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Do you really need to measure SetBack ?
Since some years we hear few customers asking, based upon sales-talks from competitors, whether FASEP Wheel Aligners measure SetBack. Our answer is "should they?".

So now please give us few moments to explain our point of view and maybe, with a little attention, you may have a better idea of how you are going to spend your money for buying a new Wheel Aligner, whether it is a FASEP one or a competitor.

What is SetBack
How Set Back affects the behavior of a vehicle (part I)
How Set Back affects the behavior of a vehicle (part II)
Just a selling issue?
What is Set Back caused by
Set Back by design
Set Back to detect a collision?
Set Back or Caster problem?
When Set Back is caused by the Clamping System.
The FASEP System: no Set Back, thank you.
Without Set Back. Only the FASEP System?
Conclusions

figure 1

1. What is Set-Back?

SetBack is the angle between two wheels that are on the same axle.
In the picture 1, the front right wheel is apparently "shifted ahead" .

Measuring SetBack should  mean answer to the question "which wheel is shifted from the original position?, and how much is that shift"?

The value might be indicated as a distance (A) or as an angle (B)  on the axis perpendicular to the axis of travel.

figure 2

2. How Set-back affect the behavior of a vehicle (part I)

SetBack does not really change the behavior of the vehicle on the road.

4 parallel wheels still travel straightforward, whatever their position.

There could be other reasons for a car pulling: for example a different Caster between the right and the left wheel.
Further on this later.

figure 3

3. How Setback affect the behavior of a vehicle (part II)

The SetBack, also, does not influence the car good going on turns (the so called "rudder effect").

Actually, during the turns, the left and right wheels steer with different angles and on different centers and they are always  one before the other with respect to the traveling direction (the wheel inside travels ahead with respect to the external).
The picture 3 shows how the internal wheel travels ahead of the external by an angle B bigger then any possible SetBack angle.
The internal wheel, when on turns, can be ahead of the other by several centimeters (or degrees, if we talk about angles).
Obviously, the design of the suspensions takes this facts into account as they affect suspension performances.

We can clearly see that the few millimeters (or tenths of degree) of the SetBack (if present) are much smaller then the "shift ahead" of the internal wheel (by various centimeters) thus, proportionally, the SetBack (if present) influence on the car behavior is negligible.

4. Just a "selling issue"?

If SetBack does not influence the car good going on the road or on turns, we wonder how measuring the SetBack could be useful.

We may think, then, that it could be a "selling issue".
Let's not go into marketing or patent issues, let's continue talking on the technical side let's answer the question: "what is SetBack useful for"?

If SetBack does not help to find out the car behavior problems (specific wheel alignment measures may explain those problems!), we may hear someone saying that SetBack is useful to check frame problems and detect collisions.

We then would like to change the question from "what is measuring SetBack useful for?" into "what is SetBack caused by?"

5. What is Setback caused by?

The answers can be limited to 4 cases:
- the car maker
- a collision
- a different regulation of  right and left caster
- the wheel alignment measuring system itself

The last case may appear senseless, yet we have to ask: given a car with  real SetBack zero, could we read a SetBack measured different from zero? Later we will try to explain why the answer is yes.

figure 4

6. SetBack by design.

Are you sure that cars have a perfect "rectangular" shape, as everybody imagine?
It is enough to measure the Left Wheelbase (PS) and the Right Wheelbase (PD) with a common meter-gauge to understand how much they are different in reality, even in a new car, simply because of the car maker design.

There are a lot of examples and a lot of technical reasons.
The most known case is the "torsion-bar spring" (not to be confused with  anti-roll bar!) that we find on the front axle of Alfa75, Renault 5, Mitsubishi Pajero, Daihatsu Feroza or on the rear axle of Renault 4, Twingo, Clio.

Generally, we can say SetBack is caused by design choices regarding the suspension type (torsion-bar spring) or the transmission type (typical of the front traction with transversal motor).
In these cases, it is possible to find really different dynamic behaviors, because of a difference between PS and PD of various centimeters (comparable to the difference of advancement of the internal wheel with respect to the external one; look at the picture).

Once we "revealed" a simply forgot truth (PS differing from PD by design by few centimeters), we would like to ask:

"if the car maker designs purposely a SetBack between wheels on the same axle, why there is no specification for the SetBack value from the Car Maker to see if the SetBack value is right or not?"

and then

"why there is not any adjustment directly on SetBack, as it is designed by the car maker itself?"

We still continue to make you wonder: are you sure that you have to adjust or keep "under control" the SetBack, and that the car maker would leave a such important thing being wrong?

figure 5

7. SetBack to detect a collision?

This is maybe the the most used argument by the "sales-men of SetBack": "SetBack can show if the chassis or the suspensions have some problems!"

Let's imagine that the car under examination does not have a SetBack by design, so that the "original" SetBack should be zero, we measure the SetBack and, when it is found different from zero, we should  get some help about the chassis status.

Of course, nobody is going to buy a wheel aligner to measure SetBack in order to fix a chassis bent  because of the collision.
it would be a lot better using a car-bench built right for that scope. But we may be interested  to detect if our customer hit against the walkways or if he got hit and  damaged the suspension's elements.

Let's suppose then, for example, that the alignment indicates a SetBack of few millimeters/degrees between right and left wheel.
We now wonder which wheel is wrong. Unluckily it is not possible to measure the variation of the wheels from the original position, we can only measure the variation between the right and the left one. So detecting which wheel is wrong looks rather difficult.

Some Wheel Aligners do indicate which wheel is shifted. Actually, by admission of some manufacturers, they follow the rule that the left wheel is statistically more "protected" and the right wheel is the one hitting on the walkways: therefore they assume the left wheel position correct, and then indicate the SetBack positive or negative for the right wheel.
I think it is a quite weird rule. 

To answer to the initial question regarding a possible collision that might alter the suspension status, we prefer to consider more exact and measurable values. Inclination, caster and toe are important indicators of the good status of the suspension, also to find some gross blunder caused by a collision.

Anyway, looking at the suspension plays before going for the alignment might do a much better job then a "top-alignment-system" that simply guess which wheel is wrong.

figure 6

8. SetBack or just a Caster problem?

It is a weird thing to treat sinusitis with a medicine for the headache. But, on the opposite, we know that a sinusitis cure can represent the solution for the headache.
Simply: to solve a problem, you better look for the causes of that problem, not just for the symptoms.

This is the case of SetBack caused by a different caster between right and left wheel.
If we want to take care of an headache (I mean the SetBack) it is possible we fail, but if we treat the real illness (a different Caster), the collateral effects (SetBack) may disappear.

In fact, depending on the suspension type, a cross-caster (difference between left and right) can cause a SetBack.

With reference to the suspension scheme shown in the picture, where the caster regulation is on the lower joint, directly connected to the wheel, while the upper joint does not move, it is easy to understand that a caster regulation causes a SetBack different from zero because it changes the wheelbase. Of course, the Caster is the cause of SetBack and not the opposite. In the case of SetBack different from zero, it is better to check if Caster need to be adjusted before looking for collisions, etc. It would be like to take care of the headache instead of considering the real cause.

Obviously the Caster is one of the fundamental measures of alignment, which you can do with all FASEP aligners.

figure 7

figure 8

9. Set Back caused by the Clamping System. The Tyre changer  Test.

Before we wondered if a car with real SetBack zero may result with a SetBack different from zero? We said the answer is yes.

As a matter of fact, the sensor which reveals SetBack is linked to the wheel with a clamper.

Generally the clamper is subject to Run-out, which is compensate by electronically. In this case all is ok. The value of SetBack will be shown by the computer screen and in this situation we refer to the three cases we explained before.

Recently, by the way, we may find in the market some clamps "run-out free" which help to avoid the run-out procedure to save time and preserve the original suspension balance. According to the manufacturers these clampers do not need any run-out.

However, to check if the clamper (or the seller) tells the truth, it is possible to make the TYRE CHANGER TEST.
Keep a wheel with a good tire rim, possibly new, place it on a tyre changer turntable and lock it. Then take the clamper and attach it to the rim following the manufacturer procedure. Now rotate the tyre changer turntable and with the reference of a fixed point (your eyes cannot be sufficient) check the clamper oscillation in the point where sensors is attached,. Obviously we want to see that the clamper shaft rotates in the center perfectly. But we may be surprised!

There is a simple explanation: some clampers are designed to avoid run-out for angle offset (clamper and wheel stay parallel) but they do not maintain the effective center of rotation (clamper and wheel axis do not coincide). As a result, without run-out compensation, some values are right (toe, camber and caster), and the alignment is correct, while some other values, such as SetBack, will result wrong.

Looking at the picture beside: the blue lines (vertical) indicate that wheel and clamper are both parallels, the red lines show that clamper axis is not the same on the center of wheel axis. Without making a run-out procedure, computer will indicate a SetBack that in reality doesn't exist. The reason of this is the design of system, and it leads to the conclusion that with the "long arm" it's necessary to do  run-out if we want to see a real SetBack. Otherwise the Setback indicated might depend on the clamper and not from car.

figure 9

figure 10

10. The FASEP System, without SetBack, without SetBack problems.

So we now have a better idea of what is SetBack and where it comes from:
1. SetBack, as a change from car constructive conditions, does not really influence the car going on the road.
2. SetBack can be introduced by the car makers.
3. Car makers do not set tolerances or adjustments for SetBack.
4. it is not possible to identify the wheel in wrong position only referring to SetBack value.
5. SetBack can be a consequence of other Alignment measures.
6. SetBack can be introduced by the same Measuring/Clamping System.

Everybody can draw conclusions.
Our conclusion is that SetBack is a fake measure, that does not help in any way going a better job.

That is why FASEP has designed a measuring system, avoiding SetBack measure and implications, with a lot of advantages.

The advantages are:
- run-out not necessary in the majority of cases, thanks to clamper design and absence of sensor "long arm".
- compact and easily-handled sensors.
- less necessity to level the sensors than in "long arm" sensors (in the case of "long arm" sensors, if they are not correctly leveled, Toe and Setback assume wrong values.)
- easy use of the sensors also in case of spoilers, big wheels or little wheels, lowered suspensions, etc.

11. Without SetBack. 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 did it the same way, measuring toe "along the car" instead of "across" and obviously on such systems there is not a Setback measure.

Among others, we can mention:
- Policontrol (made in Switzerland, for control lines) now used by Bosch - Nussbaum aligners.
- Powercontrol (made in U.S.A., for car manufacturer's assembling lines)
- B-Dyna (made in Japan, for control lines)
- G-Swat (made in Japan, for control lines)


Latest added to the "along the car" approach:
- John Bean 3D (Balco, Sun and Hofmann are the same machine)
- Hunter 3D
- Bosch aligner introduced in Automechanika 2002.

12. Conclusions

I am that when looking for buying a new Wheel Alignment system, maybe an expensive one, every customer was advised that buying a system without SetBack measure was a big mistake because SetBack measure was to be considered fundamental for a proper adjustment of wheel alignment.

And we are also sure that many of these costumers thought it was a true statement and possibly few of them based their buying decision on this particular fact.

Soon, when they will start looking for a new Alignment, they may hear that there are new great technologies, 3D,etc., but there will be not SetBack.

So, was SetBack really useful?

Finally, we would like to ask a question to the owners of a "long arm" aligners:
when you really wanted to make a good alignment job, in your experience, was Setback a critical issue you had to take care of?
We should be glad to discuss the case under a technical point of view.