| Most recumbent bikes and HPVs in general are designed for
ergonomics, or low aerodynamic drag, or both. Because of this, the chain line is often not
a straight shot between the crank chain ring and the driving gear cluster, causing the
chain to be "bent". The are two methods of bending the chain line. The first is
to use a large pulley to redirect the chain. The advantage to this is a simple one chain,
one derailleur system. It's so simple I don't even have to explain it.
The second method is
to use an intermediary set of gears somewhere between the front chain ring and the driving
gear cluster. The advantages to this design are relatively low friction and the ability to
"step up" the gearing. This step up in gearing allows the use of smaller driving
wheels to achieve high speeds, without the use of hard to find 73 tooth chain rings,
or power robbing internal geared hubs. The disadvantage is that there are now two chains
to manage. This usually means a secondary derailleur
to manage the tension in the front
chain, as well as chain guides or "keepers" everywhere. In this page I will explain how to make these intermediary gears, which are
commonly known as a "jackshaft".
You
can make a jackshaft using either a Shimano style freehub, or an older
Suntour style freewheel. The
first example is using a freehub.
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First remove the gear cluster
assembly. You may need the special Shimano cluster too to accomplish
this, as well as a chain whip tool. Most freehub gears clusters
are riveted together, but it's pretty easy to drill out the rivets and free
them up.
Next remove the hollow axle. After
the axle is remove there is still a thin metal cap over the
bearings. You can pry that out with a screwdriver. Remove the axle
bearings so they don't bounce around underfoot. |
| Next loosen the hollow hub bolt
counter-clockwise a couple turns with a
10mm metric Allen wrench.
This keeps everything together while you wrestle with the bearing
cup. |
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The freehub needs to be taken
completely apart so the pals can be removed. This allows will allow
the freewheel to spin in either direction.
It may be necessary to make a tool
like this from 1/8" thick steel to loosen the bearing cup. The notch
is not needed, that was just a feature of the metal scrap I found. |
| Insert your tool into the notches in
the bearing cup, and twist clockwise. |
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Here's the hub with the hollow bolt,
and the bearing cup removed. |
| The inner guts of the freehub can
then be removed. Make sure to take this all apart on something
clean, as some of the bearing are going to fall out.
Remove the palls
(the things that go clicky clicky),
and the wire keeper that holds them in. Chase down the bearings and
put them back into place, and reassemble the freehub. |
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Cut the threaded section off of the
hollow bolt. It will be used as a spacer. You will need to use a cut
off tool or dremel rather than a hacksaw, as the hollow nut is made
of hardened steel. Make a thick washer, which will go on the bottom
of the freehub. This mid-drive
will be used as part of a human powered boat drive, so it looks a
bit different that a bike mid drive, but the same concepts apply. |
| Build up the new mid-drive. The
parts pictured left to right on this axle are an axle bolt, a spacer
made from a chunk of tubing, and axle spacer, the thick washer, the
freehub, another spacer made from tubing and the right side nut.
I wrapped some electrical tape around
the axle to space it out to the inside diameter of the hollow nut.
This ensures that the freehub is properly centered on the axle when
it's all tightened down. |
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Here is the completed intermediary
jackshaft. I used a couple of spacers between the two gears to
ensure they were far enough apart that the chains would not rub. I
made a big lightweight spacer for the remaining space on the hub
with some handy aluminum tubing. |
| Here's how to attach the freehub to a bottom bracket cup, without having a machine shop at your
disposal. First you need a steel bottom bracket cup
from a cheesy bike with a non-sealed-bearing-BB (item 1). Then you need
to rip apart a perfectly good rear cluster, remove the pals so it turns
both directions, and put it back together (item 2). Hack the threads off
the hollow star nut that held the carrier to the hub so you can use it
as a sleeve (item4). Use an old solid bike axle (item 3), plus a bunch
of axle spacers and washer to complement... |
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Slide the axle with the "item 4" sleeve
into the carrier. An axle nut and a spacer and another nut are to the
right of the sleeve. The additional spacer and sleeve will be used to
mount the derailleur later. |
| Add another spacer to the back of the
carrier, to keep the axle centered in the hole back there, and to space
it out a bit from the BB cup. |
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Add the big washer, and another spacer.
This fills in that huge hole in the BB cup, and centers the axle in
there.
Note that there is a certain amount of
slop between axle and the sleeve, and between the spacers and the BB cup
hole. You can wrap the axle with electrical tape to make the sleeve fit
snugly, and when it's all tightened down it will be plenty strong. |
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Add the BB cup, a couple more washers, and
a nut. Tighten to a torque of one grunt.
Uh. That last nut sitting there forlornly is
left over.
Move along now... |
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Suntour Freewheel Jackshaft |
| You
can make also make a jackshaft using a freewheel, which is an older type of gear cluster, commonly made by Suntour. You'll need a chain whip to remove the gears.
Even if the wheel is a throwaway, leave
the spokes attached to the wheel until after you are done removing
the freewheel. It's much easier if you have something to hold onto
while you are grunting away, removing gears that have been gunked in place for the past 20
years. As you can see in the picture below, I cut the spokes before removing the
freewheel body, making it much harder to remove. |
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You'll need to completely disassemble
the freewheel, to remove the palls. This will allow the freewheel to spin
smoothly in either direction. Depending on how tight it is, you may need some
special tools to disassemble the freewheel itself. If you don't have the
special tools, you can just mount the freewheel body in a vise and use a
punch in one of the two holes in the bearing keeper to
tap the keeper loose. Note that loose is clockwise. Be careful when you get it loose, there
are approximately 1 million ball bearings inside. Clean everything well. Use some
grease
to stick the ball bearings in place when you are reassembling it. Wash the grease out with
a lightweight oil. Grease is too heavy a lubricant for those little balls. |
| The threads that held the freewheel onto the hub are the same
as in a bottom bracket. The picture below shows the back of the freewheel and a bottom
bracket retainer. |
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After you screwed the bottom bracket retainer into the
freewheel, you can just screw that into a bottom bracket. |
| If your project happens to have a
bottom bracket in the right place, you are lucky. If you are not so
lucky, you can buy plain bottom bracket shells, hack them up, and braise them right to the frame. If you are
unsure of the jackshaft placement, you can mount the remnants of the bottom bracket to an adjustable clamp. Note that by using larger gears you
will attain higher power transfer efficiency. The smaller gear in the photo below is just
there to retain the larger gears.
You will need a secondary derailleur
to keep the front chain
from popping off. I recommend mounting it as close to the intermediary gears as possible. |
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