Multiple Driveshafts and System Balancing

System balancing means assembling the whole driveline, with joints, and placing this assembly in the balancer as if it were just one shaft. This balance method is required with two-driveshaft systems. However, it may be a bit impractical with 3 shafts or more. So in this case, system-balance two shafts together, and balance the third separately.

Balance At 3,000 RPM Versus 500 RPM

Most small shops making driveshafts for body builders only have balancers that spin to 500 rpm. Ford driveshafts are balanced at 3,000 rpm, mainly to find damaging vibration resonances occuring only at these higher speeds. Vehicles today, with overdrive feature, can see over 5,000 rpm in normal use. An imbalance beyond specification at this speed can be destructive to the drivetrain. It is therefore recommended that the vehicle modifier pursue actions that ensure balance accuracy to 3,000 rpm, if the vehicles they will be used on are likely to be operated at highway speeds. The following suggests ways in which the vehicle modifier can meet this recommendation:

a. Dana Corporation is a possible source for driveshafts, and associated components, meeting Ford specifications. Their address and phone numbers are listed in the "REFERENCES" section of this bulletin.

b. The following must be ensured if only a 500 rpm shaft balance is available:

^ A straight shaft with uniform metal thickness

^ Use of highly-toleranced U-joints, as sourced by the original equipment vehicle manufacturer (joints from auto-parts stores are not likely to meet the required tolerances).

^ Possibly restrict the applications of these driveshafts to vehicles normally operating at slow speeds


Eddy-Current Type Brake Retarders

These brake retarders add more work to the drivetrain, requiring that the joints be upgraded to improve their life. Adding a brake retarder is not the only consideration that may require upgrading U-joints. Torque, speed, vehicle use, as well as other modifications, also go into the formula for sizing components. Therefore, it is recommended that the vehicle modifier refer to the "Spicer Drivetrain Selector Guide" for proper sizing of driveline parts.

DO NOT add an electrical grounding strap between the vehicle frame, and the transmission housing or support crossmember. Excessively high current has been found to run through the E40D transmission during cranking and braking when this is done with the ground between the vehicle frame and retarder missing. This high current surge is potentially damaging to internal transmission components. Follow the grounding instructions from the brake retarder supplier explicitly.

Assure proper alignment of the driveline after retarder installation. Be observant of angles in both the plan (top) view and the side view.

It is important to determine if there is a real need for a brake retarder. Some fleets are finding that driver training, in lieu of a retarder, may be more cost-efficient.

Minimum Operating Angle Of 1/2 Degree

A slight angle is required to prevent U-joints from wearing out from brinelling, providing a smooth flow of power through the driveline. It is similar to intentionally putting a pre-load on a wheel bearing.

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.

Using The Frame Siderail To Obtain A Reference Angle

The long, straight frame rails on an F-Super Duty Stripped Chassis have sufficient accuracy to use for a reference angle, and specific instructions published for this vehicle, which ask the modifier to use the frame rail as a reference, should be obeyed. However, this bulletin, and referencing Dana literature, does not require measuring a frame reference angle (all requirements reflect driveline angles relative to each other).

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/flange, 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.