It is of the utmost importance to buy a bike that fits you. If the bike doesn't fit, don't buy it, even if you find a bike that's really great at a low price. A bike that doesn't fit will annoy you every time you ride it, and will get in the way when you get out on the trails.
First, I recommend that you read Peter White's excellent article on bicycle fit.
Now I'll give some specific tips about getting a mountain bike that fits you well. There are two main things to consider when getting a bike that fits you. They are height of bike (usually determined by seat tube length) and length of bike (usually determined by top tube length).
Bike frame sizes usually bear some relation to the length of their seat tubes (i.e., they tell you something about the height of the bike). But what is the length of the seat tube? The entire length of it, or from say the middle of the bottom bracket to the middle of the top tube? Haro bikes often have curved top tubes, does their frame size number take that into account? Are the seat tube extensions in Kona bikes included in the frame size? Different manufacturers do things differently, so don't treat the numbers as any definite indication of what bike will fit you best. You may think a frame size sounds too small or large, but what's important is how you feel when you're on it. The only thing that's certain is that given the same model of bike in the same year, a larger number will mean a larger bike.
To start with, you need to ensure that you adequate crotch clearance. Generally, you should be aiming at a minimum of 3 to 4 inches. Because mountain bikes are designed to allow this sort of crotch clearance, they are generally very long for their height. Even if you aren't planning on riding your mountain bike offroad (there are reasons to do this: mountain bikes make good touring bikes, especially for shorter people) you must allow a large amount of crotch clearance to get the appropriate saddle to handlebar distance. If you know the size of a road bike that fits you, you should be getting a mountain bike that has a "frame size" (seat tube height) of at least 2" smaller.
This is only the beginning. Now you must look closely at the relative position of the handlebar with respect to the saddle. The best position for you depends on how upright you like to be when riding. Do you want a stretched-out, low-handlebars position (best for maximum speed)? Or an upright position (better for control on downhills and more comfortable for longer rides)? If you're like most of us, you'll want something in between.
If you're not sure what position will suit you best, you will want to ride a few bikes around to see what you like. The main thing you should look for is feeling balanced. You shouldn't feel too stretched out. This can put too much weight on your hands, which will make your hands tired and can make the bike likely to nosedive (throwing you over the bars) going downhill and off dropoffs. On the other hand, you shouldn't be too upright, as this can lead to a feeling of being cramped (if the handlebar is too close). Also the higher center of gravity can give you less control over the bike since you can't maneuver your weight around well enough. As a general guide, most of the people I ride with are into recreational cross-country riding, and they usually have handlebars that are at the level of the saddle or a bit below. If you want to race, you may want the handlebars lower.
The relative position of handlebars and saddle are affected by several things: the length of the top tube, the height of the head tube, the angle and reach of the stem, and the type of handlebars. Different bikes can offer quite a difference of positions, so try out quite a few bikes before you buy. If you like a more upright position, look for a bike with features like a shorter top tube, a stem that is higher (larger angle between head tube and stem, which makes the handlebars higher) or shorter (which brings the handlebars closer to you), and riser bars.
Fisher Tassajara with original geometry: good for short torsos Fisher Kaitai with Genesis geometry: good for long torsos
Keep in mind that most bikes are designed with the average person is mind. However, some people have much longer legs than average, some have much longer torsos that average. It used to be part of bike lore that women usually felt to stretched out on bikes because they tend the have proportionally longer legs and shorter torsos, but it turns out that this isn't the case: the real problem is that smaller bikes (which smaller women need to ride) tend to have proportionally longer top tubes than larger bikes, so the handlebars tend to end up further from the saddle. (For more on this see Buying a bike that fits a woman.)
If you have a short torso with respect to your height, or if you are small and thus ride a small bike (which tend to have proportionally longer top tubes), you'll want to find ways of bringing the handlebars back towards you. It may be simply a matter of putting on a shorter stem (this is easy to do as MTB stems almost always have well-positioned bolt to let you swap them without disturbing anything mounted on the handlebars). If this doesn't work (you find that the steering is too quick with the shorter stem, or the bars are still too far away) you'll have to look for a bike with a shorter top tube. You could buy a smaller size bike to get a shorter top tube. This usually works reasonably well, as you'll end up with more crotch clearance. You can run into trouble, however, with finding the handlebars too low, which if it's not too low may be fixed by changing the stem for one with a higher rise. Note also that different bikes tend to have different lengths of top tube, so you can choose one that suits your needs better. For example, you might consider the Fisher Tassajara over the Fisher Kaitai because the former have shorter top tubes.
If you are small, finding a shorter top tube can be made easier by the womens specific bikes that many manufacturers are making. These bikes have several adaptations for the smaller riders that work as well for shorter men as for short or even average-sized women. They have shorter top tubes, suspension settings for lighter weight, sometimes brake levers with reduced pull.
However, don't assume that all bikes labelled as "women's specific" are going to be appropriate for a women or small man rider. Some of them, for example the Trek and Juliana bikes, are very well thought out fpr the needs of the smaller rider. Many so-called women's specific bikes are way too heavy and offer a ridiculously upright riding position, or really don't have short top tubes. To see a more thorough discussion of women's bikes and components, see What to look for in a women's bike.
If you have a long torso with respect to your leg length, you may find the distance to the handlebars too short. One thing you could do is buy a larger framed bike to get a longer distance between you and the handlebars, but this is not at all recommended. Crotch clearance is essential for safe off-road riding. You can replace the stem with a longer stem, but on many bikes this results in slow steering, as it increases the distance from the end of the handlebar (where your hand rests) and the pivot point (the end of the stem that meets the head tube). However, recent trends in mountain bike geometry (pioneered by Fisher with its Genesis geometry) are in your favor. The trend involves somewhat longer top tubes, which enables you to have a shorter stem (which speeds up the steering) and still have the handlebars the same distance from the saddle. In your case, you can then put a longer stem on to get the handlebars out to a good position.
Leg Length MTB Crank Length 60 to 65cm 155mm 66 to 70cm 160mm 72 to 75cm 165mm 75 to 78cm 167.5mm 79 to 81cm 170mm 82 to 83cm 172.5mm 83 to 86cm 175mm to 177.5mm 87 to 90cm 180mm to 182.5mm 91 to 94cm 185mm 94cm plus 190mm
Most modern MTBs come with 175mm cranks. This is too long for many people, and too short for some. For more info on this, see my article on cranks. The chart to the right has leg lengths (crotch to floor in bare feet) in the left column and suggested crank lengths for MTBs on the right. Note that this chart is adapted from the chart in my cranks article, by adding 5mm to each crank length in the original chart. This reflects the fact that MTBers tend to use longer cranks than road riders. Note that the industry-standard 175mm cranks actually are ideal for only a very small segment of the population. Taller than that you'll benefit from longer cranks, and shorter than that you'll benefit from shorter cranks.
Measure your leg length (the cranks article shows you how) and see where you fit on this chart, and compare this with the crank length on the bike you're thinking of buying. If the suggested crank length is a sizeable distance (5mm or more) from what's on the bike, it will benefit you to get different length cranks.
Some length cranks are easier to find than others. If your ideal crank length is 170mm, you are in luck. All Shimano cranks come in either 170mm or 175mm, so if the bike comes with 175mm cranks, ask your bike shop to swap the cranks for an equivalent 170mm crank. They may charge you a small fee to do this, especially if the cranks they take off aren't the same brand as the ones they put on, but it will make pedalling much more stressful on your knees, and it will well and truly be worth it.
If the suggested crank length is less than 170mm or greater than 175mm, it can be hard to get exactly what fits you. The shimano Deore XT cranks offer the widest variety I know of: they come in lengths 165/167.5/170/172.5/175/177.5/180mm. Even if your suggested crank length is less than 165mm, or greater than 180mm, it would be very beneficial for you to have a crank length that comes as close to your ideal as you can. I know this is alot of money (these XT cranks aren't cheap) but if you're a serious cyclist, it willd definitely be worth it. Note that it can be difficult to get these non-standard lengths of cranks. For example, 165mm versions weren't available in the UK. But you can order them from the US. See the cranks article for more info.
If you have very short legs and can't afford a 165mm Deore XT chainset, then if your bikes comes with 175mm cranks, at the very least get them swapped for 170mm cranks. It won't be optimal for you, but it will be far better than sticking with the extremely long cranks that came with the bike.
Click here for the next section (The bike buying process).Back to the beginning