Stack
The vertical distance from the bottom-bracket centre to the top-centre of the head tube. More stack usually makes it easier to place the bars higher, but cockpit parts still matter.
Pair it with reach.RideIn geometry lab
Bike geometry terms explained
Understand what each number measures, what it can suggest and where a neat comparison can become misleading.
Measurement map
Start from fixed reference points
Most frame measurements relate the bottom bracket, head tube and wheel axles. Contact-point components then place the saddle, pedals and hands around that frame.
Reference glossary
The measurements in a geometry table
Reach
The horizontal distance from the bottom-bracket centre to the top-centre of the head tube. It describes frame length without being distorted by seat-tube angle.
It is not the distance to the grips or hoods.Effective top tube
A horizontal measurement between the steering axis and seat-tube axis. It helps describe seated cockpit length, which also depends on saddle and stem position.
Sometimes called horizontal or virtual top tube.Actual top tube
The length measured along the physical top tube. Sloping frames make it less useful for comparing seated positions than effective top tube.
Check which top-tube value a chart uses.Seat tube
Usually measured from the bottom-bracket centre towards the top of the seat tube. Brands may use centre-to-top, centre-to-centre or an effective value.
The method must match before values are compared.Head tube
The length of the frame tube that carries the headset and fork steerer. It affects available cockpit height, but stack is the clearer cross-bike comparison.
Fork and headset dimensions also contribute.Head angle
The steering-axis angle relative to the ground. A smaller angle is commonly called slacker; a larger angle is steeper.
Interpret it with fork offset, trail and wheel size.Seat angle
The angle of the seat-tube line relative to the ground. A larger effective angle generally places the seated rider further forward.
Saddle setback and height change the real position.Chainstay
The distance from the bottom-bracket centre to the rear axle, also called rear centre. It contributes to wheelbase and weight distribution.
Tyre clearance and frame layout can drive the value.Front centre
The distance from the bottom-bracket centre to the front axle. It helps describe weight distribution and contributes to wheelbase.
Do not confuse it with reach.Wheelbase
The distance between the front and rear axle centres. A longer value often supports stability, but steering geometry and mass distribution also shape the ride.
Compare bikes in a similar category.Standover
The height from the ground to the top tube at a stated point. Brands do not always use the same point, so treat cross-brand comparisons cautiously.
Tyres and suspension state can change it.Bottom-bracket drop
How far the bottom-bracket centre sits below the axle line. More drop lowers the rider relative to the axles but reduces pedal clearance.
It is different from bottom-bracket height.Bottom-bracket height
The distance from the ground to the bottom-bracket centre. Tyre size and suspension sag affect it, so published and real values may differ.
Use drop for cleaner rigid-bike comparisons.Fork offset
The perpendicular offset between the steering axis and front axle, also called fork rake. Together with head angle and wheel radius, it determines geometric trail.
Suspension-fork model and travel must match the frame.Trail
The ground distance between the projected steering axis and the tyre contact point. More geometric trail generally increases self-centring, but tyre shape and handling inputs still matter.
Calculated values depend on wheel and tyre radius.Axle-to-crown
The distance from the front axle centre to the top of the fork crown. Changing it can alter the frame’s angles, stack and bottom-bracket position.
Suspension figures are normally unsagged.Stem length
The nominal length between the steerer clamp and handlebar clamp. It changes hand position and steering leverage but does not change the frame’s reach.
Angle and spacer height also matter.Handlebar width
The bar’s stated width, measured by a method that depends on bar type. Width affects control, shoulder position and frontal area.
Road bars may be measured centre-to-centre or outside-to-outside.Crank length
The distance from bottom-bracket axle centre to pedal axle centre. It affects joint angles, pedal clearance and the saddle height needed for the same leg extension.
Frame size does not always dictate the best crank length.Road, gravel and commuting
Fit before handling claims
Frame stack and reach
Use them to compare the frame’s front-end position. Then account for headset top cover, spacers, stem angle and length, handlebar reach and the position used on the bar.
Seated position
Effective top tube, seat angle, saddle setback and saddle height interact. Two frames with the same reach can place the seated rider differently.
Useful baseline
Measure or find the geometry of a bicycle that already works. Record its frame size and contact-point setup before comparing another frame.
Comfort is individual
A taller front end can support a less demanding posture, but flexibility, strength, riding duration and adaptation all affect what remains comfortable.
Mountain bikes
Include suspension and riding position
Standing position
Stack and reach help locate the bars relative to the pedals, but bar rise, roll, width and stem dimensions still shape the usable cockpit.
Dynamic geometry
Fork and shock sag change angles and bottom-bracket height while riding. Compare figures measured in the same state and check the manufacturer’s setup assumptions.
Climbing and descending
Seat angle affects the seated climbing position; head angle, trail, wheelbase and weight distribution influence descending behaviour together.
Wheel configuration
A mixed-wheel frame or adjustable geometry position can have more than one valid table. Match the wheel size, travel and flip-chip setting.
RideIn’s source-checked examples use the exact tables on the Boardman size guide and product pages. Always recheck the current manufacturer table for the bicycle being considered.