Caster setting relative to what?

MoparMap

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So I just acquired a set of the caster checking fixtures to be used with the DRB III and inclinometers, and one thing that came to mind is what exactly I want to use as a zero when measuring caster. The rake of the frame is going to affect caster angle relative to ground, but it would seem like the actual caster of the suspension would be relative to the frame rail as far as suspension geometry and how it affects stuff like camber gain on turning. Maybe I'm over thinking things a bit, but for anyone out there that actually measures caster, how are you measuring it? Do you zero your angle off the frame rail or do you just run it relative to the ground? I would think most alignment fixtures would be measuring it as the car sits, but it seems like this might be giving a value that's not quite maybe what it should be?
 

ogio855

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I'm also interested in someone with knowledge on this topic. With the rake in my chassis the natural "zero" angle relative to the chassis is about 0.9 degrees. If you set the 0.7 in relation to the ground this translates to 1.6 relative to the chassis. This causes the inner camlock to be notably more inbound than the rearward, if you assume they should be relatively equal, this would suggest the rear caster is to be set relative to the chassis plane. (The car is currently setup relative to the ground)
The caster position is also extremely sensitive to the adjustment of the camlocks. I've read a lot of people saying "leave the factory setting alone and adjust them at the same time," impossible, if you actually watch the gauge move it's astonishing how small of an adjustment affects the position.
Since the rear of the car doesn't turn, the caster position looks like it plays an effect on the bump steer settings more than anything, and this would be a pure geometry setting to make it move as intended. Which should then be relative to the chassis?
I could be mistaken, but what other point does the lower ball joint position play on the rear of the car? There is also a fairly large degree of difference that the A-arms sit in relation to each other, so the rear caster has quite a bit of change throughout the vertical travel path.
 
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MoparMap

MoparMap

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Yeah, rear caster is a very interesting topic. Generally speaking I would think they would want the car to be a neutral in bump steer as it can be, so it would seem odd that it might have caster for that reason to me. The back has shims for the inner tie rod mounts to control that, so I would have thought they would aim for as close to zero as they can get at least. The service manual actually has the targeted bump steer toe patterns and the rear is about as straight as you can get it:

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The other thing that came to mind for me was I wonder if they put in just a touch of caster on the back to better control the toe link loading or something. Positive caster will naturally provide some self centering force to the tires (the old shopping cart wheel example), so having just a touch of positive caster would try to keep the wheels pointing straight instead of pure zero which might let them wobble a bit left to right. They wouldn't actually really wobble I don't think, but it might sit there and apply an oscillating force on the toe link that would work it back and forth and could maybe prematurely wear it out? Having just a little auto center might keep the link loaded more constantly. This is all just theory in my head from what I know about suspensions though.

Really, when I think about it, camber is also going to be slightly affected by chassis rake. I have a little jig I use that just attaches to the wheels and you put a level on it to read camber, but whenever I do that I make sure I first set the jig perfectly vertical on the wheel before measuring the camber. However, the adjusters are actually going to adjust that relative to the line between the ball joints defined by the caster (at least theoretically if you adjust both cam bolts the exact same at the same time). I need to get out my old race car vehicle dynamics book and read over it again though. I'd say maybe all of this doesn't matter at all since the values are likely so small, but then again we're trying to set stuff to fractions of an angle on alignment and 64ths of an inch when we do toe. I use the comp coupe shims on my car as well instead of the cam bolts, so I can adjust stuff "exactly" at the same time. When I was setting up the caster on mine for the track this past time I would basically mirror the shims to get the caster right and keep the camber the same. So for example if I had a D shim and needed more caster I would put an E shim in the front and a C shim in the back on the same arm.
 

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