Is this geo calculator accurate?

It's the only geo calculator I've really used. It looks pretty accurate, just put in a med Bottlerocket, I don't know what the Bottom bracket drop/rise is, so I can't get an accurate stack height, but the reach is dead on according to TR's site.

BB rise/drop is the height of the BB above or below the horizonal line of the axles. That way tire size and other stuff doesn't come into play. I just figured it out awhile ago so I'm trying to sound intellegent.

i've used it a few times prior to swapping forks on my trail bike - it's been pretty damn close for me.

I've cobbled together a few formulas a while ago, trying to determine what the changes in geometry would be if I fitted another fork ... I figured somebody else could make an use of that so I put it online and it somehow got out.

Anyhow, the math -should- be quite solid, at least the a-c changing bit. the +- degree insert is a bit wacky, I didn't really know/care which angle commencal (in which I was interested at the time) specified with their inserts - so I presumed the actual headangle change is specified (which is most likely wrong, so some discrepancies can root from here, but nothing major I suppose).

The reach/stack was an afterthought, at the time when only a few manufacturers specified it. It depends on some approximations regarding fork rake, headset height and such, but it should put you within a centimeter or so. I tested it against the few manufacturers that specified the reach and stack back in the day and it always came in under .5cm difference, so it should be good, assuming that the other data is accurate. And that all of the manufacturers specify their measurements in the same way.

And yes, the bb height is actually drop/rise, as seen in the picture. It's the only measurement that makes sense.