Hiya – if I change my frame and by so doing increase reach by 20mm, can I simply shorten the stem by 20mm to compensate, all other parameters being equal ?

Mibbie.

Changing the stem length will change the handling, which may be a good/bad thing (most likely good)

all other parameters being equal ?

Problem is they can’t be. Either your seat tube angle has also got steeper or your top tube has also got longer or some other combo. I’d give it a go and see

Fair enough – head angle has gone from 69.5 to 66.5 and St angle from 73 to 72.5. Top tube has gone up from 600 to 633 effective. Stem going from 90mm to 70mm. Travel from 100 to 130mm.
Is it serious doctor ?

This is all theory based on online calculator

Why the slack seat tube angle?

Why the slack seat tube angle?

My guess is that’s the actual, not effective. If it’s a fairly recent design, the seat-tube is more likely around the 75 mark.

Nickfrog – your idea sounds reasonable. However, the slacker head-angle will mean the front wheel sticks out more relative to the steerer, so shortening the stem might make you sit further behind the front wheel than before, despite the longer reach. That might not be a bad thing.

I changed bike too recently and the difference in geometry between old and new is nearly identical to yours. I went from an 85mm stem to 70mm, which felt great. I then took the plunge and went down to a 50mm, knowing (like you) that the overall reach wouldn’t be less than on the previous bike. But I’m still not quite comfortable yet…though reluctant to go back to the 70mm stem as the handling on tight techie sections is better with the 50mm. I’m waiting for some proper riser bars in the post, so hoping they will get it feeling right again. So as others have said, you probably need to try different stems (borrow off mates etc…)