Modern bicycles present a potential buyer with an amazing multitude of gear options. From single speeds to 30-speed triples, there is a bike out there for you. With a bit of background info, you will be able to quickly recognize the pros and cons of any particular drivetrain layout at a glance.
With only one gear ratio, single speeds are the simplest drivetrain arrangement possible. The crank has one chainring that drives a single cog on the rear wheel. It offers one ratio, which may not be high enough for going fast or low enough for climbing a hill. Super simple, but limited in function. Your legs must deal with all the speed changes and varying power demands with no mechanical help from other gear ratios.
Bikes with a single chainring on the crank and a gear change mechanism in the back fall into one of two categories—cruiser or modern mountain bike. Beach cruisers or inexpensive commuter bikes use this style of drivetrain for simplicity and to keep the cost down with a 5, 6 or 7-speed shifter in the rear. You get a much wider range of gear ratios compared to a single speed and you don’t have to hassle with front shifting. Modern mountain bikes may also be found with a single chainring up front, though they typically use 9, 10 or 11 gears in back. Bikes are set up this way when simplicity and durability are high priorities. Down hill racers, freeride bikes and those intended for primarily down-slope use will be the most commonly fit with a single chainring.
Racing road bikes are primarily set up with two chainrings, as they have been since the term “ten-speed” came about. By adding a second chainring to the crank, a bike’s available gear choices double, giving a rider far more choices in gear selection AND smaller steps between each ratio than a single chainring offers. Comparatively, there is a higher “high” gear and a lower “low” gear, with a bias toward the “go-fast” end of the ratio spectrum. Recently, there have also been mountain bikes designed with a two chainring drivetrain (as opposed to their traditional three chainring/triple design). This change has been made possible by new cassettes with 10 or 11 cogs, which span a much wider range than older 5-9 speeds. Weight is saved by having only two chainrings and front shifting is more reliable.
Without question, bikes with three chainrings have the widest range of gears, from high to low, and are still a very viable layout for certain types of riding. On the road, triple cranks work well for loaded touring bikes, commuter bikes and recreational bikes designed to be jack-of-all-trades. The ratios added with a third chainring are mostly in the lower ranges, making it easier for a rider to ascend steep or long (or both!) climbs. This is the same reason that traditional mountain bikes have triple cranks. The irregularities of off-road trails can mean climbs much steeper than road-grade, while mud, rocks and roots make the travel tougher. The low gears of a triple crank allow mountain bikes to claw their way up or power their way through tough terrain. The downside to a triple crank is more front shifting, which has historically been tougher for riders to perform smoothly and reliably. Modern front derailleur designs have helped triple shifting quite a bit and there are some styles of riding that demand a triple crank. As mentioned above, advances in double crank technology means some bikes that have been triple-equipped in the past are moving toward two chainrings for shifting simplicity.
So, there is not necessarily ONE right drivetrain for your next bike. You may look at bikes designed for the same style of riding that have different chainring layouts. Now that you know some of the advantages and disadvantages of the various designs, you can test ride bikes and see which suits you best.
As always, your knowledgeable bike salesperson can help show you the differences and ask questions about your intended riding to ensure you get a drivetrain that will perform the way you want it to.