Setback question

[Lovejoy]

Not the usual sort of vehicle you deal with but had the tracking set on my Frontera by a local company. They have a John Bean set up which is a good unit but the printout shows setback of:

 

Setback Front: -23mm

 

Setback Rear: -25mm

 

Wheelbase difference: 2mm

 

Now with that amount of setback the N/S wheels should be visibly different in the arches to the O/S. I would expect the wheelbase to be different by a large amount if there was anything bent to that extent as well.

 

The car drives ok, seems more responsive since the tracking adjusted as well and doesn't pull. The thrust angle is listed as -0 degrees 7 minutes.

 

What I'm wondering is how the setback is measured and how can it be nearly an inch out without noticing anything obvious.

 

The steering wheel sits at about 20 past 10 when travelling straight but other than that it seems reasonable for the type of vehicle. I am a retired mechanic so do understand a bit about geometry and cars in general and there is no sign of damage or excessive wear anywhere. I put four new balljoints on it and had it MOT'd before the alignment.

 

I used a system called optoflex when I was working in the trade; huge projectors attached to the wheels, mirrors, crosshairs and sliding boards! The new systems are far more advanced but I don't understand it all so any clarification would be great.

 

Frontera Sport (swb)

 

1995

 

Coil rear suspension

 

16 inch wheels.

 

Print out:

 

[Tony]

Hello

Setback is used for diagnostics and axle/ drive alignment. Typically they didn't measure the castor angle, which is adjustable on your vehicle, had they done so we would probably find the cross castor is wrong. In the event the castor was a fixed angle, setback would show if the reason is in the sprung or unsprung chassis.

[Lovejoy]

Thanks Tony.

 

So if the castor is wrong that would give the setback (reading) difference? or will knowing the castor angles indicate whether it is a sprung or unsprung problem?

 

Why do so few places read castor angles? According to John beam literature it only requires turning steering wheel fully to the right and fully to left. (they have an arago) Not sure if extra work is required on other set ups but surely it's in the garage's interest to investigate all it can from the machine?

 

May go back and ask them why they didn't read castor and if they could please.

[Tony]

Thanks Tony.

 

So if the castor is wrong that would give the setback (reading) difference? or will knowing the castor angles indicate whether it is a sprung or unsprung problem?

 

Why do so few places read castor angles? According to John beam literature it only requires turning steering wheel fully to the right and fully to left. (they have an arago) Not sure if extra work is required on other set ups but surely it's in the garage's interest to investigate all it can from the machine?

 

May go back and ask them why they didn't read castor and if they could please.

 

The castor swing " left right" opens data for all the secondary angles, these are absolutely vital whether those angle are adjustable or not! Plus the acquisition of this extra data takes about 1 minute.

 

I've found the reason most do not measure the secondary angles is a combination of....

1: Ignorance for the importance of the information.

2: Most don't understand the information anyway

3: Time.

 

In your case i would triangulate the front/ rear setback, the castor positions and the wheelbase. Since your castor angle is adjustable, it's position would determine if the castor needs correction and in turn correct setback since setback is measured at the unsprung chassis.

 

4/5 shops do not measure the secondary angles, any chassis image without them is worthless, like yours. Put it this way, with all my experience i cannot conclude your complaint because the data is missing, at this point we can only speculate.

 

On a side note you paid for a full geometry and only got a partial geometry, would you be happy to buy shoes with no laces?