How do you measure Caster Angle
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Fleet Mechanic
Joined: Jun 2004
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From: Minnesota
How do you measure Caster Angle
How do you measure the caster angle at home? I put a dropped front axle in my truck and removed a few leafs. Put on radial tires ETC. I removed the shims that were under the factory axle until I can drive the truck and see how it handles. I would like to know what I am at currently.
Lead Driver
Joined: Oct 1999
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From: Kalispell, MT
The same magnetic angle indicator that you use on the diff and trans will do the job on the caster. You're measuring degrees of variation from plumb (verticle).
If you look at your spindle from the side, imagine a line thru the center of the king pin. Positive caster angle is when it leans back toward the cab. The bottom of the king pin moves toward the bumper. Positive caster allows the wheels to keep pointing straight ahead when you take your hands off the wheel. In extreme situations, it makes it hard to steer around corners.
Negative caster is when the top of the kingpin is toward the bumper. Negative or no caster makes it easier to steer, but the truck will wander around the road requiring intense concentration to keep it going in a straight line.
Once you find what you have without the wedges, get the correct wedge angle to set to factory specs. Most rigs come with a slight positive caster.
If you look at your spindle from the side, imagine a line thru the center of the king pin. Positive caster angle is when it leans back toward the cab. The bottom of the king pin moves toward the bumper. Positive caster allows the wheels to keep pointing straight ahead when you take your hands off the wheel. In extreme situations, it makes it hard to steer around corners.
Negative caster is when the top of the kingpin is toward the bumper. Negative or no caster makes it easier to steer, but the truck will wander around the road requiring intense concentration to keep it going in a straight line.
Once you find what you have without the wedges, get the correct wedge angle to set to factory specs. Most rigs come with a slight positive caster.
Lead Driver
Joined: Apr 2002
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From: Littleton, New Hampshire
JC Whitney has a relatively inexpensive alignment tool for around 60 bucks that you can use to set caster and camber. Since you have a straight axle you can't adjust camber at home, but you can adjust caster with shims under the axle pads. I've use mine numerous times to set caster and camber at home on different projects. 
Vern
Vern
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Fleet Mechanic
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From: Minnesota
So if I understand correctly caster is not in relation to the vehicle? That is what I was wondering. So if a truck was perfectly level from front to back and you dropped the rear end 6" you would have positive caster?
Lead Driver
Joined: Apr 2002
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From: Littleton, New Hampshire
Originally Posted by Christopher2
So if I understand correctly caster is not in relation to the vehicle? That is what I was wondering. So if a truck was perfectly level from front to back and you dropped the rear end 6" you would have positive caster?
Vern
Senior User
Joined: Jan 2004
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From: Albany NY
What's the difference between the king pin inclination and caster? I would think they would be the same thing but the shop manual shows totally different specs. Is the king pin inclination maybe measured to the axle perches whereas the caster is measured to the vehicle?
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Fleet Mechanic
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From: Minnesota
Yes it is not a relative thing it is an abosulte. In other words what it is it is. That's good I have a magnetic angle finder already.
Originally Posted by GreatNorthWoods
That would depend on what the caster angle was before you dropped the rear end, but generally speaking that is correct. If caster was +or+ zero degrees and you dropped the rear six inches, the caster angle would now read positive. How much positive is impossible to say and would depend on the length of the truck. The shorter the wheelbase the more positive the angle would be. Does that make sense?
Vern
Vern
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Cargo Master
Joined: Jan 2005
Posts: 2,497
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From: Tucson AZ
the bottom line here 4 to 6 degrees when the truck is sitting level, be sure to check the toe-in while you at it, protect those new radials
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