Chrysler: Wheel Alignment is Now An Exercise in Futility

Nothing much has changed with suspension system design in the past 30 years
since the introduction of the abominable McPherson Strut. The McPherson
was a lame effort to mimic the struts as used in aircraft. In aircraft,
the wheel undulations are purely vertical with no change in caster or
camber. In cars this would be the ideal, but such is not the case: When the
springs flex, that is in compression and rebound, huge camber variations
occur, which have a profound and negative effect upon the directional
stablity of the vehicle.

When wheels are aligned, three angles are measured and adjusted when
provided for. These are camber, toe, and caster. For many years, caster
is now fixed and no longer adjustable by the mechanic. HOW CAN CAMBER BE
ADJUSTED TO SPECS WHEN IT CHANGES ON ITS OWN WITH EVERY BUMP IN THE ROAD?
You can see how much camber changes on rebound by simply jacking up your
car and watching how the front AND rear wheels tilt outwards. Every time a
wheel tilts out, your car wants to go in that direction, like a sudden
turning impulse. Now, bounce the front end up and down and you will see
that the wheel tilts inward on spring compresssion. Since each wheel must
move independently, these little turning impulses occur in both directions
and completely at random. Besides, what do you think this scrubbing does
for tire life? With every bump, some of your tread goes to rubber dust.

It should be noted that all alignment angles are calculated to a point
contact with the roadway surface. See any auto textbook for the
visualization. This is very critical, because no tire has the ideal point
contact, but an oval footprint.

Back in the '70s when tires were close to half-round in cross section (-80
aspect ratio) the center of pressure of roadway contact was closer to
ideal, a point. Even as camber varied back and forth from the set value,
the center of pressure varied little laterally from the center sweet spot
because the tire tread rolled tangentally with respect to the road. Try it
with a cardboard cutout of a half-circle and you will see that a plus or
minus 5 degrees camber change only moves the contact point a little from
the reference zero degree position, the usual point at which camber is set.

Now, in the '00s tires aspect ratios are typically in the -60s with tires
as low as -35 aspect ratio. Now think of tires as rectangular in cross
section instead of half-round. So, when the camber changes just a little,
the center of contact moves from the center to the edge, unlike the small
movement as with the -80 aspect ratio tires. The turning impulse has now
been amplified many times over. Try this with a cardboard cut-out cut to a
rectangle with rounded edges to model these -45, -40, and -35 tires and you
can see on a tabletop what happens dynamically. The slightest tilt from
perfectly vertical puts all the pressure on the very outside edges of the
tire - a huge lateral movement from the ideal center sweet spot. Besides
the alignment difficulties presented by the low, wide tires, hydroplaning
is another, even more serious issue. Some of them have radical tread
designs to shed water accumulating under the ski-like tires; this will no
longer work when the tread wears down and even the best of them become
hydroplaners. I predict insurance companies and lawyers will act on this,
with insurance rates going sky high and lawyers working overtime on product
liability suits. You ain't seen nothing yet. This is going to be bigger
than the SUV rollover debacle.

Now for the driving test. A late 70's water-cooled VW used 155/80-R13
tires. Once aligned to factory specs, one could take the car up to 90 mph
on a straight and level road, release the grip on the steering wheel
(guarding it for safety) and the car would go STRAIGHT DOWN THE ROAD, with
no tendency to lead left or right. The car would stay in its lane for at
least 5 seconds (about 500 feet). Even earlier cars, those built in the
'50s with -92 (6.70-15 and similar) profile tires with sloppy steering
gears could duplicate this excellent straight line stablity test. I believe
this is now impossible to do with any car having low aspect tires because
even if perfectly aligned to specs, the slightest bump will set up a drift
causing a wander. D-C is now even offering a driver wake-up gimmick to
catch his wandering car before it can have a head-on.

It is my belief that the -80 profile was the best compromise for tread wear
life and directional stablity. The VW water-cooled was capable of 100,000
miles on the rear tires and 60,000 on the fronts, mixed city and highway
driving. Reference: Michelin All Weather, lowest priced line. Tires were
in the range of $30 to $50 for the life of the car, which turned out to be
about 13 years.

I invite a legitimate tire company engineer or a top D-C engineer not
afraid of his job, as well as experienced senior citizen drivers to confirm
the validity of the above assertions and conclusions. If so confirmed, it
is essential that the suspension systems of all cars fitted with the extra
wide, extra low tires be redesigned to zero camber/toe change with wheel
undulations OR a prompt refitting of all wishbone and McPherson strut cars
to -80 or higher profile tires. Those of you who have been mislead into
purchasing cars fitted with -75 and lower tires need to check for
directional stability and hydroplaning tendencies and make warranty claims
(to the car manufacturer) if unsatisfactory. While I do not suggest
unreasonabless on anybody's part, it is not unreasonable to act if it is
felt the car companies have long been aware of the complications of going
lower and lower profile and completly ignored them for the satisfaction of
style and style alone in an effort to push planned obsolence and boost
continually sagging sales. At least, is more benign to demand proper
smaller diameter wheels and high profile tires than to have to sue big time
after a crash due to an unroadworthy car.

In article <com>,
Nomen Nescio <com> wrote:
 
[snip]

Off your meds?