3 Speed Hub As Jackshaft Question.

No hurry at all Anne. I have no plans at this time for doing this, but with 4 or 5 salvaged SA 3 speeds sitting on a shelf and one or two more still on old bikes, I keep thinking there will be a good use for these and that they have too much potential to just throw away. While I've very interested to see what CB2 & Brown do with their new fixie hubs, like you I have serious budget constraints which pretty well eliminates buying a new one. The thought that we might alter an old one to turn into a jack shaft transmission for free is pretty compelling. Verbal descriptions and diagrams are helpful, but photographs can tell so much more. Thanks. How is summer progressing?
SB
 
silverbear said:
No hurry at all Anne. I have no plans at this time for doing this, but with 4 or 5 salvaged SA 3 speeds sitting on a shelf and one or two more still on old bikes, I keep thinking there will be a good use for these and that they have too much potential to just throw away. While I've very interested to see what CB2 & Brown do with their new fixie hubs, like you I have serious budget constraints which pretty well eliminates buying a new one. The thought that we might alter an old one to turn into a jack shaft transmission for free is pretty compelling. Verbal descriptions and diagrams are helpful, but photographs can tell so much more. Thanks. How is summer progressing?
SB
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After suffering some very hot and humid days last month we seem to be moving into cooler weather which is good because it means I can actually be active and doing things without feeling like I'm melting.

Like you Silverbear I have several SA 3 speed hubs that I salvaged from the council tip and I would like to see what could be done with them :)
 
OK heres a pic for concept on the old Shimano hub. Its needs a 20T coaster cog to get the proper ratio. It will be a bit different in operation than the SA fixie hub but will look and mount the same. Am waiting on cogs for the SA.

On the SA the normal drive input is used by the motor and the hub outer drives the back wheel. On the Shimano the outer hub is driven and the normal input drives the back wheel. Disadvantage to the Shimano, is engine will not pedal start. On a CG have to use the pull start option. Advantage old 3sp hubs are every where cheap.
No disadvantages to the SA fixie other than it relatively high$$.

Heres a pic of the build it will go on at some point. Plenty of room behind the seat post for the hub mounting. Gearing will be such that a top speed of 32mph will be reached at a reasonable rpm of 6500. Cruise will actually be just under 30 at 6000. This is close to stock single speed CG gearing, but not having to rev it so hard to get there. The other two speeds will be lower(1&2).
 

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Thanks CB :) I think I have a Shimano hub somewhere amongst all my spare bits. I'd pretty much discounted it because I'd heard they were supposed to be fragile. Perhaps I should look at having a little experiment with it.
 
Yes I have heard they are fragile, and this one had a broken shift key(easily made another). Its was more of a proof of concept thing. I would assume a SA would work in a similar fashion and be more robust. If you have an SA clamp the axle in a vise(protect the threads!) turned around as if it was mounted on the left and turn the hub it should then drive the input sprocket. Count the turns and see how many usable gears you have. When the Shimano is reversed it has only two speeds. Still a high and low range on the cheap is pretty cool!
 
So I ran a bunch of #s thru the gear calculator I have and the results are pretty cool. The backwards turning 3sp offers a .25 overdrive as well as a 1:1 direct. That means in hi 6000 engine rpms equal 7500 output rpms to the rear wheel. In low range or 1:1 engine =output. if your engine turns 7500 the output would be a whopping 9375 driving rpms in OD! Hub might not like that! The cool thing is the drive ratios can be easily changed just by changing the coaster sprocket(output) the inner sprocket is a fixed value (in my case 21t). I think coaster sprockets range from around 15t up to 22t so a wide range of ratios are possible. No need to fool with engine or wheel sprocket changes Just change the one coaster sprocket and adjust the chain or add or subtract a few links. One could have premade chain additions to make it easier. If the outer support arm for the chain side of the hub was a bolt on then sprocket/chain changes would be pretty quick.

There was close to a 7mph jump between the two ranges with one of the combos I ran thru the calc. using my 6000 engine rpm standard. Be nicer to have more speeds but this hub came out of the recycle pile for free, that's a great price! This would be a good simple trans for a road bike that needs higher cruise speeds using a basically stock tuned engine.

The 3sp fixie hub should be just right!
 
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Need to correct the 9375 rpms at the drive hub, that's incorrect. Its the equivalent to 9375 crankshaft rpms, the hub is effectively multiplying the engines output by 25%. The engine is turning 7500, but the hubs output is being driven(through the stages of reduction) as if the CG engine was driving the rear wheel directly from its sprocket at an engine rpm of 9375 The hub is actually turning less than 1000 rpms. This is of course affected by the size of the sprockets used on the hub. These figures assume a 1:1 ratio through the hub.
 
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OK got the sprockets in and mounted on the new SA S3X fixie 3speed. I didn't get the chain lines as close as I wanted but it will do, plenty of chain clearance though. The driven sprocket is a 22t and the driver is a 23, just worked out that way. Will overdrive the wheel by a small percentage. Im running an MTB wheel which is dished toward the right. Combining that with the standard CG dished sprocket also dished to the right and I think I will have good a good chain line for the rear wheel.

The hub has 3speeds the first is 62% of the input, second is 75% and third is 1:1 This computes to 32mph@6500rpm, a good speed at a fairly relaxed rpm. top speed in the gears(40t rear sprocket) 1) 20 2)24 3)32. Knowing that my bike will ride tractably at 10 mph as a single speed , means in low It will be happy at 6mph. Great for some off road riding I want to do. Now its a relatively easy job to make the mount for the hub and get it "chained up".
 

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