Caster measurement inconsistencies?

[rickinAZ]

When I first got my JLURD, I installed adjustable LCAs, set to 0.7" longer than stock, to offset the 2.0" spacer lift and address the common issue with Jeep’s “one-size-fits-all” stock lower control arms. At the time, I measured the caster to be between 6°-7° and was happy with both the measurement and how the Jeep tracked. Fast forward five years to this week, when I had my first alignment done at Discount Tire. While they only adjust the toe, they provide laser measurements for toe, camber, and caster as well. I was surprised to see their caster reading at around 5.25°. This made me consider getting the LCAs lengthened. Just in case, I took three separate measurements at home using an analog gauge and two iPhone angle-finder apps (I didn’t use a physical digital gauge). I measured at the usual two spots: the side of the pumpkin and under the steering knuckles. In all cases, I came up with results between 6°-7°, with the minor variance due to my shaky hand.

So, what could explain the big difference between Discount Tire’s reading and my DIY measurements?

FWIW, here is Chatgpt's opinion:

Bottom Line: The most likely explanation is that the Discount Tire alignment machine is just not dialed in for lifted or modified Jeeps. While their laser readings might be off for caster, your DIY measurements, done in a more relevant way (directly off the axle and steering knuckle), are probably the more accurate reflection of your vehicle’s true caster angle. Unless you’re experiencing handling issues (like wandering or instability), I wouldn’t worry too much about the alignment reading at Discount Tire.

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[jadmt]

go with the actual alignment rack measurement...remember when you do it at home your surface is probably not 100% level as most garage floors have some slope for drainage etc...the alignment rack is going to be level. also .7" longer control arms seems pretty excessive for a 2" lift. as far as the chatgpt's opinion remember if your garage floor is sloped 1° your 6° becomes an actual 5° or 7° depending on which way you were facing.....

 

[rickinAZ]

Jeff, I agree that the alignment rack is more accurate than my garage floor, but keep in mind that in colder climates garage floors are sloped to drain snow melt, while here in Arizona we get so little moisture that builders typically pour them flat.

There are two other factors to consider:

• The general consensus is that stock Rubicons benefit from the Mopar lift kit’s 24.5" LCAs, which bring caster closer to an optimal 6° instead of the sub-5° they typically have from the factory. With my 2" lift, my LCAs are adjusted 0.2" beyond that 24.5" baseline. Intuitively, the extra 0.2" doesn’t seem like much—if anything, it feels insufficient, which is why I’m wondering if I actually need more length.

• On the other hand, if the 0.7" longer control arms are truly excessive, that would push caster even higher, since increasing LCA length directly increases caster.

 

[BDinTX]

Are you zeroing your gauges on the floor before taking readings? I do that and still have variation in measurements. I chalk it up to margin of error in the digital gauge (only goes to 1/10ths) and a not perfectly flat garage floor. So I measure a few times, mentally drop the outliers, and swag an average.

Personally, I wouldn’t mess with it if it drives fine the way it is. But I have also actually broken stuff (twice) trying to “fix” something just because it was outside my normal OCD tolerance range.

 

[oceanblue2019]

Get a $20 Klein digital inclinometer. Put it on the floor under axle to measure slope. Make note of reading. Then put on side of the pumpkin on the machined flat and make note of reading. Correct for the floor slope and add 6 degrees - this is your caster.

 

[rickinAZ]

Personally, I wouldn’t mess with it if it drives fine the way it is. But I have also actually broken stuff (twice) trying to “fix” something just because it was outside my normal OCD tolerance range.

Good Advice. Most of my OCD on this can be attributed to the fact that my JL has been out of commission for a week with a bad A/C compressor. In the mean time I've been driving my wife's Audi SUV and when I jumped back into my Jeep, it suddenly felt like I was driving a farm tractor. Who'da thunk?

 

[rickinAZ]

Followup from OP:

Following @jadmt and @oceanblue2019's suggestions, I picked up an affordable digital gauge, zeroed it out against the floor, and took another measurement. I got 6.6° at the knuckles and 89.5° (which translates to 6.5°) at the pumpkin. This matches the 6-7° range I saw with two different iPhone apps and a manual analog angle finder.


Bottom line: My caster is where I want it to be, the laser measurement (5.25°) from Discount Tire isn't calibrated for lifted Jeeps (as ChatGPT pointed out in post #1), and using my wife’s Audi SUV as a reference vehicle wasn’t the best idea for a seat-of-the-pants comparison. [but...we all knew that ]

 

[Ratbert]

In the mean time I've been driving my wife's Audi SUV and when I jumped back into my Jeep, it suddenly felt like I was driving a farm tractor. Who'da thunk?

There's a shitload of people here that are convinced that their Wranglers drive incredibly well, even when compared to vehicles that actually drive well.

I'm guessing that ignorance is bliss for them.

 

[JL 2D]

There's a shitload of people here that are convinced that their Wranglers drive incredibly well, even when compared to vehicles that actually drive well.

I'm guessing that ignorance is bliss for them.

I use 17 degrees of caster, Jeep drives better than a Camry.

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