BB Height & Drop Calculator

Knowing these values can help you understand your bike's geometry, cornering stability, and pedal or crank clearance.

BB Height is the vertical distance from the ground to the center of the bottom bracket. This measurement can be taken from geo chart or measured by user.

BB Drop is how far the bottom bracket sits below (or above) the wheel axles' centerline. It can be hard to measure directly, so you can use the value from your bike's geometry chart or estimate it with my calculator.

SAG refers to the amount of compression in the suspension (front fork and/or rear shock) caused by the rider's weight when sitting in a natural riding position. It is typically measured as a percentage of the total suspension travel and plays a crucial role in how the bike handles.

Ground clearance is easy to overlook when comparing bikes, but it affects how often your pedals hit the trail. Reach, stack, angles, and wheelbase all matter; so does clearance. BB Calc estimates two measurements: Ground Clearance at the pedal axle and Crank Clearance at the crank's lowest point.

In short, using “average travel” to illustrate how the bottom bracket sags is a helpful approximation, but it does not fully model real-world suspension kinematics. On enduro bikes with the same wheel size and comparable front and rear travel, it gives a workable estimate for quick geometry checks. Averaging travel can show how setup changes affect pedal clearance and BB position, but it is not a substitute for a full suspension kinematics calculator when you need precise results. For everyday use and quick checks, I use this practical method.

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I built BB Calc for quick geometry comparisons, not to model every detail of a bike's suspension. It uses average suspension travel as a practical estimate. For more precise results, select Vertical Travel to account for the difference between suspension travel and the bottom bracket's vertical movement.

The Advanced model now includes a chainstay-based Weighted axle-line reference at the BB. The Basic model remains intentionally simpler and uses the front axle as its fixed reference.

I also added a Wheel Circumference adjustment. Rim width affects how the tire sits on the rim and can change its circumference. Use this option to tailor the calculation to your wheel and tire setup. I chose a simple Axle-to-Ground reference to keep the calculation practical.

Fork Axle-to-Crown length also affects BB Height and BB Drop. I keep BB Calc focused on geometry below the axles to keep the model simple. Forks with the same travel can have different Axle-to-Crown lengths, and manufacturers may use different measurement standards. These differences can affect how closely your results match a bike's geometry chart.

These choices make BB Calc easy to use while accounting for real-world differences between bike setups.