Bottom bracket drop vs height: keep the axle and ground references separate
Read frame-relative BB drop and ground-relative BB height without confusing either with saddle drop or pedal clearance. Account for actual tyres, geometry setting and unequal-wheel conditions.
Published 9 October 2026 · OpenBikeFit
Frame chart field guide · not to scale
Two origins. One bottom bracket.
Fictional level equal axles: 350 − drop 70 = BB height 280 mm
Both supports +6 mm → BB height 286 mm; drop stays 70 mm
Static equal-wheel construction only; not pedal clearance or handling approval.
On this page
- 01Drop starts at an axle reference
- 02Height starts at the ground
- 03A fictional equal-wheel calculation
- 04Tyre labels are not mounted radii
- 05Configuration and suspension need their own rows
- 06BB height is not pedal underside clearance
- 07Keep this drop separate from cockpit drop
- 08Build a comparison record that can be checked
Read before comparing
Inspect the scope of frame comparison
The existing free workflow keeps its own access and mechanical gates. Opening it does not create a bike or approve work.
Open toolTwo geometry rows can describe the same bottom bracket from different origins. Bottom-bracket drop is referenced to the wheel axles; bottom-bracket height is referenced to the ground. A change in tyres can alter one while leaving the other unchanged in a simple static construction. Treating them as synonyms hides that condition.
This guide turns those rows into an explicit geometry record. It does not recommend a drop value, decide handling quality, approve a wheel or fork conversion, or calculate a safe pedal clearance. Keep the complete frame setting and mounted assembly with the source before drawing any comparison.
Drop starts at an axle reference
In the familiar equal-wheel side view, draw a line through the front and rear axle centres. BB drop is the vertical separation downward from that line to the BB centre. A larger positive drop puts the BB lower relative to that reference. It is not a distance from the bottom of the crank or the lowest part of the frame.
Check the sign and label in the source. Some manufacturers call this field an offset. Do not silently turn a negative value into its absolute magnitude, and do not assume that every offset field uses the same direction. A BB positioned above a stated reference cannot be described by dropping the sign from its number.
Height starts at the ground
BB height is the vertical ground-to-BB-centre distance for the stated assembly and measurement condition. It depends on the actual wheels and tyres supporting the frame and on its orientation. A nominal tyre-width label is not a direct observation of axle height or tyre radius on a particular rim and at a particular load.
Bike Insights distinguishes reported BB height from estimates based on a wheel/tyre specification. Keep that reported-versus-modeled distinction in your notes too. A computed figure can be useful for understanding a construction while still differing from the physical bicycle. More decimal places do not make its assumptions disappear.
A fictional equal-wheel calculation
Imagine a level, rigid fictional bicycle with both axle centres 350 mm above flat ground and BB drop 70 mm below their common horizontal line. Its BB height is 350 − 70 = 280 mm. Here the equation is height = axle height − drop, because the two axle heights are equal and the drop convention has been named.
Now suppose the actual supporting radius increases by 6 mm on both ends without changing frame orientation. In that simplified construction, the axle height becomes 356 mm and the BB height becomes 286 mm; the frame-relative drop remains 70 mm. These are fictional dimensions, not tyre-size recommendations or a prediction from a printed width label.
The calculation stops being this simple if the front and rear support heights change differently. A mixed-wheel assembly or unequal tyre conditions can tilt the frame, and suspension adds another state. Do not force a two-axle sloping line into the equal-wheel equation. Preserve the manufacturer’s convention and measure or model the full stated condition separately.
Tyre labels are not mounted radii
A rim’s bead-seat diameter, the tyre’s marketed size and the actual ground-to-axle height are related but different quantities. Casing shape, rim dimensions, inflation and loading can matter to the observed support height. Do not add a catalogue tyre-width difference directly to BB height and present the answer as physically verified.
Canyon explains why its geometry discussion uses a BB offset instead of ground height when describing frame context. That source does not authorise a tyre change, and its category-level handling language is not adopted as an individual verdict here. Keep clearance and permitted wheel/tyre combinations in the exact current product documentation.
Configuration and suspension need their own rows
An adjustable frame can publish several geometry conditions. The Specialized STATUS 2 170 DH chart, for example, has separate Hi/Lo rows for BB height and drop. Use the complete matching condition rather than taking one favourable row from each. Its mixed-wheel build is also a reason not to invent one universal wheel radius from the table.
Do not combine a static chart with a loaded suspension measurement and call the difference a manufacturing error. Record whether the source is static, sagged or otherwise defined. This article gives no procedure for changing a flip chip, fork, suspension or wheel configuration; the correct model-specific instructions and physical assessment remain separate.
BB height is not pedal underside clearance
Even a known BB height does not locate the lowest physical part of the pedal in every riding condition. Crank length locates the pedal spindle relative to the BB; pedal body shape, orientation, lean, ground shape and suspension motion add other boundaries. A height row cannot certify that a pedal will avoid the ground.
For a fictional static point-only example, BB height 280 mm minus a vertically downward 175 mm crank places the pedal spindle 105 mm above level ground. That is a spindle coordinate, not 105 mm of pedal underside clearance and not a safe riding limit. Do not extrapolate it to turning, uneven terrain or an unidentified assembly.
Keep this drop separate from cockpit drop
Saddle-to-bar or saddle-to-hood drop compares contact points on a bicycle. BB drop compares the BB with an axle reference. They share a word, not an origin or endpoint. Changing a tyre support height in an ideal rigid level construction can lift the entire bicycle above ground without changing its internal contact-point separation.
The cockpit tool does not use a BB-drop row as a substitute for measured saddle or bar coordinates. The frame comparator likewise stops at declared frame and bar-clamp references. Neither model proves ground clearance, wheel compatibility or handling. Keep those physical questions outside the displayed residual rather than interpreting a small residual as an all-clear.
Build a comparison record that can be checked
Retain exact frame/year/size, the drop sign and axle reference, BB height source, actual wheel and tyre configuration, frame setting and static or loaded condition. Label derived values as modeled and keep the arithmetic assumptions with them. If a row or condition is missing, preserve the gap rather than replacing it with a nearby model’s value.
Use the free fictional frame example to see what a narrower cockpit model can and cannot claim, or keep the reference questions on the blank worksheet. Reading or opening these pages makes no mechanical change and accepts no processing consent. A chart is valuable context when its scope stays attached; it is not approval to purchase, assemble or ride a particular configuration.
Practical questions
Frequently asked questions
Is BB drop measured from the ground?
No. It uses the stated axle reference. BB height starts at the ground and ends at the BB centre for the specified assembly and condition.
Can BB height change while BB drop stays the same?
Yes in a level rigid equal-wheel construction, changing both supporting radii equally lifts the frame while preserving its axle-relative geometry. Unequal wheels, loading and suspension need separate treatment.
Can I subtract crank length to find safe pedal clearance?
That gives only a static spindle coordinate under the declared assumptions. Pedal body, lean, ground and suspension conditions still matter; it is not a safe-clearance certificate.
Is bottom bracket drop the same as saddle-to-bar drop?
No. Their origins and endpoints differ. Keep axle/BB geometry separate from physical saddle, clamp and hood coordinates.