The Stovebolt Site Forums The Shop Area Driveline caster shims

Just got the front end aligned, everything OK except the caster should be about 1.50 deg to 2 deg. Mine is at .38 on one side and .27 on the other. I have the factor 2 deg shims installed. My springs still have some arch to them, So do I find 3 deg shims or do I go with 4 deg.? Running radials. If I go with 3 deg I will be close but no cigar. I think 4 deg will be to much. What is every one thoughts 53 3100 root

If you can find them, go with 3.5 degree. I have a Moog number for them, KB8908. That should put you right about in the middle of the range.

LONGBOX55 are you sure about the #,could not find a MOOG with that # root

Ooops! Looks like I added an extra letter, and gave you the wrong number. It's K8902. I really need to have my eyes checked!

Caster is a non wearing adjustment. Truck tracks straight? Truck does not wander?

If answers are Yes and Yes then No to the shims. For it is not broke then no need to fix it.

You can change to a 3 degree shims or ad 1 degree shims to it.

No, the truck wanders and does not track every well, so I am going to look for a 3 deg or 3 1/2 deg shims. Will see what I can find. Will keep everyone informed. Thanks for the input, now the looking and the work begins. LONGBOW thanks for the correct #. root

What size/make wheels and what size tires do you have?
What PSI in the tires?

I have chevy 15 X 4 3/4 5 hole they are from a later model car and I am running 235 x 75 radials with 35 psi. root

Can I install 2 different deg shims, LH and RH, to even up the caster split for alignment? root

Hi Root,
If you have .27 degrees on one side and .38 on the other side, you essentially have no caster and your truck should steer easily but wander around like crazy. If you find some 3.5 degree shims, install them in place of the 2 degree shims, you should get a bit under 2 degrees positive caster and that will help. But something has caused your caster to be out of whack. Let's see if we can help figure that out first. Are your rear tires taller than your front tires? Do you have all 8 spring leaves on front and rear? Has your rear end been lifted (or your front end lowered) for any reason?
Jon

The difference may not be worth worrying about. If you had independent front suspension, adjusting one side higher or lower than the other side is no problem and is frequently done to compensate for road crown or slope, but with a straight axle it isn't as easy...or at least I'll say I've never been able to do it with much accuracy. Your springs are likely to compensate while your axle remains rigid. Something is causing the small difference RH/LH, but I think the more cogent question is what caused your caster angle to go to nearly zero with 2 degree shims in there.
Jon

The short answer is no. The right way to correct a side to side caster difference is to bend or twist the axle, something most front end shops quit doing in the 1960's. If the driver's side has less caster, that's a good thing. Because roads aren't as flat as a pool table, 1/2 degree less positive caster helps the vehicle "climb" the drainage crown on a road and track straight. For instance a driver's side caster of +2 1/2 degrees and a passenger side reading of +3 would be just about ideal.
Jerry

As I understand it (someone correct me if I'm wrong), as springs get old and de-arch you loose caster. It also puts your drag link out of level. Now your truck is going to handle bad, and terrible on bumps. Now, you could put more shims in to correct the caster angle, but that doesn't fix the drag link problem. The right fix would be to start with new front springs. I will admit that I'm running 4 degree shims to correct the 1 degree of caster I started with.

I may be wrong about the whole 'de-arching' thing, but that's how it was explained to me and that would explain why so many of these old trucks don't have the caster angle they should.

You're correct. First the spring/caster issue:
If you look at the springs, you'll see the front axle mounts further toward the front and not in the center of the spring. As the spring relaxes, natural tendency of the way your axle is mounted is that it wants to relax in the center and not further up toward the front...that is to say it is going to relax about the rear point where the axle is connected to it. If the axle was mounted mid-spring this wouldn't be such a problem. As this happens, the axle flattens out and may even tilt forward when the spring goes upward (when a bump is hit) and caster angle is lost in the process. If the springs get really weak, the axle may tilt forward all the time.

As for the drag link, it is probably good to understand that even under the best of circumstances it is fundamentally not a great design. Think of it as the radius of a circle with the center point being the spot where it connects to the pitman arm (since this is the spot which won't go up and down). As the springs go up and down, you can easily appreciate the end of the drag link connected to your third arm will be pulling it in and pushing it back and pulling it in again as the end of the drag link travels through its little arc. It would have fewer problems if it was a foot longer in my opinion--since the end travel would be minimized the larger the circumference of our imaginary circle would be. As your springs weaken and the axle is essentially pushed upward, the tip of the drag link is pulled backward. The further up the axle goes, the further from the center point your sector gear will be in order to keep your truck going straight. This isn't a good thing at all. The design of your steering box is such that the center point should be the tightest point with points to the right and left of that becoming looser. If the front springs are very weak, your drag link will be pulling and pushing and pulling again as the springs go up/down. And this will cause your wheels to first go left, then straight, then left again when you hit a bump...and it doesn't take much of an irregularity in the road to cause your springs to go up/down.

From my experience, the worst of these factors for wandering is caster angle. A caster lower than the truck was designed to have will always cause wandering. A caster angle of close to zero can cause dangerous wandering. You can solve the worn spring caster angle problem with shims, but about the only way you can keep your sector gear in the middle is to build an adjustable drag link.
Jon

One other thing I'll mention...as for parallel drag link and all:
The further up we go into our imaginary arc the more effect axle movement has on the movement of the third arm. If the drag link is in the center of that circle (a parallel arm), then up/down movement will cause the least amount of pull/push/pull action. If we move the radius upward or downward (but upward always in the case of worn springs), the movement of the axle will then cause an increased pull/push/pull action due to the fact the drag link end is now moving through a more severe arc at a different angle. Man, I hope that makes sense. Reading it again I'm not so certain, but moving the drag link upward will create a more dramatic movement to the point where any given upward drag link movement may cause a greater measurement of third arm swing.
Jon

Is there excess play in the steering gear box or steering linkage? Jeffrey

They need to be taken care of before doing alignments.

In addition to that, the factory tie rods and drag link ends are adjustable for preload. Both of those also need to be in spec not only to eliminate wander, but also to allow for an accurate toe setting.