How Do Gears Work?

Mechanical advantage: the advantage gained by the use of a mechanism in transmitting force; specifically: the ratio of the force that performs the useful work of a machine to the force that is applied to the machine (from Merriam-Webster Online Dictionary).

The bikes we ride are full of mechanisms designed to increase mechanical advantage. From the balance that comes from the gyroscopic effect of a spinning wheel to the levers that help to manipulate our body’s actions on both the pedals and handlebars, the bike is all about making it easier for us to move faster. More important than anything else is the advantage we are afforded by our gearing.

Apart from fixed-gear and single-speed bikes, most bikes will have a range of gearing options divided amongst one to three front chain rings and an ever-increasing number of rear cogs. Shifters come in many shapes and sizes: combined shifters and brakes on road bikes, downtube shifters on old road bikes, bar ends on time trial bikes and handlebar shifters on mountain bikes. Shifting can be mechanical or electronic. No matter how your system works, the end result is always the same: it allows you to customize your gear ratio to remain as efficient as possible given the current topography.

Have you ever actually thought about how your gears work and what they’re doing? How your gearing interacts with your power to produce speed? What is the right gear combination to use? The answers aren’t always cut and dry, but with a little understanding, you’ll be better prepared to make those decisions.

The drivetrain of the bike consists of the front chain rings, chains, rear cassette and derailleurs. For the sake of this illustration, we’re going to consider the now common compact crank set up with a 50-tooth ring (the big one) and a 34-tooth ring (the small one). Add in an 11-speed rear cassette from 11-tooth (smallest cog) to 25-tooth (largest cog). The chain is then sized as needed to complete the loop around the large chainring, large cog and through the derailleurs without too much tension.

When the bike is in the small chainring, you will “pull” 34 links of the chain for each full pedal revolution. This acts to turn the rear wheel and, in the largest cog, will pull on the 25 teeth of that cog, spinning the wheel a bit more than one full rotation with each pedaling cycle (~1:1 gear ratio). Now shift up to your large chainring. All of a sudden, you’re pulling 50 links of chain on each revolution and you’ve doubled your mechanical advantage (2:1 ratio) so that the rear wheel now rotates twice for each pedal revolution. Shift all the way down the cassette to the 11-tooth gear and your ratio is now nearly 5:1, causing the rear wheel to spin almost five times for each revolution.

So now you’re understanding what’s happening with the rear wheel. But how does this interact with speed? The distance the bike moves in one pedal revolution is the product of the gear ratio and outside circumference of the wheel. With the same gear ratios, a large wheel and a small wheel will complete the same number of revolutions, but the larger wheel will travel a greater distance because the revolutions are acting over a greater circumference.

If the largest gearing combination provides the greatest advantage, we should probably ride in the largest gear all the time, right? I’ve seen some people out on roads and trails that must have exactly that line of thinking as they grind away up a minor incline. The catch here is that moving more gear, requires a greater input, which we refer to as torque, and each of us possesses a finite amount. That torque interacts with cadence to result in power. Power, in turn, works to overcome external forces like rolling resistance, aerodynamic drag and gravity to produce speed.

Since torque will ultimately be the limiting factor for any rider, adjusting gearing to the current situation allows the rider to achieve a desired cadence that helps to optimize power and, as a result, speed. You could produce 200 watts at 50 rpms, and you could produce 200 watts at 90 rpms, and your speed would be the same, but you would feel that your legs are working much harder at the lower cadence. Over short durations, this may not be a problem, but, over time, the higher torque required to produce a given power at low cadence will lead to fatigue. Keeping cadence higher limits the amount of torque produced on each pedal stroke and instead achieves power through repetition, which is much easier for the body to sustain.

Developing good shifting habits should happen naturally if you are attuned to the pressure you feel against your feet and the tension in your legs. When it starts to get harder, shift to an easier gear. When the resistance becomes lighter, opt for a harder gear. Shifting often helps you stay in this sweet spot of cadence and torque. But there are a couple of things you should also consider when thinking about effective shifting.

The first is avoiding excessive cross-chaining, which is the condition where your chain is at opposite extremes between the crank arm and cassette (for instance, large ring and large cog or small ring and small cog). The gearing systems on bikes are designed with some overlap, so that your “big-big” gear combination is similar to some smaller cog while you are in the small chain ring. Choosing a straighter chain line whenever possible will result in less wear on the drivetrain and less cross-tension on the chain, leading to better shifts and longer life.

The other consideration is the timing of your shifts. Whether dealing with a minor bump in the road or a major climb, you want to avoid making a big shift when the chain is under full tension. For small changes, you may just ease up on the pedals for a fraction of a second as you make the shift. In this case, you are likely to shift only across the rear cogs. When the change is more extreme, such as a steep climb coming right off a flat straightaway, a common problem is a dropped chain, which often results from too much inward tension on the chain as the front derailleur moves, combined with waiting too long to shift, leading to situations wherein the chain is also under high tension. When you can see the climb ahead and know that you will ultimately end up in the small ring in the front, anticipate that change and make the shift while you’re still in the flats and the chain is under less tension. Compensate for the loss in speed by shifting a few gears harder in the rear as you drop to the small ring. This will allow you to work back up through the full range of gears on the cassette as the grade steepens.

The information above should help you to be more efficient with your shifting, but it’s also important to address the mechanical efficiency of shifting. Drivetrains contain numerous wear items: chains, cables and derailleur springs. As these things wear out, the shifting performance of the bike can change. At the least, this can lead to frustration and at the worst, it might affect the outcome of an event. Regular maintenance will help to avoid these challenges. Applying some lube to the chain and derailleur cables every week or two is a good start, along with an annual professional tune-up. Most of these will include inspection of parts and replacement if necessary, along with adjustment of any stops that may be preventing the gears from working properly.

Maybe you didn’t really need a physics lesson to learn how to shift better, but I hope this helps you appreciate the importance good shifting plays in your performance on the bike. It can greatly enhance the experience, whether your goal is to compete at the highest level or just to enjoy riding your bike with ease. It may take a bit of practice to improve your shifting, especially if you’re breaking out of old habits, but the best thing is that every time you swing your leg over the top tube, you will have a new opportunity to improve.