ROAD BIKE GEOMETRY EXPLAINED:

The angles and figures to take account of when choosing the right bike for you

Geometry charts aren’t just about picking a bike that fits you – the measurements involved play a pivotal role in defining a bike’s personality.

When launching a new or updated model, brands will often refer to tweaks in the geometry. But what exactly does it all mean? And how can you use the geometry charts to inform your own bike-buying decisions?

Stack and reach

Stack and reach are the two most consistent measurements that will help determine if the bike will put its rider into a racing position with a long reach and short stack (aggressive), or one that will be comfortable for long, all-day rides (endurance/relaxed) thanks to a short reach and high stack.

Stack is the vertical distance from the centre of the bottom bracket to the mid point at the head tube. Reach is the horizontal distance from the centre of the bottom bracket to the middle of the head tube.

In the past, people often relied upon checking out the top tube and head tube, but according to bike fitter at Soigneur London Tim Allen, this approach is “a bit outdated and misleading. When it comes to the top tube, the angle of the head tube and seat tube will have an impact – if a bike has a 55cm top tube, and a 70º angle, and another has a 73.5º angle, effectively the reach on the 73.5º bike is going to be longer.”

Head tube length is equally variable.

“Once, a standard fork length was 370mm, and you could compare head tube lengths. Now, more bikes have different fork lengths. A gravel bike or endurance bike that’s got disc brakes and more tyre clearance will have a longer fork to cater for that, so the head tube is going to become shorter but it doesn’t increase the drop,” Allen comments.