| Wheel Alignment Modern
cars are very sensitive to wheel alignment. In the dim past of motoring,
when tires were poor and speeds were slow, the exact alignment of a car’s
wheels was not critical. In the days of solid axles the alignment was
set at the factory and the only thing that could be altered by the
average mechanic was the toe-in. If camber or caster had to be altered
you needed a blacksmith, not a mechanic.
As design progressed and independent front suspensions became
universal, camber and caster needed to be reset on a regular basis. The
most infamous car I know of was the ’62-’65 Chevy Nova. Hitting
anything, even a big bump in the road would throw the suspension out of
alignment. Today, with most cars possessing independent suspension at
both ends, all 4 wheels need to be aligned.
First, a little background on the why and how of wheel alignment.
Wheels need to be aligned for two reasons; good handling and good tire
wear. Good handling means that the car tracks in a straight line without
continual correction of the steering wheel and when the brakes are
applied, or a corner is taken, the car behaves in a predictable manner
and exhibits the maximum performance the tires were designed for. Good
wear means all 4 tires last as long as possible, in some cases 80,000
miles (realizing that tire rotation is almost always required to achieve
this). The ideal is for all tires to always be in contact with the road,
always rolling, never skidding forward or sideways. Modern suspensions,
properly aligned, can come remarkably close to this goal. Even more
ideally, engineers would like to achieve the minimum loading possible on
each tire to achieve this. A tire must be loaded enough for the rubber
to grip the road surface. Any loading beyond this minimum makes the tire
wear faster than it should and increases the power needed to move the
vehicle (gas mileage and performance suffer). Aerodynamic (ground)
effects that add load with increasing car weight and active suspensions
that prevent (or even create opposite) lean in turns are methods that
achieve some of this.
There are two aspects of wheel alignment that most people never
consider. Powered wheels must rotate at different speeds and turning
wheels must turn at different angles.
When a car goes around a corner the outside wheels need to rotate
faster than the inside wheels or one wheel has to spin. Differentials
split power in such a way that the powered wheels can turn at two
different speeds. In the early days of differentials one wheel
essentially freewheeled while the other drove the car. With the advent
of limited slip differentials both wheels could apply power. Today the
best limited slips (Torsen and certain viscous drives), along with
electronic traction control, do an excellent job of getting the maximum
power to both wheels of the driven axle(s) without causing tire
slippage. In the early days of limited slip differentials (E-Type)
multi-plate clutches were used to transfer the power to the wheel with
the most traction. This worked pretty well on dry pavement, but could be
pretty dangerous on patchy wet pavement; esp. after the clutches had
worn some and engaged rather abruptly. If you accelerated hard while
turning and one tire hit water some petty violent fishtailing could
result as the differential transferred power form one wheel to the other
in rapid order. This is where electronic traction control can be a
life-saver.
The turning wheels face an additional problem. The wheel on the
inside of the turn must make a sharper angle than the outside wheel
because it will be following a smaller arc. About the turn of the 20th
century one Rudolph Ackerman, an English publisher, worked out a system
for achieving this. If an imaginary line drawn through the steering
knuckle’s axis of rotation and the end of the steering arm intersected
the center of the rear axle, then each front wheel would turn the
correct angle in a turn and avoid any tire scrub. Although the Ackerman effect is not adjustable, a good wheel alignment
technician will measure it. If the car is out of spec it means the
steering arms or some other component of the front suspension is bent
and must be replaced.
Before I get to the three alignment characteristics that most of you
are familiar with I’d like to discuss two characteristics that can
adversely effect the handling of a car even when it’s aligned to
specs.
Steering Axis Inclination (SAI) is the angle from vertical of the
axis about which the front wheels rotate.
The SAI must be equal (within ½° on both
sides of the car, irrespective of the camber angle. If the SAI is not
equal the scrub radii will not be equal and strange handling will
result. What is scrub radius? If the point of intersection of the steering axis with the road is
outside the centerline of the tire a condition of negative scrub radius
is created. This condition creates a force toward the center of the
vehicle and tends to make the vehicle more stable, especially under
braking. Almost all front-wheel-drive vehicles use negative scrub
radius. If the steering axis intersects the road outside the centerline
of the tire we have positive scrub radius. Many rear wheel drive cars,
esp. older ones, have positive scrub radius. Which is better? Zero scrub
radius is the ideal. It gives the best tire wear and the easiest
steering. Stability under braking can be achieved in other ways. Citroen
was famous for its center-point steering in the 50’s. That’s just
another name for zero scrub radius. With the advent of larger diameter
wheel rims suspension designers now have more freedom to put the
steering axis where it should be and still maintain other desirable
suspension characteristics. When cars had 13 and 14 inch wheels it was
pretty difficult to achieve zero scrub radius. Needless to say, if you
replace the stock wheels you need to make sure you don’t move the
centerline of the tire. You must specify a rim offset that is the same
as stock. Custom wheels come in different offsets to achieve this and
can be made to order with the proper offset if necessary.
Thrust Angle and Thrust Line are defined by the rear suspension’s
relationship to the centerline of the car.
The Thrust Angle is determined by the toe- in of the rear wheels. If
they do not have equal toe the vehicle will pull in the direction of the
side with the greater toe-in. The Thrust Line of the car will not
coincide with the centerline of the car. This can be compensated for by adjusting the front wheel toe-in, but
that is not the correct way to align a car. A similar situation can
exist at the front end if the horizontal axis of the wheels is not
perpendicular to the centerline of the car.
This is known as Setback. The
upshot of this is that a car must have the same wheelbase (maximum
acceptable variation is about 1/2 inch) on both sides in order to be
correctly aligned and a correctly done alignment must start with the
REAR suspension!
Now for the 3 parameters most of us associate with wheel alignment.
Caster is the most complicated of the three. Caster is the offset of
the horizontal (rotational) axis of the wheel from its vertical
(turning) axis. The main thing caster achieves is steering stability. A
wheel that has caster will only roll easily in one direction. It resists
turning. Think of a dresser with casters on its legs. Normally the
casters point in four different directions and the dresser is hard to
move. Give it a push, or turn all the wheels in the same direction and
the dresser will easily roll in that direction. A bicycle is another
example. The fork is angled. This induces straight-line stability. A
side benefit is a smoother ride. When the wheel hits a bump the force
vector is ahead of the frame so the full impact is not felt by the
rider. Automobiles also benefit from these characteristics of caster.
However, too much caster (typically more than 2°
) makes a car reluctant to turn and twitchy when it does. Many cars with
independent rear suspension also have caster in the rear wheels.
Camber is the offset from vertical of the tire’s centerline. Logic tells us that camber should be zero for the best tire wear. If
cars only traveled in a straight line and never had very much power
applied to their tires this would be true. But cars go around corners
and power is applied, sometimes to the front wheels. In the early days
cars typically had zero camber on the rear wheels and positive camber on
the front. All other factors being equal this resulted in understeer,
the condition whereby it takes more turning of the wheel the harder a
car corners and if the power is reduced the car tends to go in a
straighter line. A safe situation for most drivers. With the advent of
radial tires it became apparent that a little negative camber could
increase cornering power, especially in front-wheel-drive cars, and tire
wear was not appreciably increased because radials tend to conform to
the road surface. The reason that negative camber increases cornering
power is because the outside tires in a corner are very heavily loaded.
If they are tilted over (which increases with body roll) they only
provide traction on their outside edge. Put in a little negative camber
and the whole tire will have contact with the road even when the car
leans. Some tires, such as Hoosier racing slicks, like as much as 3°
of negative camber. More typically 1/4° to
3/4° is specified. Just as with caster,
independent rearends have a camber setting. An independently sprung Jag
running on street radials responds best to 3/4°
negative rear camber.
Toe-in has already been discussed so I won’t dwell. Toe-in makes a
car go in a straight line without wandering. Too much toe-in wears tires
out at a rapid rate and costs gas. Some years ago GM tried for that tiny
bit of extra mileage by setting many of their cars for zero toe-in. On a
perfectly flat, smooth road (GM Proving Grounds) it was OK, but in the
real world their cars wandered too much. Rear suspensions also require
toe-in for vehicle stability, although the amount maybe different from
the front.
As the static and dynamic loads on a car’s suspension change the
alignment of its wheels changes. Early solid axle cars suffered from
wheel base changes as one wheel hit a bump. This had the net effect of
turning the vehicle ever so slightly. Many modern cars still suffer from
bump-steer. This happens because the steering tie rods don’t describe
exactly the same arc as the lower suspension arms. As the wheel moves up
the shorter arc of the tie rod pulls the steering arm in and the wheel
turns out. GM and Chrysler cars of the 50’s were famous for this.
Solid axles suspended on leaf springs also suffer from bump steer. As
the natural arc of the spring flattens with cornering load the axle and
wheel move back, lengthening the wheelbase on that side. For the same
reason the unloaded side tends to shorten the wheelbase. The famous
Jaguar independent rearend would suffer a worse fate if it were not for
the huge rubber biscuits at the front of its trailing arms. Because the
axle hub can only move vertically and the single trailing arm has to
travel in an arc the rubber biscuit allows the trailing arm to
"stretch" its length. If you would like to see an actual plot
of the alignment changes in an XJS front end see www.mich.com/~kroppe/camber.html
and www.mich.com/~kroppe/caster.html
and www.mich.com/~kroppe/toe.html
So now you’re ready to take your Jag for a 4 wheel alignment. What
do you look for to be sure your car is aligned properly?
Before you even go to the shop you need to make sure that all tires
have even wear and are properly inflated. If tires are worn unevenly the
car cannot be properly aligned.
Tires are OK so you have to find a shop that has an infrared or laser
alignment rack with a computer printout. Alignment men still exist that
can do a beautiful job with a tape measure, a bubble level and a toe-in
jig, but they would have to charge you more for an inferior job, so why
bother?
Once your car is on the rack the first thing the technician should do
is check every suspension and steering component for excessive wear.
This includes wheel bearings and springs.
The suspension checks out so now the tech sets the ride height. All
manufactures specify a ride height. It assumes a certain preload on the
suspension and is usually achieved with suspension clamps
but can be done with sandbags on the floor.
After the ride height is set the tech clamps a sensor to each wheel
and calibrates it. He clamps the steering wheel in a
straight-ahead position so the front toe-in reads correctly and he locks
the foot brake so the caster measurement reads correctly.
If alignment is necessary the tech must start with the rear wheels.
Herein lies the rub. Most rear suspensions aren’t adjustable. The only
thing on a Jag rearend that’s adjustable is the camber and that
involves removing shims from between the halfshafts and the inboard
brakes. A tedious and expensive task. If the toe is out of spec
components must be replaced. Heating and bending is NEVER recommended.
Many front ends cannot be adjusted for camber or caster. This is
especially true of McPherson strut suspensions. Aftermarket adjustable
camber plates for popular cars, like Mustangs, are available. Toe-in is
readily adjustable and with the steering wheel locked straight ahead the
tech will adjust both tie-rods equally. Jag steering racks have a
centering hole, but if your car doesn’t the center of the rack (or
steering gearbox) must be found and if the steering wheel is not in the
straight-ahead position it must be removed and installed properly.
Following is a checklist of everything the tech should do and the
order in which he should do it.
- Check tire pressure and wear patterns
- Check all suspension components including springs and wheel
bearings.
- Preload the suspension.
- Install and calibrate a sensor on each wheel.
- Lock the steering wheel and the foot brake (brake lights should be
disconnected at the fuse so they don’t burn out).
- Get readouts of all the parameters front and rear.
- SAI and Ackerman angles.
- Wheelbase on both sides of the car
- Camber and toe-in, front and rear
- Caster on the front
- Computer printout of measurements and calculated data such as
Thrust Angle.
So what should you have to pay for a quality 4 wheel alignment? If no
components need to be replaced there’s a good chance that the only
thing that needed adjusting was the front toe-in. This can cost as
little as $35. If all the adjustments possible need to be made, as with
a ground-up restoration, you can figure on $150 and up.
Fender Covers Among your essential tools you probably have a pair
of fender covers. I actually carry a pair in every car I own that
requires leaning over the fender to work on the engine. Because I’m a
cheap bastard I bought the Kragen specials for $9.95 a pair, the ones
that float away in the slightest breeze, slip when you look at them and
flutter onto the floor from the draft of the cooling fan when you rev
the engine. A couple of paper clamps per cover is the solution to your
and my problem
Metric Bolt Grading Everybody
knows how to read the strength of SAE bolts. For auto use you want Grade
8. The head of the bolt has 6 radial lines on it. For light duty
applications you can use Grade 5. Never use Grade 2 or ungraded bolts.
So what are the metric equivalents and how do you read them? A chart is
yours to printout and hang on the garage wall Don’t forget to use Grade 8 nuts with those Grade 8 bolts!
5 speeds Many times people ask me about the technicalities of
putting a 5 speed in an XK or an E-Type. My answer in a nutshell is,
"It ain’t cheap and it ain’t easy". Figure on $5000 by the
time it’s finished. If you do the labor yourself and have some welding
and machining capability you maybe able to cut significant dollars off
this. Some of the problems: shifter doesn’t line up, clutch linkage
doesn’t work right, speedo can’t be installed, or is very expensive
to install, gear ratios are wrong and the rear axle needs to be
re-geared. If you have an XK you can put an overdrive trans in and even
if it’s the old Moss box, this is the easier way to go. If you have an
E-Type the Getrag, as supplied for the XJ-6 in Europe, seems to be the
easiest way. Don’t believe it when the kit maker says it’s a
bolt-in. Just ask Gil Borgardt about his experiences with his 120. |
