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How do you set up the pinion angle on an IRS?

I figure you can not have the rear pinion to point the normal 3 degrees upwards on an IRS setup, because then the up and down wheel movement will not be vertical?
And then the engine must stay horisontal too? To avoid vibrations?

Or am I wrong here?
 

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in a perfect world, everything should be perfectly in line, both from a side view and a top view

but in reality this isnt always possible, so instead just keep the pinion parallel with the output shaft on the tranny (no matter how high or low the differential is) This way the harmonics caused by the u joints cancel each other out, because both joints are at the same angle

on a conventional driveshaft, when its angled up / down / left / right, the design of the yokes causes it to experience slight changes of rotational speed

this is why the yokes on the front and back of the driveshaft are installed certian ways compared to each other, to help cancel that this effect as much as possible

for an example of misaligned drive shaft vibration, drive a 4wd truck that has a solid front axle, that vibration in the steering wheel is a result of the u joints in the system

this is also why they named Constant velocity (CV) shafts what they did, THIER design doesnt cause the shaft to change speeds that slight amount, allowing them to be used in situations where the 2 joints are never in proper alignment
 

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Here is my answer from "clubhotrod"...

Pinion angle really has no bearing on an IRS. The housing is mounted solid to the frame so it doesn't move with the suspension. There may be some designed into the housing to frame mounts but this would be minimal and only for u-joint longevity.
"Rotating" the housing, as you say, for pinion angle will have no bearing on half-shaft or wheel movement as this is controlled by the suspension arms. You could actually have the pinion pointing straight up and the half shafts wouldn't care...
Mark
 
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