What you may not have realized at the time of your purchase (two years ago) was that the tire selection then was still slim pickings.
I’ve been watching the 14”-16” performance tires disappear from the marketplace for years.
My BMW E30 and it’s 4 lug 14” wheels are caught in the crosshairs as well, forcing me to upgrade to larger wheels in order to get better tires.
The thing is, you can have the greatest suspension setup in the world, but without the tires to hold the road, the suspension is reduced to nothingness.
Back in the day the XJS came with Pirelli (was it) P7’s, they were one of the high performance tires of the time. They sucked, but that’s another story.
If the suspension is in good nick and the car is aligned properly it would not need to be rechecked as it should yield the exact same angles.
Neither Camber, nor Caster or Tow will change with a different rim or tire size.
Now, if a different setup would perform better with the different wheel and tire size is an other question, and I imagine very difficult to assess.
I stand corrected.
Not only is that good to know, but it also saves a lot of work. Just slap on the new wheels and tires, and as long as there are no rubbing issues your good to go.
I could be wrong but if one uses the proper mid-laden tools jag recommends front and rear then there should be no reason to be concerned about driver, passenger or fuel tank weights. If the suspension is locked in place with the proper tools while aligning suspension why would the height or geometry move regardless of weight from seats or gas tank.
3 Likes
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
25
That is correct. No reason to be concerned about wheel size, either.
FWIW, I noticed the T.S.B. for post face-lift alignment (I think someone posted it earlier somewhere on the forum) doesn’t mention the need for putting any weights in the seats. It basically says to check the ride height (i.e. as is) at front and back to make sure it is correct, and then go to town on your alignment. IIRC, the front measurement s supposed to be 6", with a .2" range tolerance. The rear is something around 6.2", with a similar tolerance range.
I am not in a position to further argue the point because I cannot seem to find the source of where it was that I got that piece of info. I certainly didn’t make it up because I thought that it sounded good. I will keep looking and report back if I can find it.
In the mean time, most people do not have, or are even aware of the existence of mid laiden tools. Do you suppose that they could benefit from weighted seats and a full tank of petrol?
The mid laden tools are very easy to fabricate and their specifications available in the Book and elsewhere, so this should not be a problem.
In the absence of, weights and a full tank would help, but it would be only an approximation because, as far as I know, there is no ride height data with the tools in place.
The mid laden tools compress the suspension, and, especially in the rear, quite a lot, more than a full tank would do.
The point is that Camber changes with suspension travel, and in the rear it’s more than 1° from one extreme to the other.
Tolerance is +/- 0.25°, so without the tool it’s just guesswork.
Interestingly (well, maybe) the X300 ROM (leftover from my XJR days) mentions the ‘mid-laden’ tools briefly but then goes on, elsewhere in the section, to simply say that ride height should be verified before alignment and gives xxx-specs for the various models as well as location for measurement. I recall that +/- 10-12mm was shown as a tolerance range.
So we have the acceptable range of ride height together with the alignment specs themselves being expressed as an acceptable range.
But getting back to the older cars I am quite sure that the alignment specs were changed a least a couple times in the late 80s-early 90s for Series III sedans and pre-facelift XJSs…suggesting the the previous specs were not carved in granite.
What I’m leading up to is that, IMO, good results can be obtained over a range of alignment settings. Certain characteristics might be optimized by tweaking the settings to one end of the allowable range.
How much a particular owner fusses over this is variable, obviously
I think that alignment specs changed because tire technology advanced and wheel / tire sizes changed as well.
I would imagine for example that a wider tire would call for lesser negative Camber to minimise wear and a lower profile tire lesser Toe-in as it would flex much less.
Yes good results could be obtained, most important that both sides are equal, but would require lots of trial and error plus a couple of sets of tires to spare as there is no way to measure tire ware until it happens, and a bad alignment can decimate a brand new tire in just a couple of thousands of kilometres.
Jaguar, and any other manufacturer, would have spent quite a lot of time and effort to arrive at the recommended specifications having specialized tools and equipment, to what they thought at the time would be the best compromise between handling, tire ware and comfort.
Not so easy for an average tinker like us…
