[350TSS]

The seat base also got 2 additional CF laminations and additional CF along the vertical sides of the base itself. I plan to use the design for the attachment of the original seat, namely, at the front, a fork that engages with the transverse bridge across the frame and a sprung fore and aft latch at the rear of the seat. A key element of this design is that the seat base has to have the fork engaged and then pressed down onto shaped pads that sit on the frame rails so the latch at the rear engages and has enough tension to prevent disengagement or the seat base wobbling loose.

For this to work on my CF seat base and for it to sit “right” on the frame both aesthetically and ergonomically (I am 6’ and want the seat pad to sit as high as possible so my arthritic knees are not at too acute an angle with the balls of my feet on the rear set foot rests) I have to determine quite precisely the following:
a) The height above the lower edge of the seat base of the top of the fork that engages with the frame bridge
b) The distance between the latch pin at the rear and any “stop” that prevents the fork at the front of the seat base moving further forward
c) The height above the lower edge of the seat base and the bottom of the rear latch pin and its lateral positioning relative to the shape of the seat hump
d) The required positioning of the pads, fore and aft and vertically of the 4 x pads to be bonded into the seat base that rest on the frame rails, there is a lateral component here as well otherwise the seat base will not sit square on the frame rails.

There are three complications, first is that the front 150mm of the seat base is not flat and slopes upward towards the petrol tank which means that my front fork which can only be attached to the underside of the sloping seat base has to be quite accurately positioned fore and aft.

The second is that the very rear portion of the truncated frame rises at an angle of c 35 degrees from the horizontal but the rearmost 35mm is bent to fall from the horizontal by about 15 degrees. I had always intended that the seat base would sit on the frame rails so that the base would rise at say 30 degrees with the rear most portion of the frame being mostly covered by the lower edge of the seat hump.
The third is that I extended the front of the seat base by about 50mm to give a bit more room between the rear of the tank and the base of the seat hump and I do not know how long the seat base should be so that the hump does not look disconnected and looks “right”. It is one of those areas where form and function have to be integrated, if it does not look right then it is not right.

Sorry for such a long and complicated explanation but I find that writing it down helps me to resolve the issue. I now know I cannot start to fit the seat until the petrol tank is finished, the rearmost edge of the tank finishes in a downward arc and the front of the seat base will have to follow that arc. The length of the seat base can then be determined such that the hump is not too close to the tank or alternatively hanging out in the breeze. At that point I can determine my datum for all subsequent measurement and fabrication / moulding, namely the positioning of the top surface of the fork which touches the rearmost part of the frame bridge. All other dimensions will flow from knowledge of that point. I think rubber, either foam or something more resilient, will be my friend in making sure that the seat sits securely and in the right position both fore and aft and laterally.