Bikefit basics: Demystifying geometry gibberish

So, you’re looking to replace your bike. The one you have fits you just fine, but for whatever reason you fancy a change (maybe Dura-Ace 7900 doesn’t cut it now that 9000 is doing the rounds, and that 600 OCLV carbon lay-up is just soooo 2013…) and have set about doing your research, which let’s face it, for many of us is one of the joys of being a 21st century consumer. Whether it’s comparing features and benefits of 28 ever-so-slightly different models of hi-fi amplifier, or deciding how many different gearbox modes you need in your new motor before cross-referencing against best available PCP deals, we just love it.

And there’s so much more to enjoy as the years go by. Back in the day, a couple of issues of Autocar or What Hi-Fi? and you were done, but now we’re free to make the DYOR experience a totally immersive one, with virtually limitless opinions that you can access to your heart’s content, or at least until you’re forced to stop surfing the internet in the wee hours because the other half is getting suspicious.

Kim Jong UnTempting though it is to go off on a rant about un-verified internet reviews which, like everything else in cyberspace, have probably been manipulated┬áby the Kremlin or agents of Kim Jong Un (a man desperately in need of a few bike rides, by the way), I’ll restrain myself for the time being and concentrate on the subject in hand. Geometry.

You’ll be wanting to match the geo of your new bike to that of your old one, which we have ascertained fits you like a glove with everything falling to hand as perfectly as a nuclear button falls beneath the finger of a malignantly-narcissistic US president, so you set about studying geometry charts and sounding out opinions on top-tube length and such like from your riding buddies. In reality, the chances are you won’t have to actively seek an opinion as your friendly neighbourhood grommet-sniffer will probably have already proffered one, which could be a millimetre-accurate stat, or might be a more general warning about avoiding certain brands of bike because all their models share similar fit characteristics (“I can’t ride a Cervelo because they all have long top-tubes” was one jewel of enlightenment I received recently which tested my powers of self restraint to the full). Whatever the source of your informed opinion though, or perhaps just to help you as you stare at a mass of largely meaningless numbers and angles, I would urge you to ignore the bulk of that geometry table you’re studying and just look at two figures: stack and reach.

These two numbers, which are now stated for pretty much every road bike by pretty much every manufacturer, tell you all you need to know when comparing the geometry of one bicycle to another, but what are they and why are they so useful? First the what. Viewing the bike from the side, stack is the vertical distance from the centre of the bottom bracket to the centre of the top of the head-tube, while reach is the horizontal distance between these same two points. Used in conjunction, therefore, they give you an exact description of location of the top of the head-tube in relation to the bottom bracket, which in turn gives an exact description of where the handlebars are going to fall relative to the bottom bracket, and by extension the saddle and pedals.

Obviously, handlebar location can vary depending on spacer stack and stem length, but the point is that if you have two frames with identical stack and reach characteristics, and you set up using identical spacers and stem, then the handlebar centre will be in the same position relative to bottom bracket. Assuming you retain the same saddle height and set-back (the distance of the saddle nose behind the bb) on both these imaginary bikes, the bars will be in the same position relative to your derriere too. So if, for example, the object of your new desires has a stack figure 10mm greater than your existing ride and you currently run 20mm of spacers then no problem, just knock 10mm of spacers out and your position will be pretty much unchanged (I say “pretty much” because, if we’re going to nit-pick, removing those spacers will also move the bars roughly 3mm further away due to the angle of the headtube).

But wouldn’t top-tube and head-tube dimensions give you the same information, I hear you cry while stifling a yawn. Well no, they wouldn’t. The top-tube dimension (or more accurately, effective top-tube dimension, i.e. how long it would be were it horizontal, which they seldom are these days) gives a broad indication of the “length” of a bike, but the latter will be influenced by the angle of the seat-tube too, with one degree of difference in seat-tube angle equating to approximately 10mm of difference in top-tube length. And head-tube length throws up even more potential for variation, as bottom bracket drop (the distance of the BB below a horizontal line drawn between the wheel centres) can vary significantly, as can the length of the front forks, and these factors both influence the “height” of the front end. Okay, so by looking at all the figures and doing some maths then you can make a comparison between frame geometries, but why make life hard on yourself, when stack and reach tell you all you need to know?


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