Bicycle fit & cockpit comparison

BikeFitDelta: bike geometry & cockpit comparison calculator

The continuous target is within the sampled stem range 9.6 mm clamp offset View comparison
Free, no accountRuns on your device Millimeters, not a fit scoreTransparent methodology ↗
The comparison studio

Two bikes. One contact point.

Saved on this browser

Local only. No cloud sync. Export a JSON backup to keep a separate copy.

EXAMPLEIllustrative starting measurements, not a verified bicycle. Replace them with your own.

Edit measurements ↓View comparison ↓
01 / Set up your bikes

Start with what you know.

AReferenceYour current position
BCandidateThe bike to compare
Frame geometry From the manufacturer’s chart
Frame stackVertical from bottom bracket
Vertical distance from the bottom bracket center to the top-center of the head tube. Use the exact frame size and year.
Vertical distance from the bottom bracket center to the top-center of the head tube. Use the exact frame size and year.
Frame reachHorizontal from bottom bracket
Horizontal distance from the bottom bracket center to the top-center of the head tube, not handlebar reach.
Horizontal distance from the bottom bracket center to the top-center of the head tube, not handlebar reach.
Head tube angleFrom the horizontal
Head tube angle measured from the horizontal. Use degrees from the manufacturer geometry chart.
Head tube angle measured from the horizontal. Use degrees from the manufacturer geometry chart.
Cockpit setup Use the installed measurements
Total steerer riseFrame top to stem-axis center
Total distance along the steerer from the frame stack point to the stem-axis center. Include headset cover, spacers, and the stem clamp-center offset. Not just loose spacers.
Total distance along the steerer from the frame stack point to the stem-axis center. Include headset cover, spacers, and the stem clamp-center offset. Not just loose spacers.
Stem lengthCenter to center
Center-to-center stem length. Integrated or proprietary cockpits may not follow this model.
Center-to-center stem length. Integrated or proprietary cockpits may not follow this model.
Stem angleRelative to perpendicular to steerer
Stem angle relative to perpendicular to the steerer, not the ground. Enter the installed orientation; a negative value lowers the stem.
Stem angle relative to perpendicular to the steerer, not the ground. Enter the installed orientation; a negative value lowers the stem.
How do I measure total steerer rise?

Include the headset cover, spacers below the stem, and the stem-axis center offset. For a symmetric clamp, use half its height.

Helper starts with example values. Verify the actual parts before applying.

Total 40 mm

This helper assumes a symmetric clamp. Use the manufacturer’s axis offset otherwise. Spacer limits are not checked.

How does tire size shift the stack?

A larger tire raises the axle and, with it, everything measured from the bottom bracket. Reference values, verify for your case: computed from public tire diameter charts relative to a 25 mm road tire, holding rim size fixed.

Stack offset 0 mm

Assumes the same rim and no other geometry change. Casing shape and rim width shift the real number.

Quick-fill from a typical frame size?

Illustrative generic road stack and reach by size label, not a specific manufacturer or model. Replace with your exact frame's chart values before relying on the result.

Choose a size above, then apply it to a bike.

Sets frame stack and reach only. Head angle, steerer rise, and stem stay as entered.

Changes calculate automatically. Nothing is saved until you choose to save.

02 / See the difference

Your candidate is higher and farther forward.

Calculated locally
Horizontal change+7.9 mm

farther forward

Vertical change+5.4 mm

higher

Clamp offset9.6 mm

straight-line distance

RAD (Rider Area Distance)+9.1 mm

candidate 785.9 mm vs reference 776.8 mm, bottom bracket to clamp, frame only

Cockpit overlay

Same origin. Same scale.
Reference and candidate cockpit overlay, equal X and Y scale Magnified cockpit detail. Reference clamp: 471.5 mm forward, 617.3 mm high. Candidate clamp: 479.4 mm forward, 622.8 mm high. Circle and solid line identify the reference. Square and dashed line identify the candidate. Only measured geometry is drawn. 300320340360380400420440460480500520540540560580600620640 Reference clamp: 471.5 mm forward, 617.3 mm high Candidate clamp: 479.4 mm forward, 622.8 mm high Forward from bottom bracket · mm Height · mm
Reference solid / circle Candidate dashed / squareFrame top → stem axis → bar clamp
03 / Explore a closer match

Small changes. See what moves.

Geometry only

Candidate steerer rise stays at 30 mm. Try a sampled stem combination; compare the remaining offset.

Closest sampled option 90 mm / -10°

2.8 mm remaining offset

Alternative 1 90 mm / -6°

4 mm remaining offset

Alternative 2 100 mm / -10°

9.1 mm remaining offset

Continuous target 91.3 mm at -8.4° Not necessarily an available or compatible part.

A cockpit calculation cannot guarantee fit, comfort, safe component installation, or handling. Validate the setup with a qualified fitter or mechanic. Sampled options do not check spacer limits, steerer engagement, component availability, or cable routing.

Exact coordinates & calculation notes

Measured from the bottom bracket, in millimeters.

Handlebar-clamp coordinates in millimeters
Coordinate ReferenceCandidateChange
X · forward471.5479.4+7.9
Y · height617.3622.8+5.4

Clamp X = frame reach − total steerer rise × cos(head angle) + stem length × cos(90° − head angle + stem angle).

Clamp Y = frame stack + total steerer rise × sin(head angle) + stem length × sin(90° − head angle + stem angle).

Total steerer rise includes the headset cover, spacers below the stem, and stem-axis center offset along the steerer. Manufacturer definitions must be reconciled first.

The inverse solution uses the vector from the candidate stem-axis center to the target bar-clamp center, holding candidate rise fixed.

The chart shows measured frame-top and cockpit coordinates, not a complete frame silhouette. Total offset is √(ΔX² + ΔY²). Bar shape, hood placement, saddle position, and measurement uncertainty are not modeled. Method 1.1.0.

Keep your comparison.

Your next bike, without the guesswork.

Saved plans stay in this browser. Shared links contain measurements. JSON exports include the method and limitations.

A fit is more than a frame size

Compare the position.
Keep the context.

Explore the methodology
01

Use the right starting point.

Frame stack and reach locate the top of the head tube. Use the exact model, year, and size, not a generic size label.

02

Account for the whole cockpit.

Headset cover, spacers, clamp height, and the installed stem all move the bar clamp. Small details add up.

03

Verify before you install.

A small clamp offset is not a fit guarantee. Bar shape, saddle position, handling, and safe installation still need a real-world check.

How the calculation works

From frame chart to clamp coordinates.

The formula, in words

Use frame stack, frame reach, and head angle for the exact model year and size. These locate the top of the head tube relative to the bottom bracket.

Total steerer rise is measured along the steerer to the stem-axis center. Include the headset cover, spacers below the stem, and the stem clamp-center offset. For a symmetric clamp, the last term is half the clamp height. Do not add spacers above the stem.

The stem then locates the handlebar-clamp center. Compare that center on both bikes, holding bars, bar rotation, hood position, and saddle position constant. This is not a hand-position or complete bike-fit model.

A worked example

With 560 mm stack, 385 mm reach, a 73° head angle, 40 mm total steerer rise, and a 100 mm stem at −6°, the reference clamp is approximately 471.5 mm forward and 617.3 mm above the bottom bracket. These are synthetic inputs, not a named bicycle or recommended setup.

Assumptions this calculator makes

The inverse solution holds candidate steerer rise fixed and solves continuous stem length and angle. Trial options sample 60–140 mm stems in 10 mm steps at −17°, −10°, −6°, 0°, +6°, +10°, and +17°. They are mathematical examples, not a verified parts catalog or installation advice.

When to use the result, and common mistakes

The same clamp coordinates do not establish the same hand position if handlebar reach, sweep, rotation, or hood placement changes. Saddle position, crank length, rider mobility, and handling are outside this calculation. A fitter or mechanic must verify the complete setup.

Common questions

Before you compare two bikes.

What does the RAD (Rider Area Distance) result mean?

RAD is the straight-line distance from the bottom bracket to the handlebar clamp, measured on the frame alone (no stem or spacers). Comparing RAD alongside the clamp offset shows whether two frames put the bar in a similar place before you even pick a stem.

Does a matching clamp position guarantee the same fit?

No. A cockpit calculation cannot guarantee fit, comfort, safe component installation, or handling. It only compares modeled clamp coordinates. Bar shape, hood position, saddle position, and installation still need a real-world check, ideally with a qualified fitter or mechanic.

Where do stack, reach, and steerer rise come from?

Use the frame stack, frame reach, and head angle for the exact model, year, and size from the manufacturer chart. Total steerer rise is measured along the steerer to the stem-axis center, including the headset cover, spacers below the stem, and half the clamp height for a symmetric clamp.

Is my data uploaded anywhere?

No. The calculation runs on your device. Nothing you type is sent to an application server. Saving a plan stores it in this browser only (localStorage), and sharing a link places only scalar measurements in the URL fragment, not on a server.

How are the sampled stem options chosen?

The inverse solution holds candidate steerer rise fixed and samples 60-140 mm stems in 10 mm steps at -17°, -10°, -6°, 0°, +6°, +10°, and +17°. These are mathematical examples for exploring nearby setups, not a verified parts catalog or installation advice.

Can I keep or reproduce a comparison later?

Yes. Export a JSON plan or a coordinate CSV, or save the plan locally in this browser. Exports record the exact measurements, method version, and coordinates so the result can be reproduced.

The fit notebook

A little knowledge goes a long ride.

Guide 01

Stack and reach are only the start

Why the same frame size can put your bars in a different place.

Guide 02

Before you change a stem

A geometry match is a starting point, not permission to install a part.

Guide 03

How stem angle and spacers move your bars

Understand the two directions hidden in a spacer change.

Built to be inspectable.Input schema & model contract ↗ Agent guide ↗