The geometry of a bicycle is a combination of key measurements that describe the general shape and size of a bicycle. Though there are some measures that are more or less useful depending on the type of bicycle, there is a generally consistent set of measurements that can be found in the geometry chart of most bicycle manufacturers and are often referred to when discussing both the fit and handling of bicycles.
As each measurement has a different impact for the rider, we’ve separated the key measurements into two main categories, though many measurements have some impact on both:
- Fit Geometry
- These aspects of a bicycle primarily impact how the bicycle will fit the rider and are the main contributors to determining size.
- These aspects of a bicycle primarily impact how a bike will handle (e.g. steering, stability, etc).
In this post we’ll be focusing on fit geometry. You can learn all about handling geometry here.
Measure: Stack is the vertical distance between the centre of the bottom bracket and the centre of the top of the head tube. This is the stack measurement provided in manufacturer geometry charts.
Impact: Stack provides the baseline height for the front end of the bike. It has the largest impact on how high the handlebars will be and therefore often determines how upright or aggressive a position the rider may be able to achieve.
Measure: Reach is the horizontal distance between the centre of the bottom bracket and the centre of the top of the head tube. This is the reach measurement provided in manufacturer geometry charts.
Impact: Reach provides the baseline for the overall reach of the rider. It is the most significant determinant of how far away the handlebars will be from the rider.
Measure: Grip stack is the total vertical measure of how high a rider’s hand position is from the bottom bracket. It takes frame stack and adds the stack height introduced by headset spacers, the stem, handlebars, and hood positioning (for drop bar bikes).
Impact: Grip stack is the most accurate and reliable way to measure the height of a rider’s hand position. It is most useful when trying to replicate the same position on two different bikes. Grip stack is not provided in geometry charts as it is reliant on how the bike is adjusted.
Measure: Grip reach is the total horizontal measure from the centre of the bottom bracket to the hand position of the rider. It includes the frame reach and adds the reach introduced by the stem, handlebars, and hoods (for drop bar bikes).
Impact: Grip reach is the most accurate and reliable way to measure the reach of a rider’s hand position. It is most useful when trying to replicate the same position on two different bikes. Grip reach is not provided in geometry charts as it is reliant on how the bike is adjusted.
Seat Tube Length
Measure: Seat tube length is the distance between the centre of the bottom bracket and the top of the seat tube (this measure has sometimes been taken at the middle of the junction with the top tube, but this is less common)
Impact: Traditionally used as the reference to size a bicycle, the seat tube length currently dictates how much seatpost a rider will have projected from the frame (depending on their saddle height). For road and gravel bikes this can determine how much flex/comfort can be achieved from the seatpost. For MTBs this can limit the length/travel of dropper post that might be usable.
Seat Tube Angle
Measure: The angle of the seat tube (projected) in relation to the ground.
Impact: Seat tube angle has a significant role in positioning the riders hips and centre of gravity relative to the bottom bracket. Steeper seat tube angles position a rider further forward, while shallower seat tube angles move a rider further back.
Though seat tube angle provides a baseline, saddle fore/aft and seatpost offset also contribute to the rider’s position in this regard.
Measure: Standover height is the vertical height measured between the ground and the middle of the top tube.
Impact: Though standover height doesn’t impact how a rider fits or is positioned on the bike,it does determine if a rider can safely and comfortably straddle the bike when mounting or dismounting.
Top Tube Length
Measure: Effective (ETT) or Virtual Top Tube length (VTT), sometimes referred to simply as Top Tube length, is the horizontal distance between the centre of the top of the head tube and the intersecting point along a projection of the seat tube (or seatpost).
Though traditionally the actual length of the top tube of the frame was measured, ETT is a more common and reliable way to take this measurement with modern frame designs.
Impact: Top tube length provides a measure of reach for the rider. It combines frame reach along with the additional horizontal reach created as a result of the seat tube angle (behind the bottom bracket). ETT length is generally regarded as a more dated method of measuring the size and fit of a bike than stack and reach measurements, but in combination can provide a more complete picture.