Gear Ratios Explained
In much of what you read about cycling and training, common cycling terminology is used in regard to gearing, but may not be so common. I would like to explain some of that terminology now.
Most of your bikes have two chain rings up front, but some have three. Some of the bikes with two chain rings have what is called a compact crank while most have a standard crank. The difference between a compact crank and a standard crank is essentially the size of the bolt circle diameter (BCD), or the distance from bolt to bolt that holds the chain rings on. A compact crank has a 110mm BCD to fit a smaller chainring whereas the standard crank has a BCD of 130mm to fit a larger chainring. The smaller chainring allows for a lower number of teeth on the chain rings. On a compact crank, the norm on the smaller ring is 34 or 36 teeth. The bigger ring has 50 teeth. On a standard crank, the small ring has 39 teeth and the big ring has 53, but sometimes 52 or 54.
On the back wheel, the gearing is called either a cassette, a freewheel (old school), or a cog set. Most newer bikes have either 9 or 10 cogs and now even 11 in a set, with the smallest having 11 or 12 teeth and the largest going up to 25 or maybe 27. The other cogs are in the middle with a standard spacing of one tooth per cog for the first 6 and then two teeth for the next ones ie. 12, 13, 14, 15, 16, 17, 19, 21, 23, 25.
As I hope you are aware, the bigger the ring in front, and the smaller the cog in the back, the harder it is to pedal, but the farther you go per pedal revolution. Conversely, the smaller the front ring and the bigger the cog in the back, the easier it is to pedal because you travel less per revolution. The distance travelled is expressed as meters development or meters per pedal revolution (the actual number is less important than the concept). There are some gear ratio combinations that cross over from big and small ring, which is why it is not always necessary to change rings on certain terrain.
Anyway, the gear ratios are expressed as the number of teeth on the front ring to the number of teeth on the cog. For example, 53×12 is big ring and smallest cog and has the greatest meters travelled per revolution on the cog set above. A 53×16 has you in the big ring and the 5th smallest cog.
It is a very good practice to try to keep the chain around the middle of the cassette, as the chain is at its most efficient angle (straight). You should try to never be in the big ring and the biggest cog in the back, just as you should try to never be in the small ring and the smallest cog. This is known as either big/big or small/small. The reasons for avoiding these combinations are numerous, and relate to the chain angle or length. They include:
- The chain has to bend sideways, and puts more stress than desirable on it;
- The chain has more friction at those two bad angles
- The chain is more susceptible to snapping with the right amount of strength, say standing on a hill;
- ￼￼￼In a big/big situation, the chain has to pull the derailleur much more than is ideal to make it reach all the way round both of the biggest gears on the bike; and,
- In small/small situation, the chain will often rub against the big ring, again causing more friction, and annoying your riding partners. The only thing you can do about this is to change out of that gear, or drop behind or go ahead of your friends so they do not have to listen to it.
Back to the compact cranks for a moment—this type of crank is not a substitute for pedaling or training. What they do is allow for an easier and more consistent cog set selection. As mentioned, the difference in distance travelled is expressed in meters per revolution , and can be seen in the chart below. In terms of equivalencies, if you are not a strong climber and feel the need for an easier ratio than 39×25, then you can see a 34×21 is very close, and a 34×23 or 25 give you much easier pedaling. It would be like having a 39×28 instead of a 25. Since you can only have 10 cogs, you would not have to give up a cog on the other end to make room or have too big a difference between some of them like 19, 22, 25, 28 as your last three. Just like that really picky Goldilocks, one gear will be too hard and the next one up too easy.
So, how or why would someone choose a standard or a compact crank? Well, simply enough, if, for example, while traipsing around Gatineau Park on your two wheeled steed, you often find yourself desperately looking to see if you have one more cog in the back to use, and you don’t, then a compact crank may be for you. A stipulation to this would be if your cadence is in the 60’s. If you were really, really struggling, then a triple ring may be your next option.
Changing crank sets is a fairly expensive proposition, so I am not suggesting you switch crank set ups, just explaining it.
Make sense? I hope this helps.
Written By Rick Hellard, Head Coach @ Zone3sports, an Ottawa based triathlon coaching service that has achieved amazing success at the long distance triathlon.