For decades, top tube length served as a reasonable metric for providing bike size estimates, but it’s an arbitrary metric and always has been. The troublesome part of its continued use isn’t that it’s arbitrary, but rather that with modern bicycles it’s irrelevant and often misleading.
A former colleague I taught fit classes with used to make a very valid, but purposefully outlandish comment: “I don’t care if you want to ride a cardboard box, as long as it has a bottom bracket and a handlebar, it can be properly sized”.
The point was to emphasize how irrelevant tubes of a bicycle are for sizing and fitting purposes.
Tri bikes not enough to convince you? Most modern full-suspension mountain bikes fall in to the same camp, where the top tube measurement is misleading.
Road bikes leveraging offset seat tubes, which are becoming increasingly common, are also a nightmare for the fabled top tube measurement.
Today’s designs are not ‘square’
The only reason the top tube measurement was ever remotely relevant was due to the ‘squareness’ of steel (and then aluminum) road frames.
They had a top tube and a seat tube that commonly measured the same and a simple beauty to the sizing process followed.
That sizing process culminated with riders standing over a frame and smacking themselves in the nethers with the frame to find an appropriate size. How elegant.
All this was largely an artifact of Italian designs and several archaic seat height methodologies, some based on the C.O.N.I. database of bike measurements that may or may not have fit professional riders (and thus was only applicable to professional cyclists). But I digress.
The reality is this: with modern carbon designs, the top tube has been a misleading measurement since the first signs of sloping tubes.
Ironically, if you asked a modern frame builder to create a new bike for you, they’d likely first ask for your stack and reach.
“Fake news — there’s ‘horizontal’ top tube,” the crowd retorts.
What about offset seat tubes? How does a ‘corrected’ top tube measurement take that in to account? Simple, it doesn’t. Not to mention the variation of measuring that theoretical top tube.
Don’t believe me? Pick five random brands and look at their geo drawings and it quickly becomes apparent how ‘proprietary’ measurements have become.
There is nothing proprietary about frame stack and reach, except the actual measurement, of course. Furthermore, it’s unaffected by anything an industrial designer can influence.
What, why and how
Stack and reach is a system of absolute measurement. It is, for all intents and purposes, ‘outside’ the system.
The measurements are relevant to the horizontal and the vertical, neither of which are typically present in bike design.
Stack and reach is essentially any point on the bike, relative to the bottom bracket, measured against true horizontal (x) and vertical (y).
These values can be calculated to the center-top of the steerer tube, the center of a handlebar, or the rear edge of an elbow pad.
Technically, you can measure to any feature, but for the sake of being relevant, those three are by far the most common locations.
It is true, stack and reach forces an entirely new batch of numbers on us. But is it really complicated? I’d argue that it’s no less confusing than T-shirt sizing for a bike, but more on that shortly.
Do you really need special tools for determining stack and reach? No, you need a tape measure and a wall (preferably a square wall).
The first step is to determine your bottom bracket location in the vertical and horizontal. Two measurements.
Frame stack and reach, handlebar stack and reach, or even elbow pad stack and reach come from two additional measurements.
The difference between the first and second measurements in their respective ‘X and Y’ coordinates is your stack and reach. Brain buster.
The most relevant application is to give shop staff, bike fit folks, or riders the ability to compare two totally different bikes’ sizing parameters, with no flawed perspective resulting from tube shapes.
It can even give a good idea of how many spacers are needed and a solid guesstimate for stem length if known stack and reach values are accessible (and they always are, if you have a tape measure and a wall).
The legendary top tube can do no such thing.
Stack and reach also gets rid of the struggle between centimeter and T-shirt-style frame sizing, which by the way is ridiculous.
I have two beefs with T-shirt-style sizing: has any company that uses T-shirt sizing ever had to sell a bike face-to-face? Being told you’re extra small or small, or any size for that matter, is a bit of a sensitive topic for a lot of people.
Also, what does your small have to do with another brand’s small? This is even more meaningless, confusing and arbitrary than top tube length.
Getting used to new numbers
When I attend a new bike launch event I look for 580/385 (for road), or the closest thing to it. This means I know which stem to bring with me, depending on how close the new bike numbers are to my known stack and reach. The cockpit is totally dialed within minutes and it feels like home.
Sure, it’s easy for me to know my numbers, but what about working in the shop where other riders don’t know their numbers? Even if you can’t look it up on the internet, it takes three minutes at most to find the numbers yourself. Remember, a tape measure and a wall.
In addition to all this easiness, there are several companies aiming to develop universal stack and reach databases. The sooner you can wrap your head around what stack and reach are, and why they’re valuable, and how they equate to rider sizes, the less likely you are to lose your way in the near future.
Now you know why “what’s the top tube length?” is a loaded and pointless question.