Mountain bike geometry can be confusing. Here we explain the most important MTB geometry numbers, what they are and what they mean. This will help you to better understand the geometry tables of the bikes you are interested in.
Head tube angle affects how steep the fork is, and is the angle between the fork and the ground. A slack head angle will be more stable, while a steep one will make the bike more nimble.
Top tube length is the horizontal measurement from the steerer tube to the seatpost.
Reach is also a horizontal measurement, that goes from the steerer to the point directly above the bottom bracket. Reach is the main measurement that affects how ‘long’ a bike feels in the riding position.
Stack is the vertical distance from the bottom bracket to the steerer. This is the main measurement of and is how ‘tall’ a bike feels. Stack height can be adjusted slightly by re-positioning the spacers around the stem. This will increase or decrease the effective stack height.
Bottom bracket height is the distance from the bottom bracket or BB to the ground. Combining this with crank length tells you how much pedal clearance you will have.
Bottom bracket drop is a different way of measuring BB height. This is the vertical distance from the horizontal line that joins the two wheel axles to the bottom bracket.
Wheelbase is the length of the bike from axle to axle. As well as top tube length, head angle and chainstay length all affect the wheelbase.
Chainstay length is distance from the BB to the rear axle. Short chainstays make tight turns easier, while long ones help keep the front wheel on the ground while climbing.
Front Centre is the measurement from the bottom bracket to the front axle. top tube length, head angle and fork length all effect the front centre measurement. For a given reach and fork length bikes with slacker head angles have longer front centre measurements.
Seat tube angle is the measurement of the seat tube angle relative to the ground. Steeper angles, that put the saddle over the bottom bracket, make climbing easier and pedalling more efficient. You can adjust the effective seat tube angle slightly by sliding the saddle forward or backwards on the rails.
Standover height is the distance between the ground and the top tube, at the point where you would naturally stand astride the bike. Low standover heights make it easier to move around on the bike while you're riding.
Geometry tables often include other geometry numbers as well, but these are the most important ones you need to look at when choosing your new bike.