I’m running a 100mm travel fork and have a seat tube angle of 73 degrees and a headtube angle of 70.5 degrees.

If I were to switch to a 120mm fork approx. what would my new angles be?

around 1/2 a degree slacker once you’ve factored sag in. I really wouldn’t worry about the specific angles anyway – particularly as it’s not uncommon for quoted geometry to not actually be what the frames really are…

I thought it would be more than that. Cheers

I’m about to do the same, so I emailed the bike company.

I had a great reply, they basically said “do it”. In fact the lad who replied said he loved the feel of my model with longer forks on the front.

The big Q now is which 120 fork

I’m getting the Magura Durin Marathon – can be switched between 100mm and 120mm.

With that small a difference, and already quite steep angles, you should be absolutely fine. Just make sure that the frame manufacturer warranties the frame with a 120mm fork.

If you made a big change you might need to put your saddle forward a few mm to keep the same effective saddle offset, but probably won’t make any difference.