Caster is the angle the C on the end of the axle has from straight up and down or 0°. Think of a shopping cart. The wheels are attached to the cart with the shaft offset about an inch. No matter what you do, when you push the cart the wheels are forced to swing around and be behind the mounting shaft. That is caster doing that. That’s why they are often called caster wheels at the hardware store. The C on each end of your axle acts like the attaching arm on the cart wheel. (actually a bit different in that cars using angles for caster) Rotate the top of C back creates positive caster. ( from top ball joint to bottom ball joint is the center of axle. That line moved forward or aft changes caster).
Move the top of the C forward and you decrease caster.
The more positive caster the more stable going straight and the easier the wheel returns to centered on its own after a turn ( just like the cart wheel does). But there is an upper limit to how much caster you should seek.
The more caster the harder it is to turn the steering wheel off center. With the old manual steering, too much caster made for hard to turn the wheel issues so Ford limited caster to offset that problem. But what you gain in ease of wheel turn you lose in wheel stability and wheel return to center. With power steering however, you could overcome having increased caster because of the “power” to turn the wheel. So best setup is increase caster with PS. Results in straight driving down the road, easier return to center after a turn, and ease to turn wheel.
So long explanation to answer your question. Yes, you can measure caster by measuring the degree the C is from 0°( straight up and down). I use an app on my phone that provides digital angle measurement. Just figure a way to make a flat spot across top to bottom of C and measure that angle.
Just so ya know, the different degreed rubber mounts rotate the axle (moving the C that attaches the wheel) by how they hold the axle in place ( the angle is created by the degrees built into the rubber). This usually is good enough to get the caster you need and is an easy fix. But, the higher your suspension lift, the worse the caster problem is (as the frame is lifted up the arms are also lifted up at the frame which rotates the axle forward decreasing caster. That’s why the big lifts lower the arms with lowering brackets at the frame mount. HOWEVER, just my opinion here, I don’t really like any of the “easy” caster increasing options ( degreed rubber, lowering mounting arms, or the offset ball joint nuts). None of these solutions take into account the piñon angle that the lift also effects. As the frame is lifted, piñon angle to drive shaft increases. If all you do is rotate axle aft with one of the easy caster fixes noted, then the piñon angle is bad. Normally not a terrible issue in that in 4x4 your going pretty slow so not a ton of stress on ujoints, but it’s not a great setup.
The degreed rubber is ok if your not lifting much but the best way to actually dial in the right caster is to reposition the C on each end of the axle. Cut, reposition, weld. That way you get the exact caster you want and can also position the piñon angle perfectly at the same time. Lots of ways to do that. James can provide details in that he has done a bunch. Anyway, maybe this long winded explanation of caster will help some folks.
As a side note, for all those folks with a 2.5” suspension lift or less, the degreed rubber works fine and the piñon angle is not gross. You’ll be fine. Although I know for sure others will disagree
