Rear Suspension
I've decided to use the bottom bracket for the rear suspension pivot. It's convenient, if a bit heavy. If the bike is a success I'm planning to mold a polyurethane bushing in place to lighten the assembly and absorb vibration.
I cut the BB axle with the miter saw and then drilled and tapped the ends to accept the stainless steel Allen head bolts depicted. I'll Loctite these in place during final assembly.
Next I fabricated a mount to accept the cut down bottom bracket
At this point I realized I needed the rest of the frame to determine the mounting points for the shock. I scaled the measurements off of the Speedmachine picture, using a vertical measurement of the back wheel diameter as my scale, and cut sections that were approximately correct in length.
Now I am going to measure the trail of the front fork and using that and my copy of Bicycling Science, determine what the head angle should be. Then I will be able to work backward from the wheelbase of the Speedmachine to determine how long my frame sections should be.
The sections shown below were all cut at 71 degrees, but the whole thing looks a bit long to me.
