Yesterday I went over the gearing options for road and triathlon bikes. Today we will tackle mountain bike gearing which can actually be a little more complicated due to the option of having two different wheel sizes.
Just like in road, there are several gearing options for mountain bike cranks. In a double ring setup you have a 22-36, 24-38, 26-39, 28 -42, 28-40, 30-42, 30-44. For triple cranks you basically have a 24-32-42 (Shimano), or a 22-33-44 (SRAM) option (as I noted above, these are slowly going the way of the Doo-Doo bird).
Specific mountain bike cassettes tend to be either a 11-32, 11-34, or 11-36. For the sake of this article and not getting too complicated, I am going to ignore the fact that you can run a 10-speed road cassette on your mountain bike.
OK, so my head just exploded. That is a total of 36 different drive-train options, 72 if you factor in 29 versus 26 inch wheels. So what are the best options? That really depends on your riding style, are you racing, cross-country or downhill, 26 or 29er, just trail riding. For this article I am going to stay primarily focused on cross-country riders but these basic guidelines will apply to trail riders as well.
The bigger 29 inch wheel essentially increases the distance the bike will travel with each rotation of the crank. A 29er wheel will basically go faster than a 26inch wheel being pedaled at the same speed in the same gear. This is important because the low gear you have on your 26inch wheel bike will be approximately 15% lower than the same gear in a 29er. That is the difference between clearing that steep climb or having to get off and walk it. So we basically need smaller gears, but how small?
For cross-country type riding, I am going to ignore the triple cranksets for the reasons I noted above. In double cranksets we will have to look at what will give us the best range while not sacrificing the minimal gear we will need for climbing. Obviously this will depend on the trails you are riding and racing, and your overall strength. For most riders, being able to climb at a minimum of ~6MPH while spinning at 90RPM is plenty low enough for most trails (at 60RPM that would be about 4MPH). Any slower than that and it is probably faster to walk the climb. A 26 x 34 gear ratio will give us a riding speed of 5.9MPH at 90RPM. You could also run a 28 x 36 and get almost the identical speed (6MPH).
What about the top end speed? We typically don’t have high-speed sprint finishes in mountain biking, and while there can be super fast descents at 35+ MPH, most of those aren’t sections that we will be pedaling down. The bigger issue with the big ring size is to find a gear that would allow you to stay in that gear as long as possible while climbing without sacrificing too much top end speed. This would allow you to minimize front shifting that can sap momentum from having to back off on the pedals slightly to allow the shift. This is minor, but can add up over the course of a two-hour race. When looking at the current options a 39 or 40 tooth big ring will allow you to ride at 9MPH at 90RPM when in the 34tooth cog; while the 11tooth cog at 100RPM would give you 30.5MPH. That is a pretty good for most cross-country situations.
For 29ers, my recommendation would be to go with a 26 – 39 with a 11 – 34 cassette, or a 28-40 crankset with a 11 – 36, or a 11 – 34 if you’re a good climber or don’t have very steep climbs in your neck of the woods.
For the 26 inch wheel folks
All of the same issues we talked about for the 29ers apply to the small wheeled bikes but the gear ratios have to be bigger since the wheel is a little smaller. When looking at the climbing gears and that ideal 6MPH at 90RPM, a 28 x 34 gear gets us pretty close at 5.7MPH. A 30 x 34 could be a good option too since that will give us 6.1MPH. For the big ring we would like to be close to that 30MPH at 100RPM I noted above with the 29ers. A 42 x 11 would get us 29.2MPH while still offering a good climbing speed in the 42 x 34 of 9.5MPH.
For 26inch wheeled bikes, my recommendation would be to go with a 28 x 42 crankset and a 11- 34 cassette. That gives you the widest usable range. A close second to that would be the 30 x 42 crankset with the 11 – 34 cassette.
One last note on mountain bike gearing
I know a lot of riders get caught up in finding the ideal setup, with perfect gear ratios, so they can be the most efficient rider possible. That is all good but keep in mind there are a lot of guys crushing the trails on single-speeds. Just check out the results of any cross-country race these days and you will see single-speed guys finishing at about the same time as the geared guys. Gearing is important, but it is still 95% rider at the end of the day. In other words, don’t use your gearing as an excuse.
**As I noted in the road article, you can play around with all the gearing options here.