Awesome Justin! I can’t wait to try it 
corbin
And the initial show stoppers
Wow, was not expecting that much of a response and interest in this, thanks everyone for all the notes!
As several people alluded, the backlash in the S3X hub was potentially going to be a big issue and I was concerned about this from the getgo, since even the tiny slack in a Schlumpf hub or a loose crank is quite disconcerting at first, though easy to get used to.
It took me a few tries to just be able to ride around the shop floor with all the play in the cranks and not fall off. That was one big issue.
The other issue was that the sprockets from a bike cassette have climbing indentations and angled teeth and are basically engineered to throw the chain off with even the slightest sideways pull. So even though I had a reasonably straight chainline, the chain would frequently start to climb up the sides of one of the sprockets, get really tight, and then snap back down, or sometimes climb right off.
I was able to minimize this by remounting all the sprockets so that they are spinning in reverse of their normal bike direction, but it would still be the case that every 100 feet or so the chain would randomly get lifted off.
Both of those were a tad discouraging, but it was still somewhat possible to ride it on the street and test the gear shifting, which was the whole point after all. And that was redeeming, because the gear changing was smooth!
Compared a schlumpf where the gear change is so high and your legs need an immediate adjustment to the new ratio, the 20-25% change in gearing in the S3X felt like nothing. And you knew right on flipping the lever when the gears would change rather than the period of uncertainty that follows each attempt at a heel button hitting on the schlumpf.
So that was encouraging for sure.
Backlash Measurements
Indeed, in direct drive and 2nd gear, I was getting about 10.5mm of play at the end of the 165mm cranks, or about 3.5 degrees. However, when I did the same measurement in 1st gear, the play was over twice as much, almost 22mm or 7 degrees.
That was interesting, since I was always mounting in first gear and found the play seriously problematic, but felt that it was less of an issue once I switched into 2nd or 3rd.
3.5 degrees of play in the pedals is definitely a lot, but once you are up to speed in the higher gears you can learn to ride while always keeping a forward pressure on the pedals and modulating that for balance, and then you don’t end up feeling the slack that happens with torque reversal nearly as often.
I don’t think any unicyclists would consider this much play acceptable, but you can get the gist of riding with it easily enough.
Ratios
Hey Rob, yes, exactly. There is an additional benefit to further gearing up to the jackshaft which is that not only does the backlash get reduced, but so too does the torque on the S3X. I’ve read some reviews of heavy riders blowing out the S3X hub from all the abuse that they get fixie riding, so by changing the gears so that the internal hub spins much faster at a lower torque we’re being a lot gentler on the transmission which is good too.
In fact my original plan was to do this and minimize backlash from the getgo, but then it looked so much easier to attach and work with small sprockets, so I thought to at least try it with those first, and who knows maybe it wouldn’t be much of an issue.
So it was back to the drawing board and the bike parts bin, this time using bicycle chainrings rather than cassette sprockets. This also takes care of the chain climbing issue since the front chainrings are all just flat metal.
They’re heavy, but SO WORTH IT! (I’ve been riding a Nuvinci on my cargo bike for the past 3 years), plus the new N360 model has shaved off a few pounds. I still have hope that a bidirectional nuvinci will materialize, and will be pestering their engineers again soon now that I can show just what we’d do with it.
-Justin
Fixie Riders
That is great news to hear, it means that feedback pressure on Sturney Archer from their target customer (= fixie riders, not unicyclists) will be to tighten up the backlash, and that will serve us well too. Hopefully this is something that they are already addressing.
-Justin
Thanks for making the measurements. That baffles me. I would expect that the backlash introduced by the hub would remain the same throughout the gear range. The tooth contact is not changing and the number of teeth engaged isn’t changing that much. If the amount of backlash changes as different gears are engaged then some part of the engagement mechanism is coming into play. If you think about it, how do you shift a fixed gear transmission if you don’t have any slop?
I hope spinning up the hub speed works for you. With reasonably sized cogs I still think you can only reduce the backlash by a factor of about three. In order to get it down by a factor of ten you’ll need 150 chain rings. I think that’s kind of unmanageable. Hopefully SA is looking into reducing it a bit.
Good work and good luck.
Wow, very impressive work. Would love to see a video of it in action!
Chainring Adapters
Anyways, a couple afternoons with the rotary table on the milling machine last week and I had a set of adapters made to accommodate the 5-bolt spider pattern of a chainring to the small 6-hole patterns on the hub and wheel:
Large Chainrings Cont’d
So here is what it looks like at the moment. I’ve got a 52 tooth chainring on the pedal crank which was the largest I could find, and a 44 tooth on the wheel side. The sprockets on the SSX hub were also changed to a 13 tooth on the input, and a 22 tooth on the output. So the overall chain ratio is now:
52/13 * 22/44 = 2.0
Slightly slower gearing than the initial build. The gearing from the pedals to the S3X is now 4:1, while the original build was 1.86:1, so in principle it should have less than half the hub induced backlash.
I was a bit worried that the larger chainrings would detract from the tidiness of the build, but it still looks OK, and some nicely made side panels should be able to enclose most of the exposed chain and transmission hub.
Nice adapters - very tidy
Is the machining “just” a hobby or do you do it professionally?
Rob
And the results
So I finished the new gearing modifications last Wednesday and took it down to the Vanuni meetup at Science World. Unfortunately the chain lengths didn’t work out as perfectly as with the first set of gears, so the wheel side had about a quarter link of chain slack when the drive side was tight.
Even with the loose chain though it was still a marked improvement over the previous attempt and totally worked. An’So rode it around too and was shifting gears on her 2nd try. Her first comment was that it should be geared to go faster and I agree! It handled super easy in the 48" ratio, and with the long cranks it seemed like 56 or even 60" would be more appropriate.
Unfortunately it was getting dark by the time all the chain fiddling was done and I didn’t get any pics of it in action.
The plan now is to modify an 11 tooth cassette sprocket so that it slides down the splines of the S3X hub to replace the 13 toother, then I’ll have a 2.36 : 1 chain ratio, so a top gear of 56", and potentially a bit less pedal backlash too. So between that, and a chain tensioner, it should be a about as good as this prototype will get.
If I do these by this Wednesday then I’ll be sure to have it at the Science world meetup again. So we can have more people test ride it for first opinions and get some long awaited footage for you too.
-Justin
If you could modify a Schlumpf hub for the wheel, wouldn’t you have a 6 speed? 
Ha that’s a funny thought. It’d be a tad more complicated than that. You’d need to mount the chain sprocket to one side of the Schlumpf, then have a separate independently rotating hub machined that fits over top of the Schlumpf shell, which would require some pretty massive ball bearings.
OK quick caliper measurement shows the shell on the Schlumpf hub is 85mm diameter, so you could do it with a pair of these:
http://www.vxb.com/page/bearings/PROD/20mm-and-more/Kit8736
It would probably make a lot more sense in this case to start with one of the standard Schlumpf mountain-drive or speed-drive bottom bracket units rather than their uni hub. Then you’ve already got the chainring installed and are dealing with a much smaller hub diameter.
Does anyone here know if these original Schlumpf products work bidirectional with a similar low backlash as their guni hub?
-Justin
Florian claimed that the bottom bracket was not suitable for use with a fixed gear bike and not bidirectional when I asked him about it years ago. I don’t know if that has changed.
Darn, I wasn’t feeling well so I missed science world last week. It’s looking really good, I’ll try and remember to bring my camera to get some video this week.
6 Speeds?
Apparently since late 2009 this was changed for fix gear compatibility:
If that’s the case then this is great, but we’d still need to find out what the backlash is like before adding that to the already problematic slop of the S3X to make a 6 speeder. The overall gear range would be awesome, but the switch between 3rd and 4th gear would be a pain to pull off while riding.
-Justin
Not only does it look tidy, the big chain rings look really cool. This is a fantastic project and it seems like you’re able to tweak it fairly quickly. Knowing what a little bit of backlash feels like, I’d really like to try your rig out and see what a lot feels like. Maybe there is a threshold and you can’t tell the difference between 1mm and 10mm.
Also, maybe Klaas is right and the main instability introduced by the internally geared hubs and sensed by the rider is the thrust exerted on the frame by the sun gear attachment. If so, this shiftable jackshaft design is the future of geared unicycles.
Just want to say that this looks beautiful!
What an amazing prototype. The larger chain rings look very cool.
Sam
beautiful project. Had to go over all the posts twice to understand how it all works together. I mainly envy you for your machines you seem to have at your disposal to do the work
Late night ride
Done and done. Apparently my dremelling skills aren’t perfect as I slightly cracked the 11 tooth sprocket when pressing it onto the splines of the hub, but she held up and I got my first real road ride in tonight. New gear ratios are 57", 42", and 35", and the corresponding backlash on the pedals is roughly 5mm, 6mm, and 10mm.
The improvement is pretty good. I went about 6km with only one UPD when I was trying to outrun a car on my tail. The only other glitch was getting called on by the cops for riding at night with no lights or helmet, which they made me walk back home to fix.
Riding on the road, this amount of backlash wasn’t a really big problem. I was trying to make a mental note whenever there was a torque reversal, and usually it was followed by several pedal revolutions with the same forwards or reverse torque. But when I was going slower on the bumpy sidewalks and needing to do frequent corrections it was a lot more of an issue.
I’ve never ridden a schlumpf coker before so the 57" gear took a few minutes for me to get used to. I’d basically flip down to 42" whenever approaching an intersection or tricky bit, then go to 57" at any decent straight stretch. There was no need to use the 35" gearing at all. It confirms what some have said that if we could choose the gearing on a 2 speed hub to be closer together, like 1.5 and 1.2, then it would match what we’d want while riding a lot better.
Overall though I’m pretty happy with how this has worked out. Next tweak will be putting on some shorter 137 or 150mm cranks.
