Back in the early eighties, when the ﬁrst mountain bikes were rolling off production lines, choosing your bike size was pretty easy. There was little difference in geometry and tube sizes, you just straddled the bike, checked you cleared the top tube by an inch or so and noted the length of the seat tube. Job done.
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Today, we have many types of bike for different terrain, riding and geometry that has continued to evolve in a way that improves the handling of the bikes we ride. No longer can you just take a look at a single measurement and be sure that a bike will ﬁt.
When we sit on a bike, we make contact in three places: our hands on the bars, our feet on the pedals and our bum on the saddle.
It’s the relative position of these three areas that governs how the bike ﬁts and several variables inﬂuence their exact location: top tube length, seat angle, distance from bottom bracket to saddle, crank length, bar height and width, stem length and saddle angle all play a part.
The right size for a particular make and model requires knowing a few basic principles, some compromises, and trial and error.
Seat tube and standover height
When a bike is listed as ‘X’ inches, what does that actually mean? In most cases, it’s the distance from the bottom bracket axle to the top of the seat tube, but it can be to the middle of the top tube or to various other places — there isn’t a universal standard.
Even if there were, it would be no indicator that you could straddle the top tube and clear it, as top tube shape and bottom bracket heights vary substantially. This all relates to ‘standover height’: an important aspect of any bike ﬁt, since it governs the clearance of your crotch!
How much you need depends on your riding style. Cross-country whippets may only need a few centimetres, while dirt jumpers would want a lot more. Many brands now list this measurement, but you can’t beat going to a shop and standing over the frame.
Saddle height and crank length
The majority of mountain bikes have 170mm or 175mm cranks, which do the job perfectly well for most riders. But if you have short legs you may ﬁnd the cranks are too long to turn without your knee bending excessively at the top of the stroke, resulting in the wrong muscles being used.
Similarly, if you’re long legged you may beneﬁt from a longer crank so you can make the most of your lofty dimensions.
For general trail riding, the saddle should be set at a position that helps you maximise power and efﬁciency. Too high and your hips will rock from side to side, wasting energy; too low and your muscles won’t deliver power effectively.
Adjust the saddle height so when your heels are on the pedals at the bottom of the pedal stroke, your leg is fully extended — this means when you move your feet to the right position, your knee won’t lock out. If you need more clearance, drop your saddle an inch or two.
Top tube length
The next most important aspect of ﬁt is the reach to the bars and top tube length. Along with the stem length and bar height, this dictates your weight distribution, the angle of your back, bike handling and comfort.
Usually the measurement is taken from the centre of the head tube to the centre of the seat tube. However, where it actually joins the seat tube (this can either be low down or just about at the top) determines the distance, so the effective top tube (ETT) length is often quoted.
With the seatpost up, a direct line is measured from the head tube horizontally across to the post, resulting in a handy virtual measurement. As with standover height, what works for you can depend on what style of bike you ride and your personal preference.
A cross-country rider may prefer a long, stretched-out position, but a beginner who has never taken a bike off-road may want to be more upright for extra comfort, with less weight on their hands and wrists.
Your reach is often a compromise between comfort, control and pedalling efﬁciency. Find what works best for you, but avoid being too hunched or too stretched out, since this can cause discomfort and back problems.
Seat angle and effective top tube length
The cranks (or bottom bracket) are never situated directly below the saddle, and for good reason. If they were, you’d be placing excessive weight on your arms to support your upper body when you lean forward.
Thus the seat tube lies at an angle, which determines how far behind the bottom bracket the saddle will be and how you’re balanced when seated. Too much can be counterproductive, but luckily the range of angles is usually quite narrow so this measurement isn’t normally that important.
If we take two bikes with the same ETT length but different seat tube angles, the slacker-angled machine will have a bottom bracket that’s further forward in relation to your saddle and vice versa. The upshot of this is that you can have two bikes with the same reach that handle differently, due to how they distribute your weight.
One of the biggest mistakes made by beginners is to slide the saddle too far back. While it may be psychologically reassuring to sit well back from the ‘attacking terrain’ position, too little weight on the front of the bike can make the steering feel vague and stop your suspension fork from compressing efﬁciently.
Sit further forward and you’ll get maximum use of the fork, full use of the front tyre tread and the bike will handle better. This is all assuming that the reach is correct for you. As a general rule of thumb, if you drop a plumb line from the centre of the saddle it should cross the chainstays almost exactly half way between the bottom bracket axle and the rear wheel axle.
Foot position and cleats
With ﬂat or platform pedals, the ball of the foot usually drops into a comfortable position above the pedal axle. However, clipless pedals can be more problematic to get right. A good place to start is to ﬁnd the ball of your foot and place the cleat directly underneath.
Once you’ve found this spot, adjust back and forth — minor changes can affect which muscles are utilised and how effectively you pedal. See what works best for you. Lateral positioning is a personal preference: a narrower stance can improve efﬁciency, but be careful that your shoes don’t hit the cranks during the pedal revolution.
The angle of the cleats should match the natural angle of your feet, which you can see easily if you use ﬂat pedals. Many of the latest clipless pedals have built-in ﬂoat, which helps your foot achieve a natural angle and is a good option if you’re unsure what’s right for you.
Experiment with the ﬁnal set-up; once you have this sorted the pedal stroke will feel ﬂuid with no twisting of the ankles, knees or hips. This can take a few rides but is worth persevering with — when you hit that sweet spot, draw a line around the cleats for reference when they need replacing.
Once you have your basic position sorted, see what adjustments can be made. The stem, bars and saddle can all usually be modiﬁed or changed to ﬁne-tune your ﬁnal riding position.
The saddle typically slides at least 10mm back and forth on the rails, and some seatposts can actually be bought with built-in set back — but be aware of how this changes your posture.
This is especially true of stems, as not only is the reach and your weight distribution changed, but also the front-end handling, because a shorter stem quickens the steering responses. This is one reason why many bikes now have longer than average top tubes and steeper seat angles but shorter than average stems.
Your hands will be in the same position but the bike will react more quickly to your input with more weight over the front. This is useful for bikes with long forks and slacker head angles, which have a tendency to make steering feel sluggish.
Similarly, wider bars can add control and leverage, but don’t go too wide or it will feel unnatural. Bars can also be rotated to factor in the back-sweep. Backswept and upswept bars (measured in degrees) govern the angle of your wrists: certain positions may instinctively feel better than others.
The controls (i.e. gear shifters and brake levers) can be slid across the bars as well as adjusted for angle and most brake levers have reach adjustment — usually a screw or cam, which brings the brake lever closer to the handlebar for easier operation.
A fairly recent development in the world of mountain biking has been the introduction of female-speciﬁc bikes and geometry.
Because women usually have different dimensions to men for a given height (i.e. longer legs, shorter upper body and the position of sit bones), the proﬁle of some models have been altered to give a better ﬁt.
Look closely, however, and you’ll see that many models have exactly the same frames as the men’s equivalent and others only vary in the ﬁne detail. Any one particular model won’t suit all women and some will ﬁnd that a man’s bike is a better fit.
Most have several things in common, such as thinner grips, lighter sprung forks, women’s saddles and narrower bars. But it isn’t just the ladies who can beneﬁt, men can also take advantage of this thinking and should check them out if they have small hands or narrow shoulders, for example.
Many of the same rules apply for the young ’uns, whatever their age, but there are also a few extra considerations to be aware of. The bikes are usually sized by their wheel diameter, from 12in up to 26in. As with an adult, top tube clearance and reach are very important.
However, unlike most adults it’s essential a child can reach the ﬂoor when seated for control and safety. Never be tempted to buy a size up for ‘growing room’ — it can be both uncomfortable and dangerous. Take a look at the controls: can they reach and operate them effectively?
It’s paramount that the brakes work well and can be easily squeezed with weaker hands. Take a look at the shifters too: are they struggling to push a thumb lever? If so, go for twist grip-type gears because many kids ﬁnd the ergonomics easier to handle.
Gears and cranks are also often overlooked, with a considerable number of bikes sporting adult length cranks and full-sized chainwheels. Look for a good range of smaller gears instead, which will ensure they don’t struggle on the inclines.
Six steps to perfect fit
When trying out a bike for ﬁtting in a shop, always wear the clothes and shoes you would normally ride in. Consider what type of bike and riding style you’re after and how that may affect your position on the bike.
Take as much information about yourself and your riding as you can with you; something as simple as an inside leg measurement can rule bikes out (or in) in an instant.
1. Stand over the bike
Straddle the centre of the top tube with your feet ﬂat on the ﬂoor and check clearance — aim for a minimum of 2in or more if you prefer a larger safety margin.
2. Sit on the bike
Adjust the saddle to full riding height. The angle should be about level to start with; excessive nose ‘up’ or ‘down’ should be avoided, but minor alterations can be beneﬁcial to comfort.
3. Reach for the bars
With a little help from a friend, keep your feet on the pedals with the cranks horizontal and lean forward to the bars. If you’re stretching too far try a smaller size; if you feel bunched up try a larger one.
Feel how the weight on your back and hands changes between different sizes. The bars should be at a height that gives you a comfortable back and neck angle — you don’t want to be craning your neck. Can they be altered or even swapped for a different model?
4. Test it out
Go for a ride! Most shops will let you test ride your shortlisted bikes, even if it’s just a spin round a car park. This can give you invaluable feedback on ﬁt and handling characteristics.
5. Tune in
How did it feel? Use the adjustments to ﬁne-tune your position and don’t be afraid to ask the shop to swap over components to your preference.
Try out different stem lengths — even a difference as small as 10mm can have noticeable effects on how the bike steers, longer stems generally feeling slower to react to your input. However, if you ﬁnd you have to make very large changes to the stem or saddle for reach, consider trying a different bike or size.
6. Final touches
Even after buying a bike there will be a period of ‘bedding in’ and adjusting to its speciﬁc characteristics. Now is the time to set and test the suspension, cleat position and bar controls, noting changes as you ride.
Bike sizing is more of an art than a science. There are no magic formulae, equations or charts that will tell you if a bike will ﬁt or suit your riding. But with some background knowledge of the fundamental principles and the time to test out a few models, you should be fully conﬁdent that you’ll ﬁnd a bike and size that’s right for you.