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Bike fit variables: No. 4 gear ratios

Stu Bowers
16 Nov 2018

Road bikes now have more gearing options than ever before, but just how many gears do you actually need?

This article was originally published in issue 76 of Cyclist magazine

The first bicycles had only one gear. By the mid-19th century this had increased to two, and the number of gears has risen consistently ever since such that today it is possible for a bike to have 81 gears (oh yes it is: a Sturmey Archer CS-RK3 internally geared three-speed hub with a nine-speed cassette and a triple chainset, since you ask).

Of course, more isn’t necessarily better. It’s the range – the ratios – that matter most in gearing, and in that regard we have never been more spoilt.

The trick is to know which combination of gears will work best for you.

Not so long ago, most road bikes came with a ‘standard double’ chainset. That is, a big ring with 53 teeth and a smaller 39-tooth ring.

Then along came the ‘compact’ chainset – popularised by FSA in the early 2000s – with 50/34t chainrings.

Since then we’ve seen the advent of the ‘mid-compact’ 52/36t chainset, and then the ‘super-compact’ 48/32t chainset.

While all this has been going on at the front of the drivetrain, at the rear there has been an equally rapid growth in the number of cassette variations.

The once ubiquitous 11-23t cassette has given way to a wide range of options, from a tiny 9t sprocket up to a dinner plate-sized 42t sprocket.

12-speed option

And with the recent introduction of the 12-speed cassette from Campagnolo, the jump between gears need not be huge, even with a very wide range.

But it’s not just the sprocket and chainring sizes that affect gearing. The size of tyres and length of cranks also have an impact, so it’s worth understanding a bit about how gear ratios are calculated.

Without getting too geeky, a gear ratio allows you to understand how far the bike will travel for every turn of the pedals, and the starting point is to simply divide the number of teeth on the chainring by the number of teeth on the cassette sprocket.

For example, a 50x11t gear equates to a ratio of 4.55, usually expressed as 4.55:1. In other words, in this gear the rear wheel will turn 4.55 times for every crank revolution. If the sprocket and chainring are of equal size the ratio is 1:1.

From here, we can work out how far the bike will travel per pedal revolution – known as ‘metres of development’ – by measuring the wheel circumference, which is where tyre size becomes an issue.

If we’re riding in a 50x11t gear, a 700c x 23mm tyre (circumference 2,098mm) will travel 9.55m per pedal revolution, while a 700c x 28mm tyre (circumference 2,130mm) would travel 9.69m.

That means with each turn of the cranks, the bigger tyre carries the bike 14cm further.

It’s worth remembering that the size of the sprocket tends to have a bigger effect on the gear ratio than the size of the chainring. Or to put that another way, a 44x9t gear is actually bigger than 53x11t.

That’s an extreme example, but it suggests that the need for large chainrings may be diminishing, and that it is feasible for most riders to get all the gears they need from a single, smaller chainring.

Rise of the 1x

This year, pro team Aqua Blue has become the first professional outfit to compete at WorldTour level using just 11 gears, the number since front derailleurs became commonplace back in the 1950s.

With a single chainring at the front and a wide-range cassette at the back (a system known as 1x, or ‘one-by’), it’s possible for a smaller number of gears to still provide enough range to race effectively.

There will be those traditionalists who insist that a bike simply won’t be able to go fast enough without a meaty 53t chainring up front, but the maths suggests otherwise.

A single 46t chainring, paired with an 11t sprocket, ridden at 100rpm (perfectly within the bounds of normality) with 28mm tyres, gives a theoretical speed of 53.42kmh. That should be more than sufficient for most riders.

‘There’s so much bravado around gearing,’ says Phil Burt, former head of physiotherapy at British Cycling and founder of bike fit specialist Phil Burt Innovation.

‘Riding a compact might have a stigma attached to it, just like riding with an upright stem, but if it means you don’t get back and knee problems and you can ride bike more comfortably, then so what?

‘You need to think about it more as a tool to do the job, not what it looks like.’

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