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Bike fit variables: No. 1 Stem length

Stu Bowers
19 Jun 2018

The stem might be a simple component, but changing it can have a profound effect on how a bike rides and feels

Located right beneath your nose, the stem is perhaps the part of a bike that you look at most while riding, but how much consideration do you give it?

There’s every chance you simply stick with whatever stem comes fitted when you buy a bicycle, but stem length affects your position on the bike, not to mention how the bike handles.

Let’s start by considering the options. Stock stems for road bikes come in lengths from 60mm to 140mm, so there’s huge scope for making changes to a bike’s reach. But that’s only half the story.

Stems also come in a wide variety of angles too, with inclines commonly ranging from +/-6° to +/-17° (+/- because stems can usually be flipped and used either way up).

This is because the stem mounts to the fork steerer, which is itself at an angle – commonly around 73° to the horizontal. That means a stem with an angle of -17° will sit parallel to the road. 

‘The textbook theory goes a little like this,’ says Anders Annerstedt, co-founder and chief designer at Rolo Bikes.

‘A long stem length is effectively a longer steering lever arm, so will be less responsive but may feel more stable, particularly at high speed.

‘A short stem will be more responsive to steering inputs but potentially a little less stable. It’s essentially the same reason a bus has a very large steering wheel, and an F1 car has a tiny one.’

Another often overlooked consideration is that a longer stem can also flex more easily, which can in extreme cases also make for more vague steering.

Difference of opinion

The sweet spot is generally accepted as being 100mm to 120mm, but not everyone agrees.

‘It’s a bit of a cliché that a too-short stem will over-quicken the handling. It’s only true to a point,’ says Phil Cavell, director of Cyclefit in London.

‘Needing a 70mm-80mm stem probably means bike sizing needs to be reviewed, but many riders are happy to ride a 70 or 80 or 90mm stem without difficulty. Conversely we also sometimes fit 130mm.’

‘We design our bikes around a 110mm stem,’ says Annerstedt, ‘but there’s no single ideal stem length. Each case is different depending on the rider and geometry. But on our bikes a 70mm stem would feel very twitchy.’

Some riders will be more sensitive to change than others, which Annerstedt puts down to how close to ideal the set-up is in
the first instance.

‘The better your fit, the more likely it is that you will notice smaller changes. If you’re starting from a poor fit in the first place then bigger changes may not be so noticeable.’

But making any positional changes, no matter how big or small, should not be done without considering the knock-on effects elsewhere.

‘Even a 10mm change in stem length can make a huge difference,’ says Jez Loftus, bike fit specialist for Trek. ‘Also be aware no single adjustment is mutually exclusive.

‘Changing stem length will alter the way a rider holds their head, which can affect muscles in the upper back, neck and shoulders, but less obvious could be shifts in knee tracking or even ankle angle.

‘You have to consider every element of the fit each time you make a single adjustment.’

Reaching out

‘The first thing you have to ask is: why are you changing the stem?’ says Cavell. ‘Often it’s the go-to place to correct postural issues. Sometimes the bars feel too far away, but it’s not always as straightforward a fix as it seems.

‘For instance, pelvic rotation is a component of reach, so a passive and weak posterior rotation – which could be a consequence of a number of things such as inflexible/short hamstrings, poor core or poor postural awareness – can lead to feeling too stretched.

‘A body adaptation may be needed, not a stem change. Pressure mapping [on the saddle] is a great diagnostic tool to establish this.

‘Another important component is centre of gravity, which also shifts depending on stem length,’ Cavell adds.

‘This will affect weight distribution, and so have a knock-on effect on braking, cornering and so on. It could also affect power production – if the stem is too short it could take the tension out of the glutes and negatively affect muscle recruitment.’

With so many variables at play, it would be best to seek advice from a qualified bike fitter. Even then, attitudes may change over time, and Annerstedt thinks we’re due a rethink.

‘I know I’m contradicting all of the pros in cycling right now, but for me the future will be stem lengths getting much shorter and top tubes getting longer, just like in mountain bikes,’ he says.

‘The frame is much better able to absorb road shocks with longer tubes and a longer wheelbase, plus it will be more stable, while reduced fork rake and a shorter stem will keep the handling sharp.

‘But as always in the bicycle industry it’s so hard to get people to try something new.’

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