The chainstay is the forgotten hero of every frame. It’s one of the two tubes—the other being the head tube—that no frame design ever eliminates. Engineers and frame builders have come up with designs that left out the top tube, the down tube, the seat tube and the seatstays. You might say the chainstays are the sous chef of this little kitchen.
So what is it?
This one, fortunately, is as easy to define as walking is. Chainstay length is the distance between the center of the bottom bracket and the center of the dropout. This axis-to-axis distance is not parallel to the ground and therefore not the same as the portion of the wheelbase measured from the BB to rear axle, because of BB drop. Were a bike to be built with no BB drop (i.e., the bottom bracket spindle at the same height above the ground as the axles), the chainstay length would be the same as the portion of wheelbase extending from the BB to the rear dropout.
Even if the chainstays do something funky, like curve around from the bottom of the BB shell as with some gravel bikes, or are elevated above the chain as with some mountain bikes, the chainstay length, for purposes of bike design, is still measured in a straight line from BB axis to rear wheel axis.
Unlike details like fork rake, head tube angle or bottom bracket drop, chainstay length doesn’t determine the bike’s character, but it influences how the bike handles and the rider’s weight distribution. With road and gravel bikes, designers usually want a 40/60 front/rear weight split between wheels. Those proportions vary a good deal more with mountain bikes.
When a builder or engineer establishes chainstay length, they do so with one eye on wheelbase as well. Chainstay length helps a designer determine the rider’s weight distribution between the two wheels. It also influences (because of its role in wheelbase) a bike’s handling. A bike with a shorter wheelbase will turn more quickly just as one with a longer wheelbase will turn along a larger radius. That analogy of sports car vs. school bus is apt as ever.
In the case of mountain bikes, chainstay length affects another important dimension of handling: leverage. That is, how easy it is for the rider to lift the front wheel. While Archimedes said that with a lever long enough he could move the world, in this instance a slightly different principle is at work; shorter chainstays move the rider closer to the bike’s fulcrum—the rear wheel’s axle, making it easier for the rider to lift the front wheel. Consider what mountain biking would be if a rider couldn’t lift the front wheel off the ground. The sport would be backcountry cyclocross, requiring the rider to dismount at every downed branch.
Touring bikes carve out an interesting exception in chainstay length. Rear panniers require some room to breathe; that is, chainstays need to be longer than usual in order to prevent a rider’s heels from rubbing up against the panniers. If the chainstays are too short and the panniers too large and a rider’s feet too big, a full pedal stroke won’t even be possible.
Chainstay length is influenced by wheel size, bicycle frame size and the bicycle type. Obvs, smaller wheels can accommodate shorter chainstays. Similarly, the larger the tire, the longer the chainstays need to be to accommodate the tire’s width. And as top tube length increases for taller riders, many (but not all) designers will increase chainstay length in order to keep the rider’s weight distribution constant across sizes.
The way chainstay length is influenced by the type of bicycle is even more critical than the way size can affect it. I’m going to stick to road, gravel and mountain bikes here, for simplicity. Road bikes meant for racing (e.g., the Trek Madone and the Specialized Tarmac) will have the shortest chainstays, on the order of 41cm. Road bikes meant for less aggressive handling (Trek Domane and Specialized Roubaix) will have chainstays a bit longer, more like 42cm. Gravel bikes, by virtue of their larger tires, need chainstays in the range of 43cm. Cross-country mountain bikes and touring bikes come next at 44cm or more. Trail bikes enter the picture around 45cm and enduro and downhill bikes will often be in the range of 46cm. These are just approximations and exceptions abound; I once owned a touring bike with 46cm-long chainstays.
Chainstay length is one of those numbers in a bike’s design that can help inform someone’s sense of how a bike is meant to handle. It is not, however, a number that would or should ever guide a buyer’s choice of bike, with one possible exception: Someone with big feet looking for a touring bike intended to carry a big load.
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