First of all, this isn't a rant (really, seriously...) It's an attempt, as usual, to educate, inform, and maybe entertain. And as usual, I'm mostly talking about mountain bikes here.
So, back in the bad old days, when you walked into a bike shop (this is how I ended up with a 17" Trek OCLV as my first ever mountain bike and caused myself what will probably be a lifetime of knee problems because the saddle was 4" too low) you were asked to throw a leg over the bike and then attempt to pick it up. If the wheels came off the floor by some minimum number of inches, the bike fit you!
That technique had obvious problems, but the idiocy lingers - bikes are still "sized" by their seat tube lengths in many cases (some have moved to the "small/medium/large" classification system).
Now, some of you serious and nerdy cyclists reading this are probably saying to yourselves "Yeah, maybe for the folks shopping at Wallyworld... but me and my hardcore pals use toptube length to figure out fit." For practical purposes, sizing with (effective) toptube length is indeed a better way to go about fitting a bicycle, but it still has some serious drawbacks for folks who want to get their bike to both fit AND ride correctly.
Why? Simple - toptube length, taken in isolation, actually doesn't tell us where the rider's weight will fall between the wheels - and that matters a lot. Ever been on a bike with the front wheel way, way out in front of you? It's darn hard to weight the wheel in a turn, and that means - yep - washout. Ouch. Obviously the front wheel can be too close to you as well - time to go over the bars at the drop of a hat. Doh.
The reasons that toptube length doesn't work that well come down to 3 things: seat tube angle, head tube angle, and fork offset. All of these affect where the wheel is positioned relative to you, and since it's no longer the bad old days (when every mountain bike on earth was 71/73 and 38mm fork offset) that wheel positioning can vary wildly. The distance I'm referring to has a name - "front center" and it's one of the most important variables in determining if a bike will work well for you.
Take, for example, 2 bikes with identical toptubes, bottom bracket heights, and chainstay lengths:
-A 24" effective toptube, 72 degree seat tube angle, 72 degree head tube angle, and 38mm fork offset (this would be an "old school" 29er geometry) results in a trail of 80mm, wheelbase of 104.9cm and front center of 63.2cm.
If we change the seat tube angle by a degree, guess what happens?
-24" ETT, 73 STA, otherwise identical: 105.7mm wheelbase and 64cm front center. 8mm might not seem like much - but that's probably 40% of the difference in front center between a "medium" and "large" frame in many lineups. All because of a *single degree* of seat tube angle. Plenty of 29er manufacturers build a "small" size frame with a steep seat angle (to avoid toe overlap) that actually has the *same* wheelbase and front center as their "medium".
Ok, fine, 8mm. Big deal. Let's say you do something with the head tube angle and offset of the fork:
-24" ETT, 72 STA, 69 HTA, 46mm offset (similar to something like a Kona Honzo, or one of my personal bikes) results in a 108cm wheelbase and 66.3cm front center. That's 31mm longer than our 72 HTA/38 offset bike, enough to be quite noticeable to any experienced rider. And all with the same 24" effective toptube - if you purchased a bike purely on TT length, you can end up with a weight balance situation you never intended.
In many cases, people just accept a long front center and wheelbase on a bike with a slacker head tube angle because they're hung up on toptube length (or because they don't realize *why* the front center and wheelbase are so long). In fact, in most cases, it's better to design around (or shop based on) the front center to keep the front wheel where you want it. The toptube might seem too short or long compared to what you're used to - but it's easy to compensate for this with a longer or shorter stem, so that your contact points (saddle, bars, pedals) stay where they need to be.
Unfortunately, of course, lots of people have been taught that short stems=DH and freeride, and long stems=XC geeks. This is idiotic, because your priorities when designing a new bike, IMO, should be:
-Make sure the trail number is what you want. #1. The wrong trail number is always going to suck. Luckily this is pretty easy to figure out.
-Put the front and rear wheels where you want them to go.
-Pick a height for the BB.
-FINALLY, make sure the rider can fit onto the bike in their ideal position (ie saddle and bars need to be in certain places). Use an appropriate head tube length/stem/bars/spacers to position the bars and KEEP THE FRONT CENTER CONSTANT. Need a 110 stem? Fine. 80mm? Also fine. You'll adjust to the very minor differences in how you use the bars in 5 minutes. Adjusting to a bike where you're not properly balanced between the wheels is a whole different can of worms and often you just can't.
Obviously there's lots of other stuff that goes into a frame design (pick the right tubes, make sure all your parts fit right, etc, etc) but if you hit all those requirements above, you're on the right track.
If you take nothing else away from this, please, just remember that sizing a bike based on toptube length is only useful if your new bike is VERY similar to your old one. Measuring the front center on your favorite bike(s) will give you a great tool to figure out what bikes you might like in the future, and it's a great way to one-up your geeky bike friends or drive away single women (or men, to be fair) who think you're an idiot at a party.
For the very bored: MTBR thread about front center.
Edit: Thanks to the anon commenter who pointed out that I used mm instead of cm at one point. Fixed, gracias!