The Stovebolt Site Forums The Shop Area General Truck Talk Checking Caster on a '72 IFS

I have the factory service manual which gives the proper factory Caster spec, but it says use your "alignment equipment" to check current alignment.

As I'm mounting this IFS on my '55 1st frame I'd like to verify Caster is close to ensure it can be properly aligned when I finish. Any thoughts how that can be done without an alignment booth here in my home shop?

you can get it close using an angle finder on the spindle. Bear in mind, the vehicle really should be at total weight for an accurate measurement. Best would be to take it to an alignment shop once it is roadworthy to get all the angles set correctly.

Bill this is a '72 Independent Front Suspension. I'm not sure where on the spindle I'd put the angle finder. If there is a surface that is parallel to the ball joints I'd sure like to know what it is.

My immediate goal is to make sure I mount it close enough to correct caster that it will easily adjust to the correct spec when I'm finished and take it to a shop to have done right.

Since posting the question I've been looking at it trying to find a way to put something between the ball joints that I could measure the angle on.

I now have some crazy ideas for making a tool, but since I'm good a over complicating things I'm hopeful someone will say hey this surface or that surface is the right one...

I had not stopped to figure out how caster is set on one of these until this afternoon. Its literally by putting more shims under one end of the upper a-arm pivot shaft than the other end.

Right now I'm picturing two l-brackets, each with the correct size hole to fit over a ball joint nut, and both mounted to a flat surface in such a way that the centers of the ball joint holes remain in alignment, and such that a line drawn on the flat surface remains perpendicular to the line drawn through the centers of the ball joint holes.

A pointer of some sort, with a pivot located where the perpendicular line intersects the ball joint hole centers line would be adjusted until it is parallel to the floor. Degrees between the pointer and the perpendicular line would represent my caster measurement. Degrees below would be positive caster.

My flat surface is perpendicular to the floor and perpendicular to the axle centerline.

I have no idea if this description makes sense to anyone but me, but it helped me to write it out. Aside from the precision required in making this thing, the lower ball joint hole needs to be 5/8ths or 11/16ths diameter and I'm not sure I have a drill bit large enough.

I see where you're coming from, been a while since I've had to align a truck with that suspension. Your idea sounds good, and should work. I would suggest that you start with getting the camber in where you want it first, which is easy to measure with an angle finder on the rotor face. Once that's set, you can shift the caster around by swapping the shims from the back to the front. Do the toe setting last, as the other dimensions will effect the toe reading.
Another option you could do would be to get an old style camber/caster gauge. Might be able to get one at a swap meet or shop equipment sale cheap, as most alignment shops do not use them much anymore. These mount to the hub once the dust cover is taken off, and can measure both camber and caster at the same time. It does take a little math work to determine the caster angle, but really isn't too hard to do. I will admit, it has been a while since I've used one, and the Hunter Hawkeye 800 has got me spoiled for doing it old school.

I did some online searching for caster guages and found them new as cheap as $100 and $250 will buy a fancy digital one. Basically all of them are really Camber guages that calculate Caster from Camber near as I can tell.

I also found a couple of DIY Camber guage ideas but simply putting my angle finder on the rotor is probably as good as any of them.

The Porsche 911 forum gave me this:

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To calculate caster, turn the wheel 20' right. Measure camber. Turn
the wheel 20' left. Measure camber.

Caster = ( CL - CR ) * 1.5
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That seems consistent with the descriptions of the tools you can buy, but I don't know if the math is really correct. Does that seem consistent with your approach?

1.5 times the change in Camber over a 40 degree arc equals Caster. I was thinking about trying to come up with the formula by going back 35+ years to my high school math classes. If I can simply trust that formula its a lot easier...

That sounds right. Like I said, been a while since I've had to actually do the math to figure out caster, the aligner does it for me.