How to size a downhill/AM bike
I'm just looking at buying my first AM/DH bike, but because the geometry is so vastly different to a XC bike it seems like the normal measurements are useless.
How do you size a bike you stand up on most of the time?
Top tube length and reach.
Top tube, the length from the centre of the fork steerer tube (at the same height as your stem/bars) to the centre of the seat tube (at the height your saddle will be when you sit on it).
Reach, draw a vertical line from the centre of the bb up, then a horizontal line from the centre of the steerer back to meet the vertical line.
Reach determines how the bike feels when you stand up. Too long and you are stretched out and floundering when the front slides, too short and you will smash your knees on the bars when you pedal stood up (and this really hurts).
It's the most important measurement, best way to measure it is to stand up on the bike and see how you feel. No need to get fancy, just lean the end of the bars against something solid (like the front desk of the shop or a wall) and stand on the bike, should tell you all you need to know.
Top tube is important, but less so than the reach as you can alter it to a degree with saddle position and stem length, plus it varies depending on the frame. Best way to test it is just to sit on the bike with the saddle at pedalling height, easy.
try this. it should give you a range for XC, AM and DH
Fit Calculator - Competitive Cyclist
you want to look at Virtual Top Tube length, aka Horizontal Top Tube length
Stand Over Height probably wont matter since DH bikes are pretty low in that area
Last edited by nauc; 09-23-2011 at 04:10 PM.
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