Compact Frame

[VeloVT]

I kind of don't get how they measure top tube lengths. shouldn't the effective length be shorter than the actual top tube length? where do they measure standover height since it actually does matter where they measure it on a sloping tube?

I'll take a crack at it (someone correct me if I'm egregiously wrong).

1) top tube is measured from where it meets head tube to where it WOULD meet seat tube if it were horizontal

2) the reason this effective length is LONGER than actual length is that BOTH the down tube AND the seat tube have slope. Put more clearly (I hope ), if the downtube had slope but the seat tube were perfectly vertical, the actual tt length on a bike with a sloping tt would be longer. But while the down tube slopes away from an imaginary vertical line bisecting the bottom bracket, the seat tube also slopes away, in the opposite direction, so as you move UP the seat tube (=as the tt gets more and more horizontal), you are effectively stretching the tt out (b/c the sloping seat tube moves farther and farther away from the imaginary vertical bisecting the bottom bracket as it goes up). I checked this out on my new compact frame and I think it's true. But I'm not mathy (my verbal SAT score was several hundred points higher than my math score ), so someone go ahead and lay in if this doesn't hold water.

3) standover -- I wonder if this varies by manufacturer? I just measured my new bike, and standover ranges from 71cm at the seat tube to 75 at the head tube. The Scott website lists it at 73 cm. This seems to be the median? If you look at the geometry diagram here http://www.scottusa.com/us_en/category/72/contessa_road (you may have to click on the "geometry" tab), it appears they take the measurement somewhere in between.

Hope that helps, and hope someone can correct any major errors I may have made ...

[madscot13]

err, I'm not great at trigonometry. let's talk real life value. do I use the effective or actual top tube value?

do you actually stand at the midline or do you stand closer to the seat making the standover an estimation?

[VeloVT]

Well, the standover and the top tube measurement are separate issues... and I think the standover is much less important than tt (though I'm tall enough at 5'6" that standover on frames that otherwise fit me is rarely an issue, so maybe I"m a little biased on this point). But in terms of reach, you definitely want to use effective top tube length, NOT the actual length.

To give some comparison on standover values, my compact frame standover ranges from 71 cm to 75 cm right at the head tube. By contrast, my old 54 cm traditional geometry frame had a standover height of 77cm, and I had at least a half inch of clearance (probably more, I've sold it so I can't check and I don't remember precisely, but it was never an issue).

I think it's possible to overthink these things -- exactly where you'll stand over the frame and whether you'll have enough clearance, given an abstract number on a page -- maybe better to just go try a few ...

[madscot13]

you are absolutely right; I should go stand over a few. the bikes that I am actually thinking of are just frames though. And I'm not really looking to buy this exact moment.

I'm just sort of curious with what they are doing with the frame and if that means they would fit me. Stand over is always an issue with me. My inseam is about 26" and so that is the first thing I look at when I am looking at frames.