For seated riding, I agree completely.

With appropriate parts you can make nearly any frame fit nearly any rider (more so with too small frames than too large ones). That's not disputed.

Whether you should is another matter.

My first bike was a 56cm i shoulda been riding 52-53cm....needless to say after some new parts id managed to size it to were i was extremely comfortable and fast, i just couldnt actually stand over it lol so i know it can be done but your much better off getting a frame that fits in my opinion

Why would you restrict that to seated geometry? I would agree for standing as well, since when you're off the saddle there are only 2 points of contact and with 2 vertices now and 1 assumingly infinitely adjustable you can make any straight line.

I guess the consensus now is largely yes to my statement. Why then does there exist bike fits that claim recommending the perfect frame for you: stock or custom geometry, and they say that on the grounds of comfort. If you talk about ride quality where the wheelbase and other measurements come into play I would agree, but comfort? I wouldn't. Probably the only advantage to that would be so that you don't choose a frame too small and use such a long stem it makes you look ridiculous.

Now don't get me wrong, I don't mean to say there's no value to bike fitting. You get value in power output versus positioning, getting the perfect position etc, but frame size recommendation (to the precise millimeter) for comfort that so many of them claim? I disagree. You can get any frame size and as long as it allows for you to dial in the optimal comfortable position for you, that will work.

That being said, this is all under the asunptionthat your bike is infinitely adjustable. Real bikes aren't.

Then of course comes the question on recommending The appropriate brand of non-custom that are already readily available on the market frames to suit you. I would disagree with this as well, cause as long as the bike frame you have in mind allows for sufficient adjustabilty to your requirements, you would be equally comfortable in both. I would say just buy any stock frame you like visually, just make sure it allows for sufficient adjustment and ride!

Would this sti be the right way to think about things?

i think that there's a few issues worth separating out here.
one is that - like cerveloguy says - bike fit involves matching an adaptable body to an adjustable frame. there's some room for variation. not everything has to be to the milimeter.
the second thing as that yes, bikes can be made much bigger, if you don't care about how they handle. there are some pictures around of bikes that really were made much bigger than they shoudl have been (including one custom BMC that looked like a giraffe). but in proving your point, they kind of disprove it, too, just because it's one of those things that winds up technically true but nevertheless undesireable.
i'm not one who's super picky about appearance on the bike, but there are things that look stoopid. 6'4" riders on a 52cm frame tend to fall into that category.
-charles

you are absolutely right.

the only things that hold you back are

1. structural integrity - huge seatposts or massive setbacks or setforwards could be problematic, same for tons of spacers

2. the aerodynamic penalties. riding a tiny bike with a wonky stem and spacers is probably less aerodynamic than one the right size with a shaped head tube

3. you can only put the stem and seat so low if the bike it is too big

4. the bike may handle horribly if you move the seat too far forward or too far back

but yeah when people say "Buy the one that fits" as the singular important thing, I usually disagree, because usually they can all fit.

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2 comments:

1- You are effectively dealing with 2 fixed points, not one, if you accept that each person has an optimal relative seat tube angle (which I beleive is pretty established). While I understand that geometrically you can raise the front end as you slide the saddle back to pivot your hypothetical "fit points triangle" around the third fixed point, the BB, and keep the same angles and side lengths, you are neglecting to account for the fact that gravity acts in a fixed direction. So as I slide the saddle back and raise the bars I am changing my relative weight distibution and the portions of the pedal stroke where I am working with gravity vs. against gravity, both of which I posit will seriously affect comfort.

2- Part of this also hinges on how you define "comfort". As other respondents have noted, you are going to generate some funky weight distributions and handling characteristics to acheive your "fit points triangle" on a non-optimal frame geometry. I would never be "comfortable" riding a bike with bad handling or weight distribution even if I am not in physical discomfort.

I agree that given enough wonky equipment (crazy seatposts, long enough saddle rails, extremely long and short stems at crazy angles, headtube extenders) you can achieve any given "fit points triangle" on just about any frame. But making the three points of the triangle the same on two different bike frames will not yield equal comfort on each.

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I comes down mainly to handling. Too long or short a stem will make the steering sluggish or twitchy. The ideal weight distribution is about 60%R/40%F; get too far off this and handling suffers (get out of the saddle and lean your weight over the front wheel and see how well you can steer/corner).

Ironically, I have a bike that is uber-adjustable, Titanflex. The beam can slide back and forth to generate almost any seat tube angle, and with a Look Ergostem you can put the bars virtually anywhere. I theory lots of people could be fit to my bike, but it would handle horribly for many of them (that's why they come in sizes).

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Another aspect is that there are finite limits to what is available in terms of stem sizes, etc.

I have been trying to "Superman" my position, and I already have the longest stem (15cm) that anyone has ever seen.

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