What Size Mountain Bike - Mountain Bike Zen
What sized mountain bike frame do you need?
This is probably one of the first and most common questions asked when considering a mountain bike purchase. It is also one of the most difficult to answer.
The frame size of a bike is usually measured as the distance from the center of the crank to the top of the seat tube - where the seat post emerges from the frame of the bike. This distance will affect the distance from your bum to the pedal and hence how far your leg will reach is important. It also effects how far the seat is from the ground and hence whether you can touch the ground with your foot when you need to stop. Most seat posts will rise quite some distance and hence this distance is flexible, however they cannot be reduced in height beyond the top of the seat post and for this reason you must ensure that the bike you purchase will at least allow you to touch the ground and pedals safely so you can ride the machine.
You measure your inside leg as the top of the inside of your thigh to the bottom of your foot.
As a very simple guide the following guide to calculating mtb frame to inside leg measurement sizes may help:
Frame size = Inside leg measurement - 13 inches. (For road bikes this would be - 11 inches, but this site is not about road bikes!) So for a 30 inch inside leg, a 17 inch frame would be recommended.
Frame size also affects the distance from the handlebars to the seat too, and this will effect how much reach you need to be able to control the steering and brake etc. Bikes are made generally to be suitable for the most common body proportions so a bike that fits your leg will generally fit your body too.
Ultimately the only real way to make sure a bike fits you will be to go try it out. No shop worth its oil would prevent you from sitting on the bike and really should let you ride it up the street or even take it on loan for a day or two to make sure it suits you. Another way would be to put any new purchase on hold for a while and hire different bikes for your next couple of trips out. You will know straight away if a bike 'fits', if you feel a little odd or uncertain about a bike at the end of a ride then it is probably best to try something else.