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Then I could angle the pinion back up by using longer front shackles..... So where's the problem doing it this way?
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A longer shackle, in the front, is going to make the front end feel "unstable". That's the best way I can describe it. I rode in a buddy's Heep before and after he did the shackle swap (front to rear shackle), and there was a definite difference in the feel of the ride.
Also, keep in mind that angling the front pinion up also means reducing the caster angle of the axle. This will affect your steering response and straight-line stability.