I confess.  When someone would ask me “what cassette are you using?” or “is that a compact crank?” I would shrug my shoulders and change the subject.  But curiosity finally caught up with me, and I sat down to figure it out.  So here is the bike nerd’s guide to gearing, along with an Excel spreadsheet you can download from the WCC website to plug in your own data and explore the impact of the many gearing choices you can make on your bike: Gearing Cadence and Speed.xls

First some basics (please skip ahead if you already know this stuff).  There are two sets of gears on your bike – the front, or crankset, and the rear, or cassette.  The crankset is connected to your riding shoe - for every revolution your foot makes, the selected chainring on your crankset makes one revolution.  The cassette is connected to the rear wheel – for every revolution the selected sprocket in your cassette makes, your wheel makes one revolution.  The larger the chainring the longer it takes to make one pedal revolution for a given level of effort, resulting in it feeling “harder.”  The opposite is true for the cassette – for every revolution you turn the chainring, the smaller the cassette sprocket, the more times the rear wheel will spin.  So the “hardest” or “fastest” gearing combination is the largest front chainring and smallest rear sprocket.  The “easiest” or “climbing” gearing combination is the smallest front chainring and the largest rear sprocket.  We measure the combination of gears with a single number called the gear ratio, which is simply the number of teeth on the selected chainring divided by the number of teeth on the selected rear sprocket. The larger the gear ratio, the “harder” it is to pedal, and vice versa.

Road cycling cranksets come in three basic types:  compacts, triples, and doubles, and with each type you have a choice of chainrings.  Triples, as you might have already surmised, have three chainrings, while compacts and doubles have two.  Some riders starting out in the sport choose a triple, as it gives you the widest range of gearing choices, with plenty of easier climbing gears.  As riders improve they often move to compact setups, which offer a simpler gear changing pattern and lighter weight, but with a narrower gear ratio range than the triple.  More advanced riders can choose a standard double, which generally trades off some of the easier climbing gears of the compact for more potential speed at the high end.

Each manufacturer offers many different choices, but I have chosen three popular pairings from Shimano’s Ultegra group for this example.  The size of a chainring or sprocket is simply the number of teeth on the ring (you can count yours if you don’t know what you have).  For the following examples I will use the gearing that I have saved in the original spreadsheet (which you are free to change): compact - 50x34 chainrings and 11-28 cassette, triple - 52x39x30 chainrings and 11-28 cassette, and double - 53x39 chainrings and 11-27 cassette. 

In the graph from the spreadsheet, we can see that the triple (middle bars) gives us the widest range of gear ratios (4.42x) and the lowest gear ratio for climbing (1.07), but does so with some extra weight and complexity.  The double (on the right) is designed for riders who don’t need as much help climbing (1.44) in return for more top end for speed in the flats (4.82).  A compact (on the left) is a compromise between the two, with the simplicity and lighter weight of the double, easier climbing than the double (1.21 v. 1.44), but has less aggressive gearing in the flats (4.55 v. 4.82).

So how much speed are you giving up in the flats with the lower gear ratio of the compact?  The spreadsheet lets you see this as well.  In the tables below we can see that the higher gear ratio of the double allows for 1.7 mph faster speed at an 80 cadence (30.2 vs. 28.5).  At a 100 cadence this widens to 2.1 mph (37.7 vs. 35.6).  If this isn’t a show stopper for you, a compact crank may be a good choice.

The spreadsheet also allows you to see what speeds you can achieve with a given gear ratio and cadence. Want to keep up with a ride at 25 mph in the flats?  You’ll need to maintain an 80 cadence on your compact in your 12 and 13 sprockets and large chainring.

The spreadsheet can also help you understand mysteries such as “when I move from my large chainring to my small chainring, what is the equivalent change in the rear sprockets?”

If I am on a flat approaching a hill in 50/13 (a gear ratio of 3.85), if I drop down using my chainring to 34/13 (2.62) I will get the same impact as a 50/19 (2.63) or an equivalent of 4 gears.  This varies for different setups and at different starting points.  

I hope this helps, but stop being a bike nerd and get out there and ride!