My understanding was moving the upper ball joint shims back-to-front would increase castor, but I’m finding that it made nearly no difference; see video:


Do I have this backwards? I thought, for example, if you have 2.5 deg and want 3.5, you would move the appropriate number of shims back to front to achieve more castor.

Two related questions:

1) My XJS is lump’d so much lighter in the front, though I do have lowering springs. Could this be at play here?

2) Is castor strictly for self-centering steering behavior or are there other important steering behaviors castor may affect?

Thanks

 

Here's a screenshot of the caster setting specs and procedure from the ROM. Caster helps self centering of steering, and also provides more negative camber to the loaded wheel in turns.
Because you had no shims before, I suspect your upper suspension bushings have taken a permanent set, or one of the upper control arms is bent, because of bolting things down without shims in place.
Also check the number and thickness of the camber shims. They should be identical front and rear of the attachment points. That will also affect caster if they are not identical.

, I

 

Thanks Jal, I can confirm that the camber shims are exactly the same in both front and rear positions on both sides of the car.

I hope the upper wishbone isn’t bent, but I’ll check it; I know the book has a prescribed way of checking the measurements.

Still very strange to me that going from no shims to 4 only adjusted the front castor from 2.6 to 2.8.

 

When you take it on the road for about a block it will settle to its true setting.
(';')

 

I drove 50+ miles between the time I made the adjustments to the time I went to the alignment shop to get the readings. No settling at play here, I'm afraid.

 

You've got it right as for the shim location. How are your upper control arm bushings? If they have excessive lash, that will eat up the caster you adding. Having no shims for a while probably pinched the arms inward and in such away that the bushings are spent. You need to disconnect the UCA's at the ball joint to for check for lash.

I'd recommend new bushing and the UCA's are a good candidate for poly.

- caster is very important. It is for self centering and is critical to over all steering feel. Google "caster jacking effect"

 

Thanks ics, I'm going to replace both upper ball joints and all upper control arm bushings. What is "lash"? It's a term I've seen a couple times but am not familiar with.

 

Lash is short for backlash in the mechanical context.
This wiki explains it better than I ever could.
https://en.wikipedia.org/wiki/Backlash_(engineering)

 

Thanks again - do I need to replace the control arm bump stops while I’m at it?

 

No way to tell from that video but they seldom fail enough to matter,

 

About to change the upper ball joints and control arm bushings.

Figured I’d make a video of the entire front suspension as-is to see if anything jumps out at anybody.

 

Something else I’ll add: these measurements were not taken at the “mid-laden” position.

My understanding is toe isn’t really affected by this, camber will read more negative on the ground than at mid laden height (so slightly negative on the ground is what you want, instead of slightly positive as prescribed by the book), but I don’t know what affect this has on castor.

That still wouldn’t explain the big discrepancy in left vs right castor since they both have the same number of shims, but I’m hoping someone will say that castor would be higher at mid-laden height, so at least the 2.8 I’m seeing the in driver’s front may be in spec (3.3 - 3.8) or close if it were at mid-laden height.

 

All meaurements of alignment MUST be taken with the car at the correct ride height.

 

But if I can't find a shop with the patience do deal with that, surely someone here has had measurements taken at both positions and can advise what their castor specs were when the car was on the ground vs. mid-laden.

If we know:
- toe isn't affected much, if at all, by ride height
- while resting on the ground, an XJS should have slightly negative camber by all accounts

...then surely someone here can chime in regarding what typical castor specs are when an XJS is resting on the ground. This is very useful information - especially if a few can chime in with their specs to build some data.

The reality is that some folks just aren't going to have a shop around with both high-quality laser alignment systems AND the willingness to do the Jag mid laden song-and-dance. Knowing what typical specs are while the vehicle is at its resting position would be pretty valuable for a lot of us.

 

Sweet little service bulletin I found: http://www.jagrepair.com/images/TSB/...ifications.pdf

The note at the top of page 4 reads: "NOTE: Install the correct mid-laden tools. All setting height dimensions are for reference only. XJR-S and sportspack Coupe vehicles whose normal ride height prevents the use of mid-laden tools, should be checked/set at their normal ride height."

Standard XJS castor range, according to this bulletin, is 2.0 - 6.0.

XJR-S castor spec is 3.5 (+/- .25 deg). The XJR-S ride height is about 3/4 inch lower in the front, from what I've read.

Here's my take: even though the ride height is going to be a little different between my car and the XJR-S, given the fact that Jag permitted a whopping 4 degree range in castor for the standard model, I have to think that 3.5 deg at resting height is likely still in spec for a standard XJS.

The question remains, though: if you had the alignment machinery attached and start to lift the car up by the jack points, what happens to the live castor readout on the monitor? I'm guessing it goes down. Which means castor readout would be higher at the natural resting position than it would be whilst at the mid-laden position, right?

So my 2.8 driver's front might be within the 2-6 range, but just barely if it is. Something in the 3 range would be nicer. But the 1.0 or whatever it was on the passenger side is way off. Here's hoping the ball joint/bushing renewal changes that.

One last thing I'll add - from all the research I've done, castor is to some degree preferential. So long as it's not so low that the car has no self-centering characteristics, and not so high that turning the wheel is difficult, you're fine, and so are your components. Again, just based on everything I've read.

 

Castor isn't measured directly. The front wheels are turned to a specified number of degrees in one direction and camber is measured, then the front wheels are turned to the same number of degrees in the other direction, and that camber reading is measured. The resulting two camber angle values are then subtracted from each other, and that give you the caster angle. Slight changes in ride height shouldn't change the caster calculation much, if any if done on two different days with slightly different ride heights. However, ride height needs to remain constant for a particular set of caster angles to be calculated on a given day and time.

Jon

 

Now that I didn't know. What is the specified number of degrees you're referencing? And just to make sure I understand, can you give me a generic example of measuring castor the way you described?

Thanks Jon

 

15 degrees left and right of center, subtract, multiply the difference by two. I forgot the multiplication when I discussed it above.

So start with the wheels straight ahead, turn the wheels 15 degrees one way, measure camber, turn the 15 degrees the other way past center, measure camber, subtract the smaller measurement from the larger, multiply the difference by 2, that's your caster angle.

If you watch technicians use a modern computerized alignment machine, you'll see them turn the wheels one way, then the other. The machine tells them when they are at 15 degrees. The machine automatically takes the measurement, then has them turn the wheels in the other direction, where it again measures. It automatically does the subtraction and multiplication, and displays the result. It does both sides of the car at once.

Jon

 

Ok that makes sense, thanks for the explanation Jon.

Just so I'm not missing anything, it sounds like the possible culprits for castor being off are:

- Bent upper control arms (which I don't think I have)
- Bad upper control arm bushings (my old ones didn't look bad at all actually)
- Bad upper ball joint (old one looked and felt perfect upon removal, no slop and very smooth)
- Improper placement of shims or too few shims (not the issue here)

Anything else I'm leaving out?

 

Lower control arms, lower control arm bushings, or the upright could also do it. Are you getting the same caster measurements on both sides of the car?