I am trying to align a car and made a Camber fixture that attaches to my wheel rim with a precision ground spot for a digital angle finder.

With this, camber is an easy thing to measure as you could imagine but I want to use it to measure caster too.

While most info online talks about bubble gauges with a caster bubble I can only find bits and pieces of info about the formula used to arrive at a caster number when measuring camber at certain steering angles.

From what I found, I think the proper way is to turn the wheel one way to 20* and measure its camber. Then turn the other direction, also 20* and take another measurement. One direction should read pos camber, the other, negative. Add those two numbers together and then multiply the result by 1.5.

Does this sound right?

Also, with a MacPhearson strut front suspension, isn't the rearward angle of body of the strut the caster angle?

My camber/caster tool just has you take the total difference at 20 degrees (both ways).

The professional alignment mechanic (as I was in a former life) has not the time for doing needless math when the spirits level for caster does all the work for you. It is a correct statement that one interpolates the caster from a camber change while turning the wheel through 40 degrees. You turn the table 20 degrees in, rotate the gauge to level it side-to-side, then turn the thumbscrew to center the caster bubble. Then turn the table to 20 degrees out, rotate the gauge to level it side-to-side, and read the caster bubble.

I can't help you with the 1.5 times thing, because, as you can see from above, it's a snap with the proper tools. Nobody earning a living is gonna take the time.

As for the theory of caster, although it might matter on how it gets adjusted (or IF it's adjustable at all), the nature of the front suspension matters not. There is always an axis around which the front wheels steer. If there are a pair of ball joints, it's the imaginary line that passes through the center of the upper and lower ball joints. If it's a strut, it's the imaginary line that passes through the center of the upper plate [where the strut rotates] and the lower ball joint. If it's a kingpin, it's the imaginary line through the center of the kingpin. If this imaginary line intersects the ground FORWARD of a vertical centerline through the spindle, that is positive caster. If it intersects the ground rearward of the spindle centerline, that's negative caster. Some people mistakenly oversimplify and visualize the imaginary line tilting BACK for positive and FORWARD for negative caster. In the vast majority of automotive applications, that works, but it is technically the point at which the imaginary line intersects the ground. The "casters" on your rollaway or shopping cart "seek" positive caster as the wheel trails behind the pivot point. That pivot point intersects the ground FORWARD of the vertical centerline of the wheel's spindle, and it has nothing to do with the "tilt" of anything.

Back to the 1.5 thing, it might be easier (and more accurate) to obtain an inexpensive alignment gauge. Otherwise, sorry I can't help there, and good luck.

istructions for a rebco bubble gauge, all the bubble gauges are pretty much the same so this should work.

Quote:

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Instructions for Caster/Camber Gauge

Always read at the center of the bubble.
Camber is marked in 1/4° + and - .
Turn wheels 20° to R & L when setting caster.
Find a flat, level place to set up your front end.
Attach the gauge to the spindle. If you have a magnetic adapter, be sure that the surface you attach to is machined square to the spindle and doesn't have any nicks or burrs. With the thread-on type adapter be certain the spindle threads are free of burrs. Usually there are a few extra threads on the spindle. If not, you will need to remove the spindle nut and install the adapter in its place.
Rotate the gauge until the small vial at the end of the gauge shows level.
Read the camber directly on one of the two vials on either side of the gauge. Note that they are marked + and - . Read the line nearest to the center of the bubble. Each line is 1/4°.
To read caster first turn the wheels 20° out so that the front of the tire is farther from the car. If you don't have a set of turning plates you can use the angles machined on the corners of the gauge. Turn the wheels so that the angle near the adapter end is square with the side of the car and the angle at the other end is parallel to the side of the car. Rotate the gauge until it is level.
Turn the knurled knob in the center until the caster part of the gauge reads 0°. Now turn the wheels back to 20° the opposite way. Rotate the gauge again to level and read the caster on the center vial (read to the center of the bubble). Each line is 1/2°.
Adjust the caster and camber as needed. Each time you make a change bounce on the front end to settle the suspension. Remove the gauge and reinstall the spindle nut as needed.
Note: Adjusting the caster may have an effect on the camber and vice versa. Double check all your settings before finishing. Be sure to tighten all suspension bolts when done.

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I understand the concept of caster and also know that more expensive gauges will do the math. Just trying to do it on the cheap plus understand the science behind it. I don't mind taking the 10 seconds to punch the numbers in a calculator.

The guys who made the caster gauges needed to know the math, I was just wondering if anybody on here knew what the formula was.

Turns out with some more searching I found my answer. It is indeed Camber change from 20 to 20 degrees multiplied by 1.5.

Here are instructions from a camber gauge manufacturer who explains how to arrive at caster using it. If you are interested, skip to page 9.

http://www.solotime.info/dotnetnuke/Portals/0/content/011073_smartcamber_manual.pdf

In my particular case I was doubting my higher than expected results, however turns out some aftermarket parts allow for more caster than stock, hence my numbers.

I can not find it now, but somewhere on Hunters site (they make the alignment racks) they have a good white paper going over all of the math involved in alignment - it was a good write up...

Also explained why the bubble type systems are only an approx alignment and the errors associated with them.

It was a VERY good paper - might try looking for it...

The formula is:

caster (deg) = (180 / 3.1415) * [(camber1 - camber2) / (turnangle1 - turnangle2)]

For a 40 deg change in turn angle,

caster = (180 / 3.14) * (camber1-camber2) / 40 = 1.43 * (camber1-camber2)

1.43 is sometimes rounded up to 1.5.

The formula is derived in the following publication. A number of assumptions are made to make the formula simple but still reasonably accurate.

Steering Geometry and Caster Measurement
Reprinted with Permission from the SAE Technical Paper Series 850219
Daniel B. January
Hunter Engineering Company
Bridgeton, Missouri

pdf file:

http://www.disco3.co.uk/gallery/albums/userpics/24543/steering-geometry-and-caster-measurement%5B1%5D.pdf

Thank you!