When I bought my new 29-er I spend lot of time thinking about best possible gears. The results are great and I'll happy to share my experience. I know that all mashers will laugh at this recommendation missing all the high gears . Let me describe how I got to my perfect gears – it was rather through excluding all wrong choices... FYI, I used Sheldon calculation for gears with 29x2.25 tires.
First I looked at SRAM XX1. Love the idea of simplicity and weight saving. However, I found out that the range is not good enough for me – XC spinner type. Even with 28 chainring, lowest gear is not low enough. 28/42 @80RPM gives you 7.4 km/h. I consider even lower speed worth of biking (rather walking). I know that you need it very rarely – but when you are climbing over 20% ascent in the Alps for few kilometers, lower gear is very useful. Last but not least, as you have only 11 gears, so the jumps between high gears are too big to keep optimum pedal frequency.
So clearly lowest gear needs to be 22/36, which gives you 6.8 km/h @80RPM. And even if you frequency of pedaling goes little down, still worth to stay on bike. One point: I know that there is option also for 20 chainring. However, it is quite non-standard and it is mechanically tricky to fit it on 64BCD cranks.
So now about high gear. No need for triple chainring – 2x10 gives you plenty of high gear. Typically with granny of 22, the cranks come with 36 – and so with typical cassette with smallest ring of 11, you have 36/11 and that gives you 45.6 km/h @100RPM. Unless you are racing this is more than enough. What I found out that even 36 is too much and I changed to 33 chainring that gives me 41.8 km/h, which is just perfect. Additional advantage is that I use 33 chainring most of the time and change gears just through real derailleur with perfect range - with granny just for few extreme situations.
Last but not least, I found out better spaced lower gears with Shimano cassettes, which provide11-13-15-17-19-21-24-28-32-36. (SRAM cassettes have 11-12-14-16-18-21-24-28-32-36) I spend most of the time in the middle of casette and 17-19-21 gears feels much better than 16-18-21.
Hope that this was useful. Let me know your experience!
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