What holds a classic car seat up

A sagging seat in an old car is rarely the cover's fault. The cover is the last layer of a small structure: a steel frame, something springy stretched across it, foam to shape the load, and a board or panel to keep the back straight. Understanding the order of those layers tells you what to buy and what to leave alone.

The frame

Frames are light tube or folded steel, and they take more punishment than they look capable of. Check the welds where the backrest hinges onto the base, the tabs the diaphragm or springs hook into, and the runners. A cracked hinge will pull the backrest out of square and no amount of new foam will hide it. Frames are worth stripping, wire brushing and painting while they are out, because you will not have easier access again.

Diaphragms, webbing and springs

Three systems turn up in sports cars of this era. A rubber diaphragm, hooked across the base with tension springs, gives a soft, slightly hammock-like seat. Woven webbing, interlaced and stapled or hooked to the frame, gives a firmer feel and is what many saloon bases use. Coil or serpentine springs, clipped into the frame and covered with a burlap pad, hold their shape longest but sag at the clips. All three are replaceable. The mistake is to leave a stretched diaphragm in place because it still looks intact: the rubber loses its spring long before it tears.

Foam

Foam does the shaping, not the supporting. It comes in different densities, and the density matters more than the thickness. Soft foam in a base collapses into the diaphragm and leaves you sitting on the frame edge; too firm and the seat feels like a bench. Base and backrest usually want different densities, and moulded replacement cushions for popular cars are shaped to the original, which is worth the price over cutting your own from sheet. Old foam that crumbles when squeezed has nothing left to give and should be replaced, not patched.

Boards, panels and edges

Backrests often rely on a fibreboard panel to spread the load. If it has soaked up damp and gone soft, the whole backrest becomes shapeless. Cut a new one from marine plywood or a modern board and use the old one as a pattern. Look too at the wire in the edge of the foam and the listing wires in the cover: they pull the seam into a groove, and if they are missing the seat looks blown up like a cushion.

Runners and mountings

Seized runners make a seat feel wrong even when everything above them is new. Strip them, clean out old grease and grit, check the latch and the spring, and grease them lightly. Check the floor mountings while you are there, especially the captive nuts and the metal around them, which is a known rust trap in cars of this age.

Do it in one go

Every one of these layers is easier to reach when the seat is on the bench and the cover is off. Doing the frame, the diaphragm, the foam and the runners at the same time as the cover costs a little more once and saves taking the seat apart twice. It is also the difference between a seat that looks new and a seat that feels new.