Velopedia Tools

Bike Geometry Calculator & Analyzer

Calculate the geometry. Understand the proportions. Compare real bikes.

Use published or measured bicycle dimensions to understand frame proportions, wheelbase and steering geometry. When fork offset is available, the analyzer also calculates trail, then compares the bike with distinct, known-size modern geometries in the Bicycle Archive.

Transparent calculationsCategory-aware analysisReal Archive comparisons
Evaluate a specific bike

Use this Analyzer when you already have one bicycle or geometry chart in mind. Still choosing what to consider? Bike Finder is the easier starting point.

Start with Bike Finder

See it before you start

Turn a geometry chart into a bike you can explore.

The interactive example below shows the actual Analyzer display immediately. It demonstrates the measurement map only; none of its values are loaded into your form or treated as your bike.

Interactive example

See where each measurement lives on the bike

Example only · not your bike

Loading the interactive geometry example…

Reach on this example398 mm

Horizontal bottom-bracket-to-head-tube coordinate.

Illustrative road/gravel geometry used only to demonstrate the display. Your analysis is generated separately from one real bike and frame size, then adds category-relative interpretation and closest Archive comparisons.

Analyze a real bike

Start with a real bike whenever possible.

Search the Archive, or continue directly into the blank manual worksheet below.

Recommended · no geometry terms needed

Find your bike in the Archive

Search by year, brand or model, then choose one published size to carry its available measurements into the Analyzer.

Have a geometry chart?

The manual worksheet is already open below. You will need wheel and tire size, head angle, reach and stack from one frame size; optional measurements can stay blank.

Enter the chart below

No particular bike in mind? Let Bike Finder narrow the choices

Geometry input

Build a complete static snapshot

Use measurements from one frame size and one geometry position. Optional measured wheel radii improve trail and bottom-bracket estimates.

1

Wheels

Measured axle-to-ground radius is best; BSD plus tire section height provides a practical estimate.

Front wheel
? The rim diameter where the tire bead sits. Use the ISO/BSD number printed on the tire when available; 700C and 29-inch rims are normally 622 mm.
? The distance from the rim seat to the outside of the inflated tire. If you cannot measure it, the mounted tire width is a practical estimate.
Mounted width is a usable estimate when section height is unknown.
? Measure vertically from the center of the axle to the floor with the tire inflated. This is more accurate than estimating from rim and tire dimensions.
Axle center to ground; overrides the BSD estimate.
Rear wheel
? The rim diameter where the tire bead sits. On a mixed-wheel bike, select the rear standard separately from the front.
? The distance from the rim seat to the outside of the inflated rear tire. Mounted tire width is a practical estimate when height is unknown.
? Measure vertically from the rear axle center to the floor. Enter this when front and rear tire or wheel sizes differ.
Useful for different front/rear tires or mixed wheel sizes.
2

Steering geometry

Head angle is measured from the ground; fork offset is the axle offset from the steering axis.

? The steering-axis angle measured up from level ground. A smaller number is slacker; a larger number is steeper.
? The perpendicular distance between the steering axis and front axle. It works with head angle and wheel radius to determine ground trail. Leave it blank when it is not published and Geometry Finder will still work.
Not published? Leave it blank. Only trail is skipped; bike matches still work.
3

Frame geometry

Use unsagged dimensions from the same size and geometry position wherever possible.

? The horizontal distance from the bottom-bracket center to the top-center of the head tube. It describes frame length without stem or handlebar setup.
? The vertical distance from the bottom-bracket center to the top-center of the head tube. It describes frame front-end height before spacers and stem setup.
? The published seat-tube angle measured from level ground. It is shown in the schematic when available but is not used to prescribe saddle position or fit.
Use the published angle; leave blank when it is not listed.
? Usually published as chainstay length: the direct distance from the bottom-bracket center to the rear axle center. Leave it blank when the manufacturer does not provide it.
Use the published chainstay value. If it is not listed, leave this blank.
? How far the bottom-bracket center sits below the rear axle line. On a conventional equal-wheel bike, this is the published BB drop.
Use the published value when available. Leave blank to skip BB-height and axle-line calculations while still analyzing the other geometry.
? Mixed-wheel bikes may publish a separate drop from the front axle line to the bottom bracket. Leave this blank when only one BB-drop value is published.
Mixed-wheel bikes only. Leave blank to use the rear/equal-wheel value.
? The direct distance from the bottom-bracket center to the front axle center. If it is not published, wheelbase can be used instead.
Direct BB-to-front-axle distance.
? The horizontal distance between the front and rear axle centers.
When front center is blank, the analyzer can infer it from wheelbase.
4

Archive comparison

Modern known-size bicycles are the primary comparison. Historical reference geometry is available as a secondary Archive view.

Modern comparison: Velopedia groups category, a recent model-year window and a normalized size band. Numeric and manufacturer-specific sizes are retained on the matching Archive records.

Geometry guide

How to read bicycle geometry as a system

No single number defines a bicycle. Frame position, steering geometry and chassis length interact, and their meaning changes with category, wheel size and frame size. This analyzer reports those relationships separately so you can inspect each one.

01

Frame proportion

Reach and stack describe the top of the head tube relative to the bottom bracket. Their relationship shows whether a frame is comparatively long and low, balanced, or tall for its length.

02

Steering architecture

Head angle, wheel radius and fork offset combine to create ground trail. Head angle alone cannot fully describe steering response or self-centering.

03

Chassis distribution

Wheelbase is the total axle-to-axle span. Front center and rear center show where that length sits relative to the bottom bracket.

04

Ground relationship

Bottom-bracket drop describes the crank axis relative to the axle line; bottom-bracket height describes it relative to the ground. Tire radius connects the two.

Stack, reach and frame position

Reach is the horizontal distance from the bottom bracket to the top-center of the head tube; stack is the vertical distance. Stack-to-reach ratio is useful for comparing frame proportions without relying on inconsistent size names. It is not a fit prescription. Stem length, headset spacers, handlebar reach and rise, saddle position and rider proportions determine the actual contact points.

Head angle, fork offset and ground trail

Ground trail is the horizontal ground distance between the steering-axis intercept and the tire contact point. A slacker head angle, larger wheel radius or shorter fork offset generally increases trail; the reverse generally reduces it. Trail helps describe steering geometry, but tire deformation, load, handlebar width, front-center length and speed also influence the response a rider feels.

Trail = (R × cos θ − offset) ÷ sin θR is front-wheel radius and θ is the head angle measured from the ground. The result is geometric ground trail; it does not model mechanical trail normal to the steering axis, pneumatic trail or tire deformation.

Wheelbase, front center and rear center

Front center and rear center are direct bottom-bracket-to-axle dimensions. Bottom-bracket drop resolves those diagonal measurements horizontally. On a mixed-wheel bicycle the front and rear axle centers may sit at different heights, so the analyzer can use a separate published drop on each side. It can infer front center from wheelbase or calculate wheelbase from a supplied front center.

Rear horizontal = √(rear center² − rear BB drop²)Front horizontal = √(front center² − front BB drop²)  ·  Wheelbase = rear horizontal + front horizontal

Bottom-bracket drop and height

Bottom-bracket drop is a frame coordinate measured from an axle center. Bottom-bracket height is ground-relative, so wheel and tire radius matter. More drop can place the rider lower between equal-height wheels; less drop can preserve pedal clearance. When a mixed-wheel manufacturer publishes front/rear drop, keep both values: they describe the BB relative to two different axle heights. If drop is not published, leave it blank; the analyzer omits only the calculations that depend on it.

Archive method

What the benchmark actually compares

Modern evidence first

The primary analyzer groups bicycle category, a bounded recent or selected model-year window and a normalized known-size band. It widens the year window only when the tighter cohort is too small.

Distinct geometry

Duplicate numeric geometry signatures are collapsed so repeated builds of the same chassis do not distort the comparison.

Broad evidence bands

Results use phrases such as “near the middle” or “toward the longer end.” Exact percentile scores are deliberately withheld because the source data does not justify that appearance of precision.

Historical geometry as a secondary reference

Velopedia’s historical collection preserves head angle, wheelbase and rear-center measurements for thousands of bicycles from 1993–2007, but those source records do not identify frame size and do not contain modern reach or stack coordinates. The optional Historical reference view therefore compares only those three preserved chassis measurements. It does not treat a record as Medium, XL or a fit equivalent.

What “similar geometry” can—and cannot—tell you

Closest matches are numerical neighbors across shared measurements, with reach, stack, head angle and wheelbase carrying the most influence. They are useful for discovering bicycles with related static proportions. They do not prove equivalent fit, suspension behavior, ride feel, intended use or component setup, and nominal size labels remain manufacturer-specific.

Geometry terminology references: BikeCAD — front center, reach & stack · Trek — published geometry example
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