This is Monte Knapp's '67...
This is Monte Knapp's '67 Mustang complete with hand-sculpted fiberglass front bumper and fender extensions and a custom-fabricated stainless steel and aluminum grille.
Vibration Solution
Monte Knapp, Yakima, WA: I have a '67 Mustang fastback. It has a 351 Cleveland with a T5 five-speed out of a '91 5.0 Mustang and a 3.89 Traction-Lok in a Lincoln Versailles rearend. The problem is a high-speed harmonic vibration somewhere rear of the transmission. It was completely rebuilt, the driveline has been balanced (twice), and the U-joints are new. The vibration comes in at around 70 mph and tends to get worse with speed. There is a definite sweet spot (if you want to call it that) where the vibration is at its worst, and that seems to occur even cruising down the highway. If I am going slightly downhill and maintaining speed, it totally goes away and everything is right with the world. But as soon as I level out or put in a little more power, it comes back like I flipped on a switch.
Here are some more clues: It has nothing to do with engine speed because I can put the trans in Neutral and let it idle and still have some vibration. I have checked driveline angle, and both angles coming out of the tranny and going into the rear are the same. The only problem I can come up with is that there is not enough of an angle, because it seems to be about 1 degree at each end and looks like the driveline is just about straight-on. No one around here seems to know all that much about driveline geometry.
If I add angle to the rearend, it doesn't really add angle at the tranny. The car has been lowered quite a bit, and that is the reason for the straight driveline. Is there any rule for pinion angle going up or down, or does this matter? I read somewhere that the pinion should angle up. I am considering an upgrade from the current leaf-spring setup to an airbag three-link from Air Ride Technologies, but I need the correct angles before installing this system, as it is not adjustable.
Jeff Smith: I ran into almost this exact same situation several years ago with my '65 Chevelle while on the Hot Rod Power Tour(r), I had a vibration at about that same speed, except mine was worse on deceleration from around 70 mph. The odd thing was that this vibration occurred only on Power Tour(r). We discovered that when we lowered the back of the car with tools, spare parts, and luggage, it changed the ride height, which caused the vibration. As you surmised, your problem began when you lowered the car, which directly affects driveshaft angle. My quick fix was to raise the back of the transmission by roughly 31/48 inch. This made the angle of the transmission centerline and the centerline of the pinion closer to parallel and eliminated the problem. The reason a driveshaft vibrates is because the U-joints are designed to operate at less than 3 degrees.
The driveshaft operates most smoothly when the angle of the engine and transmission are parallel but offset from the pinion angle of the rearend. Imagine you can see through the side of your car. Draw a line from the crankshaft centerline through the output shaft of the transmission. Then draw a second line straight through the center of the pinion gear. Right now, those lines are probably almost on top of each other, creating a straight line (zero angle) from the crankshaft centerline to the rear-axle pinion. What you want is for the two lines to be parallel but offset.
As a quick check to see if this solves the problem, I'd throw about a 11/44- to 11/42-inch-thick spacer between the transmission mount and the trans. But let's assume you don't have transmission-tunnel clearance to allow this. The next solution would be to use an angled spacer between the rear of the leaf spring and the rear axle. These spacers, also called wedge plates, are available through Competition Engineering (PN C7025, $21.88 from Summit Racing) and come in 2-degree increments. Placing these wedge plates between the axlehousing and the leaf spring will angle the pinion downward. This will add an angular difference between the transmission and the rear axlehousing. The wedge plates may actually need to angle the pinion upward in order to create the proper parallel angles.
Let's say this situation occurs on a coil-spring car like a Chevelle or a Fox-body Mustang. In order to change the pinion angle, we'll have to purchase a pair of adjustable upper control arms. By lengthening the arms (compared to stock), this will push the pinion angle down. Coil-spring cars don't need as much negative pinion angle, so 1 or 2 degrees is sufficient. This leads us to pinion angle. Looking at the car from the side, a street-driven leaf-spring-equipped car could need a 2- to 3-degree pinion down angle where the pinion is angled toward the pavement with the car at rest. In high-powered leaf-spring-equipped drag cars, this might be more. This negative pinion angle is used to compensate for spring wrapup that occurs when applying lots of power to a leaf spring. Under load, the front half of the spring bends into an S-shape, which is what causes wheel-hop. Adding a traction bar helps control this spring wrapup but never quite eliminates it.
If you need a more thorough explanation, there are illustrations of these angles in the story "How to Set Pinion Angle," which can be found on the Car Craft Web site at carcraft.com/howto/91758.