What is compact gearing on a road bike?

QUESTION: What is compact gearing on a road bike, and why might I want it? Many of the bikes I am looking at say they have a compact crankset. —George R.

RBR’S STAN PURDUM REPLIES: In road bike gearing, compact refers to a two-chainring crankset (the chainrings and the crank arms that turn them) where the rings are smaller than those found on the "standard" size crankset. Specifically, a standard crankset typically has a 53-tooth outer chainring and a 39-tooth inner chainring while a compact has a 50-tooth outer ring and a 34-tooth inner ring.

Standard cranksets are the most common choice for racing bikes, where speed is the primary goal. (They were also the default choice on the "10-speed" bikes of the 1970s.)

Compact cranksets are often found on bikes today intended for mere mortals, where, because of the smaller rings, they make climbing hills easier. Because the large ring on a compact crankset is slightly smaller than on the standard set, the rider can't attain quite as high a speed, but most recreational riders would seldom go that fast anyway.

Of course, chainrings are only half of the equation for what size gears you actually have, because those result from the combination of what chainring you are in and what sprocket you are in on the rear cassette. Pedaling in your largest chainring and your smallest sprocket gives you your highest gear, and pedaling in your smallest chainring and your largest sprocket gives your lowest gear.

A typical rear cassette may have a 11-tooth small sprocket and a 28-tooth large one, but on bikes with compact gearing, you can get a pretty high gear by using a cassette with a smaller-than-average small sprocket — say a 10 tooth if you’re looking at some of the newer SRAM cassettes. And you can achieve a lower gear by employing a cassette with a 50-tooth large sprocket, if your rear derailleur is able to handle that wide of a range. Depending on the number of sprockets in your cassette, you can get a 10-52 cassette that will give you a wide range of gearing, even with a compact crankset.

In general, if you’re buying a new bike, it's best to talk with your local bike staff about the kind of riding you do so they can guide you to a bike set up with the right chainring and cassette combination for your usual riding. But if you’ve already got your bike and you want to achieve lower or higher gearing — or a broader spectrum of gearing overall — you can change out chainrings (which are available with many different tooth counts) and cassettes to find the combination that's right for you.

And you are not limited to road bike components, for many mountain and gravel bike components can be mixed in with road bike set ups, and you are not limited to parts made only by Shimano, SRAM or Campagnolo.

Back in the 90s, the bike I was preparing to ride across America self-contained had a triple crank, but it came with 48-, 38-, 30-tooth rings, which I knew would not give me low enough gears for climbing mountains with 40 or more pounds of equipment and supplies on the bike. So I changed all three rings to 46-, 36-, 24-toothers. And I switched the 11-28 cassette for an 11-34 one, which meant I had to replace the rear derailleur with a longer-arm version that was a mountain bike component. But when on the long and steep climbs out west, I was oh-so-glad that I had done so.

Be aware that the "spider" of your crankset — the section within the crankset that the chainrings bolt onto — may not accept every chainring due to the spacing of the bolt holes, so you’ll want to check that before purchasing new chainrings.

I recently converted a touring bike to an ebike, which meant I had to remove the triple crankset altogether and replace it with an electric motor that has only one chainring and its own crank arms. But since that particular motor has nine assist levels, it's like having nine chainrings, and thus lots of options for climbing.

Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, lives in Ohio. See more at www.StanPurdum.com.