Bike geometry calculator

[TheBomber]

I had been contemplating a new fork for my bike but all the available offerings were a little longer with extra offset. Not being sure about the significance of the differences I started to do some calculations in Excel. It’s not easy, so a little web search found this:

bikegeocalc.com (how do I make that a clicky link?)

I’m very impressed. Takes a little while to get to grips with - I recommend a mix of the help button and just having a play. Thought it well worth sharing and interested in knowing whether others think it useful?

[Paulatic]

You copy link and paste
https://www.bikegeocalc.com/#7Unnameda0 ... 2.5G30H30Z

[531colin]

There is also a free Bike CAD from "The bicycle forest" in addition to a paid-for one.
Somebody on here uses it...."foxyrider" ?

[kylecycler]

Thanks a lot for the heads-up. Had a quick play - you need to use the Help function to begin with but I reckon once you get the idea it's excellent.

Re. the replacement fork being 'a little longer', that would slacken the head angle slightly and increase the trail, while 'extra offset' would bring the trail back down so you might end up with the same trail, just a longer front centre and maybe slightly slower handling. Also less toe overlap, if that's an issue. At least I think that's the theory.

[531colin]

"rules of thumb" ....
I think 20mm longer fork (axle to crown race seat) will slacken the head angle about 1 degree for a bike of "average" wheelbase.
1 degree change in head angle is "worth" about 10mm of fork offset, in terms of getting back to the same trail.

.....thats with head angles in the range 71/72 deg, and offsets in the range 45/55mm .....and its also from my memory, which means that I advise you to check it using one of the bike CADs because my memory is not to be relied on!

[TheBomber]

Colin - yes, it was handy rules of thumb that I can see emerging from my comparisons. And your memory is likely better than you credit: a 10mm longer fork is slackening the angles by 0.5 degrees. Though to maintain trail seems to only need about an extra 3mm of offset. Another one is that 0.5 degree reduction in seat tube angle means I’d need to pull the saddle forward 5mm. That one probably varies a lot by the rider’s saddle height though.

You copy link and paste

Thanks - easy then! I had been editing the url (as the full thing is very long) so that was probably why my link wasn’t a linky thing.