Caster Gauge vs Alignment rack

Has anyone ever had the opportunity to compare a typical bubble/digital caster gauge reading with the readings you get with an alignment rack?

For caster readings, I'm aware that steering axis inclination (SAI) needs to be taken into account, which is ignored when using a gauge -- however, I can't say what that error range is and I have seen alignment racks give you the SAI value.

I suspect that the caster gauge is oversimplified, but would still give you a reading that is within a 1% margin of error.

Attached I've included a white paper by the Hunter Engineering Company - which is a popular manufacturer of alignment racks - in which they go into great detail on how to calculate caster.

Yes, I have compared my Longacre unit against two alignment racks. You get a more precise value on the rack as they will have a digital readout to 2 decimal places, but the accuracy of the bubble gauge to the rack was comparable, as was the repeatability. The biggest variable that I have found with the bubble gauges is the angle the the person is reading them from.

@BlackRex was there a large delta between the reading you get with the bubble gauge vs the racks?

for example, you read 6.2 degrees while the rack reads 6.5.

I've used camber gauge and toe plates on a rack multiple times as general experiments, or compare the toe reading in degrees vs. mm. Numbers have matched within 0.1 degree camber.

There are 1, 2, and 3 bubble gauges, with the last also doing KPI/SAI.

While accuracy is important, I'd say repeatability is more so. By that, I mean if you check it against a full sized, calibrated electronic wheel alignment setup and it's a half degree out, you can easily remember that and change the setting you want the gauge to show IF it's consistently out by that amount. The problem is when it shows different values each time, for the same settings.

If you read the bubble correctly they where within the margin of accuracy for the longacre unit, I found more error from reading the bubble at an angle.

If I were to take bubble gauges out of the question and utilize digital gauges that are level to the leveling pads — what then?

I have gauges that provide readings to 0.01 degrees accuracy. Assuming the readouts are consistent and repeatable, I suppose I would have to take my DIY aligned vehicle to my reputable motorsport alignment shop and see what the delta would be.

My comments are from using a digital Rebco camber gauge.