Bike gears and gearing explained

Bike gears and gearing explained

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Knowledge Level: Beginner

The reason the front wheel on a penny farthing bike is so large is that the pedals are directly linked to the wheel and so, every turn of the pedals, rotates the wheel once and it travels the distance of its circumference. If it was any smaller the rider’s legs would have to spin ferociously fast to maintain a reasonable amount of speed.

Fortunately the principle of gearing means that you no longer have to risk life and limb on a penny farthing and variable gears mean that hilly terrain can be tackled with relative ease.

Ratios explained

Your pedals are attached by your cranks to your chainrings. Chainring sizes vary but a standard road set-up will typically have 53 teeth (53t) on the outer large chainring and 39 teeth (39t) on the smaller inner one. Compact gearing, with 50t on the outer and 34t on the inner, has become very popular for tackling hilly sportives and has largely superseded triple three chainring cranksets. The chainrings are connected by the chain to a cassette of sprockets on the hub of your rear wheel. Sizes and ranges will vary but with a typical sportive set-up the smallest sprocket will have 12 teeth and the largest will have 28 teeth.

So, if you are in your larger 50t chainring and your smallest 12t sprocket, what does this mean? This 50:12 gear ratio means that every turn of your pedals is multiplied by the gear to make your wheel rotate just over four times (50÷12=4.2), this is why bikes no longer have to have penny farthing sized wheels. This gear is your “biggest gear”, it’s the hardest to push but for each pedal rotation it’ll give you the greatest distance. When you hit a hill, you’ll need to shift down to a lower or easier gear.

On really steep slopes or if your legs are really tired, you might be in your lowest gear. This would be the smaller 34t chainring and the largest 28t sprocket. This 34:28 ratio would give you just over one rotation of your wheel for every pedal stroke (34÷28=1.2) and will hopefully get you up that tough hill.

On mountain bikes, where you might be tackling super steep terrain with a loose or slippery surface, the gearing is even lower. You’ll often have a 22t “Granny” chainring and up to a 36t rear sprocket. Rather than multiplying your pedal stroke, this tiny gear minimises it. The 22:36 ratio means that every pedal stroke only translates to just over half a rotation of your wheel (22÷36=0.61).

Variable gearing

Until 1937, Tour de France riders had to tackle the mountain cols without variable gears. They had two or three sprockets on their rear wheel but “changing gear” would involve stopping, dismounting the bike, loosening the wingnuts, sliding the rear wheel in the frame dropout to release the chain tension, placing the chain on the new cog, repositioning the rear wheel to gain proper chain tension and remembering to retighten the wingnuts before remounting.

Fortunately after 1937, derailleurs were allowed and riders were able to change gears while riding. You now have a front derailleur that shifts the chain between the two or three chainrings at the front and a rear derailleur that shifts the chain between the 9, 10 or even 11 sprockets at the rear, giving a massive range of gears and making riding those mountain roads a whole lot easier.

Indexed shifters

Up until the early 1990’s, gear shift levers were normally sited on the downtube of the bike and relied on friction, a fair amount of rider skill and feel to select the appropriate gear. Now though shifters are incorporated into the brake hoods and use a ratchet mechanism that controls cable tension and means that one click shifts one gear. This is known as indexed shifting and, as long as the gears are correctly set-up, means simple, reliable and hassle free gear shifts.


The latest evolution in gear shifting involves using tiny electric motors to shift the derailleurs. This totally eliminates problems such as poor shifting due to cable stretch and gives lighting quick and crisp shifts. It’s still prohibitively expensive for most riders though and only generally found on top-end bikes. However manufacturers are already starting to produce second tier electronic groupsets and in the future it will become more affordable and commonplace.


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