I was curious how important having a bicycle with 24 speeds (vs. 21 gears vs. 6 gears) is. I have an Iron Horse mountain bike with 24 speeds (3 gears front, 8 gears back). My daughter has a smaller mountain bike with 6 speeds (only 6 gears in the back). Obviously the wheel size matters a lot, but how much does having 24 speeds help *me* in particular? I know my 26″ wheels make me go faster than my daughter’s 20″, but wheel size being equal, how do various gear/sprocket combinations compare?
Being a geek by nature, I wrote a (python, of course) program to sort the speeds. Here is the output:
Front: [(0, 24), (1, 34), (2, 42)] Back: [(0, 32), (1, 28), (2, 24), (3, 21), (4, 18), (5, 15), (6, 13), (7, 11)] 0 (0:0): 0.750 1 (0:1): 0.857 2 (0:2): 1.000 8 (1:0): 1.062 3 (0:3): 1.143 9 (1:1): 1.214 16 (2:0): 1.312 4 (0:4): 1.333 10 (1:2): 1.417 17 (2:1): 1.500 5 (0:5): 1.600 11 (1:3): 1.619 18 (2:2): 1.750 6 (0:6): 1.846 12 (1:4): 1.889 19 (2:3): 2.000 7 (0:7): 2.182 13 (1:5): 2.267 20 (2:4): 2.333 14 (1:6): 2.615 21 (2:5): 2.800 15 (1:7): 3.091 22 (2:6): 3.231 23 (2:7): 3.818
What does this show?
First, the number of teeth in each gear (sprocket) for the front and back. They are sorted in reverse order front/back, since you will have the highest speed with the large sprocket in the front and small sprocket in the back.
Then, the first column represents the speed number, counting with the smallest front sprocket and each back sprocket, then the next front sprocket and each back sprocket etc. The second column shows which sprocket combination it represents (front:back). Third column is the ratio front/back (number of teeth). The higher the ratio, the higher the speed (and the harder you work).
What does this show?
Something I knew all along: I only need one front sprocket (the largest). I don’t do a lot of trail, even then I generally do 2:3 or higher, and on streets I’m using 2:4, 2:6 or 2:7. Notice how 2:0 is pretty high in the list, with a 1.3 ratio – and I feel like I don’t make any progress when I try that! This means that probably the combination of 8 sprockets in the back with the largest sprocket in front is way more than enough for me.
Again, I don’t do hard-core mountain biking, maybe if I did I’d find a need for the other two front sprockets. Can someone enlighten me if the other two front sprockets are really useful?
It all depends on the type of riding you’re doing. If you’re doing much hilly riding, having the low gears are nice for making it up a long climb. And similarly, high gears are good for going fast. That said, a triple crankset isn’t inherently better than a double — you can usually get similar gearing ratios with the double and then you get some advantage in terms of cleaner chainline.
N.B. I’m pretty much entirely a road rider at this point.
Two things: First, as Mr. Katz mentioned, to get a clean chain line, you really shouldn’t use the the largest sprocket on the front and the largest on the back (i.e, 2:0), similarly for the smallest and the smallest (0:7). It puts too much strain on your derailleurs and chain. So a 24 speed is a misnomer. It’s really more like an 18 or 20 speed. Typically the largest and smallest gears on the back have the same number of teeth whether you have 6, 7 or 8 sprockets, so it’s really gradation between low and high that you get when you add sprockets. To trained riders, it probably matters. For amateurs, it’s a way to get you to want to upgrade your gear, and reduce the integrity of the back wheel so you have to buy more of them when you hit that log or rock and bend your rims.
Second, notice that the gear ratios overlap. When on a trail, where you really need to be able to shift gears quickly when moving from one terrain type to another, you pick the front sprocket for the trail you ride, and then you don’t touch it. You use low for very hilly terrain, middle for a trail with some speed and not much climbing, and high for road riding.
I’d love to show you a few trails. There’s one at Lake Crabtree that’s pretty good, some at Harris Lake, Falls Lake, and several other places in the Triangle. It’s a lot of fun, and very different from road biking.