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"Power Point Presentation of this article" courtesy Ed Dinkler (USA)

Note: angles are measured "along the car" (front to rear) only, that is in the "travel direction".

Note: angles are measured "along" (front to rear) and "across the car" (left to right).

figure 1

1. The easy case:
Let's make a simple example with a simple measuring system.
In the picture on the left, wheel alignment system is a simple 2-wheel alignment.
In the picture you can see a simple case: wheels have 0 degrees toe (red), that is the wheels are  "straight", and the value measured by the system is also 0 (light blue) for both left and right wheel.

figure 2

2. A little more complicated: wheels are "shifted".
If wheels are shifted (that is left wheel is not aligned horizontally with the right wheel) problems start coming.
Note: The shift between wheels is evidenced by letter A in the picture. It is normally referred to as "set back". Please keep in mind that left and right wheel might be shifted due to several reasons (difference in caster adjustment, manufacturer's design, collision, apparent shift *).

Let's consider the picture.
Wheels are still "straight", that is both have 0 degrees toe (red), but sensors are measuring 1 degrees toe (light blue) with symmetric sign left and right (+/-).

Real Total Toe is = 0+0 = 0 (red).
Measured Total Toe = (+1) + (-1) = 0 (light blue).

In other words: Total Toe is measured correct (0) so that tyre wear will be correct.
Instead, by the measuring system depicted, Single Toe (left and right) is measured wrong (+/- 1 degree instead of 0) so that steering wheel will not be "straight".

Conclusion: measuring toe "across"  the car can lead to wrong measurement when you have set-back.

* apparent shift is the shift due to the wheel aligner's measuring/ clamping system and not to the actual wheel position.

figure 3

3. The easy case, measured with the FASEP system

Let's see the example 1. again with FASEP system.

FASEP system measures "along" the car.
So reference is taken "front-to-rear" and not "left-to-right".
To make it easy, rear wheel angles are yet not considered, but the rear wheels are simply taken as a reference.

In the picture left you can see: 
- wheels have 0 degrees toe (red) that is the wheels are  "straight"
- value measured by the system is also 0 (light blue) for both left and right wheel.

figure 4

4. Wheel are "shifted", and measured with the FASEP system.

Let's now shift the left and right front wheels, just like in example 2.

Wheels are still "straight", that is both have 0 degrees toe (red).
FASEP sensors, however, measuring "along" the car, are still measuring 0 degrees toe (light blue). No error.

With FASEP, Total Toe AND Single Toe are measured correctly, even in case the wheels are shifted.

This is the first point to see how FASEP can be more accurate then 6/8 sensors systems.

Let's now consider a more complete case.
To make it simple, wheel are still "straight", that is all wheels have 0 degree toe.
In the picture, an 8-sensors system  is shown and angles measurements are shown in light blue (to consider a 6-sensors system just remove the green line marked with B).

With an 8-sensors system, customers are often convinced that they can get also:
- wheel base left (E) and right (F)
- setback = E-F = (see also point A in figure 2)
- front track (C)
- rear track (D)
- offset = front track (C) - rear track (D)

For Setback please remember consideration made in point 2: Setback MUST be measured in someway when you have an "across-the-car" type of wheel alignment system just like a 6 or 8 sensors type, to avoid wrong measurements (for partial toe).
See also a more specific article: Do you really need to measure SetBack ?

For Wheelbase and Tracks measurements, just see below. 

Just see how it is absolutely impossible to get Wheelbase measured with a 6 or 8 -sensors system where ONLY angles are measured.

In picture 6, you have a shorter car, but still all angles have 0 degree, like in picture 5.
So same angles, but a different wheelbase.

How can you measure wheelbase with an 8-sensors system with only angles measured?
Just you cannot measure wheelbase. 

You can just estimate wheelbase, assuming that front or rear track is correct (is the one specified by the manufacturer).
It is just a simple trigonometry calculation, but it is based on an assumption, not on a measure.

Just see how it is absolutely impossible to get also Front and Rear Tracks measured with a 6 or 8 -sensors system where ONLY angles are measured.

In picture 7, you see a wider car, still all angles have 0 degree, like in picture 5.
So same angles, but different front and rear tracks.

How can you measure tracks with an 8-sensors system with only angles measured?
Just you cannot measure tracks.

You can just estimate tracks, assuming wheelbase is correct (is the one specified by the manufacturer).
Another assumption, not a measure.

So what? To measure wheelbase you assume tracks are correct. To measure tracks, you assume wheelbase is correct. Just too much. Did you pay for a measuring system or for an "assuming" system?

Conclusion: when you measure only angles, you have many different cars (longer/shorter or wider/narrower chassis) that can "fit" with the same angles. So you really can not know what are your car's physical dimensions.

The FASEP system not only measure "along" the car, as explained in point 3 and 4.
FASEP system is also based on two physical dimension (distances) of the car.
Two REAL measures (front track G and rear track H) that, once entered in the computer, allow all calculations to be EXACT for that specific car, with no possibility of wrong results due to wrong assumptions that all cars are the same.

FASEP system is based on ANGLES (measured) and DISTANCES (G and H).
Only one car can be the right solution.

So everything is perfectly measured and identified. No wrong assumptions.

The advantages of the FASEP system
1. small, light sensors > easy to handle, less chances of drops, less problems if dropped
2. no set-back reading influence > less chances of mistakes by the user
3. no lateral-run-out reading influence > less chances of user mistakes by the user
4. wheel alignment angles measured without "assumptions" or "estimations"