Doug Burton
Zen MBB Master
Sigma. "Dangerbike". The kid that didn't fit in. The red-headed (framed) stepchild.
I've had one of these in a box in my garage, un-assembled, for about 12 years now. When I was asked by Jim and Maria to pre-assemble about 13 of them, this one was the "runt of the litter". When another one had a defective or missing part, this one was the donor and and it received the bad parts so the other ones could be complete. So I ended up making some parts. No biggie, I like working with my lathe/milling machine.
I decided I would try to fix the issues. Among them:
Strange steering behavior
Narrow range of gearing from the i9 SRAM gearhub
Strange steering behavior
Strange steering behavior.
I attacked the gearing issue by installing a 1 x 11 Shimano M8000 system, using a 46T - 11T cluster and RD-M8000 rear derailluer, plus flat bar trigger shifter. Those work REALLY nicely. I had to install a power-side idler to keep the chain off the upper chainstay in the lowest 2 ratios (the gearhub didn't need room for the chain to move) But all that worked very well.
I tried mounting a Novosport hardshell to it, which I got to fit very nicely. But it didn't work well with the steering.
Did I mention this bike has strange steering behavior?
The Sigma has a headtube angle of 61 degrees, and the original fork has 68mm of offset. This gives 68mm of road trail and a wheelflop number of 28mm. These appear to be perfectly reasonable numbers. Yet the bike has enormous wheelflop, and combining that with the single u-joint steering, and the non-linearity of the output from the u-joint makes for an interesting ride.
Maria found a big box of Sigma parts in the warehouse during the move, and asked if I wanted it. Who am I to ever turn down free bike parts?
Turns out, a variation of the Sigma design was completed with 2 u-joints, which cancels out the non-linearity when they are phased correctly. All the research parts for this configuration were in the box of goodies from Maria. Jackpot, eh?
So I built it with 2 u-joints. Didn't work. It felt like it wanted a steering wheel instead of handlebars, because the steeting axis was now horizontal. It also closed-up the cockpit and made the bars interfere with my legs bigtime. So I undid the 2 u-joint setup and went back to the original.
Then I attempted to address the wheelflop. After much research on bikes that use around 61 degrees for the headtube angle, doing a lot of math, and reading everything A.D. Carson wrote on his Recycled Recumbents site, and reading Gardiner Martin's original Tour Easy plans, I decided to make a fork having 4.5 inches of offset, using a 700c chromoly road fork and bending it. Very hard to bend precisely. Never got it 100% right, but close enough to see the wheelflop was much better. But the resulting road trail was less than 13mm (and it pulls to the right because of the fork imprecision). Still pretty twitchy. Had to make a bolt-on adapter to carry the front brake caliper, but that was kinda fun. And it fits and works well.
I will make another, more precisely modified fork and try it again, but unfortunately the end of the story is that at least I have a lot of new cool parts to use on another project, and I know a WHOLE lot about LWB engineering that I had not experienced previously.
But you know, I had a lot of fun making and modifying parts.
I'll put up some photos if I can figure out how to navigate the file sharing system.
Be well, go fast, and have fun.
Doug
I've had one of these in a box in my garage, un-assembled, for about 12 years now. When I was asked by Jim and Maria to pre-assemble about 13 of them, this one was the "runt of the litter". When another one had a defective or missing part, this one was the donor and and it received the bad parts so the other ones could be complete. So I ended up making some parts. No biggie, I like working with my lathe/milling machine.
I decided I would try to fix the issues. Among them:
Strange steering behavior
Narrow range of gearing from the i9 SRAM gearhub
Strange steering behavior
Strange steering behavior.
I attacked the gearing issue by installing a 1 x 11 Shimano M8000 system, using a 46T - 11T cluster and RD-M8000 rear derailluer, plus flat bar trigger shifter. Those work REALLY nicely. I had to install a power-side idler to keep the chain off the upper chainstay in the lowest 2 ratios (the gearhub didn't need room for the chain to move) But all that worked very well.
I tried mounting a Novosport hardshell to it, which I got to fit very nicely. But it didn't work well with the steering.
Did I mention this bike has strange steering behavior?
The Sigma has a headtube angle of 61 degrees, and the original fork has 68mm of offset. This gives 68mm of road trail and a wheelflop number of 28mm. These appear to be perfectly reasonable numbers. Yet the bike has enormous wheelflop, and combining that with the single u-joint steering, and the non-linearity of the output from the u-joint makes for an interesting ride.
Maria found a big box of Sigma parts in the warehouse during the move, and asked if I wanted it. Who am I to ever turn down free bike parts?
Turns out, a variation of the Sigma design was completed with 2 u-joints, which cancels out the non-linearity when they are phased correctly. All the research parts for this configuration were in the box of goodies from Maria. Jackpot, eh?
So I built it with 2 u-joints. Didn't work. It felt like it wanted a steering wheel instead of handlebars, because the steeting axis was now horizontal. It also closed-up the cockpit and made the bars interfere with my legs bigtime. So I undid the 2 u-joint setup and went back to the original.
Then I attempted to address the wheelflop. After much research on bikes that use around 61 degrees for the headtube angle, doing a lot of math, and reading everything A.D. Carson wrote on his Recycled Recumbents site, and reading Gardiner Martin's original Tour Easy plans, I decided to make a fork having 4.5 inches of offset, using a 700c chromoly road fork and bending it. Very hard to bend precisely. Never got it 100% right, but close enough to see the wheelflop was much better. But the resulting road trail was less than 13mm (and it pulls to the right because of the fork imprecision). Still pretty twitchy. Had to make a bolt-on adapter to carry the front brake caliper, but that was kinda fun. And it fits and works well.
I will make another, more precisely modified fork and try it again, but unfortunately the end of the story is that at least I have a lot of new cool parts to use on another project, and I know a WHOLE lot about LWB engineering that I had not experienced previously.
But you know, I had a lot of fun making and modifying parts.
I'll put up some photos if I can figure out how to navigate the file sharing system.
Be well, go fast, and have fun.
Doug
