BMW Airhead Seat Base Rebuild.


The original BMW seat base was a composite of rust, metal, wood and fibre glass - but mostly rust.

The problem came when the grab rail was removed to start the backrest project. The screws wouldn't undo and the cover had to be removed. Eventually the screws were ground off with a Dremel tool to get the grab rail off.

This page shows the build of a new seat base.

 

Rebuilt seat

 

 

 

 

 

 

 

 

The photo on the right shows the original seat parts. The cover and foam were in reasonable condition but the base was past saving.

 

There are many methods of renovating or rebuilding a seat base using fibreglass for example but it was decided to try and produce a steel replacement from 18 gauge (1.2mm steel).

Original seat parts.

 

 

 

 

 

 

 

 

The foam was used to create an accurate outline on the steel sheet.

Tracing foam outline on to steel

Cutting the seat outline

 The steel was cut 25mm outside of the outline and then slices were cut up to the scribed line to create edge segments. Smaller segments were use on the tighter curves and a symmetrical pattern was used. 

 

 

 

 

 

 

 

 

Pliers and clamps were used to fold the edge segments downwards. Again the seat was used as a guide to get the correct angle.

Forming the edge

 

 

 

 

 

 

 

 

Each edge segment was welded to the next to create a rigid tray assembly.

Then the edge was trimmed with the grinder, using a wooden batten as a guide to give an edge lip to the seat base.

Segment weld

Edge welded and trimmed

 

 

 

 

 

 

 

 

The front curve of the seat was copied by taking a steel strip and bending it to suit. IT was then welded in place on the top side of the new seat tray.

Copying the seat curve.

Front tray curve

 

 

 

 

 

 

 

 

The front of the seat was formed by folding and welding the remaining bits of steel and the cutting / grinding it back to form an acceptable front aperture.

Tray front details

 

 

 

 

 

 

 

 

Tray was test fitted on the bike without the rear fender in place.

The front needed to slop downwards before the upward curve at the very front. So the tray edge was notched and re-welded to better follow the top frame profile.

Test fit 1

 

 

 

 

 

 

 

 

Using cardboard to create a circle segment of the right radius, two curves were cut into the tray as shown and then bent up to form the sides of the fender aperture.

Fender hole cut

Fender cut test fit

 

 

 

 

 

 

 

 

A slightly oversized straight panel was tack welded to the curved side sections.

Then it was trimmed to be an exact fit.

Seat base with fender cut-out

 

 

 

 

 

 

 

 

Back to using the foam as a guide. Steel strip was used to create the mounting points for the grab rail. The mountings were made from 25mm x 2mm steel strip bent and welded in place.

To mark the location of the grab rail screws the cover was installed and then marks made through the holes. The mountings were then drilled and M6 metric nuts welded on the inside.

Back grab rail mounting points

 

 

 

 

 

 

 

 

With the seat base in position on the bike the location of the hinges was marked.

Unfortunately due to the size of the mudguard cut out, there was no room to use screws, so the hinges had to be welded in place.

An oversized hole was drilled at the approximate location of the catch prong. Then the original part was cut from the old seat base and placed through the hole into the latched position. It was welded in situ to ensure a good fit.

Seat hinges and catch

 

 

 

 

 

 

 

 

Seat rebuild finished

To finish, the base was painted black and the foam and cover were replaced.

The cover was held in place with some impact adhesive on the underside of the ridge and with some self-tapping screws.

 

 

 

 

 

 

 

 

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