63 drivetrain angle

[Michael G.]

I'm getting prepared to mate-up a 327 with a Tremec and drop it into a stock frame. With all the discussion of driveshaft angles that I've seen, I have a couple of questions, (just so I can understand what the original design intent was):

On original mid-years, in the side view, were the axis of the engine crank, driveshaft, and pinion all intended to nominally be aligned with each other on the same center in car position, or were the engine and pinion on the same angle, but not aligned on the same axis?

What was the design angle of the crank axis relative to the frame in the side view? (I measured my stock 63 car and the engine crank axis appears to be about 3 degrees to the frame rail)

[Duke W.]

Re: 63 drivetrain angle

Common Hooke joints should always operate with a small angle - say at least 2-3 degrees which causes a slight back and forth movement of the trunnions to spread out the loads - and it's best if the angle at the axle is equal, but in the opposite direction to the angle at the transmission. This will minimize the chance of vibration problems.

In a one Hooke joint system with a small angle and constant input speed, the speed of the shaft is not constant, but sinusoidal. In a two joint system such as in a rear drive layout with equal but opposite angles at each end and constant input speed, the shaft speed is still sinusoidal, but the axle yoke speed is constant because of the equal but opposite angle at that end cancels out the sinusoidal component of shaft speed.

I believe in plan view the trans and axle yokes are nearly lined up due to the engine being mounted one inch to the right of centerline and drive pinion offset, but there is a slight angle in the side view. Engine installation angle of 3 degrees sounds about right and is easy to measure by laying an inclinometer on a valve cover, assuming the frame is on a level surface, and measuring the angle of the driveshaft will allow you to calculate the shaft angle at the trans yoke, and you can probably figure out a way to measure the angle at the axle. I've don't recall ever seeing any published design angles for these componesnts.

BTW, modern constant velocity joints get their name because operating at an angle, shaft speed exactly matches input speed at any point in the 360 degrees of rotation. In a two joint system operating at different angles this eliminates any vibration issues that might develop when operating at high angles such as when the wheels are steered significantly off center in a FWD layout.

The two Hooke joint half-shafts on the C2/3/4 rear suspension could give rise to vibration issues, but apparently in most driving situations there is not enough difference in angles to cause problems. CV joints were incorporated into the rear half-shafts beginning with the C5.

IMO "driveshaft angle" obsession is in the same category as "rod ratio" and "quench clearance" obsession - small deviations from the ideal, assuming there is an ideal, are usually not great enough to cause problems.

Duke

[Michael G.]

Re: 63 drivetrain angle

Thanks Duke. Your explanation tends to suggest that the axis of the axle yoke should ideally be parallel to the engine/trans axis and that they should ideally not be collinear, but offset somewhat from each other. Such an offset of parallel axises would result in a driveshaft not parallel to either axis, but forming equal and opposite angles with those two axises, thus canceling effect of each angle.

While that may be the case with the normal Corvette trans and axle (though I doubt it, as, without measuring, the axis of the pinion seems pretty close to parallel with the frame rail, rather than parallel with the engine axis) Let's assume, though, that whatever the case, the angles can't be all that bad (and must come fairly close to canceling each other), because the stock Corvette geometry works well.

Given the fact that it works today, I'm not sure what the effect of a longer trans will be, as a longer trans effectively shortens the driveshaft and thus increase angles, which, if I'm understanding this, could be enough of an angle change that it exceeds the 3 degrees. That could make the use of the long Tremec a new ballgame... which may be why some people report vibration with the switch to the longer trans.

My best guess as to how to approach this, while I still have a bare frame sitting here, is to make sure I set up the drivetrain (with shims, etc, if necessary) so that the engine axis and the differential yoke axis are parallel to each other, while making sure the angles between them and the driveshaft remain small (<2 degrees).

[Duke W.]

Re: 63 drivetrain angle

...sounds like you understand the issues and now is the time to make any possible corrections with the body off and the drivetrain installed.

Lowering the trans mount will help, but off the top of my head I can't come up with a way. Shims could be used to raise the trans, but that's going the wrong way.

Lowering the nose of the axle should help, and this should doable using washers/shims in the appropiate places in the front axle mount hardware.

On some solid axle installations the axle nose may appear to have a slight down angle. This helps in achieving the ideal of equal but opposite angles at both ends, at least at normal ride height. Of course, the angles change with jounce and rebound - usually in the unfavorable direction, but those are transient events.

Solving driveline vibration issues that may be due to unfavorable driveline angles (or other issues such as lack of torsional rigidity in the driveshaft) is one of those rarely heard about problems that cause engineers to gray prematurely.

Nowadays structural simulation programs will usually reveal issues during the design and analysis phase, but back in the day it was experience and engineering intuition that lead to a solution.

Duke

[Michael G.]

Re: 63 drivetrain angle

I'm revising the trans crossmember to be removable, so I have to do some fabrication in the mount area anyway, so when I get things together and measure the angles with the assembly sitting in the frame, I'll be able to determine if further changes are necessary.

All of which suggests to me that it would make sense to change to CV joints now and not worry about what happens with angles. I remember a thread some time ago where a guy had done this on a a C2, for this very reason.