There was a website that has this info. Unfortunately I never book marked it. Try a google search with different word combinations and you might find gold.

http://cycling.bsn.com/cgi-bin/ergobike.cgi?96.5

At the bottom of the page you can choose your inseam measurement. The form at the top will then be filled with average values for the various body dimensions of a racer with that inseam.

The site goes on to describe different geometries, and may give you some feeling for which frame builders have longer or shorter tt, slacker seat tube angles, etc.

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Meg.

which info, the geometry or the anatomy? It's going to make a pretty signficant difference in the terms I use in the search.

A combo of both from what I remember of the site. It was how I determined that my torso is longer than my legs and my femur is shorter than lower leg. Explained quite well. Sorry I didn't bookmark it.

here it is in simple terms:

most road race bikes are "parallel" and "square," or close to it.

by "parallel" i mean that the seat and head tubes are parallel to each other. they're both 73 degrees, or 74 degrees, or something close to it. then never vary by more than a degree and a half.

by "square" i mean that the seat and top tubes are the same length. so a 55cm bike that is both square and 73 parallel has 73 degree angles and 55cm in both top tube and seat tube lengths.

this is the case for all sizes and for all people whatever height or, if it isn't, it ought to be.

when this is not true, it's usually in the smaller sizes. it's not unusual to find a 49cm bike that has a 52cm top tube. this is for two possible reasons. first, because cycling has historically been a men's sport, and geometries favor men's dimensions. often, a 5'6" cycling male would have a longer torso and shorter legs, hence his need for a smaller seat tube and longer top tube. the other reason is to keep from having toe clip overlap problems, i.e., to make sure the front wheel axle positions the wheel far enough ahead of the bottom bracket so that the rider's foot doesn't clip the back of the front wheel when the bike's turning.

but for normally proportioned people, square and parallel is a rule of thumb.

other dimensions are almost always within 1cm of the same for just about all bikes: 41.5cm chainstay, 6cm of bottom bracket drop, 43mm of fork rake (always between 35mm and 50mm), and so forth.

tri bikes tend to have four dimensions that differ from road bikes. head tubes are shorter (by 15% to 20% or so, depending on a variety of options), so that you can get down in a good aero position; top tubes are (or ought to be) shorter by, oh, about 7%; chainstays ought to be closer to 38cm or 39cm; and seat tubes ought to be much steeper, say, 78 - 80 degrees.

but not head angles. tri bikes aren't parallel. in fact, if anything, head tubes might be a degree or two shallower.

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Dan Empfield
aka Slowman

Slowman I too have wondered about frame size and fit. I appreciate the general rules of thumb, but in looking at multiple bikes (Griffen, Guru, Felt, Elite) all seat tube angles are 76 and all chainstays are >than 39.5 cm. are you saying that the bike industry has let us down as far as ultimate handling of a tri bike? Or am I being too literal as far as the general recommendations you have made?

personally, i don't like 76-degree bikes. they don't do me much good. for a person who wants an intermediate position, fine, but i'm 74 degrees for my road bike and 81 degrees for my tri bike (this is for me personally) so what do i need with a 76 degree bike?

however, there are certain bikes, felt is an example among those you mention, with seat angles shallower than i like, but with top tubes longer than i like. these two wrongs actually do make a right, because i can "make" a tri bike out of it that fits me by just moving the saddle forward. since the chainstays and head tube are all shorter than a road bike's tube lengths, it works for me.

i don't think the bike industry has let us down, exactly, but there aren't that many companies willing to make a bike with the geometry the way i'd like it to be made.

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Dan Empfield
aka Slowman

So based on this formula if I ride a 57.5cm top tube road bike, I should be riding a 53.5cm top tube tri bike? Is that right?

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**********************
I was, now I will tri again!
...
Any time is a good time.
God Bless you my friend.

'this is the case for all sizes and for all people whatever height or, if it isn't, it ought to be'

i don't know. lemonds have a realy slack seat tube and a really steep head tube. this is my favorite road geometry.

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customerjon @gmail.com is where information happens.

"when this is not true, it's usually in the smaller sizes. it's not unusual to find a 49cm bike that has a 52cm top tube."

Anyone got any knowledge off the top of the head as to brands where the above is not true? I'm shopping for my 5'6" wife who, despite being 5 inches shorter than me, has about the same inseam, so it seems I need a bike that is switched from the typical seat tube-head tube setup referenced above, or at the very least, one that's "square" in the smaller sizes.

"or at the very least, one that's "square" in the smaller sizes."

look for a bike with dual 650c wheels.

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Dan Empfield
aka Slowman

With the advent of longer seatposts and longer body positions, you can find a bike (particularly in the smaller sizes) with a top tube as much as 3-5 cm longer than the seat tube. Then there are some that have seemingly odd measurements, such as a bike that measures centre to top of seat tube, but the top tube measurement is a measure of the seat tube centre to centre (i.e. 54 cm c-t has a 52 cm top tube), or the converse ( 56 cm c-t has a 54cm c-c seat tube and a 56mm top tube). I had a GT Vengeance that was basically a size 43 (according to conventional measurements), but with a super-stretched 53cm top tube. This did allow for a very short headtube.

In the compact bikes, I would fit onto a Giant size medium (around a 50cm top tube), but the headtube was so tall that I could not get a serious TT position (and thusly could not do my poorest Michael Hutchinson impresonation). The headtube was made for a larger size (like a 54 or 56cm).

What I am trying to say is that the traditional sizing tends to be square, even though with some of these measurements on the modern bikes seem to go otherwise. They do generally (if you know the references for size) go with traditonal sizing, with the exception of some dedicated TT frames. I say we go with Empfield's idea of getting bikes with top tube, then head tube length.

A bit of a basic question, i'm afraid, but one that's been gnawing at me as I read through all the bike geometry articles...

At exactly what point on the bike do you use to take measurements for seat angle? ...do you measure from the front of the seat, the middle or where it mounts to the seat tube? at the bottom, do you measure from the bottom bracket, or from the center of the welded tubes at the bottom? I looked at the bike geometry page, and couldn't quite make it out, or perhaps i might have glossed over it...

Thanks!