Caster Question: Relative to chassis or ground? (1 Viewer)

[K Dub]

This is probably a silly question to many of you. If this has been covered a million times already, I apologize. I've read through piles of lift threads here and haven't run across this yet, but I'll admit it may have slipped through the cracks or gotten lost in information overload. Maybe this is simply "common knowledge", but it's a mystery to me.

Is caster measured relative to the chassis or the ground? Meaning, does stinkbug exacerbate caster problems (relative to ground) or is the measurement relative to the frame? If the lift is level, then this would be the same thing, but I know I'm not the only one riding high in the back.

My truck has 850J/863J springs on "L" shocks with yellow caster bushings. It rides nice, especially compared to my previous rig, so I believe that the bushings may have done the trick for me. However, I'd like to level it out. I measure 3" lift front and 5.5" rear with an ARB+winch up front and unloaded rear. I've considered spring spacers up front, but don't want to exceed the limits for my bushings and ruin the handling. Honestly, I'm inclined to swap in 860 rear springs to drop the back down, but this question has been bothering me.

What do you say, Mud? Will lowering my rear springs effect my caster angle for the better or not at all?

 

[Beej]

Caster, as its typically discussed, is manipulated by rotation of the front axle. It shows up on your print out when you get alignment. Google is your friend here.

Im not super sure im understanding you right but the levelness of the truck has nothing to do with it. When you lift, you are also rotating the axle because the radius arm mounts are stationary.

 

[K Dub]

Caster, as its typically discussed, is manipulated by rotation of the front axle. Google is your friend here.

Im not super sure im understanding you right but the levelness of the truck has nothing to do with it. When you lift, you are also rotating the axle because the radius arms are stationary so they change the angles.

Thanks for the reply. Picture this; if I pick up the rear of the car with a forklift, the front axle will be rotating relative to the ground but not the body. Is that increased caster or not?

 

[Beej]

I feel you. Assuming the truck is weightless (so weight will not shift when you lift it with your forklift), this will not affect caster in any way.

Wait, no. Youre are thinking about this completely wrong

 

[Beej]

If you take a piece of paper; draw two horizontal parallel lines like 3" apart.

The top line is the bottom of the truck. The bottom line in the ground.

Draw another line from the middle of the top line to the far left side of the bottom line. This 3rd line is the radius arm.

Your radius arm cannot increase or decrease in length and the attachment point on the truck line cannot move. The radius line must always be in contact with the truck line and ground line.

If you increase the distance from the truck line to the ground what has to happen to the radius line? It move to the right on the ground line.

 

[Beej]

thats what happens when you lift the truck so the axle is attached to that radius line. As the radius line moves to the right on the ground line, this rotates the axle (technically it rotates the housing but whatever). This rotation affects your caster.

 

[K Dub]

I feel you. Assuming the truck is weightless (so weight will not shift when you lift it with your forklift), this will not affect caster in any way.

Wait, no. Your are thinking about this completely wrong

I'm sure that I'm wrong, that's the only thing I'm ever right about.

I get how the front lift effects caster. I see how the bushings offset to compensate and rotate the axle relative to the radius arms, that all makes sense. Lastly, I don't have a caster problem, as of now. I know that adding front spacers will decrease caster, possibly outside the tolerances of the bushings. I guess I'll just drop the rear knowing that it will either have no effect, or a positive effect.

 

[Beej]

Ok, let me give this one more shot. You have to actually draw this line. Dont try and follow the text.

Remember the radius line is attached at a fixed point on the truck line so if you raise the rear of the truck line or the front of the truck line, they will both affect the radius line but since in real world, the radius arms are a little bit closer to the front axle than the rear, front lift affects caster more than rear lift. 'Mud talks about lift as equal front and rear. And thats the right way to discuss. However, caster is affected differently by front and rear lift.

If you want to throw all this out or you dont like drawing lines. More lift = less caster.

 

[Cruzilla]

Think shopping cart front wheels..

 

[BILT4ME]

Caster is measured from a vertical line through the center of the front axle.
The front wheels turn on an axis that is tipped FORWARD of the vertical line at the top, and rearward at the bottom. This is "caster."

As you raise the rear of the truck, it will cause the caster to increase. It will also affect your toe slightly, as it is rotating the horizontal axis.

If you LOWER the rear of the truck, you will DECREASE caster because the rear of the front control arms will lower slightly. Lowering the rear will NOT affect the caster as much as lowering the front because they are working on different radius points.

 

[Tools R Us]

...
Is caster measured relative to the chassis or the ground? ...

Relative to and measured from the ground, well, measured off of a flat/level surface.

Meaning, does stinkbug exacerbate caster problems ...

Yes, lifting the rear, puts the frame at an angle, lifting the rear of the front arms, reducing the caster effect relative to the ground.

 

[K Dub]

I'm a visual person, and I don't words two good. Let's try this again with pictures.

Assuming a level lift and 0 degrees caster for the sake of simplicity on this first image.

Now when I lift the rear, the diff stays the same relative to the chassis and radius arms, etc. but the angle to the ground has changed. Is this still 0deg caster?

I hope this helps explain my question better. I know what I want to do, but this question has stuck in my craw and I want to figure it out for my own edification.

Thanks for your help and patience.

ETA, gotta refresh the page before I post again. Now to read the replies from the last hour. SMH.

Looks like my suspicion was correct, and that caster is relative to the ground or theoretical level. So the lower picture has non-0 caster due to it being a stinkbuggin J lift with no weight in the back despite still being perpendicular to the chassis.

 

[Beej]

Now when I lift the rear, the diff stays the same relative to the radius arms and nothing else.

There.

Also, awesome work with the pics.

 

[Tools R Us]

Caster is measured from a vertical line through the center of the front axle.
The front wheels turn on an axis that is tipped FORWARD of the vertical line at the top, and rearward at the bottom. This is "caster."
...

That would depend on the goal, positive or negative caster, most Cruisers prefer positive.

 

[Beej]

That would depend on the goal, positive or negative caster, most Cruisers prefer positive.
View attachment 1823098

Neutral is 0*

 

[K Dub]

There.

Also, awesome work with the pics.

Thanks. I do this stuff for a living after all.

That would depend on the goal, positive or negative caster, most Cruisers prefer positive.
View attachment 1823098

Thus a stinkbug results in increased negative caster (your image is flopped from mine) and is not ideal. When I first got the car in August, I wanted to fix the stance quickly. I figured; pop in some spacers and I'm all set. Reading about caster plates vs. bushings and the trouble of 4"+ lifts has me going the other way. I'm sticking with 33" tires, so I don't need the height, I think I'll be happier with softer and lower rear springs and leave my front end alone for now.

Thank you all so much. I love the technical level of discourse on this forum.

 

[landtank]

if you have two identical trucks with the exception that one truck has more stink bug. the truck with more stink bug will measure having less caster.

hope this helps anyone still confused.

 

[Delta VS]

Is caster measured relative to the chassis or the ground?

Short answer = yes

Long answer (I think covered above), it is an absolute relation between what’s holding the wheel in place and the ground. Moving the chassis CAN (and in almost all situations WILL) have an effect on that relation. Just remember that what is holding the wheel in place is NOT the chassis.