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Home > Support > Tech Talks > How Toe is measured on the FASEP wheel alignment system?
latest update 04/04/18

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?


figure 1
click here to enlarge


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.



figure 2
click here to enlarge


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.



figure 3
click here to enlarge

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.



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.



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?


see also: Is FASEP a 6-sensors or 8-sensors type of wheel aligner?


Fabio Boni
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