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especially why more offset (eg 51mm) allows you to run a 29'er with slacker geometry, 'pparently!
Because it reduces the trail by moving the contact patch forward and thus closer to where the projected steering axis hits the ground.
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your headtube points at a point on the ground.
with zero offset, your front wheel's contact point would be MILES behind this (we call this 'trail'), and your bike would handle funny.
by offsetting the forks forward a bit, the contact point moves forward, reducing the 'trail' behind the head-tube's-pointing-at-the-ground-point.
bigger wheels generate bigger 'trail' - more offset is needed to stop the bike feeling funny.
it took a while to get 26ers sorted, and you can't make a (good) 29er by fitting a bigger wheel into longer forks.
Larger wheel diameters increase trail. Increased trail slows down steering and makes the front end less stable - a bit like a supermarket trolley wheel waggling from side to side. On order to combat this 29ers run steeper head angles to effectively reduce trail and increase steering response and stability. However if you want a slacker head angle you need to increase fork offset or rake. So basically a greater fork offset enables a slacker head angle with bigger wheeled bikes. I'm sure it's more complex than this, but this is my understanding.
thanks
what was confusing me is that it feels a bit counter-intuitive to me. Seems as though more offset would increase trail, but actually it's the reverse as it is bringing the wheel forward