How to Use Your Bike Fit Measurements to Buy the Right Size Bike

You Have Your Fit Data… Now What?

Buying the right size bike isn’t as simple as it used to be. Forgive me for sounding like a grumpy old man, but the days where you could walk into your friendly local bike shop and leave with a bike that same day are all but gone, and the days where you’d actually talk to the person who would weld your frame before flame and flux went anywhere near a metal tube are something of a distant memory! So, how does one go about navigating the mounting perils and potential pitfalls of buying a bike in 2024/25? How do we know that the bike is going to both fit well and handle well? Fear not padawan, for there is a way…

For the sake of argument, let’s assume that you’ve either:

a) Had a bike fit and you’re looking to base a new purchase on these measurements or
b) You’d like to base a new bike purchase on an existing bike that you’re comfortable on and happy with.

Regardless of the specific scenario, there is a logical and mathematical method to make sure that you’re making an informed purchasing decision to buy the right size bike. 

The Real Numbers You Need To Use

What we’re after is a set of numbers that can be consistently compared across all bikes. Looking at the ”size” of the bike doesn’t really tell us much at all these days. A 54cm frame size in one bike will not be the same as a 54cm frame size in another bike!

We must let go of this outdated and quite frankly, inadequate frame sizing convention and instead look at the numbers that determine what shape the bike frame actually is. We’re interested in how tall things are, and how far away things are relative to a set point on the bike.

These numbers are measured on the X/Y axis from the centre of the bottom bracket spindle and are generally referred to as ”stack and reach”. We’re also interested in the angle of the seat tube, as this tells us whether or not the saddle X/Y position can be achieved. There are many important geometry measurements you need to look for when choosing your new bike, but we’re particularly interested in the following:

1. Frame Stack & Reach
2. Handlebar Stack & Reach
3. Grip Stack & Reach
4. Seat tube angle

Frame Stack” is the vertical (Y) distance, from the centre of the frame’s bottom bracket/crank to the top middle point of the head tube (where the fork passes through the frame). 

Frame Reach” is the horizontal (X) distance from the centre of the bottom bracket/crank to the top middle point of the head tube.

“Handlebar Stack & Reach” are the vertical (Y) and horizontal (X) measurements of where your bars need to be in relation to the bottom bracket. 

“Grip/Hood Stack & Reach” are the vertical (Y) and horizontal (X) measurements of where your hand grip needs to be in relation to the bottom bracket. 

“Seat Tube Angle” allows you to calculate if you can achieve the saddle position behind the bottom bracket that you need.

How To Use Stack & Reach Measurements

The frame’s stack and reach will tell you how long and tall a frame is relative to the bottom bracket (or axle) of the bike. If you know your frame stack & reach and you know what handlebar stack & reach you are trying to achieve then you can use these numbers to calculate what length stem you need and how many spacers you need under the stem, providing that the handlebar reach measurement (which impacts grip/hood X/Y) is consistent.

Compensations can be made depending on the stack & reach combination of different frames to achieve the desired handlebar / grip positions. E.g. If your ideal frame reach is 375mm and you choose a bike with 10mm less reach (365 mm), then you can compensate by fitting a stem which is 10mm longer than the one that would fit on the bike with the 375mm reach.

That’s the front end of the bike, now we look at the back end. The seatpost angle is the first thing we’re interested in here. This primarily determines whether or not your desired saddle ”X” position can be achieved. If the seat tube angle is either too steep or too slack, then your saddle position, and thus the overall fit of the bike will be compromised.

I found this great dynamic illustration of how seat tube angle changes your position from Aaron Dunfordhttp://www.fusionpeak.com.au

It is possible however to modify the ”effective seat tube angle” by changing the seatpost offset if required. E.g, if the seat tube angle of the frame you’re interested in is on the steep side (the higher the number, the steeper the seat tube angle), then it may be possible to mitigate this by using a seatpost with a greater amount of offset, or layback. Ideally, we’d like to clamp the saddle as close to the middle of the rails as possible. This isn’t only to give fore/aft adjustability, but also to distribute the load of the clamp evenly throughout the length of the saddle rails.

A seatpost with layback on the left, and a straight-up seatpost on the right

How To Compare Bikes

Now we know what we’re looking for in terms of the basic parameters that will tell us whether or not a frame is an appropriate geometry for our position, we can start to compare bikes and narrow down the search for your right size bike.

There are thousands upon thousands of bikes out there, so how do we narrow it down? Unless you have a shortlist already, this can be a bit overwhelming! There are however some handy online tools available that allow you to hone in on specific geometrical parameters. Databases such as Geometry Geeks and Bike Insights are just two that spring to mind. Whilst these sites are fantastic resources, they are user generated, and mistakes can occur. It’s always worth double checking your findings with the manufacturers website for the official geometry chart, as this will be a much safer reference point.

Wait… Read This Before You Buy…

Before you rush out to apply your new found knowledge, there are some ”bike buying rules” that I would strongly encourage you to be aware of. Rules exist to allow us all to have fun in a safe way, so deviate from these at your peril! Not really, just be aware that there will likely be consequences if you do go a bit off piste. Rafa Nadal put it so eloquently… ”decisions have consequences”.

Firstly, the more range of adjustability the bike has, the more longevity it will give you. By this, I mean that as your body changes, and it will, you want to be able to adjust your bike accordingly so that you can continue to enjoy it. Having scope to change the position in all directions (handlebar stack and reach, and saddle fore/aft at the bare minimum!) depending on how your body changes is one of the key things to look for when buying a new bike. The more limited the range of the adjustment is, the more locked in to one position you will be, which may or may not be appropriate as time passes. Generally, it’s recommended that there be some spacers situated both above and below the stem, that the stem can be both shortened and lengthened if needed without compromising the handling of the bike and that the saddle be clamped as close to the centre of the rails as possible to give adequate fore/aft adjustment. 

Secondly, and continuing on the theme of adjustability above, it’s vitally important to be aware of any proprietary component standards that a potential new bike may have that could potentially compromise your ability to recreate your position. You might find a frame geometry that works very well for your needs, but it might be rendered unsuitable on account of the lack of adjustability due to the proprietary nature of some of its components. As an example, in 2024/25, the Canyon Aeroad, Ultimate and most Endurace models all spec a proprietary cockpit that is unique to these bikes. If you don’t fit the exact stem length, stem angle and handlebar width range that the bike ships with, then the bike won’t fit you as this can’t be changed, either through Canyon themselves, or with aftermarket components.

Have awareness fro proprietary systems and parts which will limit the ease of adjusting and experimenting with your bike position. If proprietary parts are ‘out of stock’ you’re at the mercy of the manufacturer to come good.

The same diligence needs to be applied when looking at seatposts. More and more manufacturers are using seatpost shapes that are unique to their own particular frames. This isn’t an issue if said seatpost is available in the setback/offset that you need. It does however become a problem if, for example, the seatpost angle of the frame is either a little too steep or too slack and the offset of the proprietary seatpost that the manufacturer makes isn’t able to compensate in order to get your saddle positioned correctly. Round seatposts will give the most flexibility and choice. 

The other thing to consider with proprietary components is that technology and trends move very fast in the bike industry… and bike manufacturers are often quick to forget about a bike they made 4/5 years ago! There is no guarantee that a brand will continue to make a specific stem or seatpost, for example, for an outdated model. This may mean that down the line, if you want to change something about your bike position, you might be stuck if the part you need is no longer available! 

With those advisories from the ”fun police” out of the way, you’re now armed with the tools you need to make better informed bike buying decisions!  

Words by Mat (one ‘t’)


If you’ve made it to the end of this article, thank you and we hope it’s been useful for you to choose the right size new bike!

If you have any questions on your bike buying journey get in touch, we’re always an open door and there’s no such thing as a silly question: info@foundation.fit

If you’re looking for the foolproof method to buy exactly the right size new bike for you, sign up for a: Fit Before You Buy session

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