I'm not saying 6 degrees is wrong I just wanted to know the reason behind so much and didn't want to end up adding to large of a shim. My thinking is if the factory spec is 2 degrees and I add a 6 degree shim that makes it 8 degrees total one way and shim flipped 4 degrees the other way. Since I'm pretty sure these trucks have some caster from the factory I just wanted to be sure before I order my shims.
My 59' has shims from the factory, not sure what degree they are. I don't believe the axle it's self has any added caster. I haven't found anything in the assembly manual that states having caster made into the axle.
Come,Bleed or Blister somethings got to give!!! Q:"What yah runnin" A:"3/8 drive Black and Decker aquarium pump motor" (the movie HOTROD)
59' Apache 31, long term project (30 years and counting)
With no shims you have zero caster...and you really don't want that. The factory (non-power steering obviously) spec can be found in the manual but ought to be a max of 2 degrees (+/- a small fraction). If you don't have enough caster, your truck will steer much more easily but will wander all over the place and it will not straighten after a turn very well at all. Some people who think taller back tires are the answer and who never change their caster to adjust for this create trucks which are unsafe. Raising the rear lowers the caster angle every doggone time. The same thing will happen if you use smaller tires on the front. If you increase the caster to 4 degrees with manual steering, your truck will drive straighter but will be harder to turn...especially in town. The reason why is caster tilts your axle back (or front if your vehicle was built with negative caster) and so to turn it, you literally have to lift your truck up. If you want a nice visual example of caster, just look at a motorcycle from the side. 6 degrees might work just fine with power steering, but very few people anywhere will be strong enough to steer a vehicle as front heavy as a truck like this with that much caster with manual steering. And the greater the caster degree, the more wear on your kingpins, kingpin bushings, bearings, etc. That part is a given. There is a good balance which must be established. Worn out springs (leaf springs of course in this case) can also cause caster angle changes if the arch has been bastardized with time. And it can change with age, rust, use, etc. So just sticking some shims in there is only a start. Get the truck to a front end man who has some gray hair and knows what he's doing. Remember the engineers at GM didn't make it this way because they were stupid. They did this because they saw it as the best balance...and they probably never thought anyone would be driving these trucks at freeway speeds.
Also. VERY IMPORTANT. You have to be totally cognizant that there is a pin on your spring which mates with a hole in your axle. That pin is the end of a bolt which holds your spring leaves together. It is imperative that pin mates very well and very securely with that hole. If you don't pay attention to this and if your shims are not correct, they can raise the pin to the point where it just barely or worse doesn't connect with that hole. The best of all worlds is the new shims you use have the same diameter hole as your spring pin's bolt, are held in place (screwed down to the springs) with new spring pin bolts and therefore the bottom of the shim has 100% of that pin available to fit into the hole in the axle. NOT ALL SHIMS ARE MADE LIKE THIS. Some are made with a hole large enough for your pin to slip through (meaning you don't have to change pins when you change shims), and this obviously lifts the pin off the axle more as you increase shim thickness and at some point you can have wall to wall trouble. If this is in any way confusing to you, STOP and study it and ask for help.
Ask anyone who owned a 2008 Ford Escape or Mercury Mariner or Mazda Tribute. Those were prone to fail at any time...literally any time. Driving up in your driveway or going 70 in a curve. Big recall...as I remember over 1,000,000 vehicles. I'd have to be convinced the bugs had been corrected on those.
1) Do not stack additional shims on top of existing shims. Buy 6 degree shims. 2) Heed Jon G's warning about securing the new shims in place. Since they are tapered, they will walk out of position if not secured. 3) Using 6 degree shims in the truck with power steering is not equal to trying to out-think GM engineers. Adding power steering to these trucks already changes the engineering. With two degrees of castor with power steering, the wheels will not come back to the straight ahead position after coming out of a turn like a stock manual steering box does. Six degree shims will correct that, as well as give you much better feel of the road. The truck will naturally tend to go in the straight ahead position while driving.
Please take a look at this. It is a shim I removed from a wrecked truck about 40 years ago. There were 2 but one was lost along the way. Obviously it was homemade. It is tapered (which you can't really tell from this image) and was installed under the stock shim. There was no contact between the spring pin and the axle with this shim installed. Whether axle movement caused the wreck or contributed to it is an egg I couldn't unscramble because there was very little of the truck left, but in order to always remind myself of this situation, this has been in one of my tool boxes since then. I'm fairly certain the driver didn't survive. Or if he did it was a miracle.