Nitty Gritty Tech Stuff - Gears

Here is the hard core scoop on gears. I’ve been riding for years and I only now started to pay attention to how this works. There are two sets of gears on a standard bike. There is a set in the front, attached to the crank, with either two or three individual gears. These are called chainrings. Then there are the gears in the back, attached to the rear wheel. These are collectively called the cassette, or the freewheel. When you buy a cassette, all its gears are connected to each other, so you are getting a group of gears in pre-selected sizes.

Now, any of these gears, front or back, is described by the number of teeth on the gear. Cranks with two chainrings, or a double crank, typically have chainrings in two of the following three sizes: 53T (53 teeth), 42T (42 teeth) or 39T (39 teeth). Cassettes come in a wide range of groupings. To complicate matters, cassettes are also available in groups of seven, eight, or nine gears. Mike, Hans and Eric each have two chainrings and cassettes with seven gears, thus they each have 14 speed bikes. Karyns bike has three chainrings, the extra being called a granny gear, and a cassette with seven gears. Thus Karyn has a 21 speed bike.

The gear and cassette sizes for each bike are -

Mike
chainrings: 53T, 39T
old cassette: 13-14-15-16-17-18-19-21T
new cassette: 13-15-17-19-21-23-26T

Eric
chainrings: 53T, 42T
old cassette: 13-14-15-17-19-21-23T
new cassette: 14-16-18-21-24-28-32T

Hans
chainrings: 53T, 40T
cassette: 12-14-16-18-21-24-28T

Mike (Karyn's Bike)
chainrings: 52T, 42T, 30T
cassette: 13-15-17-19-21-23-26T

OK, what do we do with all of this? If you divide the chainring size by a cassette gear size, you will find out how many times the wheel goes around with one turn of the crank. So if Mike is in his 53T chainring and his 13T cassette gear, his wheel will turn 53/13 = 4.08* times for each turn of the pedal crank. This, incidently, would be his “biggest” gear. If he is in the 39T chainring and the 26T cassette gear, his wheel will turn 39/26 = 1.50 times for each turn of the crank. This is his “smallest” gear.

It is typical to state the gear combination that you are using by saying, “I am in my 53 by 13," which is written as 53x13. Lets compare the lowest gear on each bike.

Mike: 39x26 or 1.50 wheel revolutions/crank
Hans: 40x28 or 1.42 wheel revolutions/crank
Eric: 42x32 or 1.31 wheel revolutions/crank
Mike: 30x26 or 1.15 wheel revolutions/crank (Karyn's bike)

Mike has decided to ride his wife Karyn's bike on the big climbs. This is a good idea. For every crank he would make on his own bike, he would have to work (1.50-1.15)/1.15 = .304 or 30.4% harder than he will have to work on Karyn's bike when each are in their lowest gear. For every crank Hans makes on his bike, he will have to work (1.42-1.15)/1.15 = .235 or 23.5% harder than Mike (on Karyn's bike) when each are in their lowest gear. Eric will have to work (1.31-1.15)/1.15 = .139 or 13.9% harder than Mike (on Karyn's bike) in his lowest gear. This may not seem like much difference unless you think about what it would be like to have your income tax go up by 13.9%.

*Note: For you significant digit freaks, I justify calculating to three significant digits by knowing that each gear is absolutely exact. Thus a 53T is a 53.00000...T. There is no such thing as a fraction of a tooth appearing on a chainring.


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