by JWL .. Mon Jul 19, 2010
This is probably one of the most misunderstood subjects related to building a car. The transmission output shaft and the pinion input shaft should be parallel with the pinion allowed to be lower at the front by the amount of wind-up expected during operation. For a car with ladder bars or very stiff rear springs the pinion need only be down 1/2 or 1 degree. The passenger car spec from Ford was "less than 5 degrees" of pinion down in relation to transmission output. If I was building a normal street car I would set the pinion nose down about 2 or 3 degrees at normal ride height. This means 2 or 3 degrees lower than the transmission output angle. If I was to build a car which would be seeing maximum power acceleration runs, but without special control links, I would set the pinion down 5 or 6 degrees to anticipate the amount of spring wrap-up which might be encountered.

If you spend some time thinking about the dynamic situation you will realize the relationship between the transmission output and the pinion shaft are almost constantly changing as the vehicle encounters uneven road conditions and load conditions. If there is higher than normal wear it might indicate the driveshaft is running almost straight in line. This is the best location for maximum power application but the worst condition for wear characteristics.

Flathead4d .. Tue Jul 20, 2010
JWL; I've been getting more confused about pinion angles the more I read. Here's what I have. My trans is 4 degrees down. The rear end and pinion sit higher than the trans so I set it a 4 degrees up. It is my understanding that the two center lines must be parallel in order for the driveshaft angles to be equal. Does that sound right? I seem to have a vibration around 60-65 MPH even with the clutch in. The flywheel, pressure plate and disc were balanced as a unit. Also the driveshaft was balanced. Any advice you can offer would be greatly appreciated. Thanks, John

mtflat .. Tue Jul 20, 2010
Lostinthe50*s wrote: thanks guys for the valuable input. I'm not concerned with the tranny angle and I still can't figure how to measure the angle of the pinion.

I picked up a cheap magnetic angle gauge at NAPA a few years back. It looks like a big compass with one flat edge - that edge has a magnet embedded in it and the center is marked as "0". Put it on the flat surface of your diff yoke and read the angle indicated by the needle. It's really self-explanitory. Same thing on the trans output shaft - on face of the U-joint yoke.

Flathead4d - if I read JWL right he'd set your diff at 1 or 2 deg up. In operation the constant torque will then hold it close to 4, which matches your trans output shaft.

by JWL .. Tue Jul 20, 2010
Yes, the two centerlines of the shafts(trans and pinion) should be parallel when the car is at normal, loaded, ride height with a small allowance, as pointed out by MT, for the thrust wind-up.

Regarding measuring the pinion angle, sometimes this can be done by using the face of the rear end housing where the center section bolts into the housing. Other times the measurement can be made by putting a set of u-joint cups in place and measuring from the OD of the cups when the pinion is rotated so the cups are vertical. I think the 49-53 Fords used a flat faced cup which fits in a groove and bolts to the rear flange. Care must be taken to think through the angles because measuring off the face of the flange or bearing cups will be at 90 degrees to the trans output shaft.

If everything seems "right" and a vibration exists I would first check the bearing in the tail housing of the transmission. Besides the fit of the slip yoke on the splines, it also fits into a bearing sleeve in the tail housing. It is important to lube the slip yoke OD at the time of installation into the sleeve bearing and to be sure it is a good fit on the slip yoke journal. With the car jacked up take a good hold of the front of the driveshaft and try to push and pull the driveshaft up and down. This will help you determine whether or not the slip yoke is a good fit in the tailhousing.

By V8Bob ..Tue Jul 20, 2010
One component of setting up a drive line that is not fully understood is the actual working angle of the u-joint, and in the case of the common single cardan u-joint , this should be between 1/2 and 3 degrees. Set at "0" will not allow the needles to wear evenly, and over 3 degrees will cause vibration at higher road speeds, or rpm, because of the the increasing speed differential in and out of the u-joint. ( 3 degree working angle is good for +/- 5000 rpm max) The 3 degree working angle really gets confused with the common engine tilt or attitude most vehicles with open rear drive axles use, which is +/- 3 degrees. Also, when the rear (pinion) is higher than the transmission, the working angle can be easily exceeded with a rear down ( normal) tilt on the transmission. In summary, the trans output shaft, or crankshaft centerline, should be parallel to the pinion within 1 degree during normal road operation, and the working u-joint angles between 1/2 and 2 degrees, with 3 degrees max.
Rear axles located with ladder bars or 4 links have no (or minimum) torque or braking effect on the pinion angle, and are easier to set up vs axles using parallel leaf springs. JMO, based on some facts.

BillB .. Fri Jul 23, 2010
Ran into this link .. the pictures help.
Drive Line Phasing


From UTube ...An excellent tutorial from the 1930's on the principles and development of the Differential Gear.


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