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Rear Suspension – Leaf Springs

The MG Y Type is typical of its type with independent front ‘coil-spring’ suspension but a live rear axle mounted upon leaf springs. Sometimes you will hear these leaf springs referred to as cart-springs. This is not really true as the majority of carts did not have any suspension at all. A well designed leaf spring with a decent damper to control it can be made very close to perfect; not as perfect as independent suspension, but a lot cheaper to make. On the Y Type the rear springs are anchored at their leading end and swing in shackles at the rear end. Both ends have silent-bloc rubber bushes to permit movement without the need for lubrication and to also provide some insulation against road vibration. The leaf spring is just that, a collection of flat spring-steel bars clamped together in diminishing lengths; the uppermost leaf taking all the driving and braking strains.

 

The Y has an under-slung chassis that runs underneath the rear axle. The leaf springs also mount to the axle underneath it giving a low centre of gravity. Under a no-load condition this leaf spring is slightly curved with its ends higher than the axle centre. This is not universal as Austin on their A50/A55/A60 models had the rear leaf springs curving down at each end under a normal load. This often caused owners of these cars to panic thinking their springs were shot. Note that the forward end is held firmly whilst the rear end can swing on its shackle. This is because the spring is curved so the axle actually moves in a slight arc with its centre on that front mounting. Because the spring is curved, as a load is applied, it will straighten out and lengthen a little. That is why the shackle is required. Also, because the spring grows and shrinks a little every time it hits a bump and rebounds, the leaves rub against each other. This rubbing acts as a natural damping medium but it does left the lower leaf cut up into the upper ones as they age and wear. So to get round this problem MG fitted rubber pads to the ends of each leaf, termed rubber-interleaving. Very similar springs were fitted to the TD, TF, MGA and MGA; all use the same rubber bushes for the eye-ends.

 

The Y Type has six leaves with the top leaf supported its full length by the No2 leaf, then they diminish in length until No6. The leaves are drilled at the centre for a bolt to hold them all together. This bolt has a round head and it acts as a locating dowel where the spring is clamped to the rear axle. Again rubber insulation is fitted to limit vibration and road noise. The spring is clamped to the axle by four long ‘U’ bolts. To stop the leaves from wandering about in use, there is a leaf-clip riveted to the fourth leaf to hold them all in-line.

 

Leafsprings1c[1]The Y’s leaf spring is a good long one with a lever-arm damper to control it. These dampers are called shock-absorbers by some people, but again this is untrue These dampers are called shock-absorbers by some people, but again this is untrue. The shock-absorber is the spring itself, but as it will ‘oscillate’ (boing-boing-boing) if left to its own devices it requires a damper to stop them. As mentioned the leaves also act as their own damper to a certain extent.

 

The hole drilled at the centre of the spring is its weakest point. It is here the top leaf will eventually break, especially if the ‘U’ bolts are not tight. Under acceleration the torque action will cause the spring to wind-up (become an ‘S’ shape) one way, and under braking to wind up in the opposite way. (On powerful cars this is shown as axle-tramp’.) On very expensive cars the spring is relieved of these stresses by the axle having its own radius rods and a Panhard arm (as on the YA/YT). This winding up and un-winding can break the spring leaves eventually. Age will cause the leaves to settle, especially on the driver’s side. If the rubber pads have completely worn away you will lose about an inch in the car’s height; replacing them ‘jacks up’ the spring again to its previous height. As the leaves have rubber pads and there are rubber inserts under the ‘U’ bolts, as well as rubber silent bloc bushes each end, it is not adviseable to paint the springs with oil, Oil will rot all this rubber. Keeping them clean with a wire brush is better then a coat of chassis black paint is all that is required. If you are of the nervous type you might carry a spare top-leaf, but beware that that ‘centre hole’ is actually off-set a little forward, so fit the leaf correctly or your axle will not be square to the cars chassis.

Leafsprings2c[1]My beautiful picture

 

Neil Cairns.