In the Truth piece in this issue we learned that certain control arm angles in a double A-arm suspension will create the ideal tire contact patch for a particular race car. We discovered through testing and evaluation that a larger contact patch provides more traction in a tire, much like increasing the loading, but not needing to.
Having that knowledge is very important, but not knowing how to apply that knowledge would be sad. So, continuing along on that thread, let’s dig deep into the issue of cambers and camber change so that we might apply this fresh knowledge to our own car. Let’s figure out what we need to do in order to have the best tire contact patch.
Camber Change Basics
I’m going to run through this quickly because we have covered this so many times that shame on you if you haven’t learned this yet. Camber change mostly happens because the chassis and control arms move.
Most double A-arm suspensions are of the short arm/long arm design. That means that the upper control arm is shorter than the lower control arm. As such, when the wheel moves vertically, the arms move on a different radius and the upper ball joint moves more laterally than the lower ball joint. This creates a change in the camber angle.
There are two motions that will change the camber in a wheel/tire. One is vertical motion of the chassis. The other is chassis roll. In almost every form of racing, except with bumps setups, there is some combination of those two motions when transitioning into the turns.
We must consider both of those motions when planning out our control arm angles. With chassis vertical motion, in dive, both wheels move towards negative camber. In an upward motion, both wheel move towards more positive camber.
In chassis roll (roll to the right in a left turn), it is a little different. On the right side, chassis roll moves the RF wheel towards less negative camber while on the left, chassis roll moves the LF wheel towards less positive camber.
So, summing up here, on the right side, in a left turn, chassis dive and roll create opposite motions. They can be made to cancel each other out to attain zero camber change. More on that later.
On the left side, in a left turn, chassis dive and roll both move the wheel towards a less positive camber angle. This motion and the amount of camber change can be mitigated, or reduced as we are about to learn.
How Cars Are Constructed
What you will have to deal with, and the amount of work you will have to undertake is directly related to how your car is constructed in the first place. A car with plenty of adjustment for arm angles and one where it is legal to do so will be much easier to dial in as to camber change and maximizing contact patch area.
First on the list of difficult cars are the stock classes where the rules limit how much you can do and in what areas you can work. Whatever you can do to these cars to get them closer to ideal will make a huge difference in performance.
Then we have the dedicated race car chassis that was built wrong to begin with. Earlier model Modifieds, some of which still exist and are racing, were built so that the upper arm angles are all wrong for proper camber change design. Proper camber change is defined as being near zero change, or as close as you can get to that. We’ll delve into that particular design in a minute.
Road racing cars, even the most expensive types in some cases, lack proper camber change design and I have personally made changes to a few of those cars that made a remarkable difference in how they turned transforming an understeer car into an oversteer car with one quick adjustment.
One More Thing
Before we get started into the nuts and bolts of this thing, there is one more peripheral thing to understand about cambers. Each brand, and each different construction of the tire within the same brand will require a different camber in order to achieve the largest contact patch area.
This is important because if you have a dominant car where you’ve figured all of this out and then go to a track where they run Goodyear tires instead of your usual Hoosiers, you’ll quickly find that you are out to lunch due to needing different cambers.
Sidewall stiffness and the tire cord construction differences will require different cambers and some tires will just flat out create a bigger contact patch than other types. You may never get the car to turn as well with a new brand of tire as it did with your old brand. That you will have to live with.
Starting With a Blank Sheet
To make this easy to explain, let’s start with a blank sheet of paper. We can imagine we are designing a race car chassis and its geometry from scratch. We know what we have evolved into with most dedicated race cars using, from the Truth article, either the moment center theory or the jacking force theory. But how did we get there?
Our first example is a car with level upper control arms. I am going to use a simple geometry software program to find the results of camber change and report those findings to you so you can see how this goes.
We have a late model dedicated fabricated offset chassis with fairly normal conventional dimensions. The left upper arm is 10.5” long, the right upper is 9.0” in length. The right side static camber is negative 3.0 degrees and the left side static camber is positive 4.0 degrees. The left lower control arm length and angle is 15.5” and 2.5 degrees and the right lower control arm length and angles are 17.0” and 1.0 degrees. We will maintain these settings throughout the test and only make changes to the upper control arm angles.
For comparison, we measured the moment center and it is -1.3” height and -14.5” left of centerline before we dive and roll the chassis.
Zero Degrees Upper Arm Angles
With zero degrees of upper arm angles we dive the chassis 1.0 inch and roll it 3.0 degrees. If you are running conventional setups without bumps or coil bind, these numbers are very representative.
Using the above starting cambers, after the dive and roll, we end up with a left dynamic camber of positive 0.9 deg. and a right dynamic camber of negative 0.5 deg. We lost 3.1 deg. of left camber and 2.5 deg. of right camber.
The left dynamic camber is less than we really need for that tire. On the right front, we probably need a full three degrees or more of negative dynamic camber for that tire to work, so if we add the 2.5 degrees of lost camber angle to the static 3.0 degrees, we would then need to start with a negative 5.5 degrees of negative static camber. Does this sound familiar?
And by the way, we still have a moment center location of -2.0” height and -14.0” width (negative is left of centerline), which is close to what we had at static ride height.
More Upper Arm Angle
Now let’s put 15.0 degrees of angle in each upper control arm. To clarify, we are referring to an angle with the chassis mount lower than the ball joint in this case. We will adjust the lateral spacing on the chassis mount to maintain our 4.0 deg. of positive static camber on left side and the negative 3.0 deg. of camber on the right side.
Now after the 1.0” of dive and the 3.0 deg. of roll, we end up with positive 1.4 degrees of left side camber and negative 3.4 degrees of right side camber. The right side actually gained some negative camber instead of losing it.
The left side camber lost much less camber and the 1.4 degrees would work much better. Less loss and a camber more in tune with what the tire wants is a good thing. But can we do better.
Fine Tuning the Camber Change
Our right side camber is much better, but if we wanted zero change in camber, we might fine tune the upper angle so that the camber stayed the same after dive and roll. So, we change that right upper arm angle to 13.5 degrees. After dive and roll we end up with the same 3.0 degrees of right camber we started with. We have achieved the desired zero camber change for the right front tire.
At the left side, let’s add upper arm angle to say 25.0 degrees. When we do that, after dive and roll, the dynamic camber is now 1.7 degrees. We now lost only 2.3 deg. verses the 2.6 deg. when the upper angle was only 15.0 degrees.
Going a little farther, let’s say we put in 35.0 degrees of left upper control arm angle and see what happens. That means that the chassis mount is now a full 6.0 inches lower than the ball joint. After dive and roll, we end up with a dynamic camber of positive 2.0 degrees and only lost 2.0 degrees of camber. Less camber change is better, but there are structural limits to how much angle we can put into a control arm. But you get the point.
For design purposed, for this car, we can probably be satisfied with upper control arm angles of 25 to 30 degrees on the left and 13 to 15 degrees on the right. Remember that this is a sample car and what you run for a setup and the track you will be racing at will generate different dive and roll numbers. These numbers might not work well for your car.
Cambers for Bump Setups
When we look at the cambers for bump and coil bind setups, we still agree that there will be very little chassis movement when the car is on the bumps, but from the time the car leaves pit road until it enters the turns, there will be plenty of camber change going on.
We can live with this huge amount of camber change because it occurs on a part of the track where there is little, or no, lateral force and the cambers and contact patch size doesn’t really matter.
But let’s look at what camber change does take place. With our sample car, we will enter the dive and roll numbers we could typically see for a bump setup. The dive for a car with a 4.0” static ride height and a 0.5” safety factor would be 3.5” of dive. The roll for most of those cars is at or under 1.0 degrees, so we’ll use 1.0 deg.
Putting in those numbers with a left upper angle of 30 deg. and a right upper angle of 13.5 deg. gives us a dynamic left tire camber of negative 5.8 deg. and a dynamic right tire camber of negative 8.2 deg. This is horrible for the LF tire, we need some amount of positive camber, not negative. And we have excessive negative camber for the RF tire.
What we need to do is change the upper arm angles to reduce the camber change from the high amount of dive and adjust the static cambers. Let’s start with the upper control arm angles. If we go back to say 20 degrees on the left upper and 5.0 degrees on the right upper, we’ll see less camber change.
After that change, we see dynamic cambers of negative 2.5 deg. on the left and negative 5.3 deg. on the right. This is much better, but we now need to change the static cambers. If we need a positive 1.5 degrees for the left tire dynamically, then we need to add that number to the dynamic negative camber of 2.5 deg. for a total of 4.0 deg. additional static camber. Adding 4.0 to the current static camber of 4.0 deg. gives us 8.0 deg. of static camber needed to yield 1.5 deg. of dynamic camber after dive and roll.
On the right side, we can take away some of the negative camber. We are gaining 2.3 degrees on the right side after bump dive and roll, so if we reduce the static camber by that amount, we’ll end up with 0.7 deg. of static RF camber.
From all of this we have learned that the upper control arm angles dictate how much the cambers will change for a particular setup. The static settings for control arm angles and cambers will be much different for conventional setups with low amounts of chassis dive and higher chassis roll than we would use for bump setups with high amounts of vertical chassis travel and low amounts of roll angle.
What Does the Tire Need?
Early on we talked about what the tire needs for camber. We are talking about the front tires obviously because there is little we can do to adjust the rear tire cambers for a straight axle suspension. Each brand of tire has different sidewall stiffness and tire construction for the tread support and so need different cambers so that we can end up with the largest contact patch possible.
Like we also pointed out, you may be able to extract a larger contact patch from one tire brand than any other tire of the same tread width and sidewall height. Tire temperatures can point us in the right direction, but we are finding that the driver may be the best litmus test.
As for tire temperatures, a softer sidewall tire can be cambered so that the tire surface temperatures on the side closest to the inside of the turn are 20-25 degrees hotter than the surface temperatures on the outside away from the turn. This difference is achieved with higher camber settings which tend to cause the inside of the tire to flatten out.
Lower tire pressures help us accomplish the goal of producing a larger tire contact patch, but we are not advocating low tire pressures that compromise safety. We do know the trend in racing is to run as low a tire pressure as we can get away with and this is an indicator of the importance of increasing the contact patch area.
Conclusion
All of this works to point us in the direction of maximizing the tire contact patch area for our race tires. If we can understand how cambers change, what causes that and how we can minimize the change, we can work to get more traction in our race cars.
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