Single Speed Gearing
Often when talking about gearing a singlespeed, the expression "two to one" comes up. Beside the fact that this is a meaningless statement, it implys that the coice of gears is based on easy math rather than real decisions. Let's look at this.
All gearing choice and discussion is conditional on the possibility of actually finding a gear and chain combination that is actually possible to use. This discussion is covered on the Chain Length Calculation page.
32 devided by 2 is 16. So the ratio of 32 to 16 is 2:1.
34 devided by 2 is 17. So the ratio of 34 to 17 is 2:1.
We call this simple math. Any moron can do this math in his/her head with these numbers. What connection does this have with choosing a gear for a bicycle with at specific rider on a specific terrain? Absolutely none. It is just simple math. To base a gearing choice on the fact that the number of teeth on two chosen gears can be divided into the whole numbers of 1 and 2 is nuts.
Here is a table of gear ratios with the more common gearing choices highlighted for off road riding, single gear, 26" wheel. It is a quick and simple way of seeing a facet of gearing choice. It doesn't tell the whole story, but it tells us something.
As can easily be seen, a great variety of gears exist within the 2:1 and 1.7:1 range.
Real gearing is probably not the best term. Typically, this is refered to as Rollout or Developement. What makes this a better system to use is that rather than comparing one gear ratio to another, a gear is described by what it actually does. This is the amount of forward distance moved on the bike by each rotation of the crank arms.
Now we can quickly compare 26" MTB wheel gearing to 29" MTB wheel gearing.
The Chosen gear must fit on the bike. Bikes are produced with differing range of adjustments.
This chart shows how many gear choices exist, given a specific chainstay length.
Lets say you have a bike with a 16.750" Chainstay lenght and 0.250" of adjustment in either direction.
Then we separate out the gears that are actually within the range of gearing that we want.
You then end up with about 150 different usable gear combinations that produce between 135 and 175 inches of development.
Now, reduce the range of inches of development to a finer range that reflects what is actually desired. For me, That means between 145 and 155 inches.
We now have 36 different gear configurations available.
(12 21) (12 22) (13 23) (13 24) (14 25) (14 26) (15 27) (15 28) (16 28) (16 29) (17 30) (17 31) (18 32) (18 33) (19 34) (19 35) (20 35) (20 36) (20 37) (21 37) (21 38) (21 39) (22 39) (22 40) (22 41)
Many of these gears are not desirable for different reasons.
- The chainring size may not be available.
- The cog size may not be available.
- A lighter, more efficient gear is a better option.
Since cogs are generally available (or best used) between 16 and 20 teeth, and chainrings usually available in even numbers of teeth, we end up with:
Five choices within the specified range. These then get checked against the physical capabilities of the system.
For example, (16 28) and (20 36) will not fit on my Zion 660EBB. See: Chain Length Calculation page. So, now I am down to just 3 good choices for gearing within the chosen range.
Bigger Really Is Better
Larger diameter gears work much more efficeiently than smaller gears.
Personaly, I am running a 34/19 gear (148.4"). We do a lot of climbing where I live, and I'm overweight and not the strongest climber. I would love to go up to 34/18 (156.7") or 32/17 (156.1"). That would help my downhill speeds a little if I could handle the gear on the climbs, but with mud season coming, I may even have to drop to 34/20 (141.0) depending on power losses. We will see.
Interesting to note is that if I was running 29" wheels, I would be running a 31/19 as a close to equivelant gear.