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Sizing Issues(3 posts)

Sizing Issues2300 Edmontonian
Dec 1, 2002 8:19 PM
After my Trek 2300 this year, I'm looking to buy a decent second hannd bike over the internet for training.
When I bought the bike, my LBS told me that different companies have different sizes.
Like my 54cm Trek may be a 52cm Cannondale.
To what extent is that true?, considering this how do you buy your bikes or frames over the internet?
And hey, how much does sizing really matter?
re: Sizing Issuesgeeker
Dec 1, 2002 9:20 PM
"different companies have different sizes"

They measure (seat tube length) different ways, starting from center of bottom bracket. Trek measures to the top of the seat tube collar. Cannondale measures to the top of the top tube. (*Both* of these methods are sometimes referred to as 'center to top'...) Others, eg Lemond, measure to the center of the top tube ('center to center').

"how do you buy your bikes or frames over the internet"

Probably best to focus on top tube length. There's an article about this on Sheldon Brown's site. For a given top tube length, longer seat tubes (quoted sizes) will put the handlebars higher, which may be a factor. Also see Rivendell's site for an interesting sizing article.
geometry charts and measurement...C-40
Dec 2, 2002 5:59 AM
If you're buying a name brand frame that's relatively new, the geometry charts should be available on the manufacturer's website. If not, the seller should be able to measure the TT length, standover height and head tube length without difficulty. Measuring the seat tube angle is also pretty simple. All it takes is a level, a plumb bob, a scale and a little trigonometry. Protractor devices are generally too small and inaccurate to take seat tube angle measurements.

You 54cm Trek compares to other brands 52cm measured center to top (of the top tube). In brands that measure center to center, it would fall between a 50 and a 51cm.

You should not buy a frame strictly by top tube length. The vertical fit of the frame is just as important. You should also know the standover height, head tube length and the seat tube angle before buying.