Been back in the workshop
Made up my 'subfame', which gave me a chance to play with my new welder.
I decided not to bend the tube in the end, I came to the conclusion that the subframe should follow the lines of the seat rather than the lines of the frame. I had a piece of strap bent up to support the rear of the seat from an earlier mock-up and I decided to attach an H section to this to form the subframe.
Here's a slightly rubbish shot (taken with a camera phone with no micro setting) of how I notched the cross piece to fit the side rails correctly.
I held the tube in my drill vice and drilled one end with the hole saw then turned the vice through 180 degrees to drill the other end this ensured that the holes were parallel to each other. I ensured the notches were central by using an engineers square held against the tube and hole saw. If I had to do the job again I would hold the tube centrally in the vice and place some packing beneath the tube to support it during drilling. As can be seen from the photo I had a great deal of overhang when drilling the second notch and as a result had a bit more flex in the tube than I was comfortable with.
I welded the cross piece to two straight sections using magnetic clamps to hold everything true. I then mocked up the position of the subframe and tacked it in place before welding it all together.
This shows one of my better welds.....
While this shows the seat unit placed on the subframe.
The back of the subframe is held in place securely. I've got to work out how I'm going to secure the front. I made this up on my lathe.
(crap phone cam strikes again)...
The idea is to weld a plate to the unthreaded part and weld this to the cross piece on the subframe. This will let me attach the subframe to the frame using the hole at the end of the top-tube where the frame splays out.
I've also got to work out what to do where I've had to cut the front of the seat to clear the frame tubes.
Once I've got the subframe fixed I need to work out how to attach the base to it. One option is to weld some strap to the subframe and attach the seat unit with pop rivets and spreader washers.
Finally I need to think about how I'm going to cover it. At the moment I'm trying to decide between a seat 'pan' with foam and cover that would bolt onto the fibreglass base or foam stuck to the base with a cover held on with snap fasteners (in a classic XR fashion)