Back in the heyday of Cup racing, I attended track tests put on by some of the top teams at various tracks, and one of the things many of them did was make changes to the upper control arms. At that time, I didn’t quite understand why they would do this. Maybe they didn’t, either.
I think this may still be happening today, although I don’t think anyone has figured out why it does what it does. The truth is, some of those changes made the car turn better. If a team found the right control arm angle and length, and the car went faster, then the experimentation worked.
In the way we understand control arm angles today, it’s all about camber change and moment center location. When a team does make a change, hopefully they know what result they are looking for and what control arm angle will give them that result. Let’s take a look at some issues with making changes to control arm angles.
Length Affects Angle – Left Side – If you change your left upper control arm length from 10 to 11 inches, you have changed the angle if you bolted it back into the same holes in the upper chassis mounts.
For example, we have a front end with a 28 degree angle in the left upper control arm and it is 10 inches long. If we change that arm to an 11-inch arm and don’t change the mount height, the angle changes to 25.3 degrees. We would need to lower the chassis mount half an inch to get the 28 degree angle back.
If our original thinking was that a longer control arm would reduce the camber change in the left wheel, then we would be wrong, because the camber change actually stayed the same due to the change in control arm angle. Longer was better and less angle was worse. They canceled out.
Length Affects Angle Right Side – OK, so upper control arm length doesn’t really affect camber change on the left side, but what about the right side? Let’s experiment on that side and see what happens. We have an 8.5-inch right upper arm with a 12 degree angle and after dive and roll, our camber goes from (-) 3.5 degrees to (-) 3.9 degrees. We are gaining 0.4 degree of camber.
Going to a 9.5-inch upper and bolting it into the same height holes in the upper chassis mount, we end up with (-) 3.4 degrees, a loss of 0.1 degree, while the upper angle changes to 10.7 degrees. And if we maintain the same original angle of 12 degrees, our camber goes to 3.7 degrees, still a gain of 0.2 degrees of camber.
The change in length only created a moderate difference in camber change, but the angle made a greater difference in camber change: 0.2 degrees with length only and 0.5 degrees with angle and length change.
Going back to the beginning statements, it looks as though the early crew chief changes to the arm length only, and bolting the arm back into the same holes did make a significant difference in right front camber change and the handling would have also changed.
Angle Affects Camber? – In fact, on this car, which represents a typical front end layout in today’s late-model asphalt race car, the length doesn’t make much change at all on either side. The angle is what influences the camber change. So, we’ll take a look at angle versus camber change.
First, we will reduce the angle at the left upper. Remember, we started out with 28 degrees of left upper control arm angle. If we change that to 24 degrees, what does the camber change look like now? We started with 4.0 degrees of camber and ended up, after dive and roll, with 1.3 degrees with the 28 degree upper angle. Now with 24 degrees, we have 1.25 degrees of dynamic camber, only a half a tenth loss. If we go to 36 degrees upper angle, we now see 1.42 degrees of angle, less loss, but only by a tenth or so.
With conventional setups, the left camber loss is hard to improve. But what about the right front? We’ll do some angle changes to see how much angles affect camber change.
We’ve already seen where an arm length change on the right front caused an angle change that helped to reduce the camber change. Let’s just keep the same arm length of 8.5 inches and change the angle. We started out with 12 degrees of right upper arm angle and the camber went from (-) 3.5 to (-) 3.9 degrees.
Now let’s go to 16 degrees. With the same dive and roll, we end up with (-) 4.8 degrees. We have gained 1.3 degrees of camber—not the change we wanted. Now, let’s go to 8.0 degrees of right upper angle. After dive and roll, we end up with (-) 3.0 degrees of camber, a loss of 0.5 degrees of camber—again not what we want. What we really want is zero change in camber.
If we put in 10.5 degrees of upper arm angle for this car, we get a dynamic camber of (-) 3.5 degrees, or zero change in camber, which is ideal for the tire. So, it seems we need to pay close attention to the right upper arm angle.
If we make changes to the length and bolt the arm back into the same height holes in the upper, the angle difference that change caused was the reason the car either turned better or worse.
The beauty of all this is, when we put those control arm angles into geometry programs, the moment center ends up well left of centerline, a position we have come to know makes the car turn better. Now we know why.
Conclusion – For more conventional setups, we need to think about our control arm angles. Take the information we have presented and think about how your car dives and rolls through the turns and then try to maximize your camber change by making control arm changes.
Much of the information we have presented over the years about roll/moment center design leads us to the most efficient camber change, because the arm angles that produce the optimum moment center location actually also produce much better camber change.
The post Changing Control Arm Angles: On Purpose or Not appeared first on Hot Rod Network.