MTB Geometry Calculator

An angleset rotates the steering axis inside the head tube, so a −1° cup nominally slackens the head angle by one degree. On the complete bike the measured change is a little less — around −0.75° on a typical enduro frame — because the front axle swings forward and slightly up, and the front end settles once the wheels are back on the ground. Along the way the wheelbase grows roughly 10 mm, the bottom bracket drops a millimetre or two, and trail increases — which is where the calmer steering feel comes from.

The calculator shows both numbers: the cup angle you fit — Works Components cups come in 0.5° steps up to 2° — and the head angle you’ll actually measure, plus everything the change drags with it.

Fitting a 27.5″ rear wheel to a 29er drops the rear axle about 19 mm and the whole bike pivots around the front contact patch: head and seat angle slacken roughly 0.9°, the bottom bracket drops 10–13 mm, reach shortens and stack grows. The slacker seat angle and the lower bottom bracket are the two changes riders feel most — seated climbing moves rearward, and pedal strikes get more likely.

Some frames offer a flip chip to claw the geometry back. Run the calculator with your bike’s chart before buying the wheel, then again with the chip position to see how much it recovers.

Yes — everything moves together. Every 10 mm of extra fork length slackens head and seat angle by about 0.4°, lifts the bottom bracket about 3 mm and shortens reach around 5 mm. A 20–30 mm jump is enough to change how a bike climbs, which is why over-forking works — and why frame makers cap approved fork travel. The effect scales with wheelbase, so a small frame changes more per millimetre than a large one.

The same maths applies in reverse when you drop travel for a lighter, quicker-steering build — and travel, angleset and wheel changes combine, so the calculator applies them together rather than stacking rules of thumb.

It models the bike as a rigid 2D assembly built from your chart, applies the modification, re-grounds both wheels and recomputes every measurement together — and the numbers check out against known conversions. Two honest limits: it’s static geometry, unsagged like a manufacturer’s chart, and it’s only as good as the chart you enter — published values are sometimes rounded or internally inconsistent. Tyre height matters too, so tyre width is settable front and rear.

Not sure what a change would do to your bike, or whether it’s worth doing at all? Book a service at our Auckland workshop and we’ll talk it through on the bike, or shop forks, headsets and wheels once you know your numbers.