Hi Martin!
Very nice having you aboard; welcome!
... reckon larger sprockets last longer and prolong chain life. Not yet got a Rohloff, but I was thinking of getting the more expensive optional 21T sprocket, coupled with a 50T chainring.
I spent some time this evening with the Rohloff gear chart and it -- along with the SJS Cycles site's Rohloff cog offerings -- had me thinking along similar lines.
Thinking about Ian's reference to Sheldon Brown's even-even gearing combos, I wondered if the even-odd (40-17) setup might result in a wash, where wear is concerned. Yes, it is a mismatched combo in terms of tooth count, but the larger diameter compared to the 38-16 might come out the same in terms of service life even with greater wear due to the odd sizing. Certainly, the 38-16 is more economical -- that 21 tooth cog costs nearly twice as much. The 16T is £25.99, while the 17T is a modest increment upward at £29.99, and the 21T is £49.99 (USD$ $40.37, $46.59, and $77.66 respectively at current exchange rates).
The 21T cog and 50T chainring would have to effectively double drivetrain life to cost-out effectively. If chain life is factored in...maybe it would. Also, thinking about the sizing of the timing rings on my tandem, I think torque loads on the bottom bracket would be reduced as well. I went to far larger timing rings and now my bottom brackets last forever. Something for me to ponder further. One point Andy makes in his "Living With A Rohloff" articles is how the larger 17T cog contributes to longer chainlife because -- while there is no lateral stress on the chain compared to a derailleur application -- the chain doesn't have to wrap as tightly, saving wear. To get equivalent overall range with the 17T cog as the 16T also means increasing the chainring size, so the chain is stressed less at both ends (larger arcs) -- a less extreme version of what Martin is considering, but also at less initial outlay.
I'd love a low gear around 15 gear-inches, but like Martin, I'm concerned about compromising my standing with the Rohloff warranty. And, in the back of my mind, I'm a bit concerned about the potential for actual breakage out in the field, however unlikely that might be. It isn't a complete stopper, and it doesn't seem to be a problem in practice, but it really is possible to generate pretty high torque loads when starting from a stop on a 24% grade with 32kg/72lb of gear.
In earlier days ( <cough> late 1970s <cough>), I developed maps of routes that pioneered the Eugene Hill Tour. It was later taken up by local clubs and became an annual event and even a fundraiser for them. I was deep into gearing then and caught the low-gear bug (logarithmic graph paper days. Australians, do you remember Ron Shepard and his ultra-low gearing efforts/Low Gear Fellowship from that era? He used FW cogs up front
and chainrings behind). I modified a number of older cranksets to take freewheel cogs (pre-Mountain Tamer Quad days, too, though I got one of those in due course). I found it is possible to actually ride a 12-inch low gear with a load, provided the hill is steep enough. Biggest problem was catching the pedal cage with my shoe cleat -- the crankarm whipped around so fast at that point, it usually took till the third go-'round to catch it, so I started out in second or third gear, then shifted down once underway.
The thing is, as my gearing got ever lower, torque on the rear hub and freewheel components grew greater and greater. I never broke anything, but I soon had a terrible time getting the freewheels off the hub -- they would screw on so tight, they distorted the land next to the freewheel threads. I finally solved the removal problem by coating both hub and freewheel threads with molybdenum disulphide high-pressure lube and then using crushable (sacrificial) aluminum spacers between the freewheel body and the hub. As you can probably imagine, my chain life dropped dramatically, and this was in the days when a 5-sp Regina Oro was a really robust chain. heavy, anyway.
The lesson I took from these efforts is my output remained essentially the same within anobjectively narrow range. What changed was the effective torque multiplication due to the lower gearing, and I think this is what Rohloff is concerned about. I strongly suspect even the "red" or "illegal" Rohoff gearing combos don't kill the hub outright or even for a very long time or at all, unless the rider is really strong or other factors are in play (hill slope, heavy load, etc). I'm guessing these deep ratios do eat into the service life/MTBF (mean-time between failure) ratings Rohloff have specified so there is less "cushion" or safety margin before something goes Bad. This effect is bound to be more risky when it is still early days for a hub, and the gears and internals have not worn into full mesh and contact with one another. After the hub is broken in, it seems considerably less risky.
I am still boggled by the realization that all that forward torque is countered by such a small reaction-moment arm --
the thing fits inside the dropout! Pretty astonishing when you think about it, and it makes me happy Thorn have spent so much effort to craft their left-hand dropouts with great care. Kinda makes me wonder about the aluminum OEM Rohloff dropouts, but there's a lot of "beef" in them as well, and they're really thick.
Oh! While writing this, I see Neil has posted on this very topic -- talk about timely! Good having you weigh in also. Neil, I'm glad to hear the 40x17 has worked so well for you in practice, and to hear it does so on the kind of hills you reference is reassuring too. Thanks so much for your "user report" and now...boy! Now, I need to ponder all the input and come up with a spec. This is all outstandingly helpful, and I do so appreciate all your efforts and responses and kind wishes. Keep 'em coming, please! All sounds very promising to get an ideal ratio for my needs.
All the best,
Dan.
