Bicycle Geometry Explained

I think that the question i hear most from my clients is "What frame size will i be"?

The answer should be simple or so you would think, the general answer from your local bicycle retailer will more often than not be "you look around a Medium or you will be a large" Seems pretty straight forward when looking from the outside in.

Lets delve a little deeper and take a closer look at the problems with this over used gung ho approach to selling a MODERN high performance bike.

What is a Medium?

If i asked somebody this question i would most likely get the answer 54cm or 56cm and they would be correct.....but where is the 54 or 56 measured? It pretty standard to measure this from center of bb to top of seat tube or A on the diagram below.
The first thing you will notice is about this diagram is how many measurements and angles are on it and i can honestly tell you that each and every angle and measurement on this frame is far more important than the A (frame size) measurement that is the industry standard.

I believe a bicycle frame has two types of measurements to consider

PRIMARY (A measurement or angle that when changed WILL alter your bicycle position/contact points)


SECONDARY (A measurement or angle that when changed will NOT alter your bicycle position/contact points)

Let me explain. The
A measurement is a SECONDARY measurement and will not alter your position on the bike if changed, if it is 4cm longer it will merely mean that less seatpost is exposed from the frame.

Measurements B,C,D,E are all
PRIMARY measurements meaning that each and every one of them if altered even slightly can drastically change your bicycle position, unfortunately these more important measurements are rarely ever mentioned during a bike purchase.

So how do you choose the correct geometry?

if you know your current geometry and it has been correctly fitted and is comfortable simple google the geometry of that frame and find the closest match from your preferred new bicycle options.

If you are unsure if your geometry is indeed correct for you can easily book in for a pre purchase fit with us. once you have completed your fit we will draw you up your custom geometry, we can then give you some options of matching bikes that we can source that match your budget or if you prefer show you how to read the geometry which means you can spend some time shopping around or surfing the web for the best deals, the choice is yours.

What if all the measurements match but
D is 2 cm longer on the bike i am looking at purchasing? Will this matter?

Quite simply YES it will! As i tell all my clients "in Bike fitting a mm = a Mile", anybody that has cycled for a few years will know exactly what i mean.

Remember to plan before you buy, as this will insure lots of happy pain free riding and enjoyment

Craig Baldwin
Bike Fit Technician - Auckland Bike Fit