Actually the straight-blade fork does have a rake (offset) amount built-in. This is built into the fork by having the blades angled forward compared to the steerer-tube. Same effect as a curved fork since the rake/offset of the hub ends up being the same amount. Resulting in the same amount of trail on the ground. Another way to generate the rake/offset with a perfectly straight fork, like suspension forks, is to use an offset triple-clamp on the steerer-tube.

here's some links with pictures and explanations of the geometries and their handling effects:

http://www.phred.org/~josh/bike/trail.html
http://yarchive.net/bike/rake.html
http://www.velonews.com/tech/report/...es/7322.0.html
http://www.dclxvi.org/chunk/tech/trail
http://www.kreuzotter.de/english/elenk.htm
http://www.coe.uncc.edu/~gkwatkin/Di...n/chapter2.pdf

In the end, it's the trail amount that makes the biggest difference. It's the sum of all the other factors combined. The larger the trail amount, the more stable the bike, the shorter the trail, the more nimble and twitchy the bike becomes.

So if you put a fork with less rake on your bike, it increases the trail and makes the bike more stable (harder to steer). More rake/offset on the fork result in less trail will make the bike more nimble.

http://