Depending on who you speak to, we lose 10- to 20-percent of our torque due to friction losses in the drivetrain. Naturally, no one likes losing torque after spending so much money and working so hard to create this rotating force. This is, after all, the force we use to move the car and no matter which class you run, the name of the game is to move the car. So if we’re going to be good racers, one of the many questions we must ask is, “What can I do to reduce torque loss due to friction in the drivetrain?”
By: Dave Morgan
Courtesy of ProMedia Publishing
One approach to reducing friction loss, as well as vibrations, is to focus on an adjustment we normally call “Pinion Angle.” We tend to think of this difference in the angles between the driveshaft centerline (CL) and the pinion gear CL. A more correct term for this angle is to call it the “Working Angle” of the rear U-joint. However, to look at this relationship as being only between the pinion and the driveshaft would be a mistake. We also need to consider the working angle at the opposite end of the driveshaft, where it meets the transmission output shaft. In doing so, we will be considering the entire drivetrain angle.
Yet before we leave the topic of what we call this adjustment, let’s understand that to many drag racers, particularly those with leaf spring rear suspensions, the words “pinion angle adjustment” often relates to a tuning aspect of their suspension setup. For these guys, pinion angle is used to adjust how hard they hit the tire on a launch.
This is a somewhat controversial topic. For some, the reason for paying attention to pinion angle is so that we will reduce bind in the U-joints in the driveshaft. If we follow this logic to its final conclusion, then the reason racers see a difference in ET, reaction time, or 60-foot time with different pinion angles is that incorrect pinion angle is a form of bind and having this bind is a way to remove violence from the hook. Remove the bind, or in other words have the correct pinion angle, and you’ll hit the tire harder because the suspension will move that much more freely. Such racers often monitor U-joint temperatures with the same heat gun they use to measure track temperatures.
The other side of the conversation is based on the notion of harnessing the torque of the pinion gear as it climbs up the ring gear. Drag racers use this rotational force to a better effect than any other type of motorsports. In a leaf spring application, the forward half of the leaf spring is the front/rear locating device for the suspension system.
Keeping in mind that this front locator is a spring; it flexes and in doing so, can permit more vertical pinion motion, which lifts the front eyelet of the leaf spring upwards. In doing this, the differential housing is forced downward. Careful racers modulate this downward thrust with some excessive amount of pinion angle that they adjust into the car by adding, or removing, wedged-shaped shims between the leaf spring and the housing tubes of the differential. They are careful because to do this they must tickle the limits of the range of motion of the U-joints. If they exceed these limits too many times, noisy, spinning, middle-of-the-car badness follows.
The pinion-angle adjuster can be on the bottom of a ladder bar on on the top, as seen here on these Alston ladder bars. They are simply turnbuckles that can be lengthened or shortened to change the angle of the pinion. These adjusters can also be used to change preload.
Before we split our conversation into the two approaches, reduced vibrations and friction losses as well as improved hook, consider how much torque we are talking about. Let’s ask ourselves, “How much torque do we produce at the ring and pinion?” This number is often referred to as Drive Wheel Torque (DWT) and we can use the following formula to determine it.
DWT= Engine Torque X First Gear Ratio X Rear Gear Ratio X .85
We’ll consider a racecar that produces 515 foot-pounds of torque during the usual launch rpm. Many FSC readers have an engine that makes more torque than that, but we’ll keep the textbook numbers conservative. This is a T-5 manual-transmission car with a low gear ratio of 2.95 and a rear-gear ratio of 5.38.
As stated at the beginning of this article, estimates vary on how much torque we lose due to friction. We can affect these friction losses by watching our drivetrain angles. This idea is in line with the first opinion; reduce friction and you also reduce bind. We see this attitude in play when we look closely at the engine position in a tubeframe racecar. Normally, to reduce frictional losses, the tubeframe chassis builder, who does not have a stock transmission tunnel to work around, can aim the crankshaft directly at the pinion CL. The crankshaft, input and output-shaft on the transmission and driveshaft are all on the same angle. In this case, the front and rear working angles are the same and we have the least amount of resistance at the U-joints.
For this conversation, we’ll take an average of these 10- to 20-percent estimates and say our car loses 15-percent of its torque from friction. That .85 factor at the end of the above formula is one way of reducing the total by 15-percent. So, let’s plug in our numbers.
DWT=515 ft/lbs X 2.95 X 5.38 X .85 = 6947.53 ft/lbs
Now we know that when the pinion gear tries to rotate upward in this car, it is doing so with a force of nearly 7,000 pounds! (And we wonder why stuff back there flexes.) Scratch one up for the leaf spring guys because they can say this is the DWT force they use to wrap their springs up with and load against the chassis then beat the daylights out of their rear tires. But ladder bar racers and four-link guys can’t forget that they too, use this DWT. Ladder bar racers use it to raise the front rod end against the chassis. In four-link cars, the top bars pull and the lower bars push from this DWT.
If all we are going to do is think only about pinion angle, then we should at least get that relationship right. Frank Rehak, from the Driveshaft Shop commented on how many racers measure this angle incorrectly. He said, “They think pinion angle is the angle of the pinion in comparison to the level ground. They place an angle finder on the yoke of the pinion and what ever that number is; they call it the pinion angle.
“In a textbook approach, what people call the pinion angle is the working angle of the U-joint at the rear of the driveshaft. There is another working angle at the front of the driveshaft. That’s the one people don’t think about. They don’t realize there is a front working angle and a rear working angle and the relationship is about both working angles. We want them to be the same within a half-degree tolerance. Also, we want no more than three degrees of working angle for the U-joints on either end of the drive shaft. Keep in mind, I said that was a ‘textbook approach.’ I’ve seen racers do things that, by the book, should never have happened. Every car is so different and unique, that’s why we like to get involved with a racer’s project as soon as possible. ”
It would be reasonable to ask why we need to pay attention to the working angles of the U-joints at both ends of the driveshaft. The simple-looking driveshaft is a lot more intricate than it seems. This one component must transfer power and torque from one shaft to another, even when the angles between the two shafts vary and it must do this smoothly. To avoid vibrations, the front and rear working angles need to be within a half-degree of each other and as slight as possible. In OEM applications, the difference in working angles can be found to be between four and five degrees. In our high-torque standing-start applications, we look for a smaller difference, which is the three degrees Frank Rehak mentioned.
Look closely at all three photos, they are not the same. This is where you place the angle finder to determine the angle of the transmission output shaft. The angle finder is placed agains the yoke at the rear os the transmission yoke.
If we are going to consider both front and rear working angles, then we can split the combinations of angles into two types of cars. The first is the tubeframe car, which has the benefit mentioned before, the crank aims right at the pinion, working angles are the same, and friction is reduced. In this case, we compare the angle of the pinion to the angle of the driveshaft, which is concentric with the transmission shafts and crankshaft.
Here’s a maintenance tip for those racers who have a tubeframe car with the crank aimed at the pinion. One of the benefits to having a very small difference in working angles is that the bearing cups in the U-joint are rotated slightly with each revolution of the driveshaft. This causes the needle bearings within the cups to roll so that a different needle bearing will receive the full impact of the torque with each turn of the driveshaft. As the working angles become closer, this rotation of the bearing cups lessens and individual needle bearings get clobbered with the force of moving the car. The tip is to periodically remove the driveshaft and manually rotate the bearing cups in the U-joints. “That’s a good idea,” Rahek said. “Grease gets backed against the needle bearings and holds them in place while they get hammered by the torque. They will form splines into the surface of the trunnions to the point of U-joint failure.”
The second type of car is one that has the stock floor in it. The transmission tunnel of the stock floor creates some limitations on the engine placement along with ring-and-pinion location. And, surprise, surprise, some racers are placing bigger motors in their racecars with stock floors! The crank CL can not be aimed at the pinion CL so the two shafts that will be connected by the driveshaft will be at different heights to each other.
“That’s when guys get into trouble,” Rehak said. “The OEM guys have the working angles down pat for the engine/chassis combinations they sell to the general public. When a racer places a different engine in a car, he loses all that engineering that went into driveline placement.”
This means that racers, who upgrade to a bigger engine, need to pay close attention to the angle of that bigger motor, which probably does not have the same crank CL height location as the original, meaning the transmission will need to be relocated also; and that changes the front working angle, which no longer matches the rear working angle. If a bigger engine is part of your program, please refer to the sidebar, “How to measure Drivetrain Angles.”
Now we’ll return to the issue of using the rear working angle as a means of planting the tires harder. Keep in mind the environment that the driveshaft is in and the job it has to do. It must smoothly transmit massive amounts of torque between two shafts that may, or may not be aligned, remembering also that the angular relationship between the two shafts constantly changes. Finally, let’s be honest, most of you don’t race ballerinas.
Launching 3500-pounds of anything is going to resist a lot of force that is trying to move it (the name of the game.) This means that the driveshaft, like most driveline components, is subjected to two loads. These are the huge amount of torque from the engine/transmission and the enormous inertia that is inherent in a 3500-pound car. To be able to transfer lots of torque, move a porky automobile and not bust into a bunch of needle bearings is a very good quality to have in a driveshaft.
Everyone who has considered the rear working angle, no matter what they call it, realizes that the angle of the pinion changes as the differential goes through its vertical travel and that it rotates up when torque arrives. The amount of upward rotation is limited by the suspension linkages. These include the leaf spring, which has a lot of pinion rotation, the commercially available ladder bar, which has less pinion rotation than the leaf spring, but more than an equal-length four-link system. When we compare the equal-length four-link system to the triangulated four-link on a Fox-bodied Mustang, where the upper bars are shorter than the lower bars, we see that the equal-length four-link has less pinion rotation.
So when we can ask ourselves what angle should our pinion shaft be, we should also identify what we compare that angle to. If we want the rear working angle to be within a half degree of the front working angle for the least amount of vibration and friction loss, then we’d like to have the pinion at nearly the same angle as the crankshaft, and consequently the transmission output shaft. This means that we can register the angle of the pinion against the angle of the crankshaft and call that “Zero Pinion Angle.”
For cars with factory four-links such as Fox-bodied Mustangs and A-Body GM cars, racers should install adjustable upper control arms that also have turnbuckles, which can not only adjust pinion angle, but can be used to set preload and center the differential housing within the car.
Knowing how much our suspension linkage changes the angle of the pinion, we can lower the angle of the pinion at the yoke end by the same amount that the suspension permits the pinion to raise. For leaf spring cars, the pinion CL is set between 5-7 degrees down in comparison to the crankshaft angle (keep your temperature gun handy.) Ladder bar cars normally run three degrees down in relation to the crank. Equal-length four-link bars run one-to-two degrees down in comparison to the crank angle. Unequal-length four-link cars will see as much as four-to-five degrees in relation to the crank (find a buddy with a leaf spring suspension and a temperature gun.)
How to measure Drivetrain Angles
To reduce frictional loses and vibration, we need to compare the working angle of the front U-joints versus the working angle for the rear U-joints. We want the two working angles to be within a half-degree of each other and as slight an angle as possible, less than three degrees is normally recommended in drag racing. The exception is in cars with leaf springs where the spring wrap exceeds three degrees and cars with triangulated four-links where the upper bars are shorter than the lower bars.
There are three angles that we are concerned with in our driveline. These are:
- The driveshaft angle, which is exactly what it sounds like and is the easiest to measure. This is the angle of the driveshaft, which normally spans downward from the tail of the transmission to the pinion in the differential.
- The front working angle is a comparison between the angle of the output shaft and the angle of the driveshaft. For this, we’ll need to measure the angle of the yoke at the rear of the transmission, and then compare that to the driveshaft angle.
- The rear working angle is a comparison between the angle of the pinion gear and the angle of the driveshaft. For this, we’ll need to measure the angle of the yoke at the front of the pinion, and then compare that to the driveshaft angle.
For example, let’s consider a car with the transmission angle at 3.5 degrees, driveshaft angle at 4.5 and actual pinion angle at 3 degrees, as shown below. The front working angle is 1.0 degrees, while the rear working angle is 1.5 degrees. This alignment will probably work, but is at the edge of tolerance, which is a half-degree difference between the two working angles. Both working angles are also less than three degrees, which is considered the limit of the range of motion available from the U-joints.
Electronic Angle Finders
Electronic angle-finders, also known as inclinometers, are becoming more popular these days. They are easier to read, are very accurate and all of them include a hold-function. This helps collect readings when the part to be measured is in some difficult-to-reach area of the car. The angle can be measured, then after pushing the hold button, the reading is held until the gauge can be removed from the angled surface and into the daylight, where the degrees can then be read. Here we’re measuring the potential pinion angle of a four-degree leaf spring shim and a one-inch lowering block.
Here is a demonstration of another way to measure the angle of the pinion centerline (CL) if the driveshaft and U-joints are not installed in the car. Place the angle-finder on the forward edge of the yoke, which in theory, if not in practice, should be 90-degrees to the pinion CL.
Other opinions about Pinion Angle: The Racers Speak
Extreme Street hitter Bill Trovato says that he hasn’t put much thought into the importance of pinion angle. “I’m not sure it plays as an important roll as many people think, but I am always open to the idea and will probably investigate it further. For now though, Trovato feels his winning and record setting Xtreme Street Starfire, works just fine. “I do what makes sense to me and for the car. To me, pinion angle doesn’t factor that much into the way my car leaves way the car hooks so I don’t play around with it,” he said.
Mark Artis, who not only races in Nostalgia Super Stock, but also builds many of the cars that race in the class says pinion angle makes big difference in the setup of any chassis regardless of if it’s leaf spring, ladder bar or four link set-up.
Brian Metz, who not only serves as Crew Chief for Troy Coughlin’s Pro Street entry, but also runs Metz Performance, says pinion angle is extremely important when you want to control wheel speed. “ I adjust according to what type of wheel speed I see. We set a base pinion angle up and then adjust it accordingly once we get to the track,” Metz said.