HOW DO I FIND A BIKE THAT'S THE RIGHT SIZE FOR ME?
The most important aspect of your new bike is the correct fit. It doesn't matter how much you are choosing to spend or what wonder material you are choosing for the frame, if it doesn't fit correctly you'll not be comfortable. If you're not comfortable, you'll not go out and ride!
The first measurement you need to take to determine the the correct frame size is your inseam measurement (this does not equal the inseam measurement of your jeans!). This measurement can be accurately taken by standing in your sock soles with your back to a wall. Place a book between your legs and raise it until it is tight against your crotch. The top edge of the book to the floor is the meaurement you're looking for. FOr the purposes of this guide we're going to work in centimetres, so if you've measured in inches, multiply by 2.54.
ROAD BIKE FRAMES
To calculate the road bike frame you require, multiply your inseam measurement by 0.67. This will give you the seat tube length you require for your new road bike frame. Note that this measurement is accurate for centre-to-top (centre of bottom bracket to top of top tube) of seat tube measurements. If the bike is measured from centre-to-centre (centre of bottom bracket to centre of top tube) simply subtract 1.5cm.
Note that many modern road bike frames use a 'compact' geometry. this shortens the top tube, seat tube and seat stays. This gives a stiffer frame with lighter weight for the same equivalent traditional geometry frame. All manufacturers will give you an equivalent 'tradtional' geometry seat tube length with their compact frames. For instance a 47cm or 'small' giant TCR has an equivalent frame size of 52 cm in traditional terms.
MOUNTAIN BIKE FRAMES
For mountain bike frames, the same calculations as the road bike frame are performed, but 10-12cm are taken from the final seat tube measurement. Many riders choose slightly smaller frames if they do a lot of technically difficult riding.
ADJUSTING YOUR BIKE TO PERFECTION
One big question I get asked is 'How high should my saddle be?' The easy way to get this correct is to sit on your bike as you normally would and place your heels on the pedals. Your saddle should be high enough to give your leg a full extension at the bottom of the pedal stroke and but low enough to prevent your pelvis from rocking from side to side. Once you find the correct saddle height, mark the post with a small screwdriver at a point just below where the seatpost sits.
The recommended stem length should allow the handlebars to obscure your front hub in your natural riding position. Your position may change as you spend more time in the saddle (e.g. many people find their hamstrings lengthen and they position their saddle higher) and your stem should be adjusted accordingly.
Many people have recommended buying a couple of used stems on eBay to try the different lengths and rises available. You can easily sell the ones that are not the correct length and invest in the expensive carbon one you're lusting after, safe in the knowledge it's the correct length!
The ideal bar is as wide as your shoulders are broad, so measure between your shoulder joints. Many mountain bikers choose slightly wider 'riser' bars for extra control. This must be balanced against the danger of snagging the bars on terrain normally ridden.
For road bikes 170mm cranks come standard on bikes with frames up to 54cm, 172.5mm cranks on bikes with frames between 55 and 61cm and 175mm cranks on bikes with frame larger than 62cm. Mountain bikers generally use cranks 2.5mm - 5mm longer than they would on their road bike.
ADJUSTMENTS FOR COMFORT
Modern carbon forks and frames dampen a lot of the road vibrations which can be felt through the grips on your bike. If you still experience discomfort, it is worth investing in padded or 'gel' inserts which sit beneath the bar tape on your road bike and insulate you some some of the vibrations.
Equally, your might be experiencing a 'numb bum' from either a poor position on the bike, or an inappropriate saddle. You will recognise that you are sitting correctly on the saddle when you can feel that most of your weight is resting on your 'sit bones', 2 protrusions at the bottom of your pelvis. If you are sitting quite upright (e.g. on a city bike) you will require a wider saddle than if you have a flatter back position (i.e. road racing). A saddle is a very personal choice and once people find one they like that suits them, they will transfer it from bike to bike.
People with smaller hands often benefit from adjustable reach brake levers. These are available in a wide variety of designs and a numerous price points.
If you have any further questions on bike sizing, just drop me a line via the eBay community website (ID: green-wheels), I'm happy to help.