[xj-s] IRS cage motion/rear wheel steering

[mike90]

I posted on this in the racing forum, but thought I’d give this a
try on the XJS forum:

I have been studying the cage motion/rear wheel steering problem
for a bit, and I can come up with
the following approaches:

Using a modified trailing arm idea (these all replace the OEM
trailing arms with arms that tie to a point close to the center of
the car, along the same axis as, and in the same plane as the inner
pivot of the swing arm):

Andrew Robertson’s XJ/454 (JagLover’s photo album)

Rob Beere Racing’s A-frame, with adjustable camber and toe-in.

Terry’s Jag (for E-types only?) that ties the tieplate to what
appears to be the trailing arm mount points, and ostensibly also
has the A-frame trailing arm mod.

JCNA’s tech index feature, which shows the tieplate bolted to the
frame (? or are the bolts shown the place where the revised
trailing arms attach?) via a large T shaped plate, which is also
bolted at the trailing arm points, and which has revised trailing
arms. This looks to be E-type only, too (although the concept
ought to be applicable to XJS’s).

There’s the approach of FiBoy’s, which is to use an XJ40 part to
rigidly tie to the center of the cage, and mounts to the front
trailing arm mounts. Handles cage tilting motions, resists yaw
(rear wheel steering), but does not provide wheel thrust reaction
at the old trailing arm attachment point.

There’s the Harvey-Bailey approach which augments the trailing arm
setup by adding a chassis stiffening X-brace under the cage, and
some straps (?) to tie the cage to resist tilt motions under
accel/deccel. Does not address rear steering. Targeted at
convertibles, for which additional chassis stiffining is important
(early convertibles).

Finally, I have noted that there are vendors who offer (like Beere,
for example), uprated bushings for the trailing arms. What’s the
point of this? If the trailing arms are the cause of rear-wheel
steering (because they pull the wheel forward as the suspension
compresses or extends beyond the at-rest position), wouldn’t you
want MORE compliance at these points, and not LESS, so that the
tendency toward pulling the wheel would be lessened? Making the
compliance LESS (e.g.,stiffer) would seem to encourage MORE rear
wheel steering effect, and not LESS. On the other hand, more
stiffness at these points means that the at-rest and near at-rest
yawing tendency of the cage, especially under uneven traction
conditions (one wheel is getting traction, the other is getting
much less) is lessened.

For a street car, it seems to me that solutions that address cage
tilt under accel/deccel are more important than solutions that
address rear wheel steering. The XJ40-based solution that FiBoy
uses would seem to do both, although it would seem that the wheel
thrust loads, especially if one has uneven traction would put a lot
of stress on the center of the cage.

Any thoughts?

Thanks!

Mike–
Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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[mhewitt]

In reply to a message from mike90 sent Sun 14 Jan 2007:

mike -
what problem do you perceive that you are attempting to
address?–
77 daimler double six vanden plas/91 jaguar xjs convertible
sydney/nsw, Australia
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[mike90]

In reply to a message from mhewitt sent Tue 16 Jan 2007:

The first problem of concern (for me, on a street car) is cage tilt
under accel/deccel. As I indicated, I have had to replace two
broken cage mounts because of this issue. The second problem of
concern (although I don’t rate it very high, at this point), is the
so-called rear-wheel steering problem, which is due to the trailing
arms.

The question I was asking in this post is, what, if any, good does
it do to replace the trailing arm bushes with stiffer bushes? At
least one vendor offers ‘higher durometer’ bushes for the trailing
arms, and I am aware of at least one lister who uses delrin bushes
on the hub end of the trailing arms.

Why I ask this question: the trailing arms clearly provide a limit
on the cage motion, tilt-wise, and so, one would think that stiffer
bushes would improve the control even more. The arms also limit
the amount of yawing motion that the rear cage can experience, when
uneven traction (side to side) is encountered. However, if those
same arms are responsible for rear-wheel steering, wouldn’t stiffer
bushes tend to worsen this effect? Perhaps the added stiffness
doesn’t change the effective length of the arms enough to really
impact the rear-wheel steering effect.

That’s what I was interested in knowing.

Thanks!–
The original message included these comments:

what problem do you perceive that you are attempting to
address?

–
Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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[Kirbert (Author of the Book, former owner of an '83 XJ-S H.E.)]

mike90 wrote:

However, if those same arms
are responsible for rear-wheel steering, wouldn’t stiffer bushes tend
to worsen this effect?

The amount the trailing arms try to steer the rear wheels doesn’t
change with the stiffness of the bushings; the length of the trailing
arms and the travel of the rear suspension doesn’t change.

What might change is how much the trailing arms SUCCEED in steering
the rear wheels. If we were to presume that the subframe mounts were
really stiff and the trailing arm bushings were really soft, then the
trailing arms might TRY to steer the rear wheels but the bushings
merely flex instead. If this were the case, then yes, installing
stiffer bushings in the trailing arms would cause more rear-wheel
steering.

But this is not the case. In OEM configuration, both the subframe
mounts and the trailing arm bushings are very soft. So under any
sort of load at all, you get a considerable amount of flex in all
directions. Any rear-wheel steering caused by the radius of the
trailing arms is probably lost in the wandering of the rear end due
to everything just flexing every which way it wants.

Personally, I wouldn’t worry about rear wheel steering until AFTER
you’ve stiffened up both the subframe mounts and the trailing arm
bushings and observed just how much of a problem you think it is.
It’s not accidental; Jaguar put that rear wheel steering in there for
good reasons. Racers tend to prefer less rear wheel steering than
stock – possibly even none at all – but most street cars with IRS
have some level of rear wheel steering designed in.

– Kirbert

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[mike90]

In reply to a message from Kirbert sent Tue 16 Jan 2007:

Kirby:

I don’t mean to say that the arms change length, physically,
rather, the ‘effective length’ of the arm changes (e.g., what I
mean by this is that the distance, as measured from the bolt center
attaching to the forward end of the arm, to the bolt center
attaching the rear end of the arm to the hub, can change. This is
because there is compliance in the load string (e.g., the bushes).
This distance, and where it becomes maximum, in relation to the
position of the hub (is it at rest, compressed, or extended, that
is, are we travelling straight ahead or turning hard in one
direction or another), bears on the amount of rear wheel steering,
at least it seems to me that is does.

I agree on your point: I don’t think the rear wheel steering issue
is of much significance on a street car (if you’re going that fast,
and executing that level of control input to the car, you probably
shouldn’t be doing it on public roads…). But, the cage tilt
problem, it seems IS a potential problem for street cars.

I’m just trying to sort out the best way to approach the problem.
Is it to get stiffer bushes for the trailing arms? Stiffer mounts
for the cage? Some sort of structural restraint tied to the cage
(compliantly, I hope)? At what point has it gotten good enough?
These are the questions I am thinking about.

My interest is to get that 5-speed kit installed in the car when
the weather breaks…using THAT will cause some serious cage
loading, I am suspecting.

Thanks,

Mike–
The original message included these comments:

change with the stiffness of the bushings; the length of the trailing
arms and the travel of the rear suspension doesn’t change.
Personally, I wouldn’t worry about rear wheel steering until AFTER
you’ve stiffened up both the subframe mounts and the trailing arm
bushings and observed just how much of a problem you think it is.

–
Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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[Kirbert (Author of the Book, former owner of an '83 XJ-S H.E.)]

mike90 wrote:

But, the cage tilt
problem, it seems IS a potential problem for street cars.

I’m just trying to sort out the best way to approach the problem.

My interest is to get that 5-speed kit installed in the car when the
weather breaks…using THAT will cause some serious cage loading, I am
suspecting.

Here’s what I’d suggest: Get some stiffer subframe mounts. Get poly
bushings for the rear end of the trailing arms only. Install a set
of new OEM bushings at the front end of the trailing arms, but
install them rotated 90 degrees so the holes in the rubber are
oriented to the sides rather than fore and aft. Not much money or
effort involved in these minor changes, and chances are very good
that they will avert problems.

Once you’ve tried it out, then you can evaluate whether more changes
are necessary. Whatever you opt to change at that point is likely to
be significantly more involved and expensive, so it helps to get the
optimum baseline for evaluation to begin with.

BTW: with that 5-speed installed, you might wanna be looking into a
torque link for the engine. There has been a discussion about a
simple mod to the LH motor mount that will work; basically, drill a
hole through it and install a bolt with rubber bumpers that will
prevent the two halves of the mount from pulling too far apart.

– Kirbert

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[mhewitt]

In reply to a message from mike90 sent Tue 16 Jan 2007:

mike -
here (& maybe there, I dunno), we call what you’re calling
tilt, tramp

usually only a problem for hoons who like to do burnouts…

the two ‘problems’ (i’d call 'em features, not problems…)
work together - tramp & rear-wheel steering

to illustrate, imagine two scenarios:

1. cage rubber mounts replaced with solid metal mounts
2. radius arms replaced with rose-jointed links

in the first instance, you’d have no tramp, no rear-wheel
steering, increased nvh, & the radius arm bushes would
deflect like crazy as the suspension travels

in the second, tramp would affected depending on where in
the suspension stroke you were & rear-wheels steering would
be increased; i’d not expect any material effect upon nvh

there’s a third possibility: scenarios 1 & 2 used together;
in this instance, the suspension would lock up, or something
would break…

harvey bailey makes an anti-tramp system; I haven’t seen it,
but being hb, i’d assume it works

don’t know that i’d want to second-guess the designers &
entirely remove rear-wheel steering from the equation, though

jaguar’s own sport kit retained the bushings in the radius
arms, although I believe they were stiffer (one by dint of
materials, the other by dint of installation orientation)

whether this was to provide a modicum of anti-tramp, to
reduce (or possibly, as you posit, to increase) the
rear-wheel steering characteristic, or a combination of the
two, I dunno

as for yawing, i’d imagine the limited-slip diff would take
care of that

also worth bearing in mind is that, with good condition
mounts & bushes, we’re talking tiny movements here anyway…–
The original message included these comments:

The first problem of concern (for me, on a street car) is cage tilt
under accel/deccel. As I indicated, I have had to replace two
broken cage mounts because of this issue. The second problem of
concern (although I don’t rate it very high, at this point), is the
so-called rear-wheel steering problem, which is due to the trailing
arms.

–
77 daimler double six vanden plas/91 jaguar xjs convertible
sydney/nsw, Australia
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[Charles]

In reply to a message from Kirbert sent Tue 16 Jan 2007:

I have ripped out both sets of bushings on my track car. I
have gone to delrin bushings for the rear most bushings, as
Kirbert suggested, much stiffer. I then tried rotating the
front bushings 90 degrees, but they ripped the first time out
on the track.

The delrin bushings have helped the track car, and my street
car. I can offer them if you need a set. Rotating the front
radius arm bushings may help on the street, but on the track,
they won’t hold up unless you get poly bushings, which I’m
not sure can be had, although once were offered from Keisler.

Charles
88 V12 Track
91 V12 Street–
The original message included these comments:

Here’s what I’d suggest: Get some stiffer subframe mounts. Get poly
bushings for the rear end of the trailing arms only. Install a set
of new OEM bushings at the front end of the trailing arms, but
install them rotated 90 degrees so the holes in the rubber are
oriented to the sides rather than fore and aft. Not much money or
effort involved in these minor changes, and chances are very good
that they will avert problems.

–
Charles
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[Kirbert (Author of the Book, former owner of an '83 XJ-S H.E.)]

Charles wrote:

I have ripped out both sets of bushings on my track car. I
have gone to delrin bushings for the rear most bushings, as
Kirbert suggested, much stiffer. I then tried rotating the
front bushings 90 degrees, but they ripped the first time out
on the track.

I understood the idea of rotating the forward bushings 90 degrees was
used by Jaguar on the higher-performance models (Sportspak or some
such). I wouldn’t expect that rotating them would make them any more
likely to get ripped out. How are you avoiding ripping them out?
Did installing them non-rotated make them last? Or did you give up
on the OEM bushings altogether and go some other way?

The delrin bushings have helped the track car, and my street
car.

How stiff is Delrin? Is it like a really stiff rubber, or is it like
hard plastic? Harder than poly? As hard as nylon?

I can offer them if you need a set. Rotating the front
radius arm bushings may help on the street, but on the track,
they won’t hold up unless you get poly bushings, which I’m
not sure can be had, although once were offered from Keisler.

I’ve never seen Keisler’s poly bushings, but I’ve always doubted that
poly could flex as much as these bushing must without ripping apart.
IMHO, the solution here would be to redesign the bushing. Rather
than relying on the flex of the rubber, devise a center piece that’s
ball-shaped. Mold the poly around it so that it swivels on it like a
ball-and-socket joint. I think I recall Keisler’s bushing did
something like this. I think it’d last forever, would hold up to
racing, and it’d be 100x stiffer than the OEM bushing. Of course,
stiffer might not be good – it might be a good idea to resort to the
soft OEM bushings at the rear end of the trailing arm.

– Kirbert

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[Dick_Maury]

In reply to a message from Charles sent Wed 17 Jan 2007:

SNG Barratt out of England sells or sold the urethane large end
bushings in two versions, street and track. We have a set on the
XKE that we run the slalom with and they are very stout. Haven’t
torn up yet.–
The original message included these comments:

The delrin bushings have helped the track car, and my street
car. I can offer them if you need a set. Rotating the front
radius arm bushings may help on the street, but on the track,
they won’t hold up unless you get poly bushings, which I’m
not sure can be had, although once were offered from Keisler.

–
Dick Maury , Rebuild Dept., Coventry West, Inc.
Lithonia, GA, United States
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[Charles]

In reply to a message from Kirbert sent Wed 17 Jan 2007:

Delrin is very hard, hard nylon. The rubber bushings used to
get ripped all the time. Now these ore fine. But I do wonder
if they are causing the front bushings to rip more easily. We
are not having luck with the OEM front bushings, no matter how
they are rotated. We are thinking of going to a triangulation
to the rear with a Heim joint and getting rid of the trailing
arms, but I am shuttering at the cost as I had to replace the
engine this year.

So in the end, I’m not sure what the correct way to go is with
the bushings.–
The original message included these comments:

such). I wouldn’t expect that rotating them would make them any more
likely to get ripped out. How are you avoiding ripping them out?
Did installing them non-rotated make them last? Or did you give up
How stiff is Delrin? Is it like a really stiff rubber, or is it like
hard plastic? Harder than poly? As hard as nylon?
something like this. I think it’d last forever, would hold up to
racing, and it’d be 100x stiffer than the OEM bushing. Of course,
stiffer might not be good – it might be a good idea to resort to the
soft OEM bushings at the rear end of the trailing arm.

–
Charles
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[Charles]

In reply to a message from Dick Maury sent Wed 17 Jan 2007:

I’ll give them a try. The only alternative is to get rid of
the trailing arms completely and triangulate the rear as we
have seen in the links earlier in the posts.

Thanks.–
The original message included these comments:

SNG Barratt out of England sells or sold the urethane large end
bushings in two versions, street and track. We have a set on the
XKE that we run the slalom with and they are very stout. Haven’t
torn up yet.
Dick Maury , Rebuild Dept., Coventry West, Inc.

–
Charles
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[mike90]

In reply to a message from Charles sent Wed 17 Jan 2007:

Thanks for the tips on where to find these bushes in poly and
delrin! I am running poly from Sng on the front end with good
results, and I had to go online to see what the latest Sng stuff is
(my old edition paper catalog didn’t have anything for the rear
except neoprene cage mounts).

On the issue of ripping out the trailing arm mounts: is
triangulating the rear the only way out of this? I thought that
FiBoy’s idea using the XJ40 A frame was pretty neat. The only
concerns I would have on this approach are 1) this does not take up
the hub thrust loads (but is this needed?) the way trailing arms
would (whether conventional, or pointed back to the centerline of
the car and pivoted there), and 2) It complicates dropping the cage
(or at least it seems to, to me).

One other approach I have been considering, is to use a body
stiffener brace like that used on the XJS-C, which mounts to the
body fore and aft of the tie plate of the cage, and runs just
under the cage. Now here’s the proposed change: fit a couple of
large-end radius arm bushes to plates mounted a) on the bottom of
the tie plate, and b) to the upper-facing side of the stiffener
brace. Hmmm…this would control cage tilt, stiffen the body, and,
if the bushes are located properly in number and geometry, it ought
to provide the possibility to dispense with the trailing arms
altogether. What do you guys think?

Mike–
The original message included these comments:

I’ll give them a try. The only alternative is to get rid of
the trailing arms completely and triangulate the rear as we
have seen in the links earlier in the posts.

–
Mike, 1990 5.3 XJS Convertible, ‘Caterwaul’
Lakewood, OH, United States
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[Charles]

In reply to a message from Dick Maury sent Wed 17 Jan 2007:

Dick,

I was going to order 2 sets of rear bushings(large and small)
for the car from SNG Barratt, in Poly. With shipping and tax,
it would be over $600. It may be an option, but if I still
have to tie in the diff to the body by building a cruciform,
it might end up being the same as building a whole new set up
by triangulating it.–
The original message included these comments:

In reply to a message from Charles sent Wed 17 Jan 2007:
SNG Barratt out of England sells or sold the urethane large end
bushings in two versions, street and track. We have a set on the
XKE that we run the slalom with and they are very stout. Haven’t
torn up yet.

–
Charles
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[Charles]

In reply to a message from Charles sent Thu 18 Jan 2007:

Another option I just found is the HBE anti-tramp kit that
Roger has on the AJ6 site.

Does anyone have it for their track car?–
Charles
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[TMack]

Thanks for this Kirby! “It’s not accidental; Jaguar put that rear wheel steering in there for good reasons.” I can now give up on chasing vanishingly small amounts of rear-wheel steering…

[gregma (Greg (1988 XJ-S V12 Lucas))]

Now that I’ve replaced rear shocks with Bilstein, replaced trailing arm bushings (rubber), and have new tires on the XJS that I’m restoring, I can give finally push the car a bit.

I swear I can actually feel the rear end steering itself into the turn as I push the car harder on a turn…very amazing engineering!

[tobmag]

I have posted my modifications before but here we go again;

[Aristides (Aristides Balanos)]

@gregma

I swear I can actually feel the rear end steering itself into the turn as I push the car harder on a turn…very amazing engineering!

Yes you can…!
I know nothing about racing, but I wonder why one would want to eliminate that feature on a street car. It’s amazing how well this 30 year old, 2 tonnes and 6 meters long XJ corners, even compared to today’s cars.
And let’s not talk about its incredible ability to swallow any bump or pothole and render any road glass smooth.