What does 1 by mean in cycling?

What does 1 by mean in cycling?
VIngegaard powering forward during the 2023 Tour de France

It’s more and more common in this era of modern cycling to see riders opting for a “1-by” setup, compared to a “2-by” setup. This simply means that instead of having the usual two cassettes on the front of the bicycle (the ones close to the pedals) the rider will only use one. The rider gets less versatility in their gearing, but achieves a lighter weight bike that has less chance of “dropping”. Dropping refers to when the chain leaves the cassette. If a dropped chain occurs, a rider has to stop, get off their bike and replace the chain on the cassette. Dropping the chain often occurs in high torque situations or changing gears at inopportune times (say at a low cadence). Dropping is also more likely when riding a 2-by setup because of extra gears involved.

It’s still possible to drop your chain on a 1-by though. Primoz Roglič knows this all too well, as he dropped his chain on the final mountain time trial of the Giro. Roglič opted to use the setup for the lighter weight and given that they could build the gear ratio specifically for that climb. Roglič’s bike ran over a pothole, and his chain dropped due to the vibration, and he had to stop and replace it.

1-by setups are more common in races where the terrain is rather similar, and the risk of chain drop quite high. So not only did that include Roglič’s mountain time trial stage, but also a number of sprint Flemish classics fit the bill, such as Paris Roubaix. Roubaix has a high likelihood of the chain dropping due to the cobbles, and a rather flat course otherwise. Many riders will opt for a 1-by setup in this situation.

1-by setups are also quite common in cyclocross where not only is the chance of the chain dropping exceedingly high, but having only one cassette up front provides less surface area for dirt and mud to get into and jam up the drive chain.