rms13

I am a little taller then 5'10' and pretty equally proportioned. I generally ride a 54-55cm road frame and find a 54.5cm effective top tube to be my sweet spot. When I started to get more seriously into cycling I went through three frames that kind of fit but ultimately didn't feel "perfect" so I moved on from them and have ended up with two bikes that are really dialed in for me.

My wife has a 50cm frame bike and I was doing some work on it for her and needed to test ride it and amazingly I found that by raising the seat very high and making some minor adjustments I could comfortably ride the bike. Now I only road around the block but I kind of felt like if I really needed to make that bike fit me as my daily rider I could make it fit. kind of ridiculous when I think about how much time and effort I have spent dialing in what is the perfect fit geometry for me

Crescent Cycle

Well there's only 3 contact points. Saddle, pedals and bars. If you can get those in the right X-Y coordinates you can make a frame fit. Sometimes you don't have enough adjustment to do that. Then you have issues like stem length and steerer height and setback and such which have more limited adjustment and can affect handling. Then there's other issues of fit, like saddle shape, saddle angle, foot shims, crank length, bar width, bar shape, hood placement etc.

There's also the lack of difference between M/S and S frames. Lots of small frames don't really have much less reach than the medium frames. Half or more of the top tube reduction usually comes from steepening the seat tube angle. That affects how far you can set back the saddle but does nothing for reach. That means a size or two can be fixed with a 10mm longer stem, which is within the range of normal adjustment. Frames also get more compact in the smaller sizes, the seat tubes will shrink more than the head tubes. Heat tubes will often only be a little lower, and you can often get your position with an upturned stem and spacers.

But it isn't optimal, and for many people, they have a smaller range of 2-3 frame sizes that can be fitted to them without resorting to extreme solutions. For odd sized people, sometimes it is hard to even get 1 size to fit. You just happen to be very average and have more flexibility to choose from different frame sizes.

Bandera

Plootering about town close enough is good enough.
A proper fit makes a difference when distances are long, efforts are high and precise handling is required.

-Bandera

elcruxio

Yes this.

Fit is more than just the three contact points. How you are situationed on the bike will have an effect on the handling. A too small a frame will suffer from being relatively front heavy and will require an excessively long stem (which in turn again contributes on the front bias).

Crescent Cycle

Nah, contact points come first. It isn't like pros don't use goofy long 140mm stems to achieve their stack and fit requirements. While 100-120mm might be what some fitters aim for for optimal handling, it's more important that you can get the fit you're aiming for at all. Optimal handling just explains why one size of frame is a better fit than another for your given contact points. You can still make something work, and pros can make silly long stems work for their pro races, not just around town.

If you have a normal -6 100mm stem and a +10 120mm stem plus 5mm spacer, the 120mm stem will add 7mm reach and 41mm stack. Take a Scott Foil or something and the XS 49cm has 379mm reach and 509mm stack. Compare that to M 54cm 385mm (+6) reach and 549mm (+40) stack. Well you can see that you can make up the difference between some XS and M sizes with using only normal stems.

As far as being front heavy, there's some merit to it. But if you balance your center of gravity over the bottom bracket, wheel bases don't vary much and neither do chainstay lengths. That means that the BB is almost in the same place between the front and rear wheel on a small bike as a big bike, and with the same contact points, so is the weight distribution. For my Scott Foil example 49cm has a wheelbase of 978mm, and the 54cm 979mm with both having 405mm chainstays. That 1mm difference doesn't make a whole lot of difference in weight distribution between front and rear if your reference point is the bottom bracket.

Reach from the steering axis? That varies from handlebar to handlebar and shifter unit to shifter unit. Those can easily throw in a few centimeters of variance between bar reach, bar width, bar angle, hood position, hood shape etc. If you move your hands from the hoods to the bends does the bike handle that much worse? Because you've just moved your hand position 100+mm

More of a concern is how stem length and weight distribution interacts with the handling characteristics of a specific size of frame. On most frames head tube angles and trail are not static and vary from frame to frame. Different manufacturers make frames with different quirks too. One manufacturer might make a 50cm that fits and handles very similarly to another manufacturers 54cm.

Part of this as I explained earlier is that small frames are really more like not quite so small medium frames. You would have a harder to jumping a 2 size gap at the big end.

And of course if it was really that terrible then all the odd sized people would be unable to ride anything but custom bikes. There are optimal size bikes for people, but it is based on more variables than virtual seat tube size and you can often make something less than optimal work fine. Lots of people who can only fit one size frame at the extremes of the adjustment ranges have to do it.

Of course this is the opinion of someone who isn't a pro fitter and doesn't get paid $100/hr. Take it with a grain of salt.

fietsbob

equal to What?

for how long?