caster vs. pinion angle relationship/formula

Getting ready to add RE control arm drop brackets to a '93, and was wondering what the relationship is between pinion angle and castor. I know there is a formula that can be used to figure caster if the pinion angle is known but don't remember what the formula is.

Also looking for ideas on a good starting point for caster when I add the drop brackets. I'm sitting at about 6.5" in the front with adjustable arms, cross over steering, and a shortened and raised (at the axle end) track bar.

Thanks for any info!!!

For some reason the number 9 sticks in my head - OEM caster should be negative 7 degrees and the pinion angle is about 2 or 3 degrees positive...

I really cannot remember as it has been a year or so that my front was still operating with that relationship...

Matt

Shoot for 5 to 5.5 degrees neg. at that lift height - possibly 6 with the drops

I have the full information that you require on my site in the how to section that will help you out.

I was just about to give you this same site. I found a lot of useful information by reading these aritcles. Thanks GO JEEP

You want 7-8* degrees positive caster.

Matt said:

Shoot for 5 to 5.5 degrees neg. at that lift height - possibly 6 with the drops

Click to expand...

5° to 6° as stated, but you want positive castor, not negative

On the high pinion D-30, the split is about 9 1/2° so:

9.5 - pinion angle = castor angle.

MaXJohnson said:

5° to 6° as stated, but you want positive castor, not negative

On the high pinion D-30, the split is about 9 1/2° so:

9.5 - pinion angle = castor angle.

Click to expand...

9 is pretty close to 9.5

When you say positive caster - I am lost - the lower ball joint should lead the upper ball joint by 5 or 6 degrees...

The line connecting the centers would be tilting toward the rear of the vehicle - correct?!

How is this positive if the verticle plane passing through the center of the axle tube while looking form the side would have the line running from the bottom left to the upper right on the drivers side

You go the other way - trust me I did this before I knew what I was doing (you gotta learn some how) perfect pinion angle makes for disasterous handling; I drove about a block with ~2 degrees POSITIVE caster and barely kept the thing on the road.... Even my FSM lists it with a dash for neg

Matt

Matt said:

9 is pretty close to 9.5

.... Even my FSM lists it with a dash for neg
Matt

Click to expand...

Wasn't trying to correct you on the 9 degree thing, just drilling down for those that might want the added accuracy. I use an even 9° myself most of the time.

You are correct in your description of castor, but the amount of rearward tilt of the steering axis is a measure of "positive" castor. Having an "amount " of something is positive or more than 0. Positive castor is expressed in terms of how much rather than by the direction of the tilt of the axis.

As for the FSM, either thats a hyphen or the FSM is wrong.


Here's a couple of paragraphs from an article I'm (slowly) working on. Sorry to exclude figure 2 It's on another PC.

"The front wheels steer about an axis defined by the upper and lower ball joints. Castor (also spelled caster) is the amount of forward or rearward tilt of this axis. For positive castor, the top is tilted rearward off vertical; for negative castor, the axis tilts forward.
You can see in figure 2 that the contact patch of the tire is behind where the steering axis intersects the road or driving surface. This “trail” results in an aligning torque that centers the steering in the straight-ahead direction. With several degrees of positive castor, you can let go of the steering wheel in a turn and it tends to return to center. Without enough castor, the steering will not return to center and the vehicle will tend to wander, requiring constant steering correction. Too much positive castor can result in heavy steering, shimmy and tire wear. Too much negative castor could easily result in a sudden and unexpected meeting with a tree. Variations in the castor setting of the left and right wheels will cause the vehicle to pull towards the wheel with the least amount of positive castor."

I didn't feel like I was being corrected on the 9 - just remarking that the number stuck in my head was pretty close

As for the pos/neg thing; hows about we agree to dissagree! - I think it just comes down to prespective (terminology based on a reference point) I like (and have always used/been told) the negative value as my usual frame of referance is the direction of vehicle travel...

Do with that what you will....

Matt (stiring the pot :angel: )

I can personally attest to the soundness of go-jeep's method. Spent the time, measured and took to an alignment shop to verify "one last time". I used an angle finder accurate to the degree and was within .20 degrees of 6.5 degrees (which is where I thought I was). For me go-jeep's info has been the single most valuable bit of how-to I've found, and I have chosen to lift 7" and have been through all the issues typically associated with that. So Tim, to really help out with not just knowing (cuz that's half the battle) pinion vs. caster to be honest its more of a diagnostic tool for me. I don't know if I CAN run "proper" caster b/c of the resultant pinion angle and u-joint operating angle limits. I have (just confirmed by the shop yesterday) 6.30 left and 6.52 right degrees caster and mad front driveshaft vibes, really unsettling at around 60mph. I don't have a non-disconnect HP D30 front axle with a lockrite, no hub conversion. OK rambling but my point is that at your height I *think* (based on my experience and what I have read/heard) it'll be your pinion that dictates your caster due to the need to actually drive the vehicle at highway speed Knowing where you are just helps and lets you know when you have gone too far perhaps.

HTH

Pete

xxxj-va said:

I can personally attest to the soundness of go-jeep's method.

For me go-jeep's info has been the single most valuable bit of how-to I've found

Pete

Click to expand...

"go-jeep's method" was copied from a post here on two years ago. No offense to you Pete, but it pisses me off every time I see him jump on and refer someone to his site for alignment info.

Several years ago, I got a good case of Death Wobble after assuming that my local NTB tire store set my castor correctly. I search the internet high and low, including go-jeeps website for information on driveway alignment tips. Toe-in is a no brainer, but no one seemed to have a good method for checking castor. One evening the fixed relationship between the knuckles and pinion hit me like spotting a gold nugget in a mountain stream. It's so obvious when you think about it, but no one seemed to have considered it.

Subsequently, I measured and experimented with my Jeep and zeroed in on the 9 1/2° relationship on the XJ's D-30 axle. Two years ago this month, I posted this method in response to an alignment or Death Wobble thread. It was only a short while later that Marcus put the information on his site.
He has admitted to me in an e-mail that a friend of his saw the method on the internet and relayed it to him. He also agreed with my request to add a credit to the source (my post on ) to his article. I pointed out an error in his math and he made the correction. Later, I pointed out that the 9 1/2° split only applied to the XJ high pinion version of the D-30. He added that too.

I've always thought Marcus had one of the better Jeep websites on the net, but he's lost my respect by refusing to step up and give credit where credit is due.

Perhaps he's bucking for a job at the New York Times.

Matt said:

As for the pos/neg thing; hows about we agree to dissagree! - I think it just comes down to prespective (terminology based on a reference point) I like (and have always used/been told) the negative value as my usual frame of referance is the direction of vehicle travel...

Click to expand...

People can "agree to disagree" on matters of opinion, but not on matters of fact. Fact is fact, and only one of two versions can be correct.

In this case, when the upper ball joint is farther to the rear of the vehicle than the lower ... which is what we want ... the correct terminology is POSITIVE castor. It is negative if the upper ball joint is farther forward than the lower. If you have been told anything else, you have been misinformed.

Click here for caster simplified...

MaXJohnson said:

"go-jeep's method" was copied from a post here on two years ago. No offense to you Pete, but it pisses me off every time I see him jump on and refer someone to his site for alignment info.

Click to expand...

The reason people refer to his site so much (myself included) is that it's organized and easy to find (or link to) the information you're looking for. If we knew exactly where the info was published on I'm sure many would refer directly to it here instead, but we're left with endless searching through forum posts looking for info we may never find on the XJ site to end all XJ sites. ...and if we do find it in the forum we don't know how credible the info is.
Maybe some officials could be in charge to set up a non-forum XJ "Bible" page here on where all, far and wide could reference to.

Let's be constructive, not point (or wave) fingers. :cheers:

Sorry Max, I really was referring to the information there, if its yours than thanks to you for figuring it out and to this board and go-jeep for making it available to me. I absolutely HATE front suspension issues b/c there are so many variables and I prior to the info I found on that site there was always one "unknown". I think you are mentioned on the site regarding the HP D30 offset vs. LP, anyway thanks!

P

Just a quick note......

don't mean to muddy the waters here, but Max has also done some pretty extensive research on LCA UCA length and geometry senarios, has been extremely helpful to folks trying to figure out how to dial in certain aftermarket adjustable arms to achieve the best lengths for caster and pinion angles.....

I remember seeing some kind of chart he made a while back, looked pretty neato......

maybe that info will help you too, don't know if you've experimented with control arm lengths yet.

Matt: I'm cool with the "agree to disagree" thing, but in this particular instance, you could have a handful of guys telling some newbie to run "POSITIVE" castor and an equal number suggesting "NEGATIVE" castor. Then what is the guy suppose to do? He could take the average and run 0° or he could run down to the local library and discover that normal convention indicates that "more castor" is "more positive". Then he would be in line with all the alignment shops across the country as well. Having it both ways is just too confusing.

xxxj-pa: No apology necessary. I agree with you 100%. Go-jeep's site is very helpful and as I mentioned, one of the better Jeep related sites on the net. But, I quit recommending it because of the way he's dogged me on the alignment info. IMO, this opens all the material on his site to suspicion. Like I said, he needs to step up and do the right thing.

JEEPZZ: You're right, and IMO, is the Number 1 site on the web for the Jeep XJ & MJ, but is weak when it comes to providing technical articles and content. Hopefully, that will come in time. For now, there's many goods sites out there with great info that we can refer people to. These sites should respect their sources.

The finger pointing will continue until Marcus comes clean and does the respectable thing. Until then, everytime I see him post, I'm going to follow it up with a reference to his dishonesty and lack of integrity.

Beezil: Thanks for the postscript. Pro or con, your input is always respected and welcome

Wow Max you you carry a lot of grief. As stated by you I did not receive the information about the caster/ pinion relationship but I got it from the XJlist were I have been a member for many years. Now it is possible that they learned it from you and I also explained that it my return email a few years ago. I also said that if it meant so much to you and were the only one that could substract a known caster angle from a pinion angle coming up with the useful fomula I would add that to my site and for you to just tell me exactly what you want written. You never ever replied so thought that you were happy with the way things were.
I was under the impression that this forum was the giving and sharing of information for the good of all. That is what I made my site for as do it for that reason alone. I wrote the whole section on doing alignments many years ago as I worked in a 4x4 shop where we specalized in alignments for 4x4's. I thought it would save other some time and money knowing about how to do it as had noticed a high number of people that were fed up with shops that do not touch the caster ever!
I have limited my posts here recently as felt that getting attacked for ideas and suggestion was not the spirit that I like to post under. Hope this will now change now I know the cause.
I will add your credit regardless if I think you came up with it or not as it does not worry me at all. I had no idea that it would cause so much grief and am truely sorry for that.

Hope we can get back to what it important.

Err......
1. Castor is an oil.

2. I can't say anything nice about the submittal of articles to , so I'll leave it at that and hope that the submittal process has been remedied. Tech info should roll in after the simplification and end of the clusterf**k that it was as of 6 months ago.

Max... Put my DAMNED hubcaps back in your sig, man!

Eagle said:

People can "agree to disagree" on matters of opinion, but not on matters of fact. Fact is fact, and only one of two versions can be correct.

In this case, when the upper ball joint is farther to the rear of the vehicle than the lower ... which is what we want ... the correct terminology is POSITIVE castor. It is negative if the upper ball joint is farther forward than the lower. If you have been told anything else, you have been misinformed.

Click to expand...

I look through some other stuff I had and eveything else I have listed a value with no pos or neg sign next to it - (leading me to agree the positive value is the more common and correct terminology)

Oh well - I guess my head will just have to deal with the fact that a positive value makes absolutely no sense to me...

Correct factory range is 7.5 - 8 degrees for XJ/MJ and a 6" lifted vehicle should have between 5 and 6 degrees depending on drivability and pinion angle sacrifice...

Matt