Your bike stem is one of those unobtrusive components that punches way above its weight. Considering its size, the stem has a huge influence over how comfortable you are on your bike. It influences steering, stability, comfort and endurance, so getting it right is one of cycling life’s essentials.
Threadless stems are measured in mm, with typical lengths of between 70mm and 120mm. Some pros run 130mm stems as they ride bikes one size down for weight and fit. Us mere mortals aren’t so flexible!
Stems also use angles too, but these have less of an influence on how a bike handles. So for this post at least, we will concentrate only on stem length.
Bike manufacturers use complicated geometric calculations to deliver a middle ground with regards stem length. Much depends on top tube length, so each manufacturer has their own ideas on the correct length.
As a stem can influence handling, a stem length offering the most comfortable ride is often offered when buying off the shelf. It is then up to you to tweak it to your particular comfort level. As with all aspects of road cycling, much is down to personal preference, flexibility and fitness. There is no one-size fits all.
How stem length influences handling
So let’s take a quick look at how stem length influences how the bike handles.
The longer the stem, the more of your weight rests on the front wheel. This adds stability but can also slow down the steering slightly. The extra stability is useful in descending or if you use aero wheels as it can offset the wind. This is often why pros will ride smaller bikes with longer stems.
Conversely, a shorter stem lightens the front wheel and speeds up steering response. Get it too short and the bike can become twitchy and nervous. Some people like those lightning fast reactions, others do not.
The other, often overlooked, element of stem length is that of the steering arc. A longer stem lengthens that arc while a shorter stem does the opposite. That twitchy response mentioned above is because of this shorter arc. Tiny movements in the body, especially shoulder and arms can cause more movement in the steering giving that twitchiness. A longer stem offsets that by needing more input from the rider to steer.
How to tell if you’re riding the right stem length
The “right” stem length is going to be the one you’re most comfortable using. It will be the one that doesn’t cause neck or shoulder pain, that allows you to ride on the hoods and drops without squashing your lungs or kneeing yourself in the elbows. Much depends on the bike size, top tube length, seat position, your body and arm lengths and how flexible you are.
The most widely accepted way to see if you have the right stem length is to look down at your front wheel when you’re out riding or on the turbo. If, when in your normal position on the hoods, the front hub is obscured by the bars and you are comfortable in this position, this is a good stem length. If the hub appears in front of the bars, the stem may be too short, if it appears behind, it may be too long.
The caveat with all that is the bit about being comfortable. Your body will tell you when something isn’t working. If you’re getting shoulder or neck aches on longer rides, you may have the wrong setup. If a friend watching you or a video of yourself fit shows you’re overstretched or too upright, change the stem or move the seat forward.
When, or if, you change your stem length you have to give it a couple of weeks or a hundred or so miles to bed in. Your body is great at adapting so will need time to decide if it’s comfortable or not. Don’t try a different length and then go for one ride and decide it’s no different. Give it time.
Selecting stem length is an important part of a bike fit. It is something you can do yourself or as part of a professional fit. Once you get it right you’re instantly comfortable with your riding position and how the bike handles. That’s the place every rider wants to be.