I plan to build rear wheel with SA 3speed with coaster brake in near future. Currently my bike (cheap roadster-like ss) has 36t chain ring. My goal is to set gearing pretty low. For example 36x22t or maybe even 32x22t with 28/40-622 rim/tyre. That gives about 30gi at low gear.

There not much of info about gearing limits on forums. Particularly SA3spd. There is only advice like setting 3rd gear as main flat gear. But what about high torque leverage?

Anyone has an experience and knowledge of durability of that setup?

I have a Brompton 3 speed with a 2 speed crank , Swiss Mountain drive..
(it's a 2.5x reduction gear so as if a 50-20 crankset or 54-21.6t' .and a 15t cog..
so low in both is like 17")
I've had, & used it like that, occasionally in low range for 8 years.
which is to say lower than what you propose..


(21.6 '/,15 x .75)x 16" 3rd is hub high gear , crank in low range, crank arms turn faster than the chainring ..

4th is hub low gear, crank in high range , direct in the crank gearbox.. arms & chainring turn at same rate

Planet gears never disengage, they are applied differently, depending on the pawls engaged.. 3/4, 1:1, or 4/3..

so far, so good ..





...

.....you can buy higher tooth count cogs for a SA 3 speed online. I;ve never used anything larger than a 22, but I've seen them up to 24. Someone might sell a lager one if you do a dedicated search. With an IG setup, you don't need to worry about derailleurs and shifting, but you probably ought to use a 1/8" chain so your chainwheels should also reflect that choice, or you'll probably get some noise as the chain moves around on a narrower one.

Sheldon Brown's
Derailer Gear/Internal-Gear Calculator includes all the commonly encountered IG hubs, so you just plug in your chainwheel and cog numbers, choose your hub, and hit calculate. It's pretty simple.

Most SA 3 speed hubs are hard to kill.

I've read that 2:1 is recommended as a limit, but can't remember where. On the other hand I've thought about it and I think it would take stronger legs than mine to break a SA hub. I've been riding 40:18 for years with no issues, internals always looked fine when I opened them up.

There's a reason why in the early MtB years that the IGH path wasn't tried more then it was. The internals would not hold up to the forces and shocks off road use (and the needed low gearing). Having said that I've run a SA AW alloy shelled hub for many years with a 36T front and a 22T rear with no on road issues. Andy

My original manual from the 50ies says 46/22 max. I run one with 46/23. It still works.

Yea, it all depends on what you are doing ,, JRA spinning,just on top of the gear, modest force in the pedals ,

or hammering hard.. jumping on them..

Old school: 2:1.

Modern thought: 'My thinking is that if you need to gear lower than the recommended 2:1 then you aren’t pushing the pedals as hard as someone who doesn’t need to gear that low. Sheldon Brown even went below 1:1 on one of his bikes. Downshift for hills.' - from Bikesmith Design & Fabrication, who offer 26T 3-speed cogs.

I have made larger sprockets for IGHs following the advice at sheldonbrown.com:

"It is also fairly easy to modify any Shimano cassette sprocket that doesn't have a built-in spacer. These are available from 14-34 teeth. Shimano cassette sprockets have the same internal diameter as those used with internal gear hubs, but have 9 splines instead of 3. With a suitable grinder, 6 of the splines need to be removed, and the corners of the remaining three rounded off. A thin spacer washer may also be needed, because the cassette sprockets are a bit thinner than the stock sprockets."

I have used a metal file to remove 6 of the 9 splines from a 28T Shimano cassette sprocket, then shape the remaining 3 splines to fit the slots on the IGH driver. As noted, the only tricky bit is making sure you have spacers to keep the sprocket running true. My first attempt with a sprocket from a 9SPD cassette was too thin and would wiggle as it turned. I conduct my testing on S-A AW hubs, because even if I end up damaging one, there are always a dozen more going wanting at the co-op.

I built what I called a Dingle 6 drive with a 52-42 crankset and both 28 and 18 sprockets on a S-A AW 27" wheel (gear changes between the two 3-speed ranges require moving the chain by hand). My conclusion was that 42-28 was even lower than I needed, but I have not yet gone back to the project to try a narrower gear pairing (I am planning to try 48-42 chainrings with 24-18 sprockets). I rode up a 7% grade hill with moderate pedaling on the 42-28 gearing and did not note any problem with the hub.

-Will