To be comfortable to ride, your bike must be approximately the right size. Approximately is close enough because most bikes come in just three to six different sizes. You fine-tune the fit by adjusting the position of the saddle and handlebar, but it needs to be in the right ballpark to begin with.
There are three basic criteria for a correctly-sized bike, all of which you can determine without expert knowledge: the top tube must be low enough for you to stand over it; the saddle's position must allow you to pedal comfortably; and the handlebar's position must suit you. There's some leeway with these dimensions, as we all have a 'fit window' within which we can be comfortable. The dimensions will vary between bike types – for example, a road bike has a different handlebar position from a roadster.
Take with a pinch of salt any bike-fitting formulas that depend on static body measurements, such as your inside leg length. These formulas tend to be extrapolations from bike dimensions that seemed to work for young professional racing cyclists on road bikes. Middle-aged cycle commuters on hybrids were not consulted.
There is a better, but not infallible, way to estimate whether a given bike should fit. It's based on the fact that a manufacturer's medium size bike in any range is designed to fit Mr Average or (if it's a women's specific bike) Ms Average. Manufacturers can sell more bikes if their average size model fits the average cyclist, because there are more people in the middle of the height bell curve.
Bike size means seat tube length. While the effective top tube length is a better indication of whether a bike will fit, seat tube length is the de facto standard. So 54cm bike is one whose seat tube is 54cm long.
You and Mr Average
The average size bike in a range is the one listed as Size M or, if not designated as such, the arithmetic mean of the different sizes. To find that, add the sizes of the different models together and divide by the number of different sizes. So if a bike comes in 50, 52, 54, 56, 58 and 60cm sizes, the mean is 55cm. Mr Average should fit size M or a notional 55cm model.
To determine which size fits you, you need to compare yourself against Mr Average – unless you're buying a women's specific bike, in which case you'll compare yourself to Ms Average. In the UK, Mr Average is about 176cm (5ft 9in), while Ms Average is about 162cm (5ft 4in). If you are average height, simply pick size M or the nearest size to the mean. When the mean is the middle of two sizes, either should fit, given some fine-tuning.
If you are not Mr (or Ms) Average, the important thing to know is that bike sizes vary half as much as height. To find the size that should fit you, add or subtract half the difference you are taller or shorter than average height to the size of the average bike. So if you are 180cm tall and the mean or size M model is 55cm, you want the 57cm model; 180cm minus 176cm, divided by two, is 2cm, which you're adding because you're taller. If you are 160cm tall and are buying from the same range, you want the 47cm model.
This method will get you a bike that's approximately the right size. Depending on your proportions, flexibility, or preferences, you might be happier on a bike that's one size either side.
You can't beat visiting a bike shop in person to try a bike for size. Many shops will let you have a short test ride and some will offer a bike-fitting service. If you can't leave the shop with the bike, or don't want to pay for a bike fit, you can still get a good idea of whether a bike will fit you. Stand over the bike first. Got clearance? Good.
Now put the saddle to approximately the right height. You'll need someone to hold the bike steady for this. Put the heel of one foot on the pedal and backpedal until the cranks are in line with the seat tube. Your extended leg should be just straight. Adjust saddle height until it is. When you pedal normally, using the ball of your foot, there will be a slight bend in your leg at maximum extension.
Finally, check the handlebar position. Sitting on the correctly-adjusted saddle, take your hands off the grips (flat bar) or brake hoods (drop bar) and hold them loosely behind your back without changing the angle of your torso. If you can sit there comfortably, your handlebar position is okay; if you can't, you'll probably be carrying too much weight on your hands when you're cycling, which will lead to aches and pains. Try again with the stem in its highest position on the steerer and/or flipped over to provide more rise. No luck? You may need a shorter, higher-rise stem. Sliding the saddle back on its rails can help too, as a forward saddle position tends to tilt you forwards. If nothing works, you may need a different style of bike entirely.
If you're buying online, you won't be able to try out a bike like this. So check what the online shop's policy is regarding returns. Can you easily swap it for a different size if you get the wrong one? Even if you get the right size, you will probably need to fine tune the fit yourself. So online shopping is a better option if you already know your size and you're confident about making adjustments to the bike.
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