I recently put a two speed hub on my 6 yo daughter’s bike. It has a centrifugal clutch that engages the high gear when the wheel spins at a certain speed. I started thinking that this would be ideal for a uni hub if you are only going to have two gears anyway. The mechanism is super simple, and the engagement speed can be adjusted by bending a spring inside the hub. There’s no chance of a false neutral, and it is a smooth shift.
Maybe this could bring down the cost of a 2 speed uni hub.
That’s ingenious. No need for cables or shift buttons. The centrifugal forces do the shifting for you. An automatic gear change system that even a child can use. The unicycle purists wont like this. Lol.
If it’s actuated by RPMs, the challenge might be to find a maximum RPM you’d want to pedal in low gear. Sometimes I’ll pedal fast through short areas instead of shifting, and also I might want to pedal fast on uphill sections where the gear would just bog me down.
Also I guess the question of freewheeling would have to be addressed…
It is the SRAM automatic coaster that I put on my daughters bike. The auto shift is very smooth, and it is not abrupt and scary. In concept the best scenario would be to make a new hub with a clutch from this hub married to a Schlumpf.
The hub shifts based on wheel speed, not pedal rpm’s. For a normal uni these two things are the same, but not on a geared hub.
As for the spinning uphill scenario there are two factors for downshifting. First the wheel speed needs to slow down enough for the weights to disengage the planet gears. Second, you need to “lighten” your pedal pressure to let them release. So, if you want to spin up a hill you can do it in either gear based on how you approach the hill. It is amazingly intuitive.
I have a SA fixie three speed on my cross bike. It is really a nice piece, but I’m not sure how durable it would be in a uni. I overhauled it a few weeks ago and I was pleasantly surprised to see how clean it was inside after a couple thousand miles. So maybe it’s better than I am guessing.
It might be as simple as a sprag clutch in place of the coaster brake driver. This would give you a 1:1 reverse regardless of what forward gear you’re in. As long as the wheelspeed remains fast enough for the weights to stay engaged the hub won’t shift down while back pedaling.
Please forgive my ignorance and if this may be a dumb question but I was wondering if anyone has ever replaced the axle from a stock bike hub like the SRAM Automatix listed above with one that could be used to mount bearings and cranks instead of a sprocket or cassette. Is that possible?
I think this hub may be a possibility for a couple of reasons. First unlike a normal geared hub the axle is solid, and without a key way. Normally geared hubs use a key that slides in the axle to change the gear. Since this hub shifts on its own that is not a problem. The hub as it is was designed around a fixed axle, but I don’t see why you couldn’t have a moving spindle and fix the driver. To fix the driver tabs could be welded on so that it would replace the lower bearing cap on the uni frame. That would solve a few problems on its own.
As a proof of concept the big stumbling block would be making a sun gear that would attach to a uni axle. The easy way to do it for testing would be to cut the tapers off of a BB spindle, and weld them onto the existing SRAM axle so that you could use the existing sun gear. If it worked, a more robust version would have to be made from scratch around a larger diameter crank spindle. I just don’t see a rear hub axle being able to withstand cranking directly against it for very long.
Your description is not entirely clear to me because I’m unfamiliar with the term “driver” in this context. One difficulty in trying to do these things the easy way is that, with a sun gear attached to the axle and the planet cage fixed (to the frame), the ring gear rotates backwards. Also, if you want to run an axle through that hub that spins freely, the bore has to go through everything and you still have to have material for attachments and bearing seats.
I assumed there were two drivers in the hub one that the chain drives, and the other that pushes the brake blocks apart. The chain driver is in fact built into the planet carrier. I just found a picture of the insides of the hub, and it looks like it doesn’t use a driver to spread the brake blocks, but roller bearings and ramps.
You’re, no doubt, correct that the hub would run in reverse if the planet carrier was fixed, but couldn’t you just put the hub on backwards?
FWIW I don’t intend on trying to make this hub work for a uni. I just thought that the mechanism itself is so simple that the same concept could be applied to a uni hub. Still, even if it’s just a bench test to see if the concept could work I think there would be ways to rig it up.
If you put the hub on backwards, but driving the sun, you would still be driving the hub forward. Harper was pointing out that the driven sun gear runs counter to the driven planet carrier. The only thing amiss here is that I think the gear may be reduced if the sun is driven.
I guess it would seem a bit out of control to have the hub shift unexpectedly, but as with many things you would get used to it. As long as the wheel speed is fast enough to engage the weights it won’t shift down. If it slows down enough to release the weights you are probably going slow enough that low gear is called for.
I just tested it, and you’re right about the direction. Unfortunately, I’m right too about the gear reduction. So it would need a complete reworking in order to drive the planet carrier with the spindle, and fix the sun gear.