Update: March 2015
I've got well over 1,000 miles on this commuter bike now. Here's what it looks like:
It's still running fine. The hub (and the bike in general) has no problems and has needed no maintenance other than the initial work I did after I first got it (mentioned below).
Update: June 2014
Seattle's had a warm sunny summer and I've been riding my bike to work a lot. I've now got about 750 miles on this bike and hub. It's running great and I've been really enjoying this bike. It's comfortable, fast and simple. My daily work ride has some steep hills. I don't need a wide range of gears but there's no way a single speed would do the job. I installed mustache bars for a more upright ride position.
About a year ago I put one of these on Emma's bike. Meanwhile I wanted to have a bike for riding to work on nice days. Goals: fast, comfortable, simple, reliable, cheap. I like the bare simplicity of fixed gear bikes, but they're impractical (lots of hills around here) and unsafe (no freewheeling, can't lift pedal in corners, etc.). The S2 hub seemed ideal for this kind of bike. It has the simplicity of a single speed, but it freewheels and has 2 speeds, internally geared. You shift gears with a quick 1/8 turn backpedal.
The S2 requires you to buy or build a wheel. I opted to build. Ran the numbers in my spreadsheet to get spoke lengths, ordered the spokes (14 gauge), and built the wheel. I reused an old rim I had so my cost was the hub and spokes. For most people, the cost will be much higher, either an entire wheel or paying someone to build it. That's really the biggest drawback to this hub: getting it on your bike means the hassle of replacing or rebuilding your rear wheel.
All internally geared hubs apply counter-rotating force against the wheel axle. Thus the axle must be bolted to the frame - it must be secured so it can't spin. You cannot use a rear quick release. Furthermore, the axle has flattened cutouts with notched and tabbed washers that lodge in the rear dropouts to prevent the axle from spinning. You must install these and make sure they're secure.
The S2's low gear is 1:1, the high gear is 1.38:1. Thus whatever sprockets you pick define your low gear, then you get a high 38% taller. This means if you're putting it on a bike that is already a single speed, and you use the same sprockets, the ratio you already have will be your low gear.
I computed what I wanted in gear inches first, then worked it backwards to sprocket sizes. I wanted 50" for the low gear because there are some steep hills around Seattle. That puts my high at 69". With the 700x28 tires, this meant using 38 front and 20 rear. I started with 38-22, which is about 10% shorter, after a few months found that I wanted a slightly taller gear. I find myself wishing the S2 had a bigger jump between ratios. 38% sounds like a lot but it's really only 2-3 gears apart on a normal bike. When you only have 2 speeds you need a bigger jump between them and 50% would be nicer. That would give me a 70" high gear with a 47" low, which would be perfect.
Most geared hubs have a maximum rear sprocket size to avoid putting too much torque on the internals. The S2 specs say that max size is 22 teeth.
The S2 hub feels heavy, but it's light considering the equipment it replaces. You eliminate an 8-10 cog rear sprocket, multiple front chainrings, dual derailleurs, dual shifters and cables. Add all that up and you end up lighter than when you started. If you're putting it on a bike that was already a single speed, it will be a little heaver than it was. But not much, and it's weight centered around the axle, which doesn't contribute to rotational inertia.
Operation and Shifting
The S2 comes in 2 versions: coaster brake and freewheel. I was tempted by the coaster brake for minimalism - get rid of the brakes, cables and levers. But I opted for freewheel for 3 reasons:
The S2 shifts by pedaling backward about 1/4 turn. The exact amount varies depending on your gear ratio. Each 1/4 turn backward toggles the gear: low to high to low to high etc. So if you spin the pedals backward it shifts back and forth; when you stop you won't know what gear it's in.
And that is a bit of a problem: knowing what gear you're in.
There is no explicit indicator but there are 3 ways to know, easiest and best first:
At first, shifting was not reliable - it took 3-5 tries to get it to shift. After about 60 miles of riding it persisted. I took apart the hub to see if I could improve the shifting. I'm glad I did. Inside the hub are 2 spring loaded pawls, 180* apart, that engage teeth for high gear. One of the springs wasn't properly tensioned, so that pawl wasn't engaging. I retensioned the springs so both pawls engage firmly. I also regreased it with Shaffers 221 #2 which is higher quality, longer lasting, and water repellent. Now it shifts great - 95% of the time it shifts on the first try.
It took a while to get used to avoiding accidental shifts. For example, coming up on a curb we tend to backpedal to position for launch. This can cause the S2 to shift, launching you in high gear, which is a pain. It takes some time riding to get used to doing this right, but once you get used to it, it's alright.
Eons ago, I had a bike with an old 3-speed internally geared hub. The efficiency was poor. In top gear it felt like pedaling underwater due to the internal friction in the hub. The S2 is nothing like that - it feels fast and efficient in both gears. The low gear is direct drive, so efficiency is virtually 100%. The top gear runs through the internal planetary gears but I can't feel any drag. Unlike a derailleur system, the chain line is always straight, which provides ideal efficiency.
I'm a strong cyclist and while I use the S2 mainly for commuting so I'm riding easy on it, I have ridden hard on this and it quite is solid (so far). Before I rebuilt the hub it occasionally made a loud "ping" as I pushed on the pedals right after a shift. Seems that was due to the unsprung pawl. It's been 500 miles since I rebuilt it and it's never done that again. With nearly 1,000 miles on this hub it runs smooth, efficient and quiet.
I did have to adjust the axle cones a few times. At first they were too tight, so I loosened them a bit. After 20 miles or so they got loose, so I tightened them down. After another 20 miles or so they got loose again, so I tightened them again. Since then they've held their adjustment.
The hub came apart easily enough and seems solidly built with smooth tolerances. Though too much grease was used constructing it, and the grease wasn't quite viscous enough so some of it had migrated out onto the hub & spokes. I found there are 2 brass colored pawls connected to the center section of the hub, which are spring loaded to engage with sprocket teeth along the inside circumferance of the hub. There is a circular spring that makes these tabs pop out to engage the teeth. That spring was unevenly tensioned, so one of the the tabs was rather limp, not engaging the teeth. I retensioned the circular spring so both tabs were popping out more strongly for a positive engagement. I left much of the original grease intact but wiped away the excess. I regreased the entire hub with Schaeffer's 221 #2, my favorite all-around grease. It is a synthetic blend, wide temp range, high pressure, water repellent, with a tackifier agent to resist migration.
My S2 hub now shifts reliably and consistently, and the occasional "pings" are gone. With the new grease I'm more confident about longevity, and not worried about water contamination.
Update and Warning
I've read reports of people's S2 hubs disintegrating or generally breaking.
BUT... there are key differences between these and the ones I got:
Any freewheeling (non-coaster brake) S2 hub you buy now or in the past couple of years, should not have these problems.
Dave McCraw (whose site I've linked to above) mentions another interesting internally geared 2-speed hub. This one automatically shifts. I'm skeptical of the real-world utility and robustness of such a device, and he says it's pretty much useless out of the box, requiring internal disassembly and adjustment. But then he didn't like the S2 Hub at all, which is no surprise since his disintegrated. He says the SRAM works well after the adjustments. But it's also clear from his reviews that he'd like the S2 better if it didn't break... and the new ones don't.