Sweptline.ORG

I'm totally confused after reading about this spring spacer. My 64 D100 without power steering and my 70 D100 with factory power steering have this spacer. The thick part of the wedge is towards the front on both. This SIDs axle with CPP steering has the thick towards the rear. Can some explain if this is correct and why. I'm wondering if the one on the SIDS is this way for the CPP steering? 1st 70 with PS, 2nd SIDs with CPP and 3rd 64. Craig

There are a lot of things that change castor. Maybe the rear bumper is dragging on the SID's truck and they stuck the wedge in the other way to reduce castor and loosen up the steering. It is all about how it steers and how you want it to, and sometimes a little compromise...

Fat side forward is correct. Rearward will make steering squirrley.

The direction of the caster shim is not always in one direction or always in the other. The direction or orientation of the shim is completely dependent on the caster angle that you're starting with and what you (or the factory) are attempting to achieve or correct.

My 65' Utiline was literally @ zero caster from the factory. As a result, when I added caster shims, I have the thick section to the rear to achieve the correct 4 degree back angle (top of the spindle rotates backward toward the cab). Many people get confused on this since they think if they add the thick section at the front, that the springs will conform to the axle pad. In reality, what happens is the opposite in that the axle rotates according to the spring position with the new shim plate angle attached. In other words, the springs do not move. The axle moves (rotates) relative to the springs. You can see this when you add the plate and begin tightening the U-bolts.

- EM

If the top of the spindle rotates backward, then the steering wheel will not return as easily to center after making a turn. Think of a shopping cart. The front wheels ALWAYS track to the center BEHIND the spindle because of the caster angle. That's why they're called "casters".
When your truck caster is tilted to the back, rather than forward, the wheels are trying to turn either right or left, rather than tracking straight ahead. Think of trying to push a shopping cart and trying to keep the front wheels in FRONT, rather than behind. That is pretty much impossible.
On the contrary, the more forward the caster is tilted, (opposite of what EM stated) the more force is applied to center the wheels in line with the rear wheels.
That's why the factory added the shims for the power steering system with the thick side forward, to override the power system and return the wheels forward after making a turn.
Is there a FSM spec that shows the caster tilted back at the top like that?

I'm not suggesting that the opposite that I went with is never used. In fact, positive caster is often used on off-road or for heavy hauling.

From one authority:

Positive Caster

If you’re going for this option, then you’ll be angling your steering point pivot from the front to the back of your vehicle, which can be achieved by using a suspension kit and coil-overs or shimming a solid axle. When the pivot axis is tilted backwards so that your top pivot is further forward than the rear pivot, your car’s wheels will re-align more quickly, while creating more drag and friction on the road; this makes it easy to maintain a straight line when driving in a particular lane.

Negative Caster

By contrast, a negative caster can involve forward tilting, so that the rear part of your pivot point is more pronounced; this means that you can often turn your wheel further from a straight ahead position, which can help with banking and corners. You might also be able to steer with less effort. However, negative caster can cause suspension issues and pulling motions to the side if it is too great, and can consequently be viewed as being risky.

Again, they are both used and work well but are used based on the end use of the vehicle in question. My road-based truck drives and turns much better with the positive caster I went with.

- EM

A few other points to consider:

- The geometry of a "caster" is obvious....the pivot point leads the wheel which is negative caster, ever noticed how the wheels wobble when you push them?

- Look at a bicycle or motorcycle. There is a great deal of positive camber in the fork "rake" or angle so they go straight and true. Turn the front wheel around and the stability is lost.

- Because they are real easy to see, look at any newer car under the hood and note the position of the Mcpherson struts or those with coil sprung suspensions. The spindles are all leaning toward the rear of the car. Again, Positive caster.

- The least caster I've seen spec'd was +1.75. So I've not yet had a car with negative caster even though my Utiline was near zero.

- EM

I drove the truck today and it seems ok but, I won't know until I get the power steering pump hooked up. I was advised to leave it as is. The mechanic that set it up says its correct. I'll drive it for awhile and see.

earlymopar wrote:The direction of the caster shim is not always in one direction or always in the other. The direction or orientation of the shim is completely dependent on the caster angle that you're starting with and what you (or the factory) are attempting to achieve or correct.

My 65' Utiline was literally @ zero caster from the factory. As a result, when I added caster shims, I have the thick section to the rear to achieve the correct 4 degree back angle (top of the spindle rotates backward toward the cab). Many people get confused on this since they think if they add the thick section at the front, that the springs will conform to the axle pad. In reality, what happens is the opposite in that the axle rotates according to the spring position with the new shim plate angle attached. In other words, the springs do not move. The axle moves (rotates) relative to the springs. You can see this when you add the plate and begin tightening the U-bolts.

- EM

This is correct. I had it backwards.

That's funny stuff. I installed mine thick side toward the cab and at cruising speed the steering is very lite. Similar to a bed full of crap or like I was pulling a trailer with a heavy tongue weight.

How was the steering changed at a stand still.

MadMC63 wrote:That's funny stuff. I installed mine thick side toward the cab and at cruising speed the steering is very lite. Similar to a bed full of crap or like I was pulling a trailer with a heavy tongue weight.

Yup, easy to lose control in bad conditions when the steering is setup like that. Turn the shim around and it will be the way the engineers at the factory designed it to be.
On a side note, when one changes something on their vehicle to something other than factory spec/eqpt, and there is an accident, if the investigators discover the change, that person may become hugely liable for damages, injuries and death. Steering and brake system alterations especially so. So, don't crash!

MadMC63 wrote:That's funny stuff. I installed mine thick side toward the cab and at cruising speed the steering is very lite. Similar to a bed full of crap or like I was pulling a trailer with a heavy tongue weight.

Shim direction is only one of the variables. How much caster you have is the real question. Did you check it before and after adding shims?

No! I didn't spend anytime checking the geometry. I placed the shims on my vehicle based on the donor vehicle as seen in the provided picture. I plan to get into all that late next year As far as the "stand still" at rest position with no forward or backward motion there is no effect as the shopping cart quite well illustrates.

For what it's worth, my '64 factory power steering truck had the shims mounted thicker side in the back, and had factory inspection marks that indicate it was that way originally. But regardless, measuring the angle is the only way to go, too many variables on a 50 year old vehicle.

I know this is an old thread.. but was researching what shims to use when putting the new leaf springs in. I guess the only way to do it is to get it all in and then measure when it is on the ground.

Adding this reference to the thread: http://www.hotrod.com/articles/0902sr-s ... ght-axles/

"Caster is the tilt of the king pins towards the front or rear of the car. Forward tilt is negative caster, rearward is positive. Most cars with solid front axles use positive caster because it prevents road wander and helps keep the wheels pointed straight ahead (drag race and Bonneville cars often use extreme caster for high-speed stability)."

"Although most early cars with straight axles used positive caster as with most alignment specifications there was a range of adjustment and the amount varied among the manufacturers. Ford specified 4 1/2 to 9 degrees, Chevy called for 2 1/2 to 3 1/2, and Mopars used 1 to 3 degrees."