Seat & Subframe

Yet another crazy weekend pushing the BMW R80 build towards the finish line. This weekend (with the help of Ren and Sam), I managed to cover a tonne of ground. I’ve now all but completed the electrical side of things (a post on that coming soon) and have the first (tricky) part of the seat sorted as well!

In this post, I’ll cover much of the process behind fabricating the custom rear subframe and seat pan. The game-plan was to create a frame tube extension to run from the top frame tube, further back towards the rear wheel. With a fair bit of planning, we got to preparing the extension.

We started with a thick steel tube section and used a press to squeeze it into an oval shape to match the BMW R80’s existing frame tube.

Frame Tube Extension

Frame Tube Extension

Once we were happy with the extension we got to marking out positions for the crossbars that would provide strength and mounting points for the seat pan.

Marking Out Slots For Crossbars

Marking Out Slots For Crossbars

With the positioning of the crossbars marked out, we got to cutting slots into the tubing to seat the crossbars.

Cutting Slots Into Frame Extension

Cutting Slots Into Frame Extension

It took a little bit of work but once the slots were uniformly cut, we seated the crossbars ensuring that they were level for welding.

Crossbars Seated

Crossbars Seated

Crossbars Seated For Welding

Crossbars Seated For Welding

The crossbars were sitting level, so we proceeded to weld them in place.

Welded Crossbars In Place

Welded Crossbars In Place

In order to add some further strength and stability to the crossbars, we welded up the under side of each crossbar/frame extension join.

Welded Under Side Of Joins To Add Strength

Welded Under Side Of Joins To Add Strength

In order to support the rear-most section of the seat pan, we added further cross-braces to tie things off.

Rear Subframe Complete

Rear Subframe Complete

With the subframe ready we got to cutting a thick steel seat pan and rolled a significant “kick” towards the back.

Seat Pan Rolling

Seat Pan Rolling

We tidied up the edges and the seat pan was all but ready.

Seat Pan Almost Ready

Seat Pan Almost Ready

Now that the seat pan and subframe were fabricated, the next thing to do was tie them together!

Seat & Subframe Welded

Seat & Subframe Welded

With the custom subframe and seat pan tied together, it was time to test mount the new piece.

Test Mounting Custom Subframe / Seat Pan

Test Mounting Custom Subframe / Seat Pan

I feel like the result, thanks to Ren’s workmanship, is nothing short of amazing. We’ve still got a bunch to do to fabricate the seat itself, but this was the hard part… And it is now done.

Rene9ade Custom Motorcycles (where I’m working on my BMW R80) has covered the fabrication process of my subframe and seat pan also. If you’re looking to get this kind of work done or need a hand progressing your build, they’re definitely worth talking to.

I’ve received a lot of questions from people following this build asking “what are you doing with the seat?” – I hope this post sheds some light on my vision for this. Please by all means let me know what you think!

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