Hello All, thanks for accepting me onto this forum. I am not a Rohloff owner at the moment, BUT my retirement bike is in the making and I'm at the point of choosing a chainring/sprocket combo for the Rohloff.
Being new to the Rohloff world, it's been a steep learning curve, and I've found this thread fascinating (yes, I have read all 23 pages!). I wonder if somebody would kindly answer a few questions.
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Q1 - Yes. For example, a 34 chainring and 17 sprocket has a ratio of 2.0. And a chainring of 36 and sprocket of 18 would also have a ratio of 2.0.
Q2 - Correct. Bigger chainring means you have less tension on the chain. And bigger chainring and bigger sprocket will have a smaller articulation angle as the chain unwraps off of the sprocket and wraps onto the chainring. Both tension and articulation angle are a function in chain wear.
Q3 - Smaller crank arm length means you will want your gearing to be a bit lower, otherwise it would increase how hard you have to push on the pedals in your first gear. I think it would not change anything in other gears, as you would likely just automatically use the gear that you want to use. In other words, if you pedaled that bike with 175mm cranks in gear 11, with 165mm cranks you might choose to use gear 10, without even thinking about it. But your first gear remains your first gear.
I have a 170mm crankset on one bike, the rest of my bikes are 175mm crank arm lengths. I notice no difference, but I would expect that my cadence might be a bit higher with the shorter crankarms. I however have not attempted to measure that.
Where you say the lower end is where you are matching to, I think that is the right plan. If you need a higher gear, unless you are racing you do not REALLY need a higher gear. But if you need a lower gear, you do REALLY need a lower gear. That is why for my touring chainring size, I set it based on the cadence that I wanted at a specific speed that I felt was my minimum speed to maintain balance in my lowest gear.
If your new bike will be heavier than the Litespeed, which most certainly is the case, you might want a lower gear than you have on the Litespeed as you will be lifting more weight up the hill. You probably noted when reading previous posts that I use a 44T chainring with an unladen bike around home, but when I load it down for a bike tour with my camping gear, I use a 36T chainring instead to get lower gearing. The 44T chainring needs four more chain links, I use a second quick link for one of those four. If you want to change gearing later, with chain drive that is easy to do with a different chainring and adjusting number of links in the chain. Belts, not so easy.
That said, I think the Thorn Mercury has specific chainring and sprocket combinations that are needed for a small bottom bracket eccentric. But I do not own a Mercury, I could be wrong on that.
There are lots of things you will notice when you get a Rohloff bike after using derailleur bikes. The thing that I noticed most was that the Rohloff which uses two shifter cables must have some slack in the cables. If I am in gear 10, my shifter will easily move 9.5 to 10.5 with cable slack. But a derailleur bike, as you know you always have tension in the cables. If you do not have enough slack in a Rohloff shifter cable, when you shift you might land somewhere between the gears within the hub, which does not work so good.
Thanks for the quick reply. I'm glad that I am at least on the right track! I'm really looking forward to this bike build (I'm not building it, but it's nice to have some autonomy on the component selection, etc.), and especially for the Rohloff hub part of the decision, I want to make sure I get the gear selection right, or approximately so.
I hear what you are saying re: the belt option. I was originally attracted to the simplicity of it, especially the part about not having to clean it - but after much research, and especially after reading this thread, I realised (a) cleaning an IGH chain is nowhere near the faff of cleaning chain, derailleurs, jockey wheels, etc., and (b) if you get the gear ratios wrong with a belt, it's an expensive mistake to correct!
Your comment about getting the bottom gear right has really made me think more about that subject. I definitely agree with your comments about top end speed - these days, if I can go at 30 mph on the flat it would be a miracle, and going downhill I'm not that bothered if I exceed 30 mph due to pedalling harder, or due to gravity. Regarding bottom end, I'm not 100% sure I should be aiming for lower gears than the Litespeed, though it seems logical. Here's my reasoning:
On the Litespeed, unloaded, I run out of gears, and steam, and balancing ability, around the 20% gradient mark (I need to check this, but I think it's about right). In the 22 (front) x 33 (rear) combination, on 26 inch rims and 2.2 inch tyres, pedalling at 60 rpm, I'm doing 3.1 mph. Generally, when it gets really steep (approaching 20%), I do drop from 60 rpm and pound the pedals a bit, but this is where I usually stop. I'm not actually sure what is the primary cause of me stopping - lack of leg strength or lack of balance (the front wheel rises from the floor, and I never was good on a unicycle).
Assuming balance is my limiting factor, and I can't balance below 3.1 mph, would a lower gear really help me? (thinking aloud here)? Is it that, with a lower gear, I'd be able to maintain 3.1 mph with a higher cadence (bearing in mind that, as you said, shorter cranks usually mean higher cadence)?
