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Written January 26, 2023
(using bike gears, using bicycle gears)
This article is intended for novice bike riders and is intended to be a summary of my experience and opinions on how to effectively use gears on a bicycle. It's not scientific or necessarily good advice, so take it on at your own risk. This article assumes derailer / derailleur gearing systems and does not cover internally geared hubs or other drivetrains.
Gears on a bike provide a way to vary the number of rotations of the back wheel for a given rotation of the crank (the big gear at the front which the pedals attach to). Modern bicycles generally have one or more chainrings attached to the crank (the "front" gears), and typically 7-11 sprockets on the rear wheel (the "back" gears). The number of gears of a given bicycle can be represented with the form FxR where F is number of front gears, and R is the number of back gears. For example, a bike may be 2x9, 3x7, 1x9, or even 1x1.
The most common drivetrain seen on modern bikes makes use of derailers (or derailleurs). These devices are usually present for both the front and back gears.
The front derailer is usually attached to the seat tube (roughly vertical tube where the seat goes into), above the front gears on the right-hand side of the bike (from riders perspective). It is a small cage which the chain passes through. When the rider shifts gears, the derailer moves side-to-side, "de-railing" the chain from one front sprocket/chainring onto another.
The back derailer is usually attached to the bottom of the frame on the right-hand side of the bike (from riders perspective), and sits below the back gears with the chain running through two "jockey wheels" - smaller gears that serve to keep the chain tensioned and running straight. This is because when the chain is on the smaller sprockets, there will be more slack in it which needs to be tensioned. Like the front derailer, it moves side-to-side when the rider shifts gears, and moves the chain from one sprocket to another.
Both front and back gears can be measured in terms of the number of teeth they have. The numbers vary by make, model, and type of bike. For example, a road bike may have two chainrings (at the front) with 50 and 34 teeth respectively. This is described as a 50/34 crank. The rear gears are typically packaged as a "cassette", and may have 10 sprockets/speeds with a range of 14-28 teeth. This is described as a 14-28T 10 speed cassette.
When describing gears, the larger the number represents a bigger sprocket on the front, and a smaller sprocket on the back. For example, 1-1 (first gear front, first gear rear) means the smallest chainring at the front, and the largest sprocket at the back.
Gear ratios can be calculated in absolute terms. However, it is more useful to analyze ratios in terms of either gear "inches" (number of inches traveled by the rear wheel for a rotation of the pedals), or to look at speeds achieved by a gear combination at a given cadence/RPM. Note that these measures are impacted by wheel (and tire) sizes, so serve as a useful comparison tool between bikes which may have different wheels and tires. Useful calculators for these are linked below.
In theory, a rider is most efficient when doing ~80 to ~100 turns of the pedal in a minute - though this can vary between riders and based on fitness. Therefore, a rider should aim to be maintaining their optimal pedalling rate (also known as cadence) continuously in order to get the most value from their exertion.
The purpose of having multiple gears is to allow constant power delivered by a rider for a given rotation of the pedals / crank to be delivered to the rear wheel over a variable distance. This is particularly useful when climbing hills, as without gears the slower speed necessitated by the effort of moving the bike & rider up the hill results in a sub-optimal cadence - or "grinding", where the rider is forced to pedal slowly and with a lot of effort. Gradient, wind (head or tail), rider fatigue, road conditions, and a number of other factors can result in a change of gears being necessary for a rider to sustain their optimal / comfortable cadence and power output while speed of the bike changes.
I would say there are two main considerations when using gears. The first is how to choose gears, and the second is how to actually shift gears.
To choose a gear, a rider should try and maintain ~80rpm or whatever brisk cadence is comfortable for them, while exerting a constant amount of force on the pedals at a level they feel comfortable with. If it becomes too easy to pedal, the rider should move to higher gears until they hit the level of effort they would like. If it becomes too hard to pedal, the rider should move to lower gears until they find a comfortable gear.
A rider should be careful not to cross-chain. Cross-chaining is when the rider uses a big rear gear with a big front gear (e.g. 3-1) or a small rear gear with a small front gear (e.g. 1-8). This is undesirable, as it causes the chain to run at an angle (not parallel to the gears as it should be). This can wear the chain more quickly, cause skipping, and can cause the chain to rub against the inside of the front derailer.
If the rider avoids cross-chaining, this means that some gear combinations should not be used. Instead, the rider should try and find a similar ratio using a different combination of sprockets. For example, take a 3x8 geared bike. The rider should avoid using the small (high) gears in the back when in the first (smallest, lowest) ring on the front. So, if a rider reaches say 1-5 and it is too easy to pedal, the rider should avoid shifting to 1-6, and instead shift to 2-4. Conversely, if a rider is riding in 3-5 and it becomes too difficult, they should consider shifting to 2-6 rather than 3-4.
As a rider becomes familiar with a bicycle, they will learn the limits of cross-chaining (usually a grinding noise from the front derailer) and this shifting between front gears becomes natural. Yes, this reduces the number of practical speeds for a given bike: for a 3x8, it will not result in 24 usable gear combinations. However, there are overlapping combinations that result in similar ratios.
Derailer gear systems can and should only be shifted when the pedals are turning forwards, and the bike is moving forward. The chain needs to be in motion for a derailer to move it between sprockets.
Do not shift gears while the bike is stationary. When you attempt to start the bike moving again, this will place stress on the derailer and chain, and may result in a big clunk as the chain jumps sprockets, or for the chain to become jammed.
A tip is that if you are coming to a stop (for traffic lights, stop sign, whatever), consider shifting to an easier gear before stopping - while you are coasting and lightly pedalling so that it is easier to get started again.
Gear shifts should only be undertaken when the drivetrain is under light load. If the rider is working hard and exerting a lot of force on the pedals, they should pedal slower and with minimal effort (so the crank doesn't actually drive the wheel) for a rotation or a few while shifting gears, until the chain engages on the new gear combination. If the chain is under load, the gears may not shift immediately and the delay may catch the rider off-guard mid pedal-stroke.
For cleaner shifts, when changing the front gear UP, I recommend also changing the rear gear DOWN at the same time. This is less important when shifting the front gear DOWN.