When you go to the bike shop to buy a bike, it can be a difficult process, there are many different styles and sizes to choose from. So when choosing a bike we have put together a concise guide to frame size and materials. So you now have some idea of the right bike for you, all that stands between you and the freedom of London is what style of bike you want to buy.

Bikes range from £25 second-hand roadsters to £5,000 custom-made titanium mountain bikes, but you need the right bike at the right price to suit your needs, otherwise you simply won't use it. A cheap bike may be perfect for the local trip where theft could be a worry, while a good quality racing bike may be ideal for a long commute. It's worth seeing if your employer offers loans for bikes or is signed up to the Cycle to Work scheme, which may get you a discount on the price of the bike. Whether you are buying new or second-hand, be sure to try before you buy.SizeWith bicycles, size matters, and most bicycle models come in several sizes. For example, mountain bikes and hybrids are usually sized in inches from 15 in to 21 1⁄2 in, or simply XS, S, M and L, but road bikes are sized in metric centimetres, from 48 cm to 62 cm in 2-cm increments. Many models now come in versions for men and women. Women's bikes can have either a step-through frame for easier dismounting, or can be like the equivalent men's models but with altered geometry to allow for relatively shorter torsos and longer legs. There are no hard-and-fast rules on what an individual should ride, so it's useful to try a variety of models including those designed for the opposite sex.Usually, the correct frame size will be about 26 cm (10 in) less than your inside leg measurement for on-road use and 30 cm (12 in) less for off-road. You should be able to stand across the bike without your crotch touching the top tube, otherwise stopping at the lights could be painful. Allow at least 2.5 cm (1 in) clearance for bikes for on-road use, and 5-7.5 cm (2-3 in) for mountain bikes.With the saddle height set correctly - your leg should be almost straight with the heel at the bottom of the pedal stroke - make sure you are not stretched too far forward or too low down on the handlebars. Handlebar stems on some bikes can be adjusted up or down, and most competition bikes (on-road or off-road) tend to have more stretched-out positions. If the bike is too small, you will feel cramped and the saddle height may not go up far enough (there is a limit marked on most saddle seat pins). If you can't get into a position you like, consider another size, model or brand, because bike geometry varies between models and between brands.Frame materialsMost modern bikes are made of aluminium. It's light and doesn't rust, but the bikes can be less comfortable than steel ones because the frames feel stiffer. Quality steels such as cro-moly or manganese molybdenum build into comfortable frames, and Columbus, Tange and Reynolds are popular tube brands. Lower grade hi-tensile steels are used on cheaper bikes. All steel frames will rust if not cared for, unlike titanium frames, which are non-rusting, lightweight and durable but expensive. Carbon fibre offers the ultimate in low weight and high performance, but it's expensive and fairly fragile.

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