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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 01-06-02, 10:43 PM   #1
Oscar
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How do you take bike measurements? When measuring the chainstay, do you measure from the center of the crank to the dropout, or just the tube itself? Similarly, do you measure the top tune from end to end? Finally, how do you gauge if you have a high bottom bracket or not. Measure from the ground?

I have 6 vintage road bikes with varying geometries. I thought I would get scientific about about things in the garage....
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Old 01-07-02, 05:50 AM   #2
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Tube lengths alone are not an accurate guage to geometry because of variable angles, and tube angles are impossible to measure accurately.
I use a point of origin on the bike, the bottom bracket centre, and mark on the top tub exactly above this point(on masking tape).
I measure the vertical distance from the BB centre to the top of the top tube (or centre, take your pick).
I then measure from this point forward to the centre of the heat tube, and back to the centre of the seat tube. If you measure every point from the origin in X and Y, you can draw a graph of each bike frame.

BB height can vary with tyre thickness. Measure the BB to ground, and the wheel axle to ground. The difference is the BB drop, which is a more reliable figure. Is it high or low? depends on the application ,your crank length and pedal width (for cornering pedal clearance). Compare it to similar style frames from some common makes of the same size, eg canondale/Trek
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Old 01-07-02, 08:22 AM   #3
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Oscar -- check some of the manufacturers' sites. The better ones usually include a pictograph of a frame to render their statistics unambiguous.

I'm with Mike that the center of the crank spindle is the best starting reference point. Wheelbase/chainstay length can be tricky with horizontal dropouts; I would prefer to see it expressed as a useful range (e.g. "WB = 0.97-1.01m, for nominal 700Cx23mm tyres").
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