The most important thing when purchasing a bike is to get the bike frame size and setup right - otherwise you will find you ache on longer rides, and lose enthusiasm for cycling very quickly. Riding the most expensive bike in the world will not be fun if it is the wrong size for you.

As a quick starting point:

- With your feet 20 cm apart measure the distance from the crotch to the floor (you will need some help here!) Advice: find a big hardback book and push it up between your legs, while the other edge of the book is held square against a wall. Make a pencil mark on the wall at the top of the book, then measure from the floor to the mark (repeat three times for an average). Not very elegant but pretty accurate.

- Multiply this measurement by 0.67

That should be a good estimate of the frame size from the centre of the bottom bracket - the part where the pedal arms (cranks) join the bike frame - to the top of the seat post (the part where the saddle enters the frame).

Some bikes have a different geometry (eg sloping top bar) and will be measured differently - your shop or online supplier will make this clear.

(note: this is not the same as setting the correct saddle height for your bike - see below.)

Having bought the correct size bike (use the bike sizing chart here for added confidence) , there is a lot of flexibility for making further adjustments. The saddle height and position is much the most important adjustment to make.

First, make sure your saddle is horizontal. Use a spirit level if necessary. Although some cyclists prefer a saddle that slopes slightly to the front or the rear, this should be a conscious decision following experimentation, not an accidental misalignment.

Now adjust the saddle height so that your legs are almost but not quite straight when they are on the pedals at the bottom of the turn. You might prefer to start with the saddle a little lower at first, while you gain confidence, and then raise it 2mm each week until it is at the right height.

Detailed advice: setting a bike saddle height

There are many formulas and guidelines for setting the exact height and position of the saddle - none of these are perfect because we are all different sizes and shapes. I think the best advice for initially setting a bike saddle height is to multiply your inside-leg (to the floor) measurement by 0.883 and set the saddle height to be this measurement - measure from the bottom bracket to the top of the saddle, straight up the seat tube.

As a check, your heel should now only just reach the pedals when they are in the 'straight down' position. The saddle position might need slight further tweaking but it should be in a pretty good 'starting position'.

If you find you have pain at the front of your knees you should try raising the saddle a little, but for pain at the back of your knees you should decrease the saddle height.

Saddle back and forth position

Now you need to set the 'back and forth' position of the saddle. Typically this results in the saddle being pushed back almost as far as it will go. To check, sit on the bike with a plumbline to hand. Now with your foot on the pedal, which should be in horizontal position, the plumbline dropped from your knee should be in line with the axle of the pedal.

Your bike setup will now be pretty close to how it should be. Don't be afraid to try different positions and setups, but keep a note of the initial setup used so that you can quickly return to the 'original' settings.