Bicycle Mechanics - Upgrading trike IGH; where to get the right gear?
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My wife adores her 3-speed Torker Tristar tricycle, and despite my urging, she will not be swayed to try any two-wheeled contraption. She would, however, like to have a broader range of gears to choose from when tackling the hills of Austin with me. To that end, I'm investigating an upgrade to her trike's Sturmey-Archer TS-RC3 gearbox (http://www.sturmey-archer.com/products/hubs/cid/6/id/41/specs/1). It is mounted between the crank and the rear wheels, in a horizontal dropout 131mm wide, connected to the crank in the usual manner, and outputting to the rear axle via a ring-shaped gear attached to the drive-side spoke holes.
It looks like a Shimano Nexus SG-7C18 7-speed with coaster brake would just fit in the space (reports of its OLD vary from 130mm to 132mm). I don't know if (and don't think that) the spoke flange gear from the current hub would fit on the new one. Where could I find such a gear, or how would I go about having one fabricated?
11-26-12, 04:06 PM
This definitely sounds like a custom machining job. You'd have to have a sprocket made with a central hole large enough to fit around the hub shell, and either welded to the spoke flange or bolted to it with small bolts through the spoke holes. Another possibility is extending the existing range, by installing a double or triple crankset and front derailleur, with a chain tensioner to take up the slack. Alternatively you can fit two sprockets to the hub back-to-back, and shift between them using a rear derailleur.
11-26-12, 04:18 PM
Do not try a front derailleur and tensioner; do not try gears out back with a derailleur -- coaster brake and tensioners do not play nice together...
You could gear the whole system down so that top gear is a decent cruising gear with two steps down by fiddling with a smaller chainring up front or a smaller cog on the gearbox... or both. But you'd lose top end that way and not extend the range.
Or as Monster Pete suggested -- see if a local machinist can affix a cog out back like on the original SA unit. Either drilling the rivets that hold it on to get it loose for use with the Nexus hub, or starting with probably a chainring of the same tooth count and going from there.
Andrew R Stewart
11-26-12, 04:28 PM
Trailmate and Sun Bicycles both have SA 3-spd/coaster brake options. Both use a cog attached to the hub shell or spoke flange for the chain drive to the rear axle. whether either might fit your trike is a question i don't have the answer for. But I always try to find an already manufactured solution before custom fabracating something.
Do you have a LBS that can help source either brand? If you were in Rochester i'd quote you a kit in a few minutes. Getting the dimensional numbers/specs might take a bit longer.
Here's a link to the Sun Bicycles kit Page.
Good luck. Andy.
I see that they also have 5-speed options, which might be a good compromise between the status quo and a complicated custom conversion. Thanks for putting me on the trail!
11-26-12, 09:16 PM
Sturmey Archer makes a 5 speed mid drive unit http://www.sturmey-archer.com/products/hubs/cid/6/id/46/specs/1
This unit also has a reverse gear, which will only work if the cog on the axle is fixed, not freewheel.
It's listed as 139mm OLD. Never seen one up close, so I don't know if it can be narrowed by switching nuts and spacers. Most models of Sturmey Archer hubs can be fiddled with to alter OLD somewhat.
11-27-12, 03:53 AM
Trike IGH, fit a second sprocket to the Hubshell, You can Buy Sturmey 3 speeds like that , Shimano too.
If you know some place that does Laser cutting ,
I've seen custom sprockets that have been made to poprivet on the spoke flange.
and ages ago, a friend's Dad took an old steel shell 3 speed , welded on another sprocket,
used it as a Jack shaft, located up atop the seat stays
and ran 2 chains, one up to the hub with 2 sprockets, then down to the hub in the wheel.
and you could get a fabrication kludge to do that with the other 2 sprocket hub for 6 speeds.
Maybe just have a chainline that impinges on the rear Basket space , between the rear wheels.
now with a 3 and a 5 speed IGH, and 3 chain loops you could do something interesting.
Sturmey Archer makes a 5 speed mid drive unit...
This is your best option if you can get the OLD to work in the trike mid-drive cradle. The S-A 5 has just a bit wider overall gear range than the Shimano Nexus 7, but of course fewer gears. I don't know of any place that has these in stock right now, but I haven't searched extensively.
11-27-12, 01:53 PM
Then pick and change cog on the hub and chainring on the crank, 13 ~22T, to get the range the hub,
new 5 speed adds 1 overdrive and 1 reduction gear outside the AW3 range.
11-27-12, 02:48 PM
Ah yes, I didn't spot that the original hub was a coaster brake. In that case, going to any hub without an internal brake (cable or foot-operated) will lose all braking on the rear wheel. A conventional drum braked unit should work though, so you're not just limited to coaster brakes. With a coaster brake you also can't run any kind of sprung tensioner or derailleur hybrid setup, as was pointed out.
I went ahead and ordered the Sun S-A 5-speed conversion kit from Niagra Cycle Works via Amazon (http://www.amazon.com/gp/product/B0063R1ZDU). As tcs noted, it has a range wider than the Nexus 7-speed I had been considering, and has a coaster brake built-in, just as the current 3-speed does. Plus, it comes with the requisite spoke flange gear already installed. It doesn't look like it takes a degree in rocket surgery to swap it out (just a wrench and a chain tool), so I'm optimistic that we'll be briskly ascending the "big" neighborhood hill not long after she opens it up on Christmas. Thanks, y'all!
11-28-12, 04:32 PM
Braking will be just 1 rear wheel .. but is better than just the front.
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