# A caster question

Hello,

Recently I was messing around in a game called ‘BeamNG Drive’, a realistic softbody car simulator, and I happened to notice something strange… when turning the steering wheel all the way to the right or left, the inside wheel would have like 10 degrees of positive camber, but the outer wheel would have -4 degrees of negative camber. I know the wheels don’t turn at the exact same angle, since you’d have a lot of scrub. This game measures steering angle in Toe degrees, and when turning the steering wheel all the way, the outer wheel would have like 38 degrees of toe and then the inside wheel would have 45 degrees of toe. So when I would turn the wheel so the inside wheel would only turn to 38 degrees of toe, to my surprise, the inside wheel, instead of having 4 degrees of positive camber, had 6 degrees of positive camber. This naturally confused me, since in my mind, the camber would be the direct opposite (4 degrees of positive camber and -4 degrees of positive camber), no matter which direction the wheel is turning (left or right). I’m not sure if I’m just being stupid or if this is an ingame flaw of some sort. I hope that all made sense lol.

Thanks for the help!

I think what you are describing might be the “Ackerman effect”. Google it, it’s fascinating.

Yes, I have heard of the Ackerman effect, but thats not exactly what I’m trying to describe (don’t worry, I kind of worded it wrong anyway).

Let me give an example here, so lets say I’m sitting in the simulation with a car, and I turn the steering wheel all the way to the right. now lets say the outside wheel is at 38 degrees of rotation, and the inside wheel is at 45 degrees of rotation. In this scenario, the outside wheel has -4 degrees of camber, and has 38 degrees of rotation. Now, lets say I turn the steering wheel a little less, to the point where the inside wheel is at 38 degrees of rotation. Instead of the inside wheel having the direct opposite amount of camber (being 4 degrees of camber), it has more, in this case it has 6 degrees of camber. I’m trying to figure out if this is true to life or if this is some glitch in the simulation.

Gotcha. Chassis work was never something I was good at.

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Because the caster is positive, while turing right, the left front wheel gains negative camber angle and the right front gains positive camber. This happens because of the definition of caster angle and the kingpin axis inclination angle. The greater negative camber helps the tire provide more cornering traction.

If you make little models of these parts you can see the changes for yourself.

Caster angle centers the tire back to straight after it has been turned. With manual steering, small angles like 1 or 2 degrees would have been common. Now that nearly everything has power steeering, 4 to as much as 9 degrees can be used. That allows good handling from the large changes in camber when turned but keeps camber angles small when driiving straight.

As suggested, look into Ackermann steering as well. The inside tire has to turn to a greater angle because it is closer to the center of the turning radius. It is referred to as steer angle rather than toe angle. Toe is the difference from straight ahead for the tire when the car is going straight.

The big changes in the steer angle are caused by the arm on the steering knuckle. Both arms are angled inward meaning that the angles will be different when steering. Turn right and the right wheel will turn sharper. The left one would have much longer travel available to it, which would make it have less effect. The right one would pull faster. Turn the other way and it’s the same type of effect but in the other direction.

But that is Ackermann steering again and not the question he is asking. He is asking about camber change with steering angle, not the difference is steering angle between the inner and outer wheels.

Yes, I understand how caster works (or at least I think), but the question I’m asking is exactly why the inside wheel in my description would have a larger differential in camber to the outside wheel when at the same rotational angle. In my mind, it seems like a glitch, let me give an example of what I think should happen.

So lets say you’re on a bike. You turn the handlebars 90 degrees to the right. The tire has lets say -10 degrees of camber. Now, lets say you turn the handlebars 90 degrees to the left. The wheel now has 10 degrees of camber. In my mind, this is what should be happening in the simulation, where the wheel always has the exact opposite amount of camber when turned the opposite direction at the same steering angle (which in this case would be 90 degrees). But in the simulation, this doesn’t happen. With a car, like the explanation I gave above, has an effect where I’d turn the steering wheel to the left (or right) to its lock, and the outside tire would have -4 degrees of camber and the inside tire would have 10 degrees of camber. Of course, the ackerman steering effect comes in here, because the inside tire is turning more than the outside tire. Lets say the outside tire is at 35 degrees of rotation and the inside tire is at 45 degrees of rotation at full lock. Now lets say I turn the wheel a little less to the point where the inside tire has 35 degrees of rotation, instead of it having the exact opposite of what the outside tire had at the exact same position, which would be 4 degrees of camber, it has 6 degrees or more of camber. Hopefully I worded it right this time, and thanks for the replies!

Ok, the angles will not be the same because the car is a mirror image left to right, not an identical image… the steering axis leans in from bottom to top. That is the steering angle from vertical as viewed from the front. That angle affects the camber gain and is different side to side. If the left wheel is turned right at 38 degrees, the camber gain will not match the right wheel, turned right at 38 degrees because of that angle. It is real, not a glitch. Now, camber gain on the right should match the left if turned LEFT 38 degrees.

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Alright, I kind of see where you’re coming from. So, instead of the effect being tied to the caster angle, its tied to the steering axis. I think I’m understanding this…

Ok, I watched a video, and from what it seems, KPI/SAI actually makes the camber positive when cornering, on both the inside and outside tires. Adding KPI and Caster together means that you get positive camber on the inside wheel, and a small amount of negative camber on the outside wheel. Am I understanding this correctly?

EDIT: So from what I see, the inside wheel will have a lot of positive camber, because the caster is able to work with the SAI, and then the outside wheel has a small amount of negative camber, because the caster has to, in a sense, fight the SAI, because the SAI wants to have positive camber, while the caster wants negative camber. So basically, if a car had 0 SAI angle, and some caster, the inside wheel and outside wheel would both have the direct opposite amount of camber at lets say 38 degrees. Correct?

Yes, that is correct. But the car would be a bear to drive…

The centerline of the tire would scrub around the intersection point of the SAI and the ground. Every bump you’d hit would try to tear the steering wheel out of your hands.

SAI is there to get the steering center close to the centerline of the tire. You’d still like as small an SAI as possible but run some pretty decent amount of caster so you get more camber gain in cornering.

Keep in mind, camber gain also comes from the suspension design itself. If the ball joint is lower than the a-arm bushings on a strut car, there will be some camber gain added to the caster induced gain as it compresses into a turn.

If there are 2 A-arms - short upper and longer lower, you can design more camber gain into the car. So add up caster induced camber and suspension induced camber and you’ve got the tire pretty well positioned on the outside for traction.

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Alright, I believe I understand now. Basically, all the SAI does is reduce the scrub radius, and it also adds positive camber no matter which direct you’re turning. So adding this with caster would give you a proper self-straightening setup, and also would provide a large amount of positive camber to the inside wheel, while controlling the outside wheel’s negative camber. I’m still sort of confused as to how the SAI causes positive camber, but thats probably something I’ll search up on my own time. If I can’t find anything about it, I may come back to this thread.

Anyways, thanks for the help!

EDIT: Ok nevermind, I know why the SAI causes positive camber.

What do you think about those vehicles that look like dune buggies but have one wheel in back and two wheels in front? They look like they would be hard to drive.