Bike geometry is one of the defining features of frame design.
The geometry of your bike affects everything from how it handles, how well it fits you, how comfortable it is to ride, how aerodynamic the riding position is, and how efficiently you can pedal, plus much, much more.
With that in mind, we’ll be walking you through the basics of bike geometry and bike geometry comparison, covering:
- 11 Key Elements of Bike Geometry
- “Sagged” Geometry
- Dynamic Geometry
Ready to get to grips with the basics of bike geometry comparison?
Let’s get started!
11 Key Elements of Bike Geometry
#1. Seat Tube Length
The seat tube length is one of the most important measures of frame size.
Seat tube length has a minimal effect on handling in itself, but determines the maximum and minimum seat height. If the seat tube length doesn’t suit the rider, it’s likely the rest of the bike’s geometry will be thrown off too.
It’s measured from the top of the seat tube to the middle of the bottom bracket.
#2. Effective Top Tube Length
The effective top tube length gives a sense of how spacious a bike will feel from the saddle.
It’s a much more useful metric than actual top tube length, which fails to account for the angle of the top tube.
Effective top tube length is measured as a horizontal line between the top of the head tube and the seatpost.
A frame’s reach is defined as the horizontal distance between the top of the head tube to the middle of the bottom bracket.
The reach determines how spacious the bike will feel when riding out of the saddle, and to a lesser extent when riding seated too. It’s another useful metric when sizing a bike.
#4. Stack Height
Stack height can almost be thought of as the vertical equivalent of reach.
It refers to the vertical distance between the top of the head tube and the middle of the bottom bracket. Stack height dictates the minimum handlebar height with no spacers being used.
#5. Down Tube Length
The down tube length is the distance from the middle of the bottom bracket to the bottom of the head tube.
As a bike geometry comparison metric, it serves a similar function to reach in that it gives a sense of how spacious the bike will be to ride. However, it’s easier to measure – which can make it more useful when wanting to make a quick comparison between two frames in a store, for example.
However, it doesn’t fully accommodate for the down tube angle (and therefore the stack height), so it still needs to be used in combination with other bike geometry elements.
#6. Bottom-Bracket Height
The bottom-bracket height is the distance from the middle of the bottom bracket to the floor.
The bottom-bracket height has a noticeable effect on how the bike feels to ride, as it helps determine the frame’s center of gravity. A higher bottom bracket means a higher center of gravity, making the bike more sensitive to tipping forwards or backwards when riding steep gradients, or when braking or accelerating sharply.
Conversely, a lower bottom-bracket height helps increase the bike’s stability.
The lower center of gravity also helps with the bikes agility when cornering, as the bike (and rider’s) mass has less distance to sink towards the ground. The physics behind this are a little complex, involving concepts such as the roll axis and roll moment (more on that here if you’re interested), but the end result is that the bike feels nippier when flicking between left and right-hand turns.
The main issue with lowering the bottom-bracket height is the risk of a pedal striking the ground while turning. One solution is to use shorter crank arms, but this reduces leverage when pedaling.
The wheelbase is the distance between the frame’s front and rear axles.
As a general rule, the longer the bike’s wheelbase, the more stable it will be to ride. Will this makes you less likely to go flying over the handlebars, it can also be a disadvantage for experienced riders in some instances as it makes advanced techniques such as manuals and nose-pivots more difficult.
A longer wheelbase also increases the required steering angle to navigate tight corners.
The frame’s wheelbase can be divided into two parts:
The front-center is the forward section of the wheelbase, measured as the horizontal distance from the middle of the bottom bracket to the front axle.
A longer front-center reduces the chances of the rider going over the handlebars under heavy braking or on steep descents by pushing the center of mass backwards. Downhill mountain bikes tend to have long front-centers for this reason.
However, a long front-centre (relative to the rear-centre) shifts more weight onto the back wheel, which can reduce front-end traction.
The rear-center is the rear section of the wheelbase, measured as the horizontal distance from the middle of the bottom bracket to the rear axle.
Most bikes have a significantly shorter rear-center than front-center, meaning the natural weight distribution is towards the rear wheel. This shifts slightly when riding out of the saddle, as more weight tends to be put through the handlebars.
#8. Head Angle
The head angle is the angle of the steerer tube relative to the floor.
A steeper head angle (more upright) increases steering responsiveness at slow speeds, at the expense of some high-speed stability. A slacker head angle increases stability and extends the front-center section of the wheelbase, but reduces the front wheel’s sensitivity to steering input.
Recently, there has been a trend for mountain bikes to feature increasingly slack head angles, as they help give the rider confidence when descending at speed – though for tight, technical trail, a steeper head angle may be beneficial.
Modern mountain bikes tend to have a head angle of around 67 degrees on average, whereas a typical road bike head angle would be in the region of 73 degrees.
#9. Effective Seat Angle
The effective seat angle is the angle from the top of the seatpost through the middle of the bottom bracket, relative to the floor.
It gives a better indication of the position of the rider’s hips relative to the pedals than the actual seat angle, which can be misleading if the bottom bracket is offset or the frame features a non-straight seat tube – as is often the case on full-suspension mountain bikes.
In these cases, raising or lowering the seat height will slightly alter the effective seat angle too.
#10. Handlebar Height
The handlebar height is the vertical distance from the floor to the part of the handlebars that the rider holds.
It can be easily altered, either by replacing the handlebars with a different style or by adjusting the height with spacers.
Higher handlebars makes it easier to shift the rider’s weight backwards, which helps on steep terrain. It can also provide a more comfortable upright riding position for long stints in the saddle.
Lower handlebars can encourage a more aerodynamic riding position and make it easier to weight the front wheel effectively for technical cornering.
Trail refers to the distance between the contact patch of the front wheel and the steering axis.
The amount of trail is affected by three factors: head angle, wheel size, and fork offset.
The trail is one of the less visually-obvious elements of bike geometry, but has a massive effect on how a bike handles. Trail creates a self-centering force that pushes the wheel back towards the straight position when turned away. Therefore, a bike with more trail feels more stable, but too much trail can make the steering feel sluggish.
However, too little trail can make the handling feel twitchy – especially when the front wheel hits a bump. This is because the bump effectively moves the contact patch further forward on the wheel, reducing the trail further and encouraging the steering angle to flick outwards.
There are two different measurements of trail:
Ground trail is measured horizontally from the front contact patch to where the steering axis meets the ground.
It’s easier to measure and is often the metric used by manufacturers when they provide a trail measurement in a bike’s specifications, but in terms of physics it’s actually slightly less relevant than mechanical trail.
Mechanical Trail (AKA “real Trail”)
Mechanical trail is measured where the front contact patch intersects the steering axis at a right angle.
It’s also known as the “real trail” because it’s the metric which directly influences the self-centering effect, but it typically corresponds closely with the ground trail anyway.
For bikes with suspension, there is another consideration: “sagged” geometry.
This refers to the fact that the bike’s geometry changes when mounted by the rider, as the suspension is loaded and compresses under the rider’s weight. This provides the “sagged geometry”, as opposed to the “static geometry” of the unloaded bike.
The amount of travel and the stiffness of the suspension dictates how much the geometry of the bike will change when sagged. Bike manufacturers are careful to accommodate for the differences between static and sagged geometry when designing frames.
Dynamic geometry is similar to sagged geometry, in that it differentiates the geometry of a bike with suspension when in use from the static geometry.
However, the sagged geometry refers to the position of the bike when stationary, whereas the dynamic geometry uses the average position of the suspension when the bike is ridden over a given section of terrain. The suspension is typically more compressed when riding than at a standstill.
Dynamic geometry is difficult to measure in practice without some sophisticated technology, so tends to be used more as a vague concept in bike setup than as a specific or precise metric.