Click Here to Begin SlideshowLast issue, we started our look at why pinion angle is important. If you point your browser back to that article, you’ll recall we stated the entire notion of setting pinion angle has little or nothing to do with traction (although improved traction can come with it due to zero bind). It’s all about maintaining the proper relationship between the slip yoke, universal joints, driveshaft and pinion yoke. Fair enough - but you also have to stay on top of that pinion angle, as you’ll see below:
We’re not done yet. According to Jerry Bickel, pinion angle should be checked and adjusted any time there are changes made to the chassis that have an effect upon the ride height of the car or the length and location of the suspension link bars. Jerry’s advice is applicable to other applications too, but in any racecar, prior to setting up the pinion angle, Bickel points out that four link instant center is set, the rear end is aligned, the chassis is at correct ride height, the weight distribution is set, the tire rollout is checked and the tire pressure is set. Keep that in mind, even if you have a simple leaf spring car such as the one in the author’s Nova (shown in the photos).
One more consideration when it comes to the driveshaft is critical speed, and it goes hand in hand with pinion angle. Critical speed is the speed at which a spinning shaft will become unstable. Mark Williams notes this is one of the single largest factors when selecting a driveshaft. When the whirling frequency and the natural frequency coincide, any vibrations will be multiplied, so much so that it is possible for the shaft to self-destruct. Another way to think of this is that if a shaft naturally vibrates at 130 times a second, and one point on the shaft passes through 0 degrees 130 times a second (7800 RPM), the shaft has hit a critical speed.
Williams advises there are several ways to raise the critical speed of a driveshaft. You can make it lighter or stiffer, or increase the diameter without increasing weight. This is the reason carbon fiber makes for a good driveshaft: Carbon fiber is stiff and light and can be made to any diameter or wall thickness. Aluminum, while it has a higher critical speed when compared to steel (same diameter and length shaft), is not quite as strong as steel. Steel, with good strength characteristics, will have a lower critical speed. It’s very important to consult with a driveshaft manufacturer prior to picking a shaft for your car.
In the end, pinion angle is critical. If you have it right, it won’t make your car hook any better. But an angle that causes serious binding under load can cause all sorts of aggravation. To make your car work, get the pinion movement under control and get the angles right when the car is under load. It’s that simple.
Leaf springs such as this can be made to hook the tire hard. Check out the back tire on this stick shift Nova. Bottom line here is, the pinion angle must still be kept under control.
Here’s a look at what seems like a pretty simple (and common) leaf spring suspension arrangement. In this case, the car is built with a set of Calvert split mono-leaf springs and a Cal Tracs traction bar. How do you adjust the pinion angle in something like this?
With a simple leaf spring car, the pinion angle can be set two ways. The first is by adjusting the pinion angle downward while the perches are being installed.
The other is to use a set of wedge plates between the spring perch and the springs. They install here (pointer). Whether you need to move the pinion nose up or down will determine the orientation of the wedge.
There are all sorts of wedge plates available. These examples are from Calvert Racing. You can buy wedge plates with different degrees of offset. These are 2-degree jobs.
