yo, Here is Ford's view on this..long, almost boring...and highly tech
TSB 92-1A-9 Guidelines for Modifying Light Truck Drivelines
Publication Date: 01/26/92
by Ford via Steve83
an excerpt;
"..LIGHT TRUCK: 1989 and after BRONCO, ECONOLINE, F SUPER DUTY, F-150-350 SERIES
ISSUE: Qualified Vehicle Modifier (QVM) Bulletin Number 14 provided guidelines for modifying light truck drivelines to help ensure proper driveline integrity. The bulletin is printed here as an added aid to the service technician.
Maximum Operating Angle Of 3 Degrees
Operating angles can be allowed to be much higher, sometimes as high as 12 degrees. To get a vehicle to operate successfully above 3 degrees often requires larger U-joints, expensive double cardan constant velocity joints, maybe a restriction to slow vehicle speeds, and other considerations. A reduction in joint life does become noticeable on joints operating above 3 degrees, if precautions are not taken.
Two shafts, connected with a single cardan U-joint and turning at a constant rpm, have no angular acceleration that could force a vibration. When an angle is made between them, the first (power input) shaft will turn at the same constant, smooth, speed; while the second shaft will now have to speed up and slow down twice every revolution (change speed 4 times per revolution).
This creates an angular acceleration in the second shaft that forces a vibration; which is acceptable if kept to a minimum. The guidelines in this bulletin limit driveline angular accelerations to 400 radians per second squared or less, which is the requirement for all Ford light truck (SAE specification is 500). Some driveline modifications have been found to have resulted in accelerations of over 11,000 radians per second squared, often resulting in damaged drivetrains in early mileage. Vehicle modifiers are encouraged to consult the sources listed under REFERENCES in this bulletin, if driveline angles are likely to exceed 3 degrees in the modified design.
Driveline angle measurements should be made with the vehicle supported by the tires, and resting on a level surface. Avoid hoisting a vehicle by the frame, since this distorts the chassis enough to make the driveline angle measurements inaccurate.
Match-Mounting Driveshafts To The Rear Axle
O.E.M. rear axles have their runout measured, then a yellow dot is placed at the high side of this runout reading. The dot is placed either on the end yoke, or on the pilot/******, of the axle input shaft. Alternately, the O.E.M. driveshafts also receive a yellow dot, which is placed on the light side of the unbalance. Upon assembly of the drivetrain, the yellow dots are matched up. This match-mounting aids the driveline system-balance. Vehicle modifiers should look for these dots, and maintain this match-up when the drivetrain is reassembled after modification. Remanufactured or modified shafts should also have their light side matched with the yellow dot on an axle, similar to the O.E.M. shafts.
Driveline Vibration Dampers
Driveline vibration dampers are sometimes added to driveshafts or axles to help with eliminating noise and vehicle harshness (NVH). If they came with the vehicle drivetrain then retain the damper with the modification.
Using Double Cardan Constant Velocity Joints To Allow Larger Angles
In general these joints, used correctly, can allow having larger joint operating angles, as much as 8 degrees. However, it is very important to note that placing this type of joint at the rear of a coupling shaft will prevent cancellation from occurring at the forward end of the shaft where the transmission joins. Without cancellation, the operating angle at this joint must be maintained at 3 degrees or less, regardless of the existence of a CV joint in the driveline system..."
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Angle Correction; "...Driveline correction methods are incorporated into virtually all of Superlift's suspension systems. For example, our rear lift blocks are either tapered or flat, depending on application, to address driveshaft angle. Our replacement springs have a tapered degree shim attached, where applicable, to accomplish the same thing. Rolling the pinion upward also restores some shaft spline contact. Sometimes the blocks' pins / holes are offset to address shaft length issues. More on driveshaft length: Generally, shafts must not be lengthened unless the suspension lift height is over 6". Check out the Installation Notes in your vehicle's section of the online catalog. Unless specifically noted in the product text or installation notes, there is normally no need for replacement or lengthened driveshafts when the lift components are installed properly.."
Source: by Superlift
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Pinion Angle Correction in a 96 Source: by boss (bossind, Steve)
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Yoke Alignment & Measurement, w/Shims @ Leaf Spring in an 89 Source: by waltman
