Caster shim location

[dubie]

After looking at another truck build I noticed that I may have my caster shim in the wrong spot.On his build, the shim was between the axle and the leaf spring. I placed mine between the leaf and the U bolts. Is this going to cause me a headache down the road?

[Katrina/10]

The shim would have no effect in the location you put it.

[mr48chev]

Originally Posted by Katrina/10

The shim would have no effect in the location you put it.

Exactly! it has to be between the axle and the spring to work. It's just a weird looking spacer when you put it on top like that.

A better explanation is that it has to be between the spring and the axle so it can give the axle what ever degrees of caster it is made for. On top it won't tilt the axle back.

[dubie]

it hit me like a brick wall when I saw the way it was done in another build. I know what I'll be doing tonight
Thanks for chiming in folks

[roger55]

So, to increase caster (go positive) does the thick side of the shim go to the front of the truck or towards the rear?

[Dan in Pasadena]

You INCREASE negative caster by tilting the top of the axle toward the rear of the truck.

I always remembered "caster" by thinking of casting a fishing rod. Your arm tilts forward and backward from the elbow.

When the axle is tilted back, the wheel wants to return to center more strongly than with less caster - thus requiring less arm effort on the driver's part to steer the truck. Hope that helped some.

[roger55]

Thanks Dan. You answered in a different way but I do take that as putting the thick end of the shim to the rear will tilt the top of the axle back and increase caster. Right?

[focusedontheprize]

Originally Posted by roger55

Thanks Dan. You answered in a different way but I do take that as putting the thick end of the shim to the rear will tilt the top of the axle back and increase caster. Right?

This is correct. Look at dubie's second photo in the 1st post - that thick end is facing toward the rear of the truck.

[dubie]

anytime you raise or lower a truck you should always adjust your caster.

[Dan in Pasadena]

Originally Posted by roger55

Thanks Dan. You answered in a different way but I do take that as putting the thick end of the shim to the rear will tilt the top of the axle back and increase caster. Right?

You're welcome. But putting the thick end of the shim toward the rear of the truck tilts the top of the axle forward - NOT what you want. Got it?
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[Russell Ashley]

Originally Posted by Dan in Pasadena

You're welcome. But putting the thick end of the shim toward the rear of the truck tilts the top of the axle forward - NOT what you want. Got it?
Posted via Mobile Device

Dan, I think you have it backward. You definitely want the thickest end of the caster shim toward the rear. This rotates the axle so that the tire contact patch is behind where an imaginary line through the king pin would intersect the pavement. This creates positive caster which adds to steering effort, but is what you want when you add power steering.

[roger55]

Thanks Russell.
That does make it clear for me.

Now, what is the best way to determine what shim I need?

Here's a little info on what I have:
The truck is a '57 and I am going to use the springs that it came with. They are 7 leaf and the height looked stock and just right to me. I've got a new CPP 400 power steering box that I'm going to install.

So from what I've read I will want about 5 degrees of positive caster. But, I think I need to know what the caster is without a shim for a starting point. Right?

[dubie]

you only require these caster shims if the truck has been lowered from factory ride height by 3" or more. I believe at 3" of drop a 2 degree shim is needed. I dropped mine 6 inches so I used a 6 degree shim. I actually only needed 5 but they came in 2, 4 and 6 degree. If you are using used stock springs, you shouldn't need any shims

[roger55]

Originally Posted by dubie

you only require these caster shims if the truck has been lowered from factory ride height by 3" or more.

I'm not dropping the height at all but I am converting to power steering so I want about 5 degrees of positive caster.

[Katrina/10]

Originally Posted by dubie

you only require these caster shims if the truck has been lowered from factory ride height by 3" or more. I believe at 3" of drop a 2 degree shim is needed. I dropped mine 6 inches so I used a 6 degree shim. I actually only needed 5 but they came in 2, 4 and 6 degree. If you are using used stock springs, you shouldn't need any shims

Why are shims only required with a drop?

[dubie]

Everything I've ever been told or read about caster has to do with putting everything back in line once you drastically raise or lower a vehicle. The shim, as explained to me, assists your alignment in steering components allowing for more clearance on your moving parts.

When you lower the front end of your car, the axle beam is tipped forwards which reduces camber. This makes the steering easy in slow speed conditions (great for off-road!), but the steering gets really twitchy at speed. Since caster is not really adjustable, the only solution is to use shims.

[Katrina/10]

But if you lower the front end, you would lower the rear the same amount, so wouldn't the caster be unaffected?

[dubie]

But the rear is adjusted by pinion angle and you compensate for the drop by adjusting that when you weld on your leaf spring saddles
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[Stepside Jim]

Originally Posted by Katrina/10

But if you lower the front end, you would lower the rear the same amount, so wouldn't the caster be unaffected?

I see what you're thinking, I agree. If you drop the front, the frame is now at a down ward angle causing the front leaf springs to be tilted along with the frame.

Assuming you drop the rear the same distance as the front, the frame would be level again, just lower. Thus, the angle of the front springs would be the same as original, perhaps not needing the caster shim.

Although I don't have the answer, I can assume a benefit. Just as Dubie mentioned, the straight line high speed driving could still benefit with the shim.

Just a thought. Jim

[Dan in Pasadena]

Originally Posted by Russell Ashley

Dan, I think you have it backward. You definitely want the thickest end of the caster shim toward the rear. This rotates the axle so that the tire contact patch is behind where an imaginary line through the king pin would intersect the pavement. This creates positive caster which adds to steering effort, but is what you want when you add power steering.

http://en.wikipedia.org/wiki/Caster_angle

I think we're agreeing but I always thought tilting the top of the axle toward the rear of the car was "negative" caster and I see from the attached link that it isn't the angle of the king pin, rather the location of the contact patch (as you point out) being in front of the king pin centerline that is considered "positive" caster.

[dubie]

You're right Jim. The caster shims are used to aid in the steering wheel returning to Center when turning as well as offering up a straight line steer when at high speeds.

Typically, when you lower a truck, you can lower the front and rear the same amount, yet you're left with the back end higher. This is just how trucks are built. They come from the factory with the same amount of suspension ability, yet the front end always carries the heavier load, leaving the back end raised an inch or 2. This is where the adjustments take place. For instance, I lowered my truck 6" all the way around, yet my front end sits 2 1/2 inches lower than the rear. I aided that a bit by using 4x4 leaf spring saddles that are almost 1 inch taller than 2wd saddles, only because I wanted that rake. So I did some research and found that 6 degree shims would correct the caster I need to make the truck drive straight and true. I learned this from a buddy of mine that has been building cars and trucks for 40 years, so he jacked sky high and others were scraping pavement.it was through his experience that I learned about adjusting your centres to make everything ride better.
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[Katrina/10]

Thanks, now I understand, if the truck is dropped with level ride height front/rear, caster would need to be increased to return to normal driving characteristics.