Off road unless it’s very tame you’ll either loose traction or ability to
climb, since off road tends to be more technical, and energy sapping, hence
MTB tend to do better at on the steeper/technical stuff as you can be less
fussy with line choice, and less likely to stall hitting roots and what
not.
I suspect that maintaining grip which means either sitting on the saddle or
hovering over it, would limit most off road climbs to 15/20% or Thereabouts
clearly be some fire road climbs or stuff closer to a road that would be
easier to a) ride up b) grip, so a max gradient closer to what you’d manage
on tarmac but that’s a lot of variables!
I have a *climb* in summer that is a difficult to clear as it’s thick sand,
it can’t be more than 3% if that but due to the sand it’s a hard slog, and
not always doable in winter the sand is wetter so it’s easier.
Roger Merriman.