Which geometry specifications determine the seating position of an MTB?

Question

MTB manufacturers release geometry data for their bikes. If I want to know if a bike has a more upright or stretched position, which geometry data do I have to compare?

Answer 1

For the position of your hips relative to the bottom bracket, you need to consider the effective and actual seat tube angles. These two angles are listed when the seat tube is bent, originates in front of the bottom bracket, or both to accommodate the kinematics of the rear suspension.

The actual is seat tube angle is exactly what it sounds like. The effective seat tube angle is the angle measured from the bottom bracket to the top of the seat post at some representative height for the bike’s design.

To get a sense of how stretched out the bike is, consider the effective top tube length and reach dimensions.

The effective top tube (ETT) is the horizontal distance from some the top of the head tube back to (a projection of) the seat tube. This describes the length of the standard seated position on the bike. The reach is the horizontal distance from the bottom bracket to the top of the head tube and describes your position off the seat while descending.

The “effective” values all of these measurements change with seat height and handlebar stack (due to seat and head tube angles).

If you like the reach/ETT, but the bars are too low, raising them with effectively shorten the reach and ETT. To raise your hand position but maintain these dimensions, consider using riser bars instead. A longer stem is also an option, but will affect the feel of steering more than riser bars.

Another thing to consider is that mountain bikes sag into their suspension. On a hardtail, this means that the seat and head tube angles get steeper and reach increases slightly. On a full suspension bike, the reach should get a little longer as well as the seat tube angle will slacken and the head tube angle will steepen.

The longer the travel, the more pronounced these effects are (hence modern MTB geometry often including seat tube angles that rival the most aggressive triathlon bikes).

Lastly, there is stack, which is the vertical distance to from the bottom bracket the top of the headtube.

If we consider MTBs as existing on a continuum ranging from XC race bikes to long-travel enduro race bikes, XC race bikes will have slacker seat tube angles, shorter reaches, and lower stock. On the other hand, enduro race bikes will have longer reaches, steeper seat tube angles, and higher stacks. This means that the XC race bike will provide a lower and more stretched position, while the enduro bike will be more upright when pedaling.

I’m not including DH or freeride bikes as the seats are not intended to be in position conducive to pedaling at any appreciable effort.