How to measure castor? (1 Viewer)

[wfd175]

I've don't recall seeing THIS aspect of castor/alignment covered before so here we go. When my OME was installed with CC the alignment report said I was at 3 deg. pos. It drove fine and still does. I had the alignment checked because of crazy tire wear recently and everything was fine, except the castor was now closer to 0. At the time I attributed it to sag.

Then tonight I had a kind of revelation while sitting in a meeting. How could my castor had become less positive if the springs were sagging a little bit?

I looked at some old pictures, and went out and snapped a picture. Accounting for tire wear a little bit, the profile views look the just about the same with respect to "amount of coil" visible on the front end. Even with tire wear, and the addition of slee sliders.

I had a thought cross my mind tonight about how to measure castor. I've seen the alignment ramp type lift at the shop, am I correct to assume that the ramps and table are level? I understand the concept of positive and negative castor. If castor is measured at the centerline of the axle, do they use the flat surface or the bolts on top of the knuckle as the centerline/level line of the axle. Is this surface perp to the driveshaft?

If all of my assumptions are true, then the higher the back of the truck is at the time of measurement, then the less positive and or more negative the castor reading would be.

Am I rationalizing this correctly?

I've read about alot of strange Castor readings in the past with different lifts. CC bushing are installed and folks Castor readings are all over the board. It seems like if my theory is how they measure castor is correct, then different weights on the same lift would vary drastically with respect to the castor reading. By the way, my rig does seem like it tracks slightly straighter with a little weight in the back.

PS I don't have an armored rear bumper.

 

[e9999]

well, on the machines I've seen - laser based, I imagine they just use the mirrors they attach to the wheels. So those measurements at least would not be influenced by a "level" or not knuckle surface. But the caster angle measurement will be influenced by any rake anyway.

 

[wfd175]

Are you saying that change in rake (stinkbug) may be attributing to my different readings. I wish I knew exactly how the measure castor. Someone mentioned a couple weeks ago that they somehow placed a leveling device on the top knuckle bolts to get a castor reading.

Also, if stickbug may create a less positive castor reading, then at some unknown limit, adding a spacer may bring increase towards the positive. By stinkbug elimination. If we could somehow get the factory rake, angle, stinkbug ratio, then there should be a factor which can be applied with respect to rear height, which increases with the heavier OME springs over stock, to accurately measure castor. Without this factor for correction, any castor measurement on a rig without stock springs in the rear and front would be at best just a guess because of the change in the ratio of front to rear height. ????

 

[e9999]

I don't actually know how they measure caster now, to be honest. I had somehow assumed they were wiggling the front tires and using the laser, but I don't know for sure...
No doubt older machines were mechanically-based, though.

 

[wfd175]

Remaining questions. Hopefully a mechanic will chime in.

How/where do they measure the castor? and since the OME 850/860 changes the stock rake noticeably with the rear unloaded without armor etc.., and with and ARB and Winch up front, do they do take the change/factor from OEM rake/stinkbug into account when measuring the castor during an alignnment?

In some cases, simply adding weight to the rear with the OME 850/860 and with other combinations of aftermarket lifts will then adjust the castor reading and possible improve handling?

 

[landtank]

they measure the caster on the front wheels. It's called a caster sweep as they turn the wheels from lock to lock and then from that calculate the caster. They don't care how level the truck is even though this will influence the readings.

 

[sleeoffroad]

The rake of the truck will have a slight effect since it will rotate the front axle forward. If anything, more rake will result in less caster (given the same truck with the same front suspension with an increase in rake. Whether that is measurable or not I am not sure.

 

[smug01]

Okay here is the engineer in me. I did some math and based on a wheelbase of 112 inches (from the FSM), here is what I calculate to be the influence of rake on caster. Positive rake is stinkbug, negative is rear sagging. Not huge, but it does have an effect.

Hope this helps!

-Craig

 

[Brunt Force Trauma]

This is completely opinion, and I may be nuts, but I thought I would throw this observation into the mix.. When I am driving up steep inclines (truck leaning back) the steering is a lot tighter on my truck. When I am going down steep inclines (truck is leaning forward) the steering feels a lot looser, and seems to want to wander more. Maybe this could be attributed to the rake of the truck changing since the weight is shifted?

 

[Tools R Us]

... It drove fine and still does.

This is what is important. I have worked on rigs that drive well with less caster and some that want more to be comfortable. Setup greatly affects it, type of springs, shocks, tires, etc. High speed stability is mainly provided by caster and toe, caster has a greater effect in turns and toe when going straight.

I had the alignment checked because of crazy tire wear recently and everything was fine, except the castor was now closer to 0. At the time I attributed it to sag.

When the rig is lowered (more weight, shorter springs, etc) caster increases. Most likely one or both of the "alignment techs" didn't properly operate the machine?

... I wish I knew exactly how the measure castor. Someone mentioned a couple weeks ago that they somehow placed a leveling device on the top knuckle bolts to get a castor reading. ...

On straight axle rigs, you can get an idea of caster by measuring knuckle or in the case of the '80, arm bracket angle and comparing it to floor angle, if not level. Properly interpreting the result requires experience and is best used to estimate change from a known setting.

 

[skibob]

Caster is measured as described above, put the alignment heads on, turn the wheel one way, set for zero, turn the wheel the other way, measure.
Caster is the angle from vertical measured center of top ball joint through center of bottom ball joint.
Caster will definitely effect steering wander and driveability.
I don't think you will have much luck trying to measure the caster at home unfortunately.
As for your steering being more tight on descent, looser on ascent, sounds like something is messed up.
Best bet will be the bushings at your control arm bushings are loose or damaged causing your axle to twist depending on up/down.
You can probably get a BIG pry bar in the gap between control arm and axle, pry around and see what moves. Replace anything that is questionable and things should get better.

 

[Brunt Force Trauma]

Would castor affect bump-steer? Mine is pretty bad.

 

[Tools R Us]

Would castor affect bump-steer?

No.

Mine is pretty bad.

What are you experiencing? With a radius arm/panhard setup, bump steer is normally not a problem.

 

[Brunt Force Trauma]

No.



What are you experiencing? With a radius arm/panhard setup, bump steer is normally not a problem.

When I hit a rough spot in the road the front end jumps over what feels like a foot. I guess thats not exactly bumpsteer. But its annoying.

 

[Pin_Head]

When I hit a rough spot in the road the front end jumps over what feels like a foot. .

This could be normal if the panhard angle is significantly steeper than horizontal. Bump steer is when your truck steers into the next lane when you hit a bump. This is not normally a problem for 80s unless the angle of the panhard and the drag link are not parallel to each other.

 

[Brunt Force Trauma]

Pretty high. So why does everyone use panhard extensions instead of some sort of drop-bracket for the panhard? Wouldnt a bracket work better?

 

[Brunt Force Trauma]

Ok I thought about it and answered my own question. If I used a drop bracket in the front, I would have to drop the drag link as well to avoid crazy bump steer.

 

[1FZj80]

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative.

A motorcycle/bicycle front fork is tilted backwards and has positive caster. When the wheel is turned the front of the motorcycle is physically lifted, therefore gravity tends to keep the wheel rolling straight. The same thing happens on a car/truck/80. Positive caster straightens the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability.

The trunion bearings create this angle. However, when your truck is lifted the axel is rotated around the arc created by the front control arms. As the arms are pushed down and the truck is lifted up, the angle is reduced.

You can measure this at home, but you need a caster jig to do it. By turning the wheels through the steering arc, the degrees of caster positive or negative can be measured. The angles are very small and the only way to measure is to look at the arc created when the wheel is turned to the right and then to the left.

 

[DiscoveredLC]

Bringing back an old thread.

I'm running 2.5" mediums on the truck with 30mm Slee spacers up front. I had a minor death wobble with the 33" BFG at's I had on not too long ago. Since then I have done a front end rebuild, and installed 315 MTR's. I still get a slight death wobble at the same speed (45-55mph) nothing crazy, just the steering wheel wiggle back and forth a bit. Its pretty annoying and I want to figure it out before it ruins the tires and causes a "loose" spot on the trunion bearings from wiggling back and forth so much (The old trunions felt loose in the straight wheel position)

Right now I have the yellow 2* bushings in, and I think this is not enough. Im wondering if I should just switch to blue 3* bushings and hope that helps, or take it to a shop and figure out what the castor is.

I was thinking about making my own castor plates since they seem pretty straight forward. Here's how I might be able to do it, but Im not an expert so tell me if this would work or not.

Take the truck to a shop and get my castor readings. Then go home, put the front end up on jack stands so its nice and stable. Take an angle reading from the top of the trunion bolts ( I have a very precise angle finder i use for tube bending.)

Say the shop tells me I have 2* of castor, I take a reading on the trunion bolts, and then unbolt the front bolt on the axle, rotate the axle back until it increases that reading by 1* (inherently putting me at 3* I hope) and then take measurements from there to make my castor plates.

I have 2 sets of from control arms. One set with stock bushings, and the current set on the truck with 2* yellow bushings. I could use the set with the factory bushings to make my castor plates.

Sound ok, or totally ridiculous?

 

[Tools R Us]

...
Sound ok, or totally ridiculous?

That will work. Once you have a starting point, relative angle change is accurate. IMHO there really isn't a need for the shop caster sweep. With the stock arms there is only so much caster adjustment available before the tie rod interferes/contacts the arms at flex and you are likely to need all of it.

I make washers from thin 1.5" strap, move the holes in the axle brackets where you want them, install and torque the bolts with the new washers. Now drive the rig, play with the toe, find, confirm the happy spot for your preference and then tack weld the washers in place.

I usually move the rear bolt holes up first. They can't move much or when the bushing deforms, the arm hits the bracket, reducing flex. Then move the fronts, using a bolt and nut (~.5") as a jack tool works well to mock it up make/hold them even while cutting the holes.