Mountain Bike Geometry Guide
Mountain Biking is one of the most diverse sports and hobbies in the world, with many mountain bikes being almost completely unsuitable to crossover mountain biking disciplines. Riding a downhill full suspension mountain bike on a day’s fast paced cross country riding for example, is a big no - the large travel suspension, and the geometry of the full suspension bike would mean the fittest of riders would be left by those riding dedicated cross country mountain bikes. The same applies the other way around, a carbon fibre hardtail mountain bike designed for skimming across terrain - would struggle with large drops and jumps on a downhill mountain bike trail - which is the natural environment of the full suspension downhill bike.
In this post, we look at geometry of both hardtail and full suspension mountain bikes and examine how designs impact performance and handling.
MTB Head Angle
Head angle refers to how steep the gradient of the mountain bike fork is. If the head angle is described as slack, you can expect a larger angle VS a steep head tube angle - which sees the forks more upright. Mountain bike geometry has given us slacker head angles in recent years, and slacker head angles are more than likely here to stay. You can expect to see bigger head angles on downhill mountain bikes, with steeper head angles on cross country bikes.
Performance wise, a slacker head angle provides greater stability - whilst a steeper head angle makes the mountain bike feel more responsive and nimble.
MTB Top Tube Size
Top Tube Length can be measured from the steerer tube to the seatpost of the mountain bike. Longer top tubes have become the norm in recent years, but this measurement and length is worked out by manufacturers to ensure sensible levels of reach and control - as top tube size measures the distance between the rider's body and hands on the handlebars of the bike.
MTB stack is the measurement of how tall a bike is; the distance between the bottom bracket to the steerer (vertical measurement only) The stack height can be adjusted using spacers on the bike's stem.
Although MTB reach covers the distance and the required reach of the arms from the seat to the handlebars, mountain bike reach is a horizontal measurement - which dictates how far the rider has to reach the handlebars. Stems can be changed to change this measurement, making the MTB either longer or shorter.
MTB Bottom Bracket Height
Bottom bracket height refers to the distance between the bottom bracket, to the ground. This can be used to work out pedal clearance. Full suspension bikes will have a longer bottom bracket height - which is required for the pedals to remain clear of the ground, if the long travel suspension on a full suspension downhill bike is compressed.
The wheelbase measurement is the axle to axle length of the mountain bike. Downhill mountain bikes have longer wheelbases for increased stability on challenging terrain, whilst cross country bikes have a shorter wheelbase.
MTB Chainstay Length
Chainstay length is the distance from the bottom bracket to the rear axle of a mountain bike. This is an important measurement, with shorter chainstays providing easier tight turns and agility, and longer chainstays helping front wheel stability on long ascents.
MTB Seat Tube Angle
Different Seat Tube Angles provide different performance, with steeper angles making climbing ascents easier, and every pedal stroke more efficient - slacker seat tube angles can be adjusted by sliding the MTB saddle forward or backwards on the rails provided.
The standover measurement is between the top tube and the ground, at the point where you would come forward off the seat and stand on the bike. The lower the height, makes the bike easier to move around whilst you are riding. Women's mountain bikes typically have smaller stand over heights.