The carbon seat

A plywood box, a lump of clay and eight days to a forged carbon shell, including the four hours where the clay cracked.

Specification / as recorded

Status: Seat and rig built, August 2023
Role: Solo. Plug, mold, layup, frame and assembly
Method: Plywood buck, carved foam, clay plug, hand laid forged carbon
Timeline: Plywood box to demolded shell in eight days
Shop: A home garage, hand tools and an open mold

Sheet 02 of 05/The plug

Making the shape

The shape came first, in three steps.

1. Build a plywood box the size of the finished seat and draw the side profile on the inside face. 2. Fill the box with expanding foam and carve it back to that line. 3. Pack clay over the carved foam and smooth it until the surface is the seat.

The clay is the part that matters. The foam only has to get close. Everything the seat will ever be, the width across the shoulders, the depth of the bucket, the angle of the back, is decided by hand in the clay, and once the mold is pulled it is fixed.

PlatePL-01

TitleWhere it started

ViewDetail

A plywood box with the seat profile drawn on the inside face in pencil.

A plywood box filled with rigid foam carved into a seat curve, with brown clay packed into low spots — Then carved back to the drawn profile and packed with clay.

A smoothed brown clay seat plug set into a wooden frame in a garage — Every shape the seat will ever have is decided here.

Sheet 03 of 05/What broke

The clay cracked

Four hours after I smoothed the plug, the whole surface had crazed. Drying clay shrinks, and a layer that thick over a rigid foam core has nowhere to go, so it splits. The photo of it is on this page because it cost me days.

The plug had to be filled, skimmed and sanded flat again before I could take a mold off it. If I did this again I would use an oil based clay that does not dry, or I would skim thin coats and let each one go off before the next.

Close up of a clay seat surface covered in a dense network of drying cracks — Four hours later the clay had dried and crazed all over. It had to be filled and skimmed before a mold could be pulled.

Sheet 04 of 05/Mold and part

Mold, layup, demold

With the plug flat I pulled a mold off it. Then the carbon: chopped tow and resin, pressed into the mold by hand, worked into the corners, and left to cure.

It came out of the mold on 15 August, eight days after I cut the plywood, still covered in release agent and with a rough flange all the way round. Trimming that back is what turns a molding into a part. The finished surface has the marbled look forged carbon gets when the fibers land where they land.

Black chopped carbon fiber and resin pressed into a blue seat mold on a wooden frame — Forged carbon uses loose tow instead of woven cloth, so there is no weave to fight around a curve this deep.

A rough black carbon seat shell just removed from its mold, white release residue on the surface — Out of the mold and still covered in release agent, 15 August 2023.

Close up of a glossy black forged carbon surface with a swirling marbled fiber pattern — Randomly oriented tow is what gives forged carbon its marbled look.

Sheet 05 of 05/The rig

Building the rig around it

The frame is timber. A steel or aluminum extrusion rig would have been faster to build and easier to adjust, but the seat existed and the frame had to fit it, so I built the frame to the seat instead of the other way round.

By 26 August the seat was bolted to a frame with the wheelbase mounted, and the rig ended up with a wheelbase, pedals and a monitor arm. Upholstery panels were cut and edged to pad the shell.

The carbon seat bolted to a rough timber frame in a garage with a sim racing wheelbase attached — With the wheelbase mounted for the first fit.