How to Use the Geometry Calculator

The geometry calculator at

was built because a static geometry chart on a product page is not enough information to make a setup decision. A static chart gives you one set of numbers — usually the “base” or “recommended” configuration. The calculator gives you every configuration, computed from the actual geometry relationships, for your specific fork and chip combination.

This is a guide to reading the output and using it to make a real decision.

What the calculator computes

The calculator takes three inputs: chip position, fork model (which determines the axle-to-crown length at each travel setting), and fork offset (42 mm or 44 mm). From those inputs it computes:

• HTA (head tube angle) — how slack or steep the front end is
• STA (seat tube angle) — how far forward your weight is over the bottom bracket
• BBH (bottom bracket height) — ground clearance at the pedals
• Trail — the key steering stability number
• Toe overlap flag — whether your foot can contact the front wheel at full turn

How HTA changes with fork travel

The head tube angle changes by approximately −0.024° per millimetre of fork axle-to-crown length. That means every 10 mm of fork travel increase makes the HTA slacker by roughly 0.24°.

The base reference is 63.9° at a 150 mm fork in the FORWARD +5 mm chip position with a 29" rear wheel. From there:

• 140 mm fork: ~64.1° (slightly steeper)
• 160 mm fork: ~63.7° (slightly slacker)
• 170 mm fork: ~63.5°
• 180 mm fork: ~63.2°

These numbers shift further depending on the chip position. CENTER and BACKWARD chip positions steepen the HTA by up to 0.3° relative to FORWARD at the same fork.

Fork offset and trail

Trail is the horizontal distance between the tyre contact patch and the point where the steering axis meets the ground. Higher trail = more stable steering, more effort required to initiate turns. Lower trail = lighter steering, less inherent stability at speed.

Fork offset directly affects trail: a 44 mm offset fork produces less trail than a 42 mm offset fork at the same HTA. At a 63.9° HTA with a 29" wheel:

• 42 mm offset: trail ~105 mm
• 44 mm offset: trail ~98 mm

That 7 mm difference in trail is noticeable in handling. Riders who prefer lighter steering and more agility at speed should choose 44 mm. Riders who prefer high-speed stability and confidence in rough terrain at speed may prefer 42 mm. Neither is objectively correct — it is a handling preference.

The calculator shows trail for both offsets simultaneously so you can compare them directly for your fork travel and chip position.

Choosing a chip position in the calculator

The calculator shows all five chip positions. As you step from FORWARD +5 mm to CENTER to BACKWARD −5 mm, the HTA steepens and the STA steepens. The wheel size constraint is shown alongside each position — FORWARD positions require a 29" rear wheel, CENTER and BACKWARD require a 27.5" rear wheel.

Use the calculator to compare two configurations side by side: for example, FORWARD +5 mm at 150 mm fork vs. CENTER at 160 mm fork. The longer fork with CENTER chip can produce a similar HTA to a shorter fork with FORWARD chip, while the STA and BBH will differ between the two configurations.

Toe overlap

The calculator flags toe overlap risk for smaller frame sizes with larger wheel sizes. Toe overlap occurs when the front wheel at full steering lock can contact the rider’s foot at the bottom of the pedal stroke. It is most common on size S frames with 29" front wheels. The flag in the calculator indicates when the geometry parameters you have selected produce a combination where toe overlap is possible — it does not mean overlap is certain, as it also depends on shoe size and crank length.

If toe overlap is flagged for your configuration, the options are: move to a 44 mm offset fork (reduces trail and wheel clearance at lock simultaneously), or if you are between sizes, consider sizing up.

Using it to decide on a fork

The calculator includes 14 fork models from Fox, RockShox, Öhlins, DVO, EXT, Intend, and Manitou, covering travel from 140 mm through 180 mm. Select your fork model and the calculator uses that fork’s actual axle-to-crown length rather than a generic travel number — because two forks at the same travel setting can have different axle-to-crown lengths, which changes the HTA calculation.

Use the calculator at

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