its usual posture, be gripped firmly so that the rubber bushes add to the suspension dynamic?

Has anyone personal experience of the silicon bush kit for a radius arm as sold by Moss?

Alan
Alan Anstead

Alan, rubber bushes are part of the suspension compliance and they are designed to flex and twist. On joints that also swivel more than a few degrees, the rubber has a tough time so metalastic bushes are sometimes used; essentially a sleeved rubber bush. Polyurethene bushes are less able to flex compared to rubber and so they are not really a direct replacement for rubber which is why I re-sized the polybushes for the front suspension of my Midget so they could act like compliant swivels over greased pins. I have never looked at silicone bushings so can't comment but silicone is more similar to rubber than polyurethene so I'd treat a silicone bush as if I was fitting a rubber bush and be inclined to sleeve it for swiveling joint or use off-the-shelf metalastics.
Nick Nakorn

once that car has been lowered and sitting in its usual posture, be gripped firmly so that the rubber bushes add to the suspension dynamic>

Yes!
Flip Brühl

The posture of the car at rest should make no difference at all to the extent to which a bush 'grips' in a trailing arm or radius arm because it's taking no weight; it will only experience compression, stretch and torsion if accelerated ie: when the car is accelerating/braking/cornering.
Nick Nakorn

And you should only tighten any rubber bushing up when the car is sitting on all four wheels so that the rest position is what you set them to. then they are much less likely to suffer from over stressing and failure because they are being expected to do more than they are designed to do.
dominic clancy

I find it hard to believe that the quarter elliptic upper links do much more than:
(a) Help transfer the fore and aft load from the axle to the body under acceleration/braking;
(b) Stop the axle from flopping around.

Whether or not the metal bushes are free to rotate on the mounting bolts can't have much relevance to the springing or damping characteristics. I await correction from the better informed.

On my Frog they are free at both ends.
Simon
SA Wood

Simon, you're right that fore and aft and slight lateral loads will be all that a metalastic or swiveling polybush will have to cope with. But a rubber bush that 'grips' the pin will also twist and act as a spring as would a silicone bush if it is formulated to mimic rubber.
Nick Nakorn

So what about rose jointed radius arms? These will not offer any resistance to up and down movement.
I await with Simon!
Motley 5

A rose joint just swivels with no compliance - it takes the same loads but doesn't cushion any road shocks... I have a mixture on my rear suspension; 'in-board' joints (those attached to the car, are all polyurethene while the outboard joints, those attached to the axle, are all spherical bearings (like Rose Joints but a different brand) so I will have a mixture of accuracy and compliance. You can also get spherical bearings set in a polyurethene/metal sandwich but they're bloody expensive.
Nick Nakorn

This thread was discussed between 15/07/2014 and 16/07/2014

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.