Just wondering, is there ever a situation where lateral fork flex is 'good'? After all, we like the fork to go up and down to maintain traction, why not side to side? e.g. it might actually mean you get less deflected off line if you hit a rock.
Dumb question, I know.
In reality it doesn't work like that because, I suspect, what happens is that you hit a rock, the deflection is amplified and you twang off in a different direction. Hit lots of rocks – hello Peak District rubble gardens – and you end up with a very vague-feeling front end that only goes very roughly in the direction you point it. Not good. The less deflection, the more precision. Or that's how I read it anyway…
The same seems to work, roughly speaking, at the rear end. I've just stuck a 10mm through bolt on my RC405 and it definitely reduces rock pile randomness and gives the whole bike a tighter feel when things get messy.
There are situations where lateral flex might be desirable – on 500GP bikes a while back, super stiff frames actually made the rear end really forgiving because some flex dampened the power to rear contact patch transfer, but on a mountain bike, I wouldn't have thought so, unless you put out 200bhp or so.
Of course I could be totally wrong, but as a Peak local who rides a lot of rocky stuff, I'd choose a 20mm axle up front every time. If you want some lateral flex, Maverick SC32s are quite entertaining 😉