Velopedia Tools

Bike Geometry

Trail, Wheelbase & Frame Geometry Calculator

Turn published or measured bicycle dimensions into useful geometric relationships. Calculate mechanical trail, wheel and bottom-bracket height, front/rear center relationships, wheelbase, stack-to-reach ratio, and steering-geometry sensitivity.

Wheels

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

Front wheel Bead-seat diameter Tire section height mmMounted width is a usable estimate if section height is unknown Measured wheel radius mmAxle center to ground; overrides BSD + tire estimate

Rear wheel Bead-seat diameter Tire section height mm Measured wheel radius mmUseful for different front/rear tires or mixed wheel sizes

Steering geometry

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

Head tube angle degrees Fork offset / rake mm

Frame geometry

Use unsagged/static dimensions from the same geometry position whenever possible.

Reach mm Stack mm Chainstay / rear center mmBB center to rear-axle center Bottom bracket drop mmPositive when the BB sits below the axle line Front center mmDirect center-to-center distance: BB to front axle Wheelbase mmIf front center is blank, it is inferred from wheelbase

Frame geometry

Your Results

Enter the bicycle dimensions to calculate steering and chassis geometry.

For the cleanest comparison

Use dimensions from the same frame size and geometry position.
Measure actual wheel radius if trail accuracy matters.
Compare unsagged geometry with unsagged geometry.
For mixed wheel sizes, prefer the manufacturer's published wheelbase and front center.
Do not interpret front-center percentage as weight distribution.

Mechanical trail

Mechanical trail is the horizontal distance on the ground between the steering axis intercept and the tire contact point. Wheel radius, head angle and fork offset all contribute, which is why quoting head angle alone does not fully describe steering geometry.

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

Wheelbase, front center and rear center

Front center and chainstay/rear-center length are direct center-to-center dimensions from the bottom bracket to the front and rear axles. With equal-height axle centers, bottom-bracket drop lets those diagonal measurements be resolved horizontally. The calculator can therefore infer front center from wheelbase, or calculate wheelbase when front center is supplied.

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

Reach and stack

Reach and stack locate the top of the head tube horizontally and vertically from the bottom bracket. Their ratio is useful for comparing frames without relying on nominal size labels, but it is not a fit prescription: stem length, spacers, handlebar geometry, saddle position and rider proportions still determine the actual contact points.

Why wheel radius matters

Measured radius

Axle-to-ground radius automatically includes the actual rim, mounted tire size and tire profile, making it the best input for trail calculations.

BSD estimate

When actual radius is unavailable, the calculator estimates radius as BSD/2 plus tire section height. Mounted tire width can stand in as an approximation.

BB height

For conventional equal-height axles, estimated BB height is rear wheel radius minus BB drop. Tire sag and mixed wheel sizes can change the ground-relative value.

Dynamic geometry

Suspension bikes rotate and translate as fork and shock sag change. Published static geometry and riding geometry are related but are not the same state.

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