Bike Gears Explained

Bike gears

How exactly do bike gears work? What are gear ratios? Why do bikes have so many gears compared to cars? What’s the best gear combination on bikes for optimal energy expenditure?

Well, I’m glad you asked. A good place to start might be what gears are actually used for. And the answer is… you use them to go forwards!

OK so a little more detail required here maybe.

Specifically, gears are used to transmit torque force through the wheel to propel us forwards. In particular, the difference in gear between the front crankset and rear sprocket gives us a mechanical advantage which can be used to make riding in different scenarios easier, or indeed, harder. Shifting occurs when we move between these different gears to change the mechanical advantage, which therefore changes the input (or effort peddling) and output (speed forwards) required in different scenarios. That mechanical advantage is closely related to the gear ratios between the front and rear cogs.

Which reminds me, technically speaking there is a subtle difference between a gear and a sprocket; namely a sprocket is two or more gears connected by a chain. But I’ll forgive you an oversight here, I’m kind like that you see…

So what are the main components required when shifting?

To begin with we have the crankset with usually two or three chainrings at the front, and we have a cassette at the rear with a varying number of gears, normally between 7 to 11 sprockets. We have a drivechain to connect them, which needs to be of a certain length depending on the gears and crankset you are using. The purpose of this chain is to transfer the power you produce when peddling into the cassette causing it to rotate, giving us movement. And lastly we have our front and rear derailleurs which actually shift the drivechain between the cogs and chainrings.

There’s a bewildering array of cranksets, cassettes and derailleurs on the market, but suffice to say, fellow cyclist’s emotions run high with even the slightest mention of Shimano and Campagnolo. Shimano is a Japanese company famous for group sets like the 105, Ultegra and Dura-Ace. Campagnolo is an Italian company providing products like Chorus, Record and Super Record.

Your front crankset will move from the smaller chainring in the midline to the larger chainring outside. Visa versa for the cogs in the cassette. The chainrings in the midline require less effort to ride but you move with less speed, and the chainrings on the outside require more effort to ride but you will be able to ride faster.

What’s that crunching sound I hear when I shift gear?

Imagine the scene – your riding your favourite route, the sun is shining, the birds are singing, when suddenly a hill the size of Makalu gets in your way. You can’t go round, you can’t go back, but its ok because you have a the heart of a lion – onwards and upwards you go.

Half way up the hill, you start to go down gears to try and make the ascent a little easier. But every time you do, you hear a crunching or grinding sound and begin to wonder if your bike is trying to tell you something.

Well, actually it kind of is.

The difference in gear between the front crankset and rear sprocket gives us a mechanical advantage which can be used to make riding in different scenarios easier

You see, all of the force you are generating to move forwards is really on the top section of the drivechain and that is why the top of the drivechain is really taught all the time, and the bottom laxer. You might be shifting gear on the cassette to an easier gear but eventually you’ll need to move to the inner chainring at the front to give your legs a break. What you are asking the derailleur to do is move that taught chain to a smaller chainring, but remember the amount of force your body can exert is much higher than that tautness.

The best thing to do is stop pushing momentarily while you shift. Exactly like changing gear on a car. And in precisely the same way, just before you stop pushing, try and give an extra push of acceleration to try and maintain torque which will keep cadence steady, between 80rpm – 100rpm. It might be hard, but in the words of Shaun T, dig deep.

Also, try and change gear earlier before you actually get to that steep climb. Remember, the early bird catches the worm. You’ll also get less crunch like this… just thinking ahead.

A side note – don’t coast when shifting because it makes it harder for the drivechain to engage the next gear. And for the love of God don’t peddle backwards because you’ll most probably end up damaging the chain. Imagine going from 5th gear in your car to reverse… not cool.

Should I try and avoid any bending in my drive chain?

Good point. It is a good idea to start thinking about the amount of bending in the drive chain that is occurring between the chainring and rear sprocket.

So consider this – your drivechain is on the smallest chainring and the smallest rear sprocket. Or your drivechain is on the largest chainring and the largest sprocket. In both scenarios, your drive chain is at its maximum bend. Remember the point of the drivechain is to transfer force from the crankset to teeth on the cassette sprocket – and on the sprocket the drivechain will only be in contact with around 5ish teeth (depending on sizes etc).

All that torsion force through such a small surface area will eventually cause damage.

Anything I can do to avoid this?

Indeed there is.

Get to know which gears and chainrings keep your drivechain straight.

What I mean by this is lets say you start at 1×1 (largest cog in cassette and smallest chainring). Now you want to go faster, so you move up to 1×2 then 1×3 and so on, until around 1×7 (in other words, you’re on the 7th cog in cassette and still on smallest chainring). In order to maintain cadence, instead of jumping to 2×7, in other words keeping the rear gear same and going up to the bigger chainring, what you can do is go from 1×7 to around 2×3. This will keep cadence constant and make an easier transition for both you and the bike.

You can also degrease the chainring regularly, just to keep things ticketyboo.

Give it a go – change both chainring and gear at the same time. But you’ll need to find the optimal for your bike as the differences in gear ratios and sizes mean this threshold will change for each bike.

What exactly do you mean by gear ratios?

You are on the ball today. The most important thing to remember is that the difference between the number of teeth on the chainring to the number of teeth on the gear in the cassette is what gives you the mechanical advantage.

Let me explain…

Imagine riding up a steep hill using your biggest chainring on the crankset with the smallest cog on the cassette. Just to make a point, lets pretend you are using the Race Face Evolve Chainring Set 104/64 mm 22/32/44 in the Bad Ass Black. That has 44 teeth in the biggest chainring. On the cassette lets say the smallest cog had 11 teeth. That means that the ratio between teeth front to back is 44:11, or to put it simpler, 4:1. That means that for every complete revolution of peddle you make the rear wheel will rotate 4 times.

Alternatively, lets say your drivechain is connecting the smallest chainring in the crankset of 22 teeth to the largest rear cog of 34 teeth. Now the ratio front to rear is 22:34, or 0.65:1. Now for every complete revolution of the peddle the rear wheel will only rotate 0.65 time, or just over half.

Whats the point of this?

Well, with a ratio of 4:1 you need to exert more force on the peddle because the rear wheel is moving a much larger distance – namely 4 times. With a ratio of 0.65:1 this will be a lot easier to peddle because the rear wheel is moving a much shorter distance, just under half a revolution.

This is where your mechanical advantage comes in.

So if you are riding a particularly steep hill, or you are just out of energy, you want as lower ratio as possible to make it easier for you to ride. In practical terms this means you want the smallest chainring and largest cog. Or if you’ve just started riding bikes to lose weight, the opposite is true, use the largest chainring to make your legs work harder.

How do I tell the best ratio on my bike?

Unfortunately its really hard to answer this without knowing what crankset and cog you are riding with. But with your new-found knowledge you might be able to work it out yourself.

For instance, the Shimano sg-x has a 53 tooth big ring and 39 tooth inner ring, which I’ve seen pros’s using for all types of terrain. You might even consider a compact chainset which is slightly smaller in diameter, for instance coming in 52 teeth big and 34 small, or 52t big and 36t inner. You can even get triple chainsets – the outer for going super-fast, the middle for steep climbs and the smallest if you are completely out of energy and just need that ‘safety’ option.

Back in the good old days, cassettes only came in sizes up to 7 cogs. These were the days when there were big jumps between the number of teeth so whenever you changed gear it sounded like your bike was about to throw up. Imagine going from a 11t to 25t cogs. These days though we have 11 gear cassette’s so the jump isn’t as much which is good for keeping cadence steady.

Just remember the gear ratio determines how far you travel forward for one complete revolution of the pedal so a smaller rear cassette and larger forward chainring means you travel further.

Kinda sounds like your avoiding the question… What’s the optimal number of teeth for a cassette and crankset?

You see those houses? I’m going to go around them for you.

Honestly it depends on the type of biking you do.

Do you race a lot? On mainly flat roads? In short and sharp busts? You probably want a more traditional crankset then, with a narrow ratio choice. Or have you only just started riding? Or do lots of steep hill climbing? Or you are just less fit? Go for a triple or compact crankset with a wide ratio choice, and use the smaller chainrings to save your legs.

Summary

Hopefully this has given you a little wiser as to the nature of gears and the relationship between the crankset and cassette. It can be confusing but with any luck, the relationship back to where the force you exert on the bike actually moves has made things a little clearer for you.

If you enjoyed this article, please share it on social media. Or if you really hate it, still share it! And leave a comment below to let me know anything you disagree or agree with.

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  • Founder of biketorpedo.co.uk... Obsessive about all things bikes and technology, Jamie decided to combine them to build BikeTorpedo to become the online hangout for UK cyclists.

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