Front-centre, chainstay length and wheelbase: direct vs horizontal distances
Locate the BB between the two axles using named projections. Explain why equal wheelbase does not imply equal front-centre, toe clearance or a matched bicycle.
Published 10 October 2026 · OpenBikeFit
Front geometry field guide · not to scale
Add projections, not diagonals.
Fictional equal axle heights: BB drop 75 mm; direct rear-centre 425 mm.
Rear horizontal = √(425² − 75²) ≈ 418.3 mm; front horizontal = 600 mm.
Wheelbase ≈ 1018.3 mm; direct front-centre ≈ 604.7 mm. Not a toe-clearance or size verdict.
On this page
- 01Front-centre has more than one convention
- 02Chainstay length is not the curved tube length
- 03Wheelbase follows the ground direction
- 04Project a complete fictional rear triangle
- 05Resolve a direct front-centre separately
- 06Equal wheelbase can hide a different split
- 07Front-centre is not a toe-clearance certificate
- 08Keep the configuration instead of mixing rows
- 09Turn the chart into checkable questions
Read before comparing
Keep the unresolved references on paper
An unfilled free worksheet creates no bike, selects no fork and certifies no clearance or installation.
Open toolA geometry table can put front-centre, chainstay length and wheelbase close together, all in millimetres. Adding two numbers feels natural, but the sum is meaningful only when both follow the wheelbase direction. Direct BB-to-axle distances can point diagonally because the BB lies below the axle reference.
This guide separates those lines and shows what they can actually establish. Its fictional side-view geometry is not a frame-size recommendation, a stability test or a toe-overlap checker. The complete assembled bicycle and exact chart condition stay important even when the arithmetic is simple enough to do on paper.
Front-centre has more than one convention
Direct front-centre joins the BB centre to the front axle centre in the declared side view. Horizontal front-centre uses the fore–aft separation between their vertical reference lines. Bike Insights explicitly documents both types. A number labelled front-centre should therefore retain the manufacturer’s arrow direction rather than silently being treated as horizontal.
The side-view convention also excludes lateral axle or BB widths; it is not a tape path around the three-dimensional frame. Identify the centres, projected direction and axle position from the drawing. If the chart supplies only an ambiguous label, keep its definition unresolved. More precise copying of the number cannot decide which of the two lines it measures.
Chainstay length is not the curved tube length
The geometry field commonly called chainstay length or rear-centre joins BB and rear axle centres in the side view. It is not the length along a bent stay’s outside surface. Its horizontal projection is a different quantity, just as at the front. Bike Insights separates normal and horizontal versions instead of treating them as synonyms.
Dropout position can be part of the declaration. An adjustable rear axle has more than one possible location, and a source may publish a nominal position or range. Preserve the setting alongside the dimension. Do not combine a shortest rear-centre row with a wheelbase from another setting and present the mismatch as an error or a new measured configuration.
Wheelbase follows the ground direction
In the ideal upright side view on level ground, wheelbase is the horizontal separation of the wheel contact references. With equal axle heights it also equals the direct axle-centre separation. For the examples here the axles share a horizontal line, so that relationship is explicit rather than inferred from a mixed-wheel or loaded assembly.
Wheelbase is not the bicycle’s full outside length including tyres. It also does not include the frame’s top-tube length or the stem. Keep wheel/support condition, steering state and frame configuration with the chart. The quoted value describes that condition, not every possible position of a suspension system or steered wheel.
Project a complete fictional rear triangle
Declare equal axle heights, BB 75 mm below their common line and direct rear-centre C = 425 mm. The horizontal rear span is √(C² − 75²) = √(425² − 75²) ≈ 418.3 mm. This is a right-triangle projection in the explicitly stated side view, not a shortening of the actual chainstay tube.
Now declare horizontal front-centre Fh = 600 mm. Wheelbase is 418.3 + 600 ≈ 1018.3 mm. Adding the direct 425 mm rear-centre instead would give 1025 mm, mixing directions and overstating the modeled wheelbase by approximately 6.7 mm. Compute with unrounded values before displaying the result; the tenths are not a claim of physical accuracy.
Resolve a direct front-centre separately
For that same fictional construction the direct front-centre is √(600² + 75²) ≈ 604.7 mm. It is longer than its 600 mm horizontal projection because it includes the vertical separation. If a table supplies this direct front-centre instead, a matched BB-drop and axle condition are needed before recovering the horizontal component.
The projection √(direct² − vertical²) requires a coherent triangle; a direct length smaller than the declared vertical separation cannot describe it. Do not erase the sign or choose a nearby dimension to make a square root work. Unequal axle heights need separately named vertical components for front and rear, not one BB-drop value applied automatically to both.
Equal wheelbase can hide a different split
Consider two other fictional horizontal-only records. Bicycle A has rear span 420 mm and front span 600 mm; bicycle B has rear span 440 mm and front span 580 mm. Both have wheelbase 1020 mm. The BB’s location between their axles differs, even though the total is identical. No handling or weight-distribution verdict follows from this invented pair.
This is why comparing only the total discards information. Matching BB-relative saddle and bar coordinates would add another set of endpoints without making these axle splits equal. A complete geometry record retains the individual spans, their conventions and the contact-point model separately. Equality at one measured point is not equality of the whole bicycle.
Front-centre is not a toe-clearance certificate
Toe overlap concerns an actual shoe or foot and the front wheel during steering, not merely the BB-to-axle distance. The crank location, shoe outline, mounted cleat/foot position, tyre and possible mudguard, steering angle and lateral relationships are additional quantities. None is supplied by one front-centre row.
This page provides no universal minimum front-centre and no suggestion to ride around a suspected interference. A straight-ahead side-view drawing cannot establish the entire three-dimensional clearance envelope. Keep the question unresolved for the exact bicycle and qualified physical assessment. Do not interpret a positive gap in a schematic as permission to assemble or ride.
Keep the configuration instead of mixing rows
Use model/year/size, axle/dropout setting, wheel and tyre configuration, steering condition, static or loaded state and the manufacturer drawing together. A rear suspension or adjustable frame can change the relationship between points. A chart-based direct length and an installed horizontal observation should remain separate records until their definitions and conditions match.
The existing OpenBikeFit comparator targets a named bar-clamp coordinate and deliberately does not model wheelbase, toe clearance or dynamic handling. Use its fictional example to inspect that boundary, not as a substitute for the missing axle information. The BB drop/height guide explains the axle-versus-ground distinction without adding an ideal wheelbase or clearance threshold.
Turn the chart into checkable questions
Retain each raw value with direct or horizontal direction, origin/endpoint, source date, axle position and any assumptions used in a derived figure. Keep reported and calculated values distinct. A blank unknown field is better evidence than a copied number from another frame size; it makes the remaining question visible rather than pretending the comparison is complete.
Use the unfilled worksheet for a paper-first record and follow the related guides when the unresolved field is fork length, trail or ground reference. No camera, saved bike or measurement upload is required to read this path. A complete dimensional explanation improves the questions you can ask; it does not deliver a frame-size choice, mechanical permission or a verified ride outcome.
Practical questions
Frequently asked questions
Is front-centre always a horizontal measurement?
No. Direct and horizontal side-view versions exist. Keep the exact drawing, direction and BB/axle references with the number.
Can I add chainstay length and front-centre to get wheelbase?
Only when the values are compatible horizontal spans under the same condition. Direct diagonal lengths need their stated projections resolved first.
Do equal wheelbases mean equal geometry?
No. Different front/rear horizontal splits can have the same total; contact points, steering geometry and conditions can also differ.
Does a front-centre number prove no toe overlap?
No. Shoe/foot, crank, tyre/mudguard, steering and lateral geometry require actual assembly-specific assessment. This guide has no universal clearance threshold.