What do the Numbers Mean? Your Guide to Geometry

Components for sure make a difference to how your bike rides, but nothing impacts your ride more than your frames geometry. There’s a whole bunch that goes into creating a frames geometry, as something as small as a degree here, or a millimetre there, can change the bikes overall handling characteristics. In this post we’ll be talking about what these numbers mean, and how they affect your frames ride characteristics.

This column won’t mean much to some you: you’ll either know what the below means, or you won’t care and just want to ride your bike. But, for those of you who are perhaps a little newer to mountain biking, or want to be able to understand the numbers we preach without glazing over, below you’ll find a basic breakdown of some of the most numbers when it comes to mountain biking geometry.

Head Angle
Head angle is one of the most talked about numbers when it comes to mountain biking and reflects the angle between the ground and the headtube. The slacker the angle (closer to zero), the more controlled your bike will feel descending, whilst losing a little ‘snappiness’, whilst the steeper the angle (closer to 90), the quicker the steering and the further forward your weight, making climbing a little easier. As a rough rule of thumb, you can expect to find trail/endure bikes with head angles that float around the mid-sixties, while XC bikes are in the high-sixties and downhill bikes in the lower-sixties.

Reach is quickly become the go-to number for most people when it comes to looking at a bike’s length. If you were to run a vertical line from the centre of your bottom bracket upwards, the reach measurement is taken between this line and the centre of your head tube. The reach measurement is representative of the length of the bike when you’re out of the saddle – which is when we’re typically pushing our bike, therefore giving a greater overall impact on how our bike handles, versus the traditional Effective Top Tube measurement which only applies when we’re seated. The longer the reach, the more room you’ll have to breathe on the bike, and more stability (in a general sense) whilst the shorter the reach, the opposite.

Wheelbase is a pretty simple one and is measured between your front and rear axle. A longer bike overall means more stability, whilst a shorter overall bike means more agility. Generally though, it’s numbers like reach and chainstay length you’re better off paying attention to.

Chainstay Length
If you weren’t aware, your chainstay is the tube that runs horizontally to the ground (in most cases) and connects your rear wheel to the rest of the bike. Chainstay length essentially contributes to the bike’s overall wheelbase and where your centre of mass is. A shorter chainstay will shorten the wheelbase, making the bike more flickable, and shift your centre of mass backwards meaning the front wheel is easier to pickup which can make climbing a little more difficult. Conversely, a longer chainstay makes the bike less manoeuvrable and puts your weight a little more central between the two wheels – meaning the front wheel is easier to keep down at the sacrifice of a little stability.

Seat Tube Angle
The seat tube angle reflects the angle between the ground and your saddle if you were to run a line down the centre of your seat post and to the ground. This essentially affects where your weight is positioned in regard to the centre of the bike, as well as where you’re positioned over the bottom bracket. To keep things simple, the steeper your seat angle the further forwards your weight is, meaning the you can climb a steeper gradient without your front wheel lifting.

Bottom Bracket Height
Bottom bracket height is another one that’s pretty simple. The bottom bracket height is measured between the centre of the bottom bracket and the ground. The higher a bottom bracket is, the higher your weight is and therefore the less stable the bike is. The lower the bottom bracket, the lower the weight is and therefore the more stable the bike is, of course at the expense of pedal clearance when conquering trail obstacles.

There’s plenty more when it comes to mountain bike geometry, but the above serves as a good crash course in wrapping your head around the numbers and should make life a little easier when it comes to trying to comprehend bike reviews and your local shop staff.