Bicycles are measured in several different ways. The usual sizing measurement relates to the seat tube length, given in centimetres for road bikes and inches for mountain bikes. The best way to measure the seat tube is from the centre of the bottom bracket shell to the centre of the top tube. Centre to top seat-tube measurements are often quoted (like in the diagram shown here), but in my opinion they aren’t as useful as centre-centre measurements because they often overestimate the size of the frame. This is especially the case with modern frames which now utilise top tubes with humongous outer diameters.
Unfortunately, with the advent of compact sizing, sizing has become even more difficult. As you can probably tell, I’m not a big fan of compact geometry bikes. The main reason for their introduction by the Giant bicycle corporation was to reduce the number of frame moulds required for the construction of their new composite bikes. Increasing frame stiffness, reducing frame weight and improving fit certainly weren’t the first considerations when the idea of compact frame sizing was conceived. I’ve come to loathe them even more when people don’t know how to quote their effective top tube & seat tube lengths.
Having said all this about seat tubes, the top tube length is generally a more important consideration when fitting a bike. Why? Simply because the length of the top tube stipulates the overall length and reach of the bicycle, which can’t be changed all that easily without changing the bike’s handling characteristics. If you add a longer or shorter stem, the steering changes noticeably. If you put the seat too far forward or backward your knees and hamstrings are more prone to injury. On the other hand, it’s extremely easy to raise and lower the saddle without affecting the geometry of the bike.
The length of the head tube is more important than ever these days, but it is often overlooked. I say it is important because with the advent of threadless headsets, the only way of adjusting the height of your handbars is to add/subtract spacers and/or flip over your existing stem (assuming it is an angled stem). Otherwise you will have to resort to buying a completely new stem with a different angle. What this means is that you should consider the amount of spacers you will need, as you shouldn’t add more than 50mm. More than about 30mm of headset spacers looks odd anyway. The problem is that production bicycle frames come as they are; if you require something different, you’d have to get a custom made frame. Although I will add that with the mind-boggling number of bike manufacturers out there today, I think 98% of the population won’t ever need a custom manufactured bike frame.
Using a frame size chart:
I’ve had a look at conventional bike frame-size charts on the internet. I thought this one was particularly useless as it doesn’t even quantify the size of the frame, apart from giving arbitrary XS, S, M, L and XL designations! So I’ve come up with my own approximation for road bikes, shown below.
Contrary to what you might think, this is not just based on pure speculation. This information is based on about 130 ‘fittings’ (since 2006) with riders who have hired my road racing bikes to tour or train here on the island of Tenerife. so far, these cyclists have ranged in height from 150cm to 203cm. The majority of these clients already have a good idea of their frame size requirements as they were quite experienced. The good thing about this graph is that it provides a reasonable estimate of the required frame size for very tall people. The only shortcoming is that I know very short people are much much better off going with smaller 650c wheels (or less), so it isn’t very accurate for people less than 150cm tall. Right now I’ve had a couple of smirnoffs, so I’ll shoot it out there into cyberspace & get people’s feedback before I modify it any further.
Of course nothing beats evaluating the correct size of a bike ‘in the flesh’. Finding your ideal bike size and position then takes time. Know that buying a frame over the internet is inherently risky… you probably don’t risk buying a pair of $100-200 shoes online without having first tried them on, so why would you risk buying a bicycle frame worth ten or twenty times that amount under the same circumstances?