driveshaft angles

92b said

When you say "goes negative" is that relative to the ground, the crank centerline, or some other reference point?

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Going negative means the pinion goes "up" in relation to the drive shaft. That's never good and will cause serious vibration and can even break u joints. The correct position for the pinion is to point down in relation to the drive shaft under acceleration, even if only 1 degree.