Two Speed Automatic Transmission

[HondaJohn]

Its been a while since I've posted anything, but I wanted to make sure it works before I put it up. I wanted to install gears on my race bike to make it useful in the hilly area of CT that I moved to, but I didn't want to stretch the frame or relocate any of the major parts. I found a transmission design originally used on a Rupp TT-500 mini bike, and was able to reverse engineer it based on online photos. It uses two centrifugal clutches, each operating a separate gear, to create a tunable two speed trans.

The first clutch mounts to the engine crankshaft, and drives a sprockets mounted to the secondary shaft. This sprocket contains a sprag bearing, or one way clutch, which locks and allows the secondary shaft to be driven by first gear. When the shaft reaches the correct rpm, the second clutch, which is mounted on the secondary shaft and driven by a solid sprocket mounted to the crankshaft, engages and increases the speed of the shaft past what first gear can manage. The sprag bearing disengages, first gear freewheels, and it stays in second until you decelerate.

I'm working on a set of blueprints and a parts list so anyone can easily recreate the one I built. I'll try to have it ready to go within the next few weeks, but until I can figure out where to put it for download you'll have to pm me for a copy. They won't be perfect, and final dimensions and fit will depend on the project and builder, but they are a good start.

[coachgeo]

interesting..... thanks. Will be interesting to see how it likes torque

[XLerate]

Yes, looks good & thanks for sharing but CoachGeo is right about the torque handling, hope for the best there.

[HondaJohn]

The design can easily handle the torque. Both of the clutches are rated to 15hp, and it uses #40 chain. This has a nominal rating of 810 pounds working strength. Load on the chain increases as the sprocket gets smaller, so torque will be at its peak when the transmission is in first gear.

The chain connecting the first gear clutch to the sprag bearing sprocket has to transmit a peak of 11 ftlbs. The 10 tooth sprocket on the clutch has an approximate chain center diameter of 1.5 inches, so torque will be multiplied by a factor of (12in / 0.75 in = 16), so the linear load on the chain will be 176 lbs. The final drive primary sprocket, with 12 teeth, has an approximate diameter of 1.875 inches, so the multiplication factor is (12 / 0.9375 = 12.. Since there is a 2 to 1 ratio between the crank and secondary shaft, the applied torque is 22 ft lbs, for a chain load of 264 pounds.

The sprag bearing, CSK25PP, has a torque limit of 68 Newton Meters, or 50 foot pounds. This gives a factor of safety of 2.3, so the limiting factor is the sprag bearing. It would have to be uprated to a CSK30PP bearing, with a torque limit of 110 Nm, to be safely used with a Yanmar L100 or clone.