Hey guys. I'm a 4th year at Cal Poly San Luis Obispo. I took singletrack vehicle design last quarter, where we design and build a bike. I wanted a trials bike so, I worked off of current designs and built this.
The parts are all taken off my dirt jumper mountain bike:
Got the tubes from Aircraft Spruce. I HIGHLY RECOMMEND them. I ordered these tubes online around 2pm on a Wednesday, and had them by 10:30am Thursday (the next morning). They will even cut your pieces to the nearest foot, which is really cool.
I did some rough FEA/stress hand calculations and 1.5" x .049" gave a pretty good safety factor, so I went with that for the main triangle (down tube, top tube, seat tube). I ballparked 1" x .049" for the rear triangle (chainstays, seatstays). I bought the headtube and bottom bracket from Nova Cycles to make sure a headseat would pound in and my existing bottom bracket would thread in.
I then started cutting and taping the mitering templates onto the tubes at the proper lengths. This took much longer than expected (about 6 hours). In order to make sure the cuts on each tube were in line with each other (around the axis of the tube) I made center lines on the tubes. The poor man's way to do this is to drag a straight edge across two similar tubes lying next to each other on the floor, just make sure they don't rotate at all while making the mark. I got the mitering templates from here.
The first day in the shop, I cut all of the miters on the main triangle. The tube notcher in our shop is a little sketchy. It is basically just a big belt sander with interchangeable rollers to make the right diameter notch. It works well for a cut or 2, but blowing out belts was way too common, so I cut the rough miters with a hack saw, and then cleaned them up with a hand file and/or a pneumatic angle grinder. It took almost 8 hours to finish all the miters.
Head tube joint mocked up.
You can see the gap between the top of the down tube and the front of the seat tube in this picture. That junction was hard to cut because there are 2 miters superimposed on each other for both the bottom bracket and the seat tube.