How to convert BB height measurements?

Some frame makers list their BB heights in inches off the ground whilst others list them as + or – relative to the imaginary straight line running across the hub axles.

Is there a way to convert the inches off the ground version (taking into account wheel diameter and tyre height) to the +/- relative to axles version?

Do you have a bike with the same size wheels as the bike you are looking at?

If so, measure from floor to centre of hub (with your tyre of choice fitted obvs).

Subtract the supplied +/- figure from the number you get.

I don’t :(. How do you convert it based off the numbers provided in geo charts?

Isn’t it just the radius of the outside of the tyre minus the figure given by the manufacturers?

Example: If a 29er has a nominal outer diameter of around 29″ and the BB drop is 2″ then the BB height is (29/2)-2. Around 12.5″.

In reverse, a 12″ BB height on a 29er would be shown as -1.5″

I know some frame manufacturers work off the tyre diameter of a reference tyre according to the tyre manufacturers’ 3D CAD drawings. The High Roller 2 2.3 is one of them and the official diameters for that are 691mm (26), 716mm (27.5), 754mm (29).

BB drop is sometimes quoted as a negative figure when it’s positive drop (but negative offset) and vice versa. It’s always actual drop with 29ers but 26 and 27.5 can be either side of axle height (static, not sagged).

For 26″ full-sus usually add the BB drop to 345.5mm, for 27.5 usually deduct the BB drop from 358mm, for 29 deduct the BB drop from 377mm.

Don’t forget to deduct the sag if comparing bikes with differing travel or where they’re designed around different amounts of sag (25% vs 35%).