Through the years we have heard various myths about the pinion angle providing bite off the corners. I have experimented with pinion angles while running a Nascar late model on a rear wheel dyno and found little in the way of power gains from more or less angle.
Here is what we know. Drive line angles that are not correct are a cause of vibration and power loss. If your race car must have drive line angles from a design standpoint, the angle of the drive shaft to both the transmission output shaft and the pinion shaft should be equal and also opposite. And, the angles should be kept to a minimum if at all possible.
When a team concentrates just on the pinion end of the drive train, things can get ugly, and here is why. A drive shaft system is somewhat complicated, but not so much that we cannot understand it. There is a video online that shows what happens when the alignment is not correct. It is almost violent if you understand mechanics and dynamics.
To view the video for yourself, just search YouTube for “Driveshaft Angle and Phasing”. There you will learn why the angles of the transmission output shaft to the driveshaft, and the pinion shaft to driveshaft angles need to be the same (equal) and in the opposite direction.
A friend of mine in the industry and I have been talking about setups for his car on and off for a couple of years and in July of last year he asked me about pinion angles. He runs a big block Northeast dirt modified. He explained that he was running about 4 degrees downhill to the front with his pinion. I asked him what his transmission output shaft angle was.
Evidently, the tranny was downhill to the rear by 6 degrees and with the pinion angle running down to the front, his situation represented the worst case scenario. Just look at the video and you’ll see why. With the angles he had, as the transmission shaft turns one full revolution, the pinion would speed up and slow down twice per revolution. At 6-7,000 RPM, this becomes obscene. In my friend’s car, it created a harmonic vibration that he always assumed to be just a part of the race car.
Here is how we can determine what we have, what to do and how to fix it. You will need to put your car up on blocks or boxes so that you can get underneath of it safely. We need to take several measurements.
The attitude of the car needs to be the same as it is when running around the race track. So, if you are running on bumps, put the front of the car down to where it would be while on the bumps. For my friend’s dirt modified, he jacked it up in the front and on the left side and down on the right just like it would be during the race.
Also consider the change in pinion angle if you run a lift arm or pull-bar. Those pieces of equipment will change the pinion angle as they move under acceleration. When power is applied to the drive train, any miss-alignment will cause the most damage. You might want to measure your pinion with the rear end at the position it will be under full acceleration.
How And What To Measure – First measure your transmission output shaft angle. There is an easy way to do that. Your engine is in line with this part. The valve covers, unless they are of a strange design, will be parallel to the block deck, which is parallel to the driveshaft which is parallel to the transmission shaft. So, just lay an angle finder on the edge of the valve cover, read the angle and write down the number. Also note which way it is inclined, down to the rear or up.
Next crawl under the car and lay your angle finder on top of the driveshaft and measure the driveshaft angle and note the inclination. Then measure the pinion angle and also note the inclination. To get the pinion angle, you might have to use a straight edge placed against the flat part of the flange with the angle finder against that.
We’re not done yet. Driveline angles can occur from any view. We have just measured the angles from a side view, but what about the top view? Now we need to look at that scenario.
To measure top view angularity, we need to do some lateral measurements to the center of the tranny shaft and the center of the pinion shaft. If you have a straight rail car and know that the rail is parallel to the centerline of the chassis, this will be easier. If not, you need to setup a string line that is parallel to the centerline.
Measure from the centerline or straight rail to each of the above mentioned shafts using plum bobs or similar devices to make sure the measurements are correct. We need to determine if the two shafts are in line, or if not, how far they are out of line.
For most offset late models, I have found the two to be out of line by 1.0 to 1.5 inches. For a 40 inch driveshaft that is out of line by 1.5 inches, the driveshaft to pinion and driveshaft to tranny shaft angles would be 2.15 degrees. That is plenty of pinion angle and almost too much by today’s standards. But the good thing is that they are equal and opposite.
If that were true for your car, then you would not need any side view angles in either the pinion or tranny. So, you could place the tranny shaft, driveshaft and pinion shaft in line with no side view angular difference. That is because the U-joints don’t really know which direction they are lined up at, only that there are equal and opposite angles in their relationship to the driveshaft.
How To Adjust The Angles – Now that we know what the angles are, let’s see how we go about making changes. Let’s assume that from a top view, the tranny is in line with the pinion. So, we only need to work with the side view angles.
For an example, using my friend’s angles, we have an engine that is downhill to the rear by 6 degrees and the pinion going downhill to the front by 4 degrees. What I recommended was reducing the engine angle, but it was going to be impossible to run it uphill to the rear, so he re-shimmed the motor mounts until he got to 2 degrees downhill to the rear.
Next, it was fairly easy to change the pinion angle because he had a three link rear suspension. Now this is going to sound strange to many old timers, but we rotated the pinion until it pointed uphill to the front by the same 2 degrees. Now the angles were equal and opposite. But, you say, “we cannot run the pinion uphill!” Yes we can. Nothing in the dynamics world says we can’t. In this case we proved it.
If you have other designs for your rear suspension, you’ll need to get creative when changing the pinion angle. For leaf spring or truck arm cars , there are wedges made to varying degrees you can use to place between the spring and the pad on the axle tube. This will rotate the rear end and pinion to change the angle to the drive shaft.
For stock four link rear suspensions, you’ll need to get even more creative. Since the alignment of the driveline is so important, it is not out of the question to cut and re-weld the suspension links to change the lengths in order to rotate the rear end and change the pinion angle. Just do it in a safe manner. Make good welds or have a professional do it.
As for my friend, the result of making these changes was the elimination of the vibration he had felt forever. He also told me that the forward bite improved. I envisioned that with the driveline out of alignment, it was like spiking the throttle on and off many times per second and that caused the rear tires to lose grip under power.
The truth is this, the speeding up and slowing down of the pinion shaft with each revolution of the engine that is due to miss-alignment happens not only on acceleration, but all of the way around the track as long as the engine is connected to the rear end.
As the season comes to a close, you might want to make a note to inspect your driveline in the offseason and take a few measurements. If the alignment is not correct, please take the time and effort to make it right. The performance gains could be substantial.
When checking your drive train alignment, put the car at the attitude that it will be when running on the race track. This could get tricky for some dirt cars, like our example in the story.