Fit first was been a philosophy I bought into pretty easily in 2008. With biomechanics being the basis of a lot of coaching and education I do, learning how to fit someone first for how they wanted to ride provided the best fit possible for each cyclist/triathlete I encountered. Now-a-days you can’t claim to be a size 54 or a large bike frame. Frames come in all shapes and sizes. However, the real question is how do you want to ride? Based on that question alone, I can use my dynamic fit unit to determine exactly what frame(s) is best for you, the proper seat post setback and front end configuration for a comfortable and stable ride.
Take a look at the photo below. During a fit session I drew this on the board to demonstrate what I mean by an athlete should not claim to be a certain size in every bike frame manufacturer. You have manufacturer X, Y and Z. All have a small bike frame size. However, the stack and reach of each small is different for each manufacturer. Manufacturer X shows a pretty average stack and reach measurement. Y shows a very short reach and a very high stack (short and tall). Z shows a long reach and a low stack (long and low).
X, Y and Z = Sample Manufacturers SM = Small Size Frame S = Stack R = Reach
What if you are not able to ride long and low? If you didn’t get fit first, what I would see is that the bike would have a ton of spacers in the front and/or a very short stem with a crazy pitch. See the picture below.
This bike rolled in some time ago. The rider was telling me that her back hurt when riding and that she rode on top of the bullhorns because she never felt stable being in aero. This troubled me because she spent all her money on a TT/Triathlon bike so she could be aero (which would allow her to be faster) but she couldn’t even get in aero and be comfortable. As for not being stable, this front end configuration is extremely unsafe. Piling the weight load of the rider over the fork/front end as such will make the trail not match well with the build of the bike. This ultimately causes a lot of instability for the rider.
If you are in the market for a new bike I highly suggest getting a fit first. Don’t spend your hard earned money on a bike that will not provide the best in comfort, drive-ability and safety. You'll achieve your goals on an amazing bike that feels like a part of you.