Finding the Holy Grail of Frame Size

In an attempt to keep us out of peril, I want to present a few common scenarios I have encountered over the years while conversing with riders in our studios during their motion capture fit.

Is this what it feels like to buy a new bike?

He who answers these questions three will be allowed to buy new bike. If you answer incorrectly, you are tossed into the gorge of eternal peril.
Bridge Keeper : I ask these questions three. What is your favorite color?  Me: Blue.
Bridge Keeper : What is your current component group? Me: SRAM.
Bridge Keeper : What is your frame size? Me:….uhhh….58…uhhhh….no 56….ahhhhhhhhhh and I fall into the gorge of eternal peril.

Is this what it feels like to buy a new bike?

Scenario #1

At one point in the cycling past, a rider had been told they are a number. Yes, a single number. Let’s say 56. That number has stuck with them since their first bike. Every bike they have bought since their first bike has been a 56. Some of the bikes over the years felt awesome and some …well felt like they fell into the gorge of peril. In scenario #1, the rider tends to look at the bike as a size number that does not change. I am a size 11 shoe and I always buy size 11 shoes. I am not growing anymore therefore why should I buy an 11.5. Well shoes do fit differently from brand to brand…don’t they? When I buy I pair of jeans, I always buy 34 length…well not always…because one manufacturer of jeans or model of jeans can fit differently.

Scenario #2

The rider went to shop A and they told him he was a 54. He went to shop B and they told him was a 56. He then did an online check and it said he should buy their custom frame. The rider now feels shop A was wrong because he is close in height to his best friend who rides a 56. He then combs the internet for a 56 something. Ultimately, he gets a 56 and it has him stretched out into the next county. Scenario #2 presents bike shop A as a villain shop. In reality, shop A and shop B are probably both right assuming they both spent adequate time with the rider to determine the size for their brand(s). With that said, Shop A carries Brand A and shop B carries Brand B. It is quite likely that a Brand A 54 and a Brand B 56 are very close in actual size. I see it all the time. Both shops can be correct in saying 54 or 56…because it is bike brand/model/year specific. To complicate things, a 56 Brand C was purchased and was out of the ballpark in reach. While he and his friend are the same height, this riders legs are longer and his torso is shorter. There is no cookie cutter approach here. Height does not translate to frame size. Is there some relation? Sure.

Scenario #3

A rider who has been riding comfortably and performing well on a “56” road bike for many years has finally decided to buy a new Time Trial/Triathlon Bike. His best friend tells him to buy a “54”. He is told to buy a size smaller than his road bike so he can be more aero. This scenario is a very common misconception and many times a costly one. I am going to leave this one here as the variables in TT bikes could be a whole post (book) on its own. Basically, this formula DOES NOT WORK :    My Road Bike “Number” – 1 Size = TT Bike Number

As part of my process fitting for a new bike, I use the Retul MUVE Dynamic Fit Bike and do a full motion capture bike fit in absence of an actual bike. No constraints. Once the data is in, saddle selected and a position is found…I load the data into Frame Finder and begin the process of comparing frames across manufacturers and models. In the end, we have a list of bikes, their sizes and configurations to go shopping with.


The key things to remember about a frame size number is that it is for a particular make, model and year. For a given make, model and year, a 56 is ( in some respects ) smaller than a 58. It may be shorter in Reach, Stack or both Stack and Reach. Ahhh, what is stack and reach you say? Well it is two numbers. It is the relationship of the center top of the head tube to the center of the bottom bracket. Wait, I have been told I was one number and now you are saying it is two numbers. Yes, these 2 numbers are more important in comparing frames especially across manufacturers and models. Several manufacturers are trying the clothing sizing model….XS, S, M, ML, L, XL. I guess that may be easier to follow than numbers. But, it still does not address sizing across brands or models within a brand. It only says, a Small should be “smaller” than a Medium. The only definitive way to compare one frame to another frame is to compare the frame’s stack and reach. In the example pictured to the right, some manufacturer models only have a couple millimeters of difference in the reach between a 51, 54 and 56.  The stack changes though. Another item of note, there is only 15mm difference in reach from the smallest frame to the largest frame in this model. Stack changes by 6cm. Almost all the manufacturers are providing frame stack and reach as part of their geometry charts. This is a good thing.

I hope this takes some of the mystery out of frame sizes and further more let you know that you are not just a number.