Selecting the proper gearing for your bike is an important part of maximizing both your performance and enjoyment of every ride. “Your gearing” as discussed here comprises two parts: the chainrings and the cassette.
The more important of the two components are the chainrings. The commonly available options are standard (53 tooth large ring and 39 tooth small ring), mid compact (usually a 52 tooth large ring and 36 tooth small ring, and compact (generally a 50 tooth large ring and 34 tooth small ring). First, you should choose your chainrings based on how strong of a cyclist you are.
There are several ways to determine your overall strength as a cyclist, but for those who properly utilize a powermeter, the choices are simplified. If you have a watts/kilogram at threshold of 2 – 3, you are most likely best served with a compact. This range is typical of road riders in the 13-17mph range and triathlon riders in the 16-20 mph range. Beginners, smaller riders, recreational riders and Cat 5 cyclists fall into this range.
If your watts/kilo at threshold is between 3 and 4.5, you are most likely best served by a mid compact. These numbers are typical of road riders in the 18-23 mph range and triathlon riders in the 20-26 mph range. Mid to front of pack triathlete riders, Cat 3-4 cyclists, and generally those considered to be strong cyclists fall into this range.
If your watts/kilo is over 4.5 you are probably best served by standard gearing. These numbers are typical of road riders in the 21+ mph range and triathlon riders averaging 26 mph or more. This is best overall bike split, Cat 1-2, “freaks on two wheels” category.
Not every rider with the same watts/kilogram is equal though. For example, a ninety kilogram rider at 4 watts/kg (360 watts at threshold) will generally need bigger gears than a 60 kilogram rider at 4 watts/kg (240 watts at threshold) So your chainring choices need to be also be influenced by not only your power to weight ratio but also your absolute power. The stronger you are in an absolute sense, the more likely you will need to step up to the next larger chainring option, and vice versa.
A final note about chainrings…. You need to know your preferred crank arm length when selecting a new crankset. So if you are simply changing the rings and you are confident in your selection, go ahead. However, if you are changing out the entire crankset, know your crank length. Crank arm selection is one of the finer points we cover during our comprehensive bike fitting process.
After you have correctly chosen your chainrings, the next step is to choose the proper cassette. This is a much easier choice, and less expensive if you initially make the wrong choice. You should choose your cassette based on the terrain you will encounter. Given the proper chainrings, generally two cassettes should cover nearly every situation a typical rider will encounter. In fact if you have the correct chainrings, you can generally purchase a narrow range cassette such as 11-23, 11-25 or 12-25, combined with a cassette more suitable for climbing such as a 12-28, 12-30, 12-32, and be done with it. You will more than likely have everything you will need.
Another factor to consider when choosing your gearing is your preferred cadence range. If you like to spin a high cadence you may want to choose more gear (i.e- smaller chainrings and/or cassettes with larger cogs). The opposite is true for riders who prefer a lower cadence, where larger chainrings and smaller tooth count cogs are preferable.
Don’t worry so much about “spinning out” or not having enough gear to pedal the fastest parts of your ride, as much as making sure you don’t run out of gear when you are attempting the longest and steepest climbs that you typically attempt. Even a compact 50—34 with a climbing cassette of 12-32 will provide a speed of nearly 30mph when spinning 90rpms in that largest 50-12 gear combination.
On a typical multi hour ride, you should use every gear on your bike, with the smallest combo being sufficient for the steepest climbs and the largest gears enough for the fastest descents. Also, take note where you “run out of gear” to help guide your future gearing decisions.
If you would like to discuss the finer points of gearing selection, crank length, and bike fit, please email or call me. A large part of my job art is helping you to buy the right thing the first time