IRS toe inquiry

XJ-S
1

Folks:

After removing the factory spec bushings in my steering rack and replacing these with poly, I carefully reinstalled that rack, paying particular attention to the parallelism of the rack main body with respect a line drawn between the lower A-arm pivots. While not done using the factory jig tool, I paid attention to the main reason this tool is specified.

Prior to changing the rack bushings, I knew I had a pre-existing toe out condition, as it identified itself by inordinate wearing of the inner sides of both tires, one much worse than the other.

After replacing those bushes, I brought the car (and with fresh tires all around) to an alignment shop I have used in the past (but all U.S. carsā€¦). This was an experience I may write about another time. They almost refused when they discovered a Jag tool was required to align (the rear suspension) the car. I made it clear that I was only after toe, and that they could tell me about caster, camber and even toe, for all four wheels when they finished the front toe. They agreed that they could do the front toe adjustment, but they did not want to touch the rear suspension at all.

Incoming front toe was: left: -0.60 degrees (0 to 0.13 degrees factory), right: -0.98 degrees, for a total toe of -1.58 degrees. Way out- in fact, you could see the outward pointing of each wheel, and also note one was a bit further out than the other.

As adjusted, the front toe was: left: 0.05 degrees, right: 0.06 degrees, total toe: 0.12 degrees.

The short story is that the car now tracks and steers with the impeccable manners and sharpness I recall from earlier days when I had run poly bushes in a different rack. So, so far, so good.

They then presented the situation on the IRS: toe: left side: -0.12 degrees (spec is 0 to 0.03 degrees, I am told), right side: +0.07 degrees, with the total toe at -0.05 degrees.

Hereā€™s the rub, though: I have not seen evidence of abnormal tire wear, let alone wear ascribable to toe misadjustment.

I have understood that the IRS cannot be adjusted for toe, and that toe issues, when they arise, evidently are due to excess needle bearing wear on the lower arms. There may be other reasons, but I am only aware of the bearings matter.

Anybody on this list have any experience with these kinds of IRS toe measurements? About 8 years ago, I changed out the diff, and thereā€™s evidence that the camber on the left side needs to be adjusted (when I swapped the diff in, I simply replaced the same shims I had carefully removed when disassembling the setup to remove the 2.88 diff- never came back to camber measurement, tho, as I probably should have done.). But toe? This must have been pre-existing from the 2.88 days, yet, I could never observe inordinate tire wear patterns on any tires I have run on this car.

I am puzzled, as the measurements ostensibly indicate a rear toe issue, but the tire wear is simply not consistent with those measurements. If anyone out there has had a toe problem on their IRS, and have solved it, I would very much like to hear about it!

Thanks,

Mike

2

Hi Mike,

First of all, do you trust the shopā€™s measurements?
Did they really settle the rear after they put the machines on?

Does the car drive straight?
Does it wonder right or left?
What happens when you take your hands of the steering wheel?

Uneven front toe does not explain the different amount of tire wear between right and left.
Could be because of front Camber, rear Camber (that most probably is off since you changed the diff), or rear Toe.
You should make yourself the ride height tools for the rear and check front and rear angles.
Only then you can reach to some meaningful conclusion.

If rear Toe is indeed out of specks itā€™s either a bent wishbone, the cage is not mounted straight on the body, or the body is bentā€¦

Here is how I dealt with my rear Toe issue.

3

Aristides:

Thanks for replying on this one-

No, I am not entirely sure I trust the shopā€™s measurements on all quantities, especially what they have reported on the IRS as regards toe.

The car tracks straight and does not wander. The steering feel is much better (as regards pointing the car) and more consistent. These remarks are for the car set up with new tires all around, comparing before front toe adjustment and after front toe adjustment. Prior to toe adjustment, the steering response was OK, but kind of ambiguous, I might say. After toe adjustment, much more direct cause and effect, precise.

I had worn the front tires after about 5 seasons of driving (summer car, gets about 1500-2000 miles per season). Worn in the sense that the inboard sides were worn pretty heavily. The last season (last summer) the tires had worn sufficiently to cause the car to pull strongly, and become quite squirrely to steering inputs. I thought the source was much more involved than just toe, but it turned out to be toe.

I have had the cage on and off the car a couple of times, cage mounts replaced and in good shape. Trailing arm mounts, especially big ends were replaced twice.

Car had not been involved in any accidents as far as I can tell, prior to my acquiring it in arounds 2002 or thereabouts.

I will spend some time on the rear suspension to see if I can sort some of this out. I believe there may very well be a camber adjustment that is required, as I replaced the diff and while carefully keeping the shims from the 2.88, I did not have camber measured. So it may be that camber on the rear at least is at issue.

Will have a look and report back-

Thanks!

M

1 Like
4

What is the reason for that? Is it in reference to the vertical plane?

(Curiosity question as I canā€™t make any useful response to your main question. Sorry!)

5

Mike -

Against my better judgment, having never driven or worked on an E-type, Iā€™m going to suggest looking at this from an axle perspective, before individual wheels.

If I understand your numbers and sign convention correctly -
Relative to the datum set up by the alignment shop:

  • your front axle previously steered 0.38 degrees right, which is significant
  • your rear axle steers left, by an amount that is out of spec but small
    The combination of the above two gave a nett right steer.
  • your front axle now steers straight (relative to somethingā€¦) so the overall nett steer is still right but greatly reduced.

Given the dynamic changes in wheel angles - especially front steer - as they gently rise and fall while you roll down the road, that nett right steer may now be small enough not to be troublesome.

I wouldnā€™t get anxious about the individual rear wheel angles without first considering where their nett centre line is pointing (I believe this is sometimes known as thrust angle). That could explain the perceived lack of rear tyre wear.

6

Hello CliveR:

This first came to my attention a number of years ago, when a VERY knowledgeable lister, George Balthrop, posted this:

[xj-s] Steering Rack Alignment - XJ-S - Jag-lovers Forums

Having looked closely at my carā€™s mechanicals, and how that tool would be used (JD 36A), I have concluded Georgeā€™s observations are probably correct.

-M

7

Without seeing the tool I can only imagine the purpose is to have the rack the same height relative to the control arm mounts each side, so the bump / roll steer is equal both sides. It sounds intrinsically desirable but Iā€™m not sure it would actually be critical since bump travel in a given road event will rarely be equal each side, and in a roll situation one wheel goes up and the other down where the steer curves will not be equal and opposite.
Very much open to correction while I go away and look at some steer curves.

8

Clive:

I wished I understood the numbers a bit better- all of the shopā€™s figures are in angular units of degrees. Figuring the toe (which is specified in inches in the Jag manuals) is TBD. I suppose it must be related to tire diameter, taken with the angle, at the tire centerline (planar centerline of the tire), but not sure.

You have offered an interesting interpretation of the net situation, and I will study this further- the outcomes tend to match the carā€™s behavior up to the point that the net steer to the right may still be present- early tests on a flat, level road suggest itā€™s pretty much spot on. Prior to the toe adjustment, the car pulled strongly to the right (once the tires were worn from the toe misalignment, that is- up to this point, really did not pull- did other things (ambiguous steering behavior is the best I can describe it).

Thrust line and so on, these are things I should set up to measure- Aristides has given some interesting ideas for home-built kit to make the measurements. Rigorous geometry, is what is needed, but otherwise, seems rather straightforward at least to measure.

Thank you!

-M

9

Youā€™re welcome, I hope Iā€™m not adding confusion.

I had a quick look at some XKE simulations I did a few months ago, in fact the steer curves look unusually symmetrical in up and down motion so I might need to adjust that comment. I need to look a bit deeper because I played around with the vertical position of the steering rack (in software) and vaguely recall the steer effects were unusually sensitive to its relationship to the upper control arm - I guessed because it is situated unusually high.
Iā€™ll be back!

Above amended to clarify reference to XKE data - I donā€™t currently have an equivalent digital model for XJS, am happy to make one if anyone has 3D dimensions for the suspension pivot points.

10

I think the figure in inches is obtained by measuring between the wheel rims - across the car, horizontally at wheel centre height. The difference between that figure at the rear of the wheel and at the front is the quoted toe value.

Anyone with a better memory than mine might correct that, itā€™s been a while since Iā€™ve used that format.

In the unlikely event that Iā€™m correct, you can convert inches values to degrees thusly:

image

where F and R are distances between wheel rims, ahead and behind centre line, and D is wheel diameter. (gives toe-in a positive value)

This is the total axle toe, of course.

11

Hi Clive,

The official name of the tool is ā€œMid Laden Toolā€ and this is its exact purpose.
Jaguar specs on Toe and, especially, Camber are given for the mid laden position.
As Iā€™m certain you know very well, Camber changes significantly from one extreme to the other of travel suspension.
The front mid laden position is almost where the car sits normally, but the rear is not.
Guess they also wanted to be sure that the car would be parallel to the ground as much as possible.

12

Here is some more

13

Aristides:

Thank you for posting the link to your very nice alignment tooling- I was very impressed when I first came across this, and remain so-

A quick question for you, and CliveR, if heā€™s interested: One thing I failed to mention on this car is that I have lowered it. This was done by 1) on the front, removing the plastic spacer rings in the spring towers (done when I rebushed the entire front end), and 2) On the rear, by adjustments of the spring seats. This was further enhanced, in the front, by running two different tire sizes on the front and rear. For this, I targeted the XJRSā€™ tire specs. While the XJRS tires were set to 16" rims, I am running the 17" ā€˜revolverā€™ rims from the early xk8, and this time around, am running 245-50-17 on the rear, and 215-50-17 on the front (my original choice was 225-45-17 and that size tire was the set that were worn due to toe issues. I changed to the 215-50 size as the stiffer wall of the 45 series is a bit much, and I was looking for a bit more compliance). These choices closely mimic the tire OD, f/r, of the XJRS. The car as a result has a slightly nose down attitude. The improvement into turns was very noticeable, though, and I rather liked that.

So it occurs to me that the removal of the spacers from the spring towers (front) and messing about with the spring seats of the IRS has changed the ā€˜midladenā€™ or mean/reference height and so the measurements of caster and camber may be up for question, relative to the Jag service manual figures. Which way would caster and camber move under such a circumstance? both increase? both decrease? I am curious, and not sure of how to reason about this.

-M

14

hello Mike

to my limited understanding, , changing the springs / dampers will change the dynamics but not the geometry of the suspension (is dimensions are the same as before, but your car is at a different point of the full travel
ā€œmid laden height toolsā€ will set the arms at the right angle, even if the height from the ground is different.
camber as probably increased a bit if the car is lower, but I have no clue about the toe change

may be you could aim to the XJRS values, as your chages are in the kind ?

15

Yes, changing ride height at the springs will change the alignment settings. Pretty much any modern car will have camber go more negative as the wheel goes up in ā€œbumpā€ motion, to increase cornering power on the outside wheel in a turn. I donā€™t have data for the XJS, below is the equivalent chart for XKE as I found enough data to build a model.

I donā€™t know which way caster would go on XJS, if the front suspension has anti-dive that would probably make caster increase slightly towards bump. Again, youā€™d need to measure or make some sort of chart or model to know for sure. XKE caster increases by a very small amount in bump travel.

Perhaps the most important quantity is steer angle (toe) change with travel. Itā€™s pretty much universal these days to have a small degree of toe out as the wheel rises in bump (roll understeer), for a feel of accuracy and assurance to the driver. I donā€™t have XJS data, the XKE has an abnormally large amount of this toe out.

Yes, if you have the appropriate curve such as the camber curve you can easily adjust your target setting for a different ride height - simply go along the curve from the datum height to the point matching your actual travel and read off the change of camber, caster etc. from the datum value.

Once you have that understanding you can check and set alignment at any ride height, of course, no need to employ tools to set a specific ride height. If anyone has dimensional data for the key XJS front suspension points I can probably make a model to give all these curves. I canā€™t do the rear because there is no template for that system in the RACE software that I use .
Some detail here https://race.software/, thereā€™s a good library of terms and parameters.
I have no affiliation with RACE, etc.

image
image1374Ɨ1030 143 KB

16

Thanks, I understand. Being somewhat lazy Iā€™d be inclined to measure ride height once and take reference measurements, so I could translate to another point on the curve and adjust without changing ride height. Would that work?

17

Iā€™m always sceptical of shop measurements. Iā€™ve noticed that the pegs of the sensors can sometimes not be securely located against the edge of the wheel rim because the tyre prevents it from being pressed in deep enough. Thatā€™s before even questioning the accuracy of the eqiupment and software.

2 Likes
18

Randall:

Agreed. This is the main reason Iā€™ve given such shops a wide berth, historically. And why I am suspicious of the IRS results in particular. The proof of the pudding is in the eating, right? I think they got the front toe reasonably correct, as the car is stable and straight on a flat, level road. And I suspect their IRS toe measurements, as I just donā€™t have any reason (based on any uneven rear tire wear) to suspect a toe problem on the rear.

Brings me back to Aristidesā€™ approach of starting from fundamentals and building up the tooling to make the measurements. I suppose I am going to have to do this, and moreover, it is a good thing, as I would really like to understand the interplay of the three quantities, toe, caster and camber, and the front/rear layout.

-M

19

It all comes down to the calibration of the equipment and, more importantly, the care the technicians take in zeroing wheel run out. All wheel heads have a compensation procedure for runout, and it has to be done on all 4 wheels. Then the tech has neutralize the suspension after each adjustment. Unfortunately, most techs just donā€™t take the time and accuracy becomes suspect.

1 Like
20

Yes, itā€™s very interesting.
Lots of info on the web on how to do it yourself.

My two cents are that you should use the mid laden tools, at least at the rear, regardless of your suspension modifications.
Rear Camber changes more than 1Ā° throughout the suspension travel.
Factory specs are -0.75Ā° +/- 0.25Ā° at the mid laden position. Without the tools, how would you calculate what to dial in?

This is not true.
If the thrust angle is not 0Ā° (parallel to the car) the rear will have a tendency to wonder in one direction and the front will have to constantly correct that. This could wear the front tires more than the rear.
Having said that, before I fixed my bent rear swing arm I had uneven tire wear both in the front and the rear, but had multiple problems.