Bike size calculator - Measure - model - analyse - change...
What are the key geometric metrics?
I have used 'Bicycling Science' as a reference for some of the formulas used in this website.
It contains completely new chapters on aerodynamics, unusual human-powered machines for use on land and in water and air, human physiology, and the future of bicycling. This edition also provides updated information on rolling drag, transmission of power from rider to wheels, braking, heat management, steering and stability, power and speed, and materials.
On a bicycle with front suspension, when you hit a bump, brake hard or go down a hill fast and get to the bottom the forks compress to absorb the shock. As they compress the trail is reduced and the bike becomes less stable. In most cases this is undesirable. These are just the times when you don't want an unstable bike. I have observed someone getting to the bottom of a steep hill on a full suspension bike and the handlebars shaking violently. This is likely to be due to the reduced trail. The cure is to fit longer forks and increase the trail all round or fit harder springs / put more air in the forks. What are reasonable figures for trail? Go to Create a map and you can load the geometry for different bikes:
Note that motocrossers use this reduced trail to good effect when cornering. One effect of braking hard at the corner and shifting their weight forward is reduced trail. The reduced trail makes the bike turn quicker. As they exit the corner they sit further back, unloading the front of the bike, lengthening the forks and increasing the trail.
When you change forks on a bicycle if the length of the forks or the offset between the steering axis and axle is different it will change the trail. You can work this out before buying and fitting them!
Bottom Bracket Height
Effective top tube, Wheelbase and Seat to Bottom Bracket
Reach and Stack
There is a great deal of information on the internet about setting up a bike to fit your body. In very general terms, the size of the bike needs to scale up to each individuals body size.
Different manufacturers use different measurements to characterise size and fit: Some manufacturers provide the 'effective top tube' measurement as most characterising overall size whilst others specify 'Reach' and 'Stack'.
You can use this website to work out what these are for your current bike to make comparisions. This is easier than measuring them directly as you can see from the diagram.
Whilst every effort has been made to ensure the accuracy of the information on this website, mistakes are possible therefore use it at your own risk!
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