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Why
you need to measure steering-related angles?
Toe-out on turns at 20 degrees
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Steering
angle: it is needed for the following purposes:
1. to measure Caster angle (typically measured with a steering angle at +/- 20
degrees)
2. to measure SAI (also known as King Pin) (typically measured with a
steering angle at +/- 20 degrees)
3. to ensure accurate re-alignment of wheels after measuring Caster and
SAI Toe-out-on-turns at 20 degrees:
1. to measure the difference in steering angle between left and right
wheel when one wheel is turned (typically at 20 degrees) Max
steering angle
1. to measure maximum steering angle left and right for each wheel
(it is typically an angle between 30 and 40 degrees)
2. to measure exact centering of steering gear box. 
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How
can you measure such angle without electronic turntables?
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As a matter of fact, in the past, Caster and SAI readings were done using a
graduated scale on mechanical turntables: a "manual" reading was
done. Caster and SAI, however, are the only angles
that can be measured in such a way, that is with an accuracy of +/- 1
degree for the steering, even being more accurate is of course better.
All other measurements (Toe-out-on-turns, lock
angle) can only be measured with a good accuracy (as a matter of fact
FASEP electronic turntables measure angles with an accuracy of +/- 0,1
degrees).
Question is then :"how to measure Caster and SAI automatically
without electronic turntables?"
Until few years ago, answer was "impossible,
you need electronic turntables".
Nevertheless, recently, most of wheel aligners are offering the
possibility to measure Caster swing using toe sensors. This cut the costs
of electronic turntables and let caster swing to be automatic.
But.. when you cut costs, you may also cut
something else..
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Advantages
and disadvantages of measuring steering angle with Toe sensors.
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The
advantage of doing Caster swing using an existing sensor (the toe sensor)
is logical. No need of a adding anything new (the electronic turntables),
no extra costs.
Advantages stop here. Disadvantages show "how
blanket is short":
1. Caster swing measured at smaller angle then manufacturer's
specification: toe sensors have a limited operating range, typically +/-5
degrees, at best +/- 10 degrees. For this reason, Caster swing is done at
+/- 10 degrees instead of +/- 20 degrees as it is required by car's
manufacturers. This increase inaccuracy of caster reading.
2. other measures are not possible:
for the same reason that toe sensor's range is limited to +/- 10 degrees,
all other measures (toe-out, lock angle) are not possible. To measure such
angle, you must have electronic turntables.
3. a big mistake in steering angle reading:
toe sensors introduce a big mistake in reading the steering angle. See
below this in details.
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Inaccuracy
and mistakes introduced measuring steering angle with toe sensor.
click image to zoom
effect on inside steering wheel is even worse
click image to zoom
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Inaccurate Caster Swing:
The first problem comes from the fact that you are measuring Caster swing
at +/- 10 degrees whereas it should be measured at +/- 20.
Measuring at 10 degrees (and usually the limit is +/- 7 degrees) means
clearly that you double (or triple, at 7 degree) measurement inaccuracy,
that is the error made in measuring the Caster and SAI angles. Measuring
shift instead of rotation:
The second problem is due to the fact that toe sensors are not "at
the center of the wheel, or, to be more precise, at the center of the
steering axis. On the opposite, toe sensors are quite far from the
steering axis, typically 30-40cm (10in or more).
This means that, when steering, toe sensor not only turns but also shift
laterally.
Picture on left side show this fact clearly.
When steering, the wheel (thus the sensor) not only turns, but also shifts
(shift is to the right, in
the example) The red sector is the portion of angle due to the lateral
shift, that the toe sensors anyway measures as a rotation.
To be more clear: just look at the
turntables when turning the wheel. The plate not
only rotates but also shifts laterally (that is why the turntables must
also be free laterally!).
Inaccurate steering angle:
The sensor cannot separate the shift (left or right) by the rotation,
therefore shift is taken as a steering angle by the toe sensors.
This leads to
an error of 1 to 2 degrees.
Thus, measuring 10 degrees steering with toe sensors might be as
inaccurate as +/- 2 degrees.
The bigger the steering angle (10 instead of 7), the bigger the error.
The shorter the car, the bigger the error.
Measuring pure rotation with turntables:
The problem described above happens only when using the sensors to
measure steering angles.
If you use electronic turntables, instead, no problem at all.
Electronic turntables, actually, only measure the pure rotation, as they follow
every movements of the wheel, including lateral shift.
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How
important is to measure steering angles (and do it well).
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The reason given to save a lot of money and allow
a "small" error to go into the measurements is listed below:
1. Caster is not such an important measure
2. Caster is not measured most of the times
3. Caster swing does not need a high precision
4. Steering angles are a "mechanic" job, not a tyre shop job. These
things might be part of the customer's way of thinking, at the time of
buying an expensive piece of equipment, where electronic turntables might
be an expensive option.
Let me tell you this.
1. tyre service is a "sudden demand" type of business. When
season starts and cars start coming into the shop, time is critical: you
must run as the next customer is waiting. Wheel alignment check is usually
added as a final check after changing tires, and it follow the same rule:
quick!
2. wheel alignment end to be offered "included" in the price of tires. This is a wrong way of doing your job: you are not only selling
tires, you are selling your expertise!
3. a wheel aligner is set to work for many years: looking at an immediate
saving and letting your job to be a little more inaccurate, will let you
be inaccurate for years. While what you need is "be fast and
accurate".
So now what?
1. customer satisfaction
Customer is happy about your job, right after changing his/her tires, not
only if tires handle properly on the road (which is obvious as they are
new, and in this respect there is probably no much difference between you
and another tyre shop) but rather if car is globally handling properly,
with no left or right pull and a straight steering wheel. To ensure
this little two things, Caster measure is a key point.
2. rapid alignment check
Rapid alignment check, mostly on the front axle alone, is due to haste,
and it only let you see if tyre wear is ok and, provided you have a
4-wheel alignment, let you see if steering wheel will be straight.
In few more minutes, measuring Caster would also let you see if the car is
pulling on one side.
Why not looking at Caster, while car is on the alignment rack, and make
sure your customer will be happy about your job?
3. importance of Caster
from statistics made possible by computerized wheel aligners, we
calculated that only 4% of tyre shop measure caster. I must assume 96% do
not understand how important Caster is.
As you may know, Caster influence "return of straight steering wheel"
after turns and influence the straight direction of a car in motion. When
a car "pulls", most of the times Caster is the problem.
4. Precision in measuring Caster
car pulls if there is a difference in left and right Caster value.
Accuracy of measure is therefore important as we need to know the
difference between left and right. If you have a left measure and a right
measure with a 0.5 degree tolerance, tolerance on difference between left
and right is double.
+/- 1 degree accuracy is not acceptable when measuring cross-caster.
5. "mechanic" job?
in today's market, mechanic and tyre shop are set to mix. Mechanic shop
are selling tires, tyre shops are becoming "quick service"
selling brake pads, exhaust, etc.
Measuring steering parameters of a car help to identify problems before it
is too late, before they become expensive.
Measuring steering related angles make sure your customer will find
his/her car in perfect conditions after changing to new tires, from A to Z.
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