In the design of the rear triangle, I figured I should focus on the rear axle first, since the design of the rear triangle is heavily influenced by how the rear axle is laid out. I had a couple of decisions to make.
The first decision was the size of the axle. The axle from the 3 wheeler was various thicknesses with step downs from center to edge. There was no way to reuse it so I had to come up with something else. I knew it had to span at least 12 inches between bearings and without suspension, it would bear the full weight of the rider, half the bike and any additional forces from bouncing around. I decided to go for a solid 1" rear axle with a 1/4" key slot. I am not completely sure the 1/4 key will be enough to keep things from spinning, but if I get desperate I can always weld stuff to the axle. I measured the width of the tire plus space for brakes on one side and a chain sprocket on the other and decided on a 14" long shaft.
The next decision was the size of the sprocket. Since I would be content with a slower top speed than what this bike had as a three wheeler, but didn't want a giant sprocket on the rear wheel, I opted for a gear about the same size as the one that was on the 3 wheeler. A 48 tooth sprocket that accepted #40 chain. I can always gear up or down by changing the jack shaft sprockets. I was a little disappointed that I couldn't get a reasonably priced sprocket that made use of the full width of the chain, but I think it will work OK.
I ordered up an axle kit, chain and sprocket and patiently waited for them to arrive.
When the parts arrived I began arranging the parts to determine the best layout and to ensure I had made the right choices. I haven't machined the splines out of the wheel hub yet so I couldn't slide that on to the shaft, but I could get a good guess at how the axle will sit in the wheel. I pretty quickly found out that a 14" shaft was not going to be long enough. Not even close. I will have to order up an 18" shaft and cut the excess off as I think 16" might not be long enough either. The first of many mistakes I am sure I will make in this process. When you build stuff like this without precise drawings, you are bound to make some mistakes.
I received the new longer axle and put it together with the hub, brake disc and sprocket. Progress.
The first decision was the size of the axle. The axle from the 3 wheeler was various thicknesses with step downs from center to edge. There was no way to reuse it so I had to come up with something else. I knew it had to span at least 12 inches between bearings and without suspension, it would bear the full weight of the rider, half the bike and any additional forces from bouncing around. I decided to go for a solid 1" rear axle with a 1/4" key slot. I am not completely sure the 1/4 key will be enough to keep things from spinning, but if I get desperate I can always weld stuff to the axle. I measured the width of the tire plus space for brakes on one side and a chain sprocket on the other and decided on a 14" long shaft.
The next decision was the size of the sprocket. Since I would be content with a slower top speed than what this bike had as a three wheeler, but didn't want a giant sprocket on the rear wheel, I opted for a gear about the same size as the one that was on the 3 wheeler. A 48 tooth sprocket that accepted #40 chain. I can always gear up or down by changing the jack shaft sprockets. I was a little disappointed that I couldn't get a reasonably priced sprocket that made use of the full width of the chain, but I think it will work OK.
I ordered up an axle kit, chain and sprocket and patiently waited for them to arrive.
When the parts arrived I began arranging the parts to determine the best layout and to ensure I had made the right choices. I haven't machined the splines out of the wheel hub yet so I couldn't slide that on to the shaft, but I could get a good guess at how the axle will sit in the wheel. I pretty quickly found out that a 14" shaft was not going to be long enough. Not even close. I will have to order up an 18" shaft and cut the excess off as I think 16" might not be long enough either. The first of many mistakes I am sure I will make in this process. When you build stuff like this without precise drawings, you are bound to make some mistakes.
One of the difficulties of this project is to design one thing while thinking ahead enough to prevent issues later on. A good example is designing the rear triangle while keeping in mind I have to eventually route exhaust around whatever I design, or that I have to come up with a way to adapt the existing foot brake or add a hand brake and both will be effected by my choice of brake and brake placement.
I am rethinking my design for the rear triangle a bit. I don't know that a horizontal tube and a vertical tube are enough for good strength. I think I will need to triangulate things a bit so I don't end up with a parallelogram which wouldn't be very strong against the fore and aft forces. I think I will add a triangle brace from the main tubing to the upper frame that the seat sits on and also an angle from from the rear bearing to the same spot. This will greatly enhance the strength of the rear end.
I am also waffling a bit on whether to use square tubing or round tubing. Round tubing looks much nicer. Less crude. But, square tubing is so much easier to work with if you don't have a tubing bender and a way to notch all the ends.
I received the new longer axle and put it together with the hub, brake disc and sprocket. Progress.
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