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Old 10-13-2018, 09:22 PM   #1
Spoolsworth
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Default Question for alignment/physics geeks out there

Alright folks,

I have a question related to suspension alignments (particularly cross-camber and cross-caster pulling) that I’ve not been able to find the answer to, so I’m coming to you guys to help me out.

The set up: we know that a vehicle will pull in the direction of the most positive cambered tire. We also know that a vehicle will pull in the direction of the least positive castered tire.

Question is: Why? What forces are acting in these situations to cause a vehicle to do this? Why, in the case of camber, if a spinning tire is leaning a certain direction, does it have the effect of pulling the car that same way? And why, in the case of caster, does a vehicle pull towards the tire with the lesser “raked” axis?

I’ve asked this question to a couple of co-workers at my shop, and also to a Hunter Engineering technician, and it quickly tumbled into a vitriolic discussion and didn’t get me anywhere.

My half-fledged theory is to think of the tires on a car like the tires on a motorcycle: in order for them to remain stable while spinning at a tilted angle it must also turn in an arc. Since we’re dealing with a car though, that simply results in a pull.

As far as cross-caster goes, I don’t have any theories yet that are satisfying enough to even mention.

Let me know what you guys think! Thanks!
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Old 10-14-2018, 12:06 AM   #2
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I wonder if the camber tendency is truly more positive or just larger contact patch. Eg if you had plus one on one side and negative one on the other if it still pulled towards the plus.
I’m suspecting the pull related to the greater rolling resistance of the larger patch.

I think of caster as being a straightening force so if you have less on one side that side will wander more though I’m not sure that would make it pull that direction
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Old 10-14-2018, 12:58 AM   #3
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A cambered tire creates a thrust force in the direction it's leaned. I think mostly because of the difference in casing flex across the tire and the inside edge creating more drag, but I'd better go back and browse a few books. This is also why it might be good to have a touch of toe out on a car with a lot of negative camber.

As far as caster, my first guess would be due to the difference in trail, and therefore a difference in self-centering force from side to side. Plus there is likely a camber force involved, and the scrub may not be perfectly centered in the tread.
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Old 10-14-2018, 01:52 PM   #4
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Originally Posted by uofime View Post
I wonder if the camber tendency is truly more positive or just larger contact patch. Eg if you had plus one on one side and negative one on the other if it still pulled towards the plus.

I’m suspecting the pull related to the greater rolling resistance of the larger patch.



I think of caster as being a straightening force so if you have less on one side that side will wander more though I’m not sure that would make it pull that direction


Uofime, from what I’ve seen published, if you had a car set up with +1.0 on one side and -1.0 camber on the other, it would pull even *harder* towards the +1.0 side than if the other side was at 0, leading me to believe that it has to do with the the direction that the tire is leaning (if both tires are leaning the same direction, the resulting pull is the most exaggerated).

Also, your point on the caster is exactly what I was thinking, and where I got confused — caster brings tires back on-center... this does not intuitively state what causes it to pull ie. when the tires are *already* centered and the car is tracking straight down the road, the centering force is satisfied, right? So why is it causing a pull?...
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Old 10-14-2018, 03:20 PM   #5
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A cambered tire creates a thrust force in the direction it's leaned. I think mostly because of the difference in casing flex across the tire and the inside edge creating more drag, but I'd better go back and browse a few books. This is also why it might be good to have a touch of toe out on a car with a lot of negative camber.

As far as caster, my first guess would be due to the difference in trail, and therefore a difference in self-centering force from side to side. Plus there is likely a camber force involved, and the scrub may not be perfectly centered in the tread.


I like what you said about the caster. “Difference in trail”, I guess, meaning that because one tire has less caster, it is trailing behind the other, and even though they’re both pointed the same direction (hypothetically), it has the effect of turning because the axis between the two tires is not perfectly perpendicular to the direction of travel? “There is likely a camber force involved” is this meaning that, with a camber involved, caster is still having an effect on the tire even when it is pointed straight ahead? I can see that in my mind, just want to make sure I’m on track.

Also, what reading resources are out there that you are referring to? I wouldn’t mind doing some reading on this stuff.
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Old 10-14-2018, 04:16 PM   #6
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Went and brushed up a little. Race car vehicle dynamics is the first book I checked.


There are two types of trail occurring with a rolling tire. First, the mechanical trail- this is caused by the caster angle, and the point the tire pivots about is ahead of the contact patch. Here is where most of the self-centering force comes from. If you have different caster from left to right, the centering forces are different. In an ideal situation, this wouldn't matter.

The other kind of trail is pneumatic trail. This comes from deflection of the tire and tread. The contact patch will lag behind the tire slightly while rolling along the ground. So even if you had zero caster, there would still be a slight self centering force.

yeah not quite
Well, those forces, and the forces between the tire and the road, act through the center of the contact patch. But is the center of the contact patch aligned perfectly with the steering axis? With stock offset wheels, it's pretty close, but there will still be a slight difference that will cause the tire to want to steer a bit. So when you have a difference in caster, these don't cancel out, and you get a pull. Probably.

As far as camber thrust, the book says it is likely caused by distortion of the tread pattern or possibly sidewall effects, and also the shape of the contact patch, but that it is not completely understood. Either way, you lean a tire over and it will want to go that way. And adding camber can make a pull from cross-camber worse, because the center of your contact patch will be offset slightly from the center of the tread.

Last edited by jamal; 10-15-2018 at 04:49 PM.
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Old 10-15-2018, 01:37 AM   #7
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Went and brushed up a little. Race car vehicle dynamics is the first book I checked.


There are two types of trail occurring with a rolling tire. First, the mechanical trail- this is caused by the caster angle, and the point the tire pivots about is ahead of the contact patch. Here is where most of the self-centering force comes from. If you have different caster from left to right, the centering forces are different. In an ideal situation, this wouldn't matter.

The other kind of trail is pneumatic trail. This comes from deflection of the tire and tread. The contact patch will lag behind the tire slightly while rolling along the ground. So even if you had zero caster, there would still be a slight self centering force.

Ok so now you might be thinking "if I'm driving in a straight line, what does a difference in self-centering forces matter?"

Well, those forces, and the forces between the tire and the road, act through the center of the contact patch. But is the center of the contact patch aligned perfectly with the steering axis? With stock offset wheels, it's pretty close, but there will still be a slight difference that will cause the tire to want to steer a bit. So when you have a difference in caster, these don't cancel out, and you get a pull. Probably.

As far as camber thrust, the book says it is likely caused by distortion of the tread pattern or possibly sidewall effects, and also the shape of the contact patch, but that it is not completely understood. Either way, you lean a tire over and it will want to go that way. And adding camber can make a pull from cross-camber worse, because the center of your contact patch will be offset slightly from the center of the tread.


That would be a nice book to have... little pricey though for my mileage.

Your explanation of the different types of trail are excellent. I also see how the center of the contact patch being off-center relative to the steering axis would make the tire tend to turn. I’m still having a bit of a hard time figuring out how caster factors in. Do you think you could explain that bit a little further?

I have come up with a theory related to what you stated about the camber and tread distortion/contact patch: if you take a tire and add camber, it’s shape goes from being ovate to more triangular. If you look at how the rolling resistance is distributed across the contact patch in a lateral fashion, it seems likely that the rolling resistance would build *with* the amount of area covered in a given cross-section of the contact patch. This would mean that, in the case of a negatively cambered tire, the inboard half of the tire would have more rolling resistance than the outboard side. The tire would have the tendency to turn in the direction of the most rolling resistance, which would be the inboard direction of the car. In the case of positive camber, the most rolling resistance would be found towards the outside of the tire, forcing the tire to turn in the outward direction of the center of the car. This is, as far as I can tell, consistent with observations. Let me know what you think!
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Old 10-15-2018, 02:48 AM   #8
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Looking at the side of the car, if you took the line from the top mount bearing to the ball joint, you have your caster angle. Extend that down and it will meet the ground in front of the tire contact patch. The distance between that intersection point and the contact patch is the trail. More caster means more trail, more trail means more leverage to keep the tire rolling straight.

On the front view, that angle between the top and the ball joint is steering axis inclination, and the distance from the intersection with the ground and the center of the contact patch is the scrub radius.


e: no not this part:
So the scrub will cause the steering force, and then the uneven cross camber will result in a different self centering force on each side, and then car wants to turn.

Last edited by jamal; 10-15-2018 at 04:47 PM.
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Old 10-15-2018, 03:50 PM   #9
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Looking at the side of the car, if you took the line from the top mount bearing to the ball joint, you have your caster angle. Extend that down and it will meet the ground in front of the tire contact patch. The distance between that intersection point and the contact patch is the trail. More caster means more trail, more trail means more leverage to keep the tire rolling straight.

On the front view, that angle between the top and the ball joint is steering axis inclination, and the distance from the intersection with the ground and the center of the contact patch is the scrub radius.

So the scrub will cause the steering force, and then the uneven cross camber will result in a different self centering force on each side, and then car wants to turn.
Jamal, I'm going to attempt to re-write what I interpreted to make sure I'm following along correctly...

Because of the scrub radius of a tire, it will have a pivoting moment. If the trail is different between the two sides of the suspension, that moment will have a different amount of force counteracting it, and thus will result in a net moment where one side will toe in or out more than the other side.

Now, the issue that I have is that the behavior of this 'moment' (whether it will cause the tire to toe in or toe out) is dependent on whether you have a positive or negative scrub radius and whether you have a FWD or RWD car and whether you are accelerating or decelerating. On the contrary, from what I know, caster pull is relatively independent of these factors, and so it seems unlikely that this would be the root cause of cross-caster pull.

Maybe I'm misinterpreting something. Let me know.
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Old 10-15-2018, 04:47 PM   #10
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No, that's right. I should have kept reading. And it's the same reason that camber thrust isn't from the shape/center of the contact patch- with a driving force instead of a drag/braking force the tire will want to go in the opposite direction.

So, anyhow, it's simpler than that. The caster and kingpin angle mean that when you steer, the outside tire goes downward slightly, and the inside tire goes up slightly. A difference in caster means that equilibrium point where the trail and the weight of the car hold the tire straight is different from side to side.
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Old 10-15-2018, 06:31 PM   #11
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No, that's right. I should have kept reading. And it's the same reason that camber thrust isn't from the shape/center of the contact patch- with a driving force instead of a drag/braking force the tire will want to go in the opposite direction.

So, anyhow, it's simpler than that. The caster and kingpin angle mean that when you steer, the outside tire goes downward slightly, and the inside tire goes up slightly. A difference in caster means that equilibrium point where the trail and the weight of the car hold the tire straight is different from side to side.


This one has me stumped... really can’t tell what’s going on here. So the weight of the car wants to pull the tire to be toe out? And the caster centering is counteracting that?
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Old 10-15-2018, 06:49 PM   #12
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This one has me stumped... really can’t tell what’s going on here. So the weight of the car wants to pull the tire to be toe out? And the caster centering is counteracting that?
A good way to visualize what he's talking about is to think about what happens to the ride height of each side when you turn the wheel. One side will raise up, and the other will dip down because the wheel is turning around an axis that isn't perpendicular to the ground. If the caster & SAI/KPI aren't equal side to side, the equilibrium point where both ride heights are equal won't correspond to "center" on the steering wheel.

Here's a video that shows what happens to the ride height (if it doesn't automatically queue up, skip forward to 13:41)


Last edited by T-37; 10-15-2018 at 07:47 PM.
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Old 10-15-2018, 08:42 PM   #13
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A good way to visualize what he's talking about is to think about what happens to the ride height of each side when you turn the wheel. One side will raise up, and the other will dip down because the wheel is turning around an axis that isn't perpendicular to the ground. If the caster & SAI/KPI aren't equal side to side, the equilibrium point where both ride heights are equal won't correspond to "center" on the steering wheel.

Here's a video that shows what happens to the ride height (if it doesn't automatically queue up, skip forward to 13:41)

Suspension Geometry - Part 1 (Camber, Toe, Caster, KPI, Scrub Radius) - YouTube


Ok, thank you for that video. That helped a lot. I think I got confused on his wording. I was interpreting the tire “going upward” and “going downward” parts backwards.

Maybe it’s just me, but that’s actually not a super simple concept for me to grasp. So because the “the equilibrium point where both ride heights are equal won't correspond to "center" on the steering wheel”, if you have a car with cross-caster or different SAIs, and you hold the steering wheel on-center, you’re actually lifting the vehicle slightly on one side, which is the force that drives the steering wheel off-center when you let it go?
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Old 10-15-2018, 11:08 PM   #14
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Wow really getting into the knotty gritty about individual dynamic movements effect on alignment.

I'm going throw a monkey wrench in here because there's one aspect that constantly nags at me and maybe I'm just over thinking it.

We have rotation on each wheel, and various aspects of the rotating mass aren't moving in the same direction. Think sitting on a chair that can spin an pedaling a unicycle and how when you lean it in various directions it spins you. Now, move this onto a moving object like a mountain bike. The leverage being created by that rotational movement has become really noticeable as bike wheels have grown larger over the years. I have to approach a corner very different to anticipate the bike wanting to briefly track in a different direction from where I want I want it to go. Now I understand that the smaller the wheel the less effect this has, but does the speed at which it rotates have an effect on our alignment during high speed cornering considering the additional weight and rpm at which a car is spinning its wheels? Could this be accentuated with caster and add to that feeling of pull toward the center? Or, like I'm guessing, I've read too much and am trying to find answers for things that aren't being discussed for a reason.
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Old 10-16-2018, 01:38 PM   #15
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Wow really getting into the knotty gritty about individual dynamic movements effect on alignment.

I'm going throw a monkey wrench in here because there's one aspect that constantly nags at me and maybe I'm just over thinking it.

We have rotation on each wheel, and various aspects of the rotating mass aren't moving in the same direction. Think sitting on a chair that can spin an pedaling a unicycle and how when you lean it in various directions it spins you. Now, move this onto a moving object like a mountain bike. The leverage being created by that rotational movement has become really noticeable as bike wheels have grown larger over the years. I have to approach a corner very different to anticipate the bike wanting to briefly track in a different direction from where I want I want it to go. Now I understand that the smaller the wheel the less effect this has, but does the speed at which it rotates have an effect on our alignment during high speed cornering considering the additional weight and rpm at which a car is spinning its wheels? Could this be accentuated with caster and add to that feeling of pull toward the center? Or, like I'm guessing, I've read too much and am trying to find answers for things that aren't being discussed for a reason.


Lol yeah you’re getting into territory that I ignore intentionally because I know thats where I’d get completely lost — effects of rotational intertia etc.
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Old 10-16-2018, 02:56 PM   #16
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Lol yeah you’re getting into territory that I ignore intentionally because I know thats where I’d get completely lost — effects of rotational intertia etc.
Yeah, I have no idea how that would all shake out. I'm sure there's some net moment about the center of mass from ALL of the rotating things, but that seems like a monumental amount of calculation.
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Old 10-17-2018, 10:09 AM   #17
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I hadn't thought about it until I went to bigger wheels on my mt bike and noticed certain changes. Got me thinking about how a 17" rim is now considered small and how going to larger wheels may influence alignment while rotating. Too much reading and too much thinking always equals too many questions.
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Old 10-17-2018, 12:52 PM   #18
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Well, no, wheel size and weight isn't really going to change the alignment. Heavier wheels and bigger tires does means more weight to accelerate in any direction, and more for the suspension to control, and that does make a difference.

Something that does happen, regardless of the weight of the wheels, is that the alignment changes from where it was sitting on the rack when you are driving. Mostly, the rubber parts like bushings and top mounts deflect under load. but things like wheel bearings, the wheel, the chassis, all flex a little too. And the tire.

So reducing all that compliance means your alignment stays closer to where you want. Spherical bearings are pretty good at that, but are expensive and harsh and generally don't hold up well to daily use but there are some ways to get that to work, like a dust boot and a thin layer of insulating rubber like with the 2011+ sti control arm bushing.

I don't have much stock rubber on my car, plus have added about as much caster as I can get without swapping to sti control arms, and I'm pretty happy with how it turns.
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Old 10-17-2018, 07:27 PM   #19
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Well, no, wheel size and weight isn't really going to change the alignment. Heavier wheels and bigger tires does means more weight to accelerate in any direction, and more for the suspension to control, and that does make a difference.

Something that does happen, regardless of the weight of the wheels, is that the alignment changes from where it was sitting on the rack when you are driving. Mostly, the rubber parts like bushings and top mounts deflect under load. but things like wheel bearings, the wheel, the chassis, all flex a little too. And the tire.

So reducing all that compliance means your alignment stays closer to where you want. Spherical bearings are pretty good at that, but are expensive and harsh and generally don't hold up well to daily use but there are some ways to get that to work, like a dust boot and a thin layer of insulating rubber like with the 2011+ sti control arm bushing.

I don't have much stock rubber on my car, plus have added about as much caster as I can get without swapping to sti control arms, and I'm pretty happy with how it turns.


I was referring to the load at angle not in relation to weight. Larger wheel while spinning takes more effort to change direction then a smaller one. Once off axis it wants to move around more. Feels like turbulence. So the thought I had was when adding caster and increasing camber is the rotational movement creating resistance that can change the alignment during a turn. We know load via compression can change toe and camber. This is load in a different form then traditional thought though still a form of load. But the more I read the more I'm thinking the wheels aren't big enough to be noticed even though they are spinning at a high speed. As someone else stated, there might be a finite amount of load but a tremendous amount of calculation would be needed to see if it even exists.

This is where I wish I was an engineer. I know the thoughts but not the terminology. Part of a functional alignment is being able to compensate for changes in geometry under load. I'm getting back into track days and want to start doing TA events in the next couple years. So I'm trying to learn all the aspects of my car, suspension geometry and proper alignment.
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