A smaller offset increases the trail, which essentially makes the bike more stable (reluctant to turn). The wheel base is also shortened, but because of the increased trail the effect is small.
Increase the offset and the trail is reduced, which make the front feel lighter and easier to turn. The wheel base is increased, but again, the effect is small.
A stock NSR has a trail of 87mm according to the spec, but my measurements say around 84mm, but it's a non stock tyre. this is measured at 23.25 degree steering angle. If you go from 35mm offset to 25mm offset with no other change then the trail increases to about 95mm. Then tip the steering back to 22.5 degrees (RS250 NF5) and you bring the trail back to 90mm. This gives stability and sharp steering. Go to 22.5 degrees with 35mm offset and you'll have 80mm trail which could make the steering a little flighty in the fast bits.
In answer to j911. If you go to 23.5 degrees (more raked out than stock NSR) with 24mm offset (97mm trail) you'll quite frankly have a bike that steers like the old boiler GSXR1100! And I had one of those so don't do it.
All these figures are approximate and just an example of how it works. If you try anything and you crasdh and burn then don't blame me.
Blame Andy. It's his website
