8 Mar 2016


Choosing the right mountain bike frame size is important for performance, comfort and safety, but can be tricky, not least because there’s no single way of measuring them.

Some companies use inches, some use centimetres, and some use simple Small, Medium and Large. All of these refer to the seat tube length. Unfortunately, not everyone takes these measurements from the same points, and some are ‘nominal’ rather than actually real.

Still others will cite ‘reach and stack’, and while these are at least consistent measurements for comparing frames, they need a little interpretation. Also, by no means does every manufacturer supply them. As a result, frames of allegedly identical size from two manufacturers can actually be pretty different. Of course you can alter reach as you update items such as barsstems, and seatposts, but the initial purchase of a bike or frameset is a single big investment and important to get right. So what should you look for?

However many forks you rock, get your frame size right and the rest will follow!

Seat Tube length
This is key in determining the overall height of the bike. It’s important to consider the extremes – you need enough tube to allow full pedaling height without using a ridiculously long seatpost. But you also want one that’s short enough to allow you to drop the saddle completely out of the way.

The lower extreme is of less concern if you’re riding cross-country, but for trail it’s worth factoring in dropper posts as well – most have ‘collars’ that effectively add around 2.5cm (1in) to the seat tube height.

A short seat tube and a sloped (often kinked) top tube adds clearance and makes maneuvering easier if you ride steep, technical trails.

This is typically measured halfway along the top tube, and is helped by slopes and kinks. You should be able to straddle the bike with around 5cm (2in) clearance – knowing that you can aids confidence, which helps reduce the risk of an off – and helps massively when, inevitably, it does all go wrong. Lower frames are also easier to throw around.

Be aware that 29ers, especially those with long-travel forks, can still be very high at the front.

Frames are generally getting longer for any given height, with a move towards shorter stems to keep the overall reach the same. The benefits are greater stability, more direct steering and greatly reduced chances of being thrown over the bars. Short chainstays keep them maneuverable.

Manufacturers usually give tables showing frame sizes against rider heights. If you find you’re on the cusp, and have an armspan greater than your height (or you ride more aggressive trails) choose the longer bike.

Your Riding Style
While you don’t want to be stretched out, flat-backed as on a road bike – see our road bike frame size feature – you need room for a longer stem and lower position for XC than if you’re choosing a trail bike.

For either style, you need to be able to remove your weight from the front wheel, so make sure the length allows you to sit upright enough to place/remove pressure on the bars at will, rather than having your torso unstoppably bearing down on them.

Speeds are generally lower and aerodynamics are of less importance off road, and for trail riding, a relatively upright position brings the greatest control and speed.

The effective length can be tuned with different length and drop angle stems, different width bars, and either layback or inline seatposts, so as long as the frame height is right, the exact length is less critical.

Perfect reach and standover height make for ideal bike fit, leading to improved confidence and ultimately, improved performance on the trail