Gone are the days of walking into a bike shop telling them your inseam and them telling you what size bike to get. At one point in time pretty much every bike company followed a very similar sizing standard where t size 56 cm in one brand was basicly the same as a 56 cm in another brand. In today’s market bike companies could not be more different than one another and crazy enough they couldn’t be more different with themselves! Most brands now offer a number of different models with varying geometries within the same size frame.
Let me explain this a little bit as I imagine it is hard to follow what I am talking about. I will use Cannondale road bikes to show you want I mean. The Cannondale SuperSix and Synapse are both sold in a 56 cm but will fit very differently. In the past the Standover Height was a very important measurement to use when selecting a size as it was the height of the top tube that you would need to straddle when standing over your bike with your feet on the ground; this is why the bike shop needed your inseam length. In our comparison both the SuperSix and Synapse have a 80 cm Standover Height making them seem the same. Another popular measurement that shops would look at is Top Tube Length; how long is that tube that you are straddling when your feet are on the ground. Once again these two bikes are exactly the same with a 56 cm length. Based on these two very popular measurements you would assume that these two bike would be the same size but you would be very wrong.
The two measurements you really want to look at are what is called Stack and Reach. The Stack will let you know how high or low you can position the handle bars and the Reach will let you know how close or far away the handle bars can be. Basically the Stack and Reach will let you know how relaxed of a position the bike will be or how aggressive the setup will be. Now this is where these two models differ. The SuperSix is a more aggressive bike that is used in racing or for an athletic rider with a 56 cm Stack and a 39 cm Reach it allows the rider to get a lower bar height and a longer reach from the saddle. The Synapse on the other hand has a 59 cm Stack and a 38 cm Reach which will allow the bars to be positioned higher and closer to the rider allowing for a more upright and relaxed position.
So as you can see just walking into a bike shop and looking at all of the bikes in the 56 cm range doesn’t mean you will walk out with a bike that fits. The smart move is to get a proper bike fit prior to shopping so you know what position you are most comfortable riding in and then finding a bike that will allow you to be in that position. It is much smarter to pick a bike that fits you rather than trying to figure out how to get you to fit on the bike.
If you think the road bike sizing is confusing then you don’t even what to get into Triathlon bike sizing where there is absolutely no sizing standard from brand to brand; a size medium in one brand could be a size small or even a large in another.
If you are going to invest a lot of money into a bike then you should be an educated investor and spend your money on something that fits you.