There's plenty of ways to get information on what bike size is best for you. You could do it the old-fashioned way and go and ride a few to see what suits, or pay a man with a jig and a keen eye to recommend a position for you. And now you can do it on your phone, too.
The SizeMyBike app takes an input of six body measurements – height, shoulder width, inseam, foot size, arm length and height to the top of your sternum – and feeds these into an algortihm that spits out bike sizing information for four different kinds of bike, including your road iron and hardtail and full suspension MTBs.
And how does it calculate what bike size you need? Well, their algorithm is derived from a stack of worthy-looking references, none of which we've ever read but with names like Multivariable Optimization of Cycling Biomechanics and Cycling biomechanics: road and mountain they sound like the kind of tomes you should be basing your sums on. Here's what SizeMyBike have to say about their process:
The SizeMyBike algorithm is based on biomechanics. The model computes a bicycle geometry which will enable you to have the 3 contact points correctly positioned. The contact points are the pedals, the saddle and the handlebar. Saddles and pedals are positioned in order to maximize the transmitted power and to minimize the metabolic costs. The optimum is reached when the 2 following conditions are satisfied. The knee should be able to bend 25° to 30° with the pedal in the 6-o’clock position (i.e maximum extension of the leg). Moreover, the axis of pedal and the bony protrusion just below your kneecap must be aligned vertically when the pedal is horizontal. The handlebar is positioned in order to reach a compromise on the following points: aerodynamics, breathing and comfort. With hands on brake hoods, this compromise is reached for trunk angle varying between 30 and 40 degrees according to whether one privileges performance or comfort.
That all sounds pretty reasonable and tallies with the kind of things we've heard in other bike fitting sessions with various suppliers. Obviously the system is only ever going to be as good as the data you put in so it's important to get those measurements exactly right, but there's no reason why a computational system such as this shouldn't be able to size the majority of people effectively. Once you've got your basic sizing there's advice in the FAQs section of the SizeMyBike website on adjusting your fit on the bike. Probably the system is most useful for getting an overview of what size frame you should be looking at before you start test riding any bikes.
Dave is a founding father of road.cc and responsible for kicking the server when it breaks. In a previous life he was a graphic designer but he's also a three-time Mountain Bike Bog Snorkelling world champion, and remains unbeaten through the bog. Dave rides all sorts of bikes but tends to prefer metal ones. He's getting old is why.