Mountain bike size: choosing your frame by height and inside leg

45 mm. That’s the difference in reach between sizes S and L of the same mountain bike, the Commencal Clash V3: 435 mm against 480 mm [1]. On paper, it hardly seems worth mentioning. On the bike, it’s the difference between feeling cramped and riding at ease, which is why a mountain bike’s size comes down to rather more than a letter.

Why size charts don’t compare

The simplest way to choose, Trek explains, is still to start from your height, using the chart each manufacturer publishes for its models. But plenty of riders find themselves between two sizes, and many are used to judging a bike by inside leg or seat tube length. Those measurements still count, but when comparing mountain bikes, Trek considers reach and the effective top tube, meaning the top tube length measured horizontally, to be more important [2].

Trek explains why reach serves as the reference from one brand to another: most trail riders spend more time standing on the pedals than sitting on the saddle. The effective top tube matters more to cross-country riders, who put pedalling efficiency first [2].

An example: three sizes of the same mountain bike

The geometry charts that manufacturers publish show what really changes from one size to the next. Here is the one for the Commencal Clash V3, a long-travel freeride bike [1].

From one size to the next, reach grows by 20 then 25 mm and the wheelbase by 22 then 27 mm, but the stack by only 5 then 4 mm. Going up a size, then, mostly means a longer bike, barely any taller. And the angles stay put: the bike’s character remains the same, and only the room it gives you changes [1].

Enlarge diagram

Between two sizes: riding style and build decide

Manufacturers give the same advice. A longer reach makes for a longer wheelbase: the bike becomes more stable, especially on fast descents, which will suit riders who prefer to plough straight through. A shorter reach makes it less stable but more agile and easier to move around, ideal for a playful style that likes getting both wheels off the ground [2]. Commencal adds the downside: on a longer bike, the greater feeling of security comes at the expense of manoeuvrability, and moving the bike takes more effort [1].

Your build counts too. Proportions vary between riders of the same height: with long legs and a shorter torso than average, a shorter reach may suit you; with long arms and a larger upper body, the size up may be a better fit [1] [2]. A test ride remains the best judge, and failing that, Commencal advises comparing the reach and stack of your current bike with those of the model you have in mind [1].

Small sizes, small wheels

Frame size can also decide wheel size. Cube, for example, applies what it calls ‘Size Split’: 27.5-inch wheels on the frames meant for the shortest riders and 29-inch wheels for the tallest, so that each size stays both stable and responsive [3]. When comparing two models, then, check that the size that fits you actually comes with the advertised wheels.

After the size, the saddle height

Once the size is chosen, the saddle still needs setting. A literature review published in 2011 listed a wide range of methods, based on leg length or inside leg, such as the LeMond method or the heel method, or on a knee flexion angle, and noted that none of them commanded a consensus. It also pointed out that overuse knee pain affects half of all cyclists [4].

A more recent systematic review, in 2022, found strong evidence for setting the saddle from the knee angle measured while pedalling, rather than from a simple formula. It adds that a change of less than 4% of leg length makes very little difference to the loads on the legs, and none to oxygen uptake [5]. In practice, a bike fit at a bike shop or with a specialist remains the surest way to start from a good position. And if your next mountain bike is electric, our guide to e-MTB range will help you choose its battery.

Children: the wheel follows their height

For a child, bike size is read first in the wheel diameter, which follows their height. The manufacturer woom warns against buying a bike too big so that it lasts: a few centimetres make the difference between joy and frustration. With the saddle at its lowest, the child should be able to put both feet on the ground without changing posture, with enough room between the top tube and their crotch; and woom advises against skipping a size [6].

The same manufacturer stresses that a child’s skill matters more than their inside leg: a very confident child can ride a bike that is slightly too big, while another will quickly struggle on it [6].

If you remember only one rule, make it this: the manufacturer’s chart gives you the size, your riding style decides between two, and a test ride always has the last word.

Worth watching

GMBN Tech runs through the key geometry measurements, reach and top tube included, that tell you which frame size fits, beyond the height chart on the manufacturer’s website.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Julien de Jurquet

Campervans, motorhomes, cycling and calculators

Julien de Jurquet writes for Outfarer about campervans, motorhomes, cycling and calculators. He reads instruction manuals to the very end and never lets a figure through without its unit, so that your choices rest on sums rather than promises.

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