A chain stay is the part of a bicycle frame that runs between and attaches to the two crank arms. The other prominent structure between these points is the seat stays.
The function of the chain stay in modern bicycles is to take rear-wheel torque from the drive train and transfer it to the down tube, ensuring that this critical structural member does not get bent out of shape by such stress (which would lead to frame failure). For this force transfer to be successful, they must connect solidly to both ends of the bike; even very subtle flex can make them much less effective.
Usually made from steel tubing, carbon fiber and aluminium alloy (among others) are also used in combination with steel tubing and others like titanium alloy, magnesium alloy and other alloys and steels to provide lightweight and strength at different price levels.
One of the most common innovations in the cycling world is implementing a hollow tube, usually aluminium alloy or steel, that can be found on top of or down by the bottom bracket (the area where your pedal spindle is located). This innovation was developed to save weight and take some stress off of the seat stays, which carry drive torque from the crankset to the rear wheel. These tubes are usually filled with pressurized nitrogen gas, but they can also be left empty if desired.
This technology has found its way onto high-end bikes such as those used in professional races and even for recreational use where saving weight and increasing performance is desired.
The seat stays (the tubes of the frame which come down from the top tube and connect to the rear wheel dropouts) are responsible for carrying drive-torque at their junction with the chain stay. While it is possible to have a drive train that transfers power between crank arms through a flexible frame, there must always be some stiffening structure in this location and on the bottom bracket if a rider ever hopes to do more than push hard on one pedal at a time. The seat stays also serve as crucial crash protection by acting as bump stops against road hazards such as those found when descending hills or during sprints from intersections.
In many cases, these tubes will attach to both ends of the bike with excellent connection points.
Before the invention of the chain stay, the most popular method for transferring power from pedals to the rear wheel was through a flexible structure often referred to as a mudguard or fender. When riding on particularly rough surfaces, this design would work well enough. Still, if pedalled with force at high speeds or any hill climb (especially during an out-of-saddle effort), this system would prove inefficient and potentially dangerous because the bicycle frame itself may bend instead of transferring torque through its axle region. This is one reason why many races are won on mountain roads; there is simply no time wasted putting weight on either pedal during out-of-saddle efforts (i.e., standing up). While the idea of having a flexible frame is not inherently bad, it has a tiny place in bicycle design compared to the alternative.
Bicycles with only two chainstays (one on each end) are equipped with what is called a double crankset (or sometimes referred to as being “two-piece” sets). While rare, they can be found on some mountain bikes. This type of set-up generally provides the rider with multiple gears for tackling steep grades or carrying speed across flat ground. However, this comes at the cost of increased weight because every time an extra bag was introduced to either drive train or another moving part, extra support must be added somewhere else in the system. This additional material does have other benefits, which is why some bicycles can carry giant gears that can accelerate a rider quickly or maintain high speeds for extended periods.
Double cranksets also have the unfortunate side effect of providing less ground clearance between pedals and the front wheel. This can be very dangerous in some cases where a cyclist is going over rough terrain at fast speeds because the sudden jolt from hitting a rock or log could cause chainsuck (when your pedal catches on your bike’s chain) or even result in throwing the bicycle off balance when you encounter an obstacle such as this. With perfect timing, there is always enough room to avoid these situations when they occur no matter what type of drive train you use. Still, when you factor in frequent out-of-saddle efforts and rough terrain, it becomes safer to avoid the problem altogether by using a bike with more than just two chainstays.