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Old 08-14-2012, 04:03 PM   #1
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Caster Help

So I need to know what everyone is measuring from when they are talking about caster angle. Name:  Caster.jpg
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Size:  38.5 KB I have read that stock Jeeps have about 7 degrees of positive caster. Is this 7 degrees the actual caster as shown in the picture above or is it from the axle flat spots circled in red?Click image for larger version

Name:	Axle.JPG
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ID:	150305 I measured from the flat spots on my axle and it is between 10 and 11 degrees towards the front of the Jeep (This means the pinion is pointing slightly upward rotating the whole axle forward). I measured a stock Jeep and the axle is between 6 and 7 degrees toward the front of the Jeep. I understand that these measurements are the axle rotation and not true caster angles. Anyway after doing a bit of trigonometry my true caster at this point is about 7 degrees of positive caster while my axle is rotated forward 10-11 degrees. Do I need my true caster at 4 degrees for my 4" lift or do I need the axle rotation at 4 degrees to bring it back into spec?

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Old 08-14-2012, 04:12 PM   #2
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The two spots you circled are not equal to the caster angle. Those are however exactly 90 degrees from the pinion angle so they are useful for measuring the pinion angle.

The top illustration is the caster angle which is that axis that the knuckle rotates about when you are steering the front tires left/right. A chopper motorcycle with extended front wheel forks has a huge caster angle. The wheel on a shopping cart turns left/right about a perfectly vertical axis which would be a zero degree caster angle.

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Old 08-14-2012, 04:32 PM   #3
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I just installed a long arrm Y-link on mine and had to get it alighned. They only charged $82.99 for a 4-wheel alighnment. well worth it, drives like new again. Its hard to check those measurments without the proper equipment. You can get in the ball park though.
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Old 08-14-2012, 07:32 PM   #4
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I have yet to find a spot on the housing that gives a reliable caster measurement equal to that of a laser alignment rack w/ roll back test.

I've tried the socket on the ball joint trick, the u-joint for pinion angle, the bottom of the lower C's, and the housing where you circled. None showed the same angle as the laser alignment, and some were not repeatable day to day.

What you can do is measure pinion angle, then get a laser alignment. The shop should give you a before and after print out with your receipt. The relationship between pinion angle and caster will be linear. So if pinion angle is 10° from horizontal and caster is 6° via alignment shop, then you know if you move pinion angle to 9°, your caster is now 7°.

Also remember what somebody else's jeep says is totally irrelevant. the tolerance is +/- 1° per side from the factory. there are housings out there with a variance over 1° per side...so 6.5° caster on the drivers side and 7.5° on the passenger side.

Make sense?
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Old 08-14-2012, 07:53 PM   #5
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Yes Jerry I do understand that it gives the axle rotation angle or pinion angle and not the caster angle, but like Unlimited said they have a linear relationship. Right now I am just wanting to get in the ballpark before I take it in for a thrust angle alignment. With my axle rotated forward 10-11 degrees my caster is at about 7 degrees. If I rotate my axle forward by lengthening my upper control arms lets say 1 degrees then the caster will also decrease by about 1 degree. I am pretty sure that I am right, but want to just make sure I have all my ducks in a row. I am pretty sure I am understanding UnlimitedLJ04.
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Old 08-14-2012, 08:02 PM   #6
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If I rotate my axle forward by lengthening my upper control arms lets say 1 degrees then the caster will also decrease by about 1 degree.
adjust caster with the lowers. the uppers have less effect due to their high position off the axle housing, plus extending the uppers will push the diff closer to the track bar. you don't want to create interference issues. keep the front uppers at stock length when possible...
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Old 08-14-2012, 08:22 PM   #7
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I only have adjustable uppers right now so I will have to just adjust them to bring the caster back into the 4 degree range needed for a 4" lift. I should only need to adjust the axle forward by about 3 degrees to make the caster angle about 4 degrees.
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Old 08-14-2012, 08:31 PM   #8
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I only have adjustable uppers right now so I will have to just adjust them to bring the caster back into the 4 degree range needed for a 4" lift. I should only need to adjust the axle forward by about 3 degrees to make the caster angle about 4 degrees.
i would buy adjustable lowers. IMHO, with uppers only, you bought the wrong arms first.

again, the lowers have a more significant impact due to their position relative to the housing. you can adjust lowers 1/4" and be pretty safe, or the uppers 2-3x more and risk clearance problems.
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Old 08-14-2012, 09:38 PM   #9
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The uppers are actually made by me as I have a lathe and mill along with the proper drill bits and taps. Basically I only have to buy two johnny joints and fab the rest. That being said I will be able to make the arms as short or as long as I need them so they will not need to be lengthened too much.
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Old 08-14-2012, 09:44 PM   #10
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Old 08-14-2012, 10:00 PM   #11
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The uppers are actually made by me as I have a lathe and mill along with the proper drill bits and taps. Basically I only have to buy two johnny joints and fab the rest. That being said I will be able to make the arms as short or as long as I need them so they will not need to be lengthened too much.
Great, so go make some lowers. Set the uppers at 15" eye-to-eye (stock length), then start the lowers at 16" (stock = 15.75") and see what caster you get. Push the lowers out till you're at the desired caster.
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Old 08-14-2012, 11:33 PM   #12
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Pushing out the lowers will increase the Caster angle and I need to reduce it. Pushing out the uppers will decrease the caster angle and I have read I am looking for around a 4 degree caster. Unfortunately the lowers would take a larger diameter tube and larger diameter tap and drills, which I do not have. I have everything for a 3/4" Johnny Joint in my garage, both right hand and let hand. I would never put a 3/4" Johnny Joint on the bottom control arm.
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Old 08-15-2012, 11:49 AM   #13
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Pushing out the lowers will increase the Caster angle and I need to reduce it. Pushing out the uppers will decrease the caster angle and I have read I am looking for around a 4 degree caster. Unfortunately the lowers would take a larger diameter tube and larger diameter tap and drills, which I do not have. I have everything for a 3/4" Johnny Joint in my garage, both right hand and let hand. I would never put a 3/4" Johnny Joint on the bottom control arm.
on 4" of lift, 15" uppers and 16" lowers should put you at 4° caster, or even less.

However, I don't know why you want 4° caster. I drove there for a while, it was squirrelly as all hell. I'm at 6.5° caster with 33s now, and quite happy.
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Old 08-15-2012, 02:29 PM   #14
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I have just heard that 4 degrees is the caster I should have with 33s. It is nothing more than things I have read. So you are at 6.5 degrees and it works well? Maybe I will have to run them and adjust as needed. I just want to bring it back into stock spec to reduce the DW chance as my wife sometimes drives my Jeep. I may need to lean on some of your experience if you do not mind.
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Old 08-15-2012, 03:55 PM   #15
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I have just heard that 4 degrees is the caster I should have with 33s. It is nothing more than things I have read. So you are at 6.5 degrees and it works well? Maybe I will have to run them and adjust as needed. I just want to bring it back into stock spec to reduce the DW chance as my wife sometimes drives my Jeep. I may need to lean on some of your experience if you do not mind.
Stock caster spec is 7° +/- 1°, so if you want to bring it back to factory specs, 4° isn't even close. The general "guide" i've read online usually says ~6° caster for 33s, and ~5° caster for 35s. I don't necessarily believe those internet rumors because I've talked to some running 7° caster on 35s & 37s, and even a guy running ~8° caster on 33s (after cutting the inner C's off and re-welding them in the desired position).

I say go by what feels good for you while driving. It's a fairly subjective thing anyway. With a LP housing you will need to make a compromise between pinion angle and caster, especially at higher lifts. The front driveshaft is very long and DC, so it can take more misalignment than the rear. That's why I said put your uppers at stock length (15"), put the lowers at 16", and push them out until you get the desired caster you want...it's about what you like, for your driving experience.

Also remember when you adjust caster you're moving toe-in slightly. The angle the knuckle is mounted on means adjusting the inner C position also causes a slight change in toe and camber. So you need to reset toe-in alignment after adjusting caster....which can be done with a tape measure.
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Old 08-15-2012, 05:29 PM   #16
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Caster on these things are not that critical for most of us with less than 4" of lift and low pinions. Its after you go above 4 you have less adjustibity or none because of pinion angle on LP. I adjusted my last TJ back and forth between a couple degrees or more and had no DS issue and I was just a hair under 4" of lift . I adjust it until I like the way it drives. You have from driveshaft and pinion in line to pinion a degree or so below the DS which I have yet to see a TJ with less than 4" of lift be an issue having it drive great and no vibes. Oncete way I like it I like I check caster by every method I have seen on the net and no matter which one you do or don't like it always comes up an acceptable number from what is recommended by tire size . This is not as hard or critical as some make it out to be . I have been doing it for many years at 57yo and my vehicles ,dont wear tires , no DL vibes very stable to drive at higher speeds. Some were indepenndent front suspension ,its just a matter of using what you have got for tools and between your ears . I used to take them in for the Pros to check alignment after I felt springs had settled on the independent front ends but 9 times out of 10 it would bedmn close so these straight axles are not difficult unless you make it that way. May not be for every one but I like playing with the alignment on my rides.
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Old 12-02-2013, 10:06 PM   #17
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hello, how do I adjust the caster about 2" on my jeep. this cannot be done with caster bolts...as they are only used for fine tuning once the springs are visually straight up and down. I was at big o tires and they didnt have a clue!!! we had to take remove the control arms bolts and caster bolts and washers...and lift up the frame so there was no stress on tires and then used a large crow bar to attempt to get caster back into position. there seems to be some pressure not allowing us to get the caster spot on. the springs are still a little bowed....I am shocked these people at Big o have no clue how to do this. please tell me the trick or special tools to do this....and what has to be removed to allow the axle to freely get the caster back into the correct spot and springs straight up and down? I did not remove the sway bar disconnects...which maybe that was the problem????? thanks in advance
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Old 12-02-2013, 10:57 PM   #18
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hello, how do I adjust the caster about 2" on my jeep. this cannot be done with caster bolts...as they are only used for fine tuning once the springs are visually straight up and down. I was at big o tires and they didnt have a clue!!! we had to take remove the control arms bolts and caster bolts and washers...and lift up the frame so there was no stress on tires and then used a large crow bar to attempt to get caster back into position. there seems to be some pressure not allowing us to get the caster spot on. the springs are still a little bowed....I am shocked these people at Big o have no clue how to do this. please tell me the trick or special tools to do this....and what has to be removed to allow the axle to freely get the caster back into the correct spot and springs straight up and down? I did not remove the sway bar disconnects...which maybe that was the problem????? thanks in advance
You are confusing terms. Caster is the relationship of a straight line through the ball joints to the vertical (assuming the Jeep is sitting perfectly horizontal...on a flat level surface). What you are talking about is axle position, and caster adjustment is not meant to adjust axle position. Having the springs straight up and down on a lifted Jeep at ride height would not be a good thing because the bump stop should be centered when at full bump....in other words, when the spring is fully compressed it should come together pretty evenly, and it wouldn't do that if the springs were perfectly vertical to start out with. The axle comes up in an arc as the spring compresses. The way to relocate the axle fore and aft under the Jeep is with adjustable control arms. Most, if not all higher lifts 4"+ come with at least longer lower control arms to control where the axle sits.

You didn't say what kind or size of lift is on your Jeep, nor how it was installed, so it makes it kind of hard to figure out what you have going on...how about some pictures?

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