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-   -   Frame sizing idiocy (https://www.bikeforums.net/framebuilders/819211-frame-sizing-idiocy.html)

Kimmo 05-19-12 07:05 AM

Frame sizing idiocy
 
Just saw a thread which reminded me of something that's bothered me a lot since pre-internet days... and it's even more of an issue these days; the OP in that thread, is he talking about compact frames? One would hope so...

But it illustrates perfectly what an utterly useless measure seat tube length is, and has always been. It can only explicitly tell you standover height; you're meant to guess about the one dimension that really matters. Sizing by seat tube seems to be one of those traditions that folks generally fail to evaluate on its merits.

So, anyway, now I can kvetch to folks across the world about it, and just maybe make a difference. Why the effing hell aren't bikes sized by top tube length, or rather effective (as if it was horizontal) top tube length?

Because, you know, that'd sort of make entire arseloads more sense.

Is anyone doing it? Can I persuade any builders to consider it?

These days, would anyone ask what seat tube length goes with a certain top tube length (I'm certain you're no stranger to the reverse question)? It's standard or compact, or something in between. Who cares how long the damn seat tube is, unless you're talking ISP.

fietsbob 05-19-12 09:58 AM

You talking handbuilt from tube set custom frames?,
or jump on it and ride it home from the bike shop

or guessing whether some Ebay bike will fit..
mass produced bicycles

MassiveD 05-19-12 02:35 PM

As far a frame sizing is concerned, who cares? What matters is fitting. Fitting can't be reduced to a number or dimension of a frame. The guy mentioned his inseam so let's talk pants. The inseam is no good without the waist, and effectively the rise could still be wrong, and a bunch of other things. If you fit off the rack it is easier, but the three methods I use are to get clothes tailored, to try stuff on till something works, or to buy stuff sight unseen on hope.

As far as sizing frames is concerned, no one number is correct. Just because something is one's suit or dress size does not mean it will fit. Both top tube, and seat tube are as arbitrary. As you point out, effective top tube length varies, so do people's preferences for saddles seat posts, bars, and stems, if anything it could be worse.

Beyond that people tend to have a body part that is hard to fit, so that will tend to drive them towards either stand over or reach.

Then there is the whole bike store dance. They need several numbers to play with so they can move those frames. If a person's reach is wrong, you sell the frame on stand over, and sell them a stem. Or the reverse.

The seat height is like the QWERTY keyboard it may not be the best design, but people are familiar with it.

By the way, I am curious what framebuilders do as far as selling their frames. Their size never enters my head, but I guess a client might care for resale. When I buy a suit, the tailor puts my name on the inside pocket, he never inserts a tag saying 44 Tall.

veryredbike 05-19-12 10:45 PM

Yeah... having fit a lot of people for production bikes... the numbers stamped on the frame are guidelines for figuring out which bike to start their test rides on.

They don't refer to much on the bike anymore... most companies (but obviously not all) will call a bike a 54 if it would fit the same person that a square geometry 54 would. They could do it by top tube, sure... but that only tells you part of it, even for the most basic fit... what about the head tube height? More and more I'm coming to discover that that's a variable that I need to pay more attention to. In the end, the V-top tube length doesn't tell you that much more than the faked tube length.

Like massive is saying, if you want a number that tells you if the bike fits when it comes to production bikes, it doesn't exist. You have to test ride the bike, or sit down with a full set of dimensions and compare to what you know works for you.

MassiveD 05-19-12 11:03 PM

The way I actually fit myself, is I draw out various frames I am familiar with, frames I own, and I have CAD for those frames. Then when I am interested in a design, I look at the numbers, and I scale a picture of the frame in CAD from knowns like rim size, or I take a test ride and jot down real numbers.

While I make frames this is all basic stuff anyone could do in sketch-up I suppose, or on brown paper.

Over time as your files increase, as your experience riding and fitting yourself increases, you have some real numbers to deal with. I always wonder about people who show up asking to be fit, many people have owned a dozen or more bikes by that time, yet every bike they approach like a blank slate.

ThermionicScott 05-19-12 11:15 PM

That's why I like how folks in C&V will often refer to a frame using both dimensions, like 62x57, often but not always C-C.

ftwelder 05-20-12 02:41 AM

Frame builders size bikes according to top tube length and experienced riders ask for them that way. These days with compact frames and a dozen different head bearing configurations/style preferences buyers of truly custom frames spec every single dimension. Some methods of construction have limits to tube angles also.

Seat tube does still matter though. I like a pretty long top tube and many builders produce bikes that are longer rather than taller.

Andrew R Stewart 05-20-12 06:37 AM

Why bother with frame references for sizing? I like using the tire size myself:) Andy. (said as an early in the day funny)

fr333zin 05-20-12 07:41 AM

I got an Airnimal Chameleon (folder) frameset last year. They come in Medium (52cm) or Large (56cm), which relates to the equivalent top-tube length. Seat height is of course determined by how much the seat stem is extended.

For me, comfort and handling has a lot to do with effective top tube length, let's call this the lounge. I like to be able to relax in the lounge. When I stop, though, it's the ballroom that becomes most important ;O)

Kimmo 05-20-12 09:04 AM


Originally Posted by veryredbike (Post 14246435)
They could do it by top tube, sure... but that only tells you part of it, even for the most basic fit... what about the head tube height? More and more I'm coming to discover that that's a variable that I need to pay more attention to. In the end, the V-top tube length doesn't tell you that much more than the faked tube length.

Like massive is saying, if you want a number that tells you if the bike fits when it comes to production bikes, it doesn't exist.

Actually, now I cast my mind back to when it first started bnothering me, I recall thinking the head tube length was probably the next most important thing to specify (chainstay length, BB drop, head angle etc are more about handling than fit), although stem angle gives you some leeway...

Effective top tube length and maybe head tube length is info I'd much rather see when I'm browsing second hand frames, for instance...

calstar 05-20-12 10:49 AM


Originally Posted by Kimmo (Post 14247285)
Effective top tube length and maybe head tube length is info I'd much rather see when I'm browsing second hand frames, for instance...

Yes. Its interesting how different folk's ideas/perspective sometimes leads to a real headslapper moment(as in "why didn't I think of that?"). Thanks for the perspective. My 14yr old nephew is looking for his first road bike(new to him but used), and although most used bikes are close to square numbers I will now definitely keep top tube length as a very important variable.I was just going to go with st length and make the tt length work by changing stem lengths, but I'll(we'll) be looking a little more critically at the tt now.
BTW, I'm excited to work with him on this project, his parents are very supportive but non-cyclists, and he thinks its a great idea to work/build up a used bike with his uncle.

Brian

unterhausen 05-21-12 12:16 PM

this is a bit of a religious argument. Some people will insist that front-center is most important. On a road bike, you really want to know how far the top of the head tube is from the bottom bracket. In the old days of horizontal top tubes and quill stems, you could get an idea of how a frame would fit by seat tube/top tube alone, but now it's a little trickier.

willum44 06-03-12 03:50 PM

I find the first place to start with to determine the size is the head tube, then the top tube.

David Tollefson 12-19-12 11:36 AM

Zombie thread!

Actually, I have my own particular beef with the current spate of "stack and reach" dogma circulating the triathlon bike industry currently. Drives me to drink!

My thoughts are that what's important are the contact points of the bike (saddle, BB compensated for crank length, and handlebar location compensated for bar reach) and how for in front of the saddle nose is the rider/bike CG. Everything else is designed from that data and the desired handling.

fietsbob 12-19-12 11:39 AM


Drives me to drink!
PROST ! :beer:

ksisler 12-19-12 02:39 PM

For the mass mass of folks, such as those beginning a search, the only number I see as useful for deciding "does this bike warrant a further look" is the "standover height" and in inches please on this side of the pond.

For my say, I'll say that all SOH's should be measured from the ground up to the top of the top tube at the top tube's highest point. Adding a note to say if the top tube is slanted, etc., is also helpful.

So if I on a burn to buy a used Trek hybrid-like bike and search the local area C/L;
-- I will find about 150 Treks for sale on a typical day
-- It would be great if the add subject line said

"TREK, used 1999 Model 730, XC, Red, 34" SOH, City/State, $price/OBO"

That would aid in quickly sorting out those available that fit the basic criteria

FWIW...
/K

MassiveD 12-20-12 03:26 AM

Problem with SOH is that I spend far less than one percent of my time standing over the bike. So I mostly just care about how it fits and handles for me. My wife is of the old school were she pushes off and swings over the saddle, she spends pretty much zero time standing over. i never liked doing that after I got traps on the pedal. Do people still do that with bindings?

What I do agree with is that real numbers are more useful to me that "sizes". In many cases the numbers the bike is sold with do not exist on the bike itself. The bikes are what they are. Stuff in a catalog is marketing. If they think people want a certain wheel base number, they give it to them, knowing everyone else is playing the same game.

ksisler 12-20-12 10:15 AM


Originally Posted by MassiveD (Post 15070001)
Problem with SOH is that I spend far less than one percent of my time standing over the bike. So I mostly just care about how it fits and handles for me. My wife is of the old school were she pushes off and swings over the saddle, she spends pretty much zero time standing over. i never liked doing that after I got traps on the pedal. Do people still do that with bindings? What I do agree with is that real numbers are more useful to me that "sizes". In many cases the numbers the bike is sold with do not exist on the bike itself. The bikes are what they are. Stuff in a catalog is marketing. If they think people want a certain wheel base number, they give it to them, knowing everyone else is playing the same game.

Agree completely; But for the scenario posed of sorting out a few bikes that are close enough to look at further, the SOH is what I would like to see (but rarely do).

Oh and in complete agreement...argh, just get me started on the sm, med, lg, xlg crap.

A while back I got wired on the issue so I traveled to and measured a half dozen bikes on the then current craigs list (circa fall 2010). The actual SOH measurement for the bikes (all with level top tubes), all listed as XL, varied from 30" to 36"... Two of the sellers admitted that they had listed there wares as XL based only on their own eyeball judgement of the bike they found next to a dumpster and scarfed up to flip them on C/L. Since both sellers were 5'7" tall at most, I had to guess that the "21 inch bikes" looked "huge to them" and thus they must be "size=XL..." What a way to waste a day! Not to say that I didn't take the one bike for $50, pulled its full Campy group off, replaced it with adequate misc Shimano parts from the bin, and sold it for $60 to a cousin needing a commuter bike.

marqueemoon 12-27-12 02:37 PM

It's far from perfect, but it's mostly good enough for people who don't care to go any deeper.

Stack and reach would be better, but the angles are not factored into those measurements.


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