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The difference between a 700c wheel with a 38mm tyre and a 26" with 2.25 is 24.7mm.
Does that mean a 2.5cm lowering in BB when chucked on a bike?
Diameter of a 700c wheel with a 38mm tyre: 698.00mm;
Diameter of a 26" wheel with a 2.25" tyre: 673.30mm.So the bb drop is (698.00-673.30)/2 = 12.35mm.
Tijs-
Diameter of a 700c wheel with a 38mm tyre: 711.00mm
That seems like a lot, I'd have guessed at more like 698mm. I've got some 38s waiting to go on, maybe I should do that and get my tape measure out :)
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[Source]
You've pulled the OD from the 635 rim table, scroll down to 622 and it agrees with me.
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673.30
While we're being pedantic, quoting an overall diameter denominated in hundredths of a mm is pretty meaningless. Even if the tyre is exactly 2ΒΌ" wide, the overall diameter depends a great deal on exactly which tyre and what rim it's on. Oh, and your source is fundamentally flawed because it simply adds twice the tyre section to the bead seat diameter and that's not how clinchers work. You quickly learn how much of problem this is if you use this method to set up your medium gear bike, because you will over run the roll out test if you set it to a theoretical 72" exactly.
gbj_tester
The difference between a 700c wheel with a 38mm tyre and a 26" with 2.25 is 24.7mm.
Does that mean a 2.5cm lowering in BB when chucked on a bike?