How
Toe is measured on the FASEP wheel alignment system ?
In other
words, in what direction are you aligning your car?
Few
customers are wondering about the "magic" way FASEP alignment
system measures toe without any cross toe measuring arm.
The fact is indeed quite simple when you think to the way an angle can be
measured on a plane: you can measure it referring to x-axis (horizontal, or
across the car) or y-axis (vertical, or along the car).
So now please give us few seconds to
explain how FASEP (and actually many other manufacturers) measures toe on a
car.
In the end, what you are supposed to do is to align the car to its travel
direction; something you may not have done so far.
The
ideal case: rectangle shape chassis, 0° toe.
A little
more complicated: rectangle shape, toe not null.
Trapezoid shaped chassis, toe not null.
Measuring
along the "y-axis". Only the
FASEP System?
Conclusions
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figure 1
click here to enlarge
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The ideal case: a rectangle shaped chassis
The yellow line is the geometric centerline of the vehicle.
Toe of the left front wheel is the angle between the wheel middle line (red)
and the x-axis or the y-axis.
In the case of a car ideally rectangle shaped, with all wheels at 0° toe,
it is very clear how the real toe of the wheel (red line) and the toe
measured on the sensor (light blue line) are giving the same value, that is
0°.
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figure 2
click here to enlarge
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A little more complicated: rectangle shaped
chassis, toe is not null.
In this conditions, if wheel is with toe-in by 1°, it is easy to see that
the sensor measuring toe along the car (y-axis) that is with respect to
the sensor placed in the rear left wheel (light blue lines), is also giivng a reading of 1°.
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figure 3
click here to enlarge
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Trapezoid shaped chassis, toe not null In
this picture, the chassis is no longer rectangle shaped, and become a
generic trapezoid shaped chassis.
In this case, we designed a front track smaller then rear track (usually
it is opposite situation). The difference between
front and rear wheels tracks is evidenced by the green lines.
Displacement of the
left rear wheel with respect to the green line (or, with respect to
the ideal position of the rear wheel if the chassis was rectangle shaped)
is called semi-track difference. The value of
toe at the left front wheel against the centerline (yellow) is still 1° (red line)
but angle "Beta" (light blue lines) measured by the front left sensor
(in relation with rear left sensor) is not 1° because the rear left
sensor is displaced to the right with respect to position in figure 2. However
angle "Alfa", given by the displacement of the rear wheel can be
easily determined by measuring the (semi) track difference.
Alfa + Beta make then the correct result of toe
angle for the left front wheel.
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Measuring toe "along the car"
(or referring to y-axis).
Only the FASEP System? Approaching
the wheel alignment measures "along the car", instead of
"across the car"
(I mean with front long arms that allow to do a transversal measure), is not a FASEP
exclusive point of view.
Other manufacturers are now doing the same way, recognizing that measuring
along the y-axis gives consistent advantages in measurement process and
accuracy, not to mention the fact that a car is traveling exactly in the
direction of the y-axis and there is where we want it to be aligned. Among
others, we can mention:
- Policontrol (made in Switzerland, for control lines)
- Powercontrol (made in U.S.A., for motoring assembling lines)
- B-Dyna (made in Japan, for control lines)
- G-Swat (made in Japan, for control lines)
Latest added to the "y-axis" approach:
- John Bean 3D (Balco, Sun and Hofmann are the same machine)
- Hunter 3D
- Bosch aligner introduced in Automechanika 2002.
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Conclusions
The way I described things here is obviously following very simple
cases and conditions of the car.
More complicated cases can easily be described just in the same way, but
taking in mind that Thrust Angle becomes important. However
the main point to understand here is that an angle can be measured always
with reference to a given direction.
Traditional aligners do measure toe from one side the other on a car, what
we defined the direction of x-axis, in other word left-to-right.
FASEP, since many years, and very recent aligners (3d generation) are
instead using a different approach, getting the same reading referring to
the y-axis, or , more simply, front-to-rear. Of
course the way to measure the angle does not change the angle.
But in all measuring systems you have always to take tolerance and
accuracy into your consideration. The question is
that your car is traveling along the y-axis, front-to-rear.
So you are supposed to do this exactly: align the car to such direction.
As simple as that.
Now, wouldn't it be better to place your measuring system in such direction
instead of left-to-right, in order to align your car exactly where it has
to be aligned, that is the travel direction? |
