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How do 4 wheel alignment gauges work?
Slimy38 - 22/9/12 at 12:14 PM

I took my tintop into Halfords to get the toe sorted, so they put those laser thingies on the front wheels, and the gauges on the back. After much screaming and shouting at the seized track rods, they finally got the front wheels to zero, with both rears showing 6 degrees. Apparently that 12 degrees between rears is because the rear axle is damaged.

However, what I didn't understand is how they are able to measure the rears? There's a laser on the fronts that measure the difference between the two, and then lasers from the front devices to the rears. But unless my GCSE maths is failing me, I didn't understand how a laser fixed to the front could tell them the toe of the rears. From what I could see, even if the rears were adjustable for toe, the gauge wouldn't actually show a different reading as the wheel moved. And the rear reading could be upset by anything that would push the wheel out, such as a spacer or the wheels offset. Or even a difference in track from front to rear.

Are they just measuring the fronts in relation to the rears, or are they actually able to measure rear toe?

[Edited on 22/9/12 by Slimy38]

britishtrident - 22/9/12 at 01:09 PM

Halfords.................................


A tool is only as good as the guy using it, did they do all the basic checks first; Ball joints, TREs, Bushes, Springs, Wheel and Tyre run out ?

12 degrees is an awful lot of toe so much it I would think even be noticeable just looking at the car and it would certainly effects on the handling, what vehicle is it ?

iceman26 - 22/9/12 at 01:11 PM

you should put the front gauges on the rears 1st and set them up then move gauges to the front and set them up

britishtrident - 22/9/12 at 01:24 PM

Smoke and mirrors

Slimy38 - 22/9/12 at 01:47 PM

quote:
Originally posted by britishtrident
Halfords.................................


Yeah I know, I was desperate...

I'd checked all the components myself, it was after a bit of a bump so I'd had to replace most of them on one side, including a banana shaped track rod. After getting the suspension fixed I'd adjusted toe as much as I could with a tape measure and straight edge but it still felt 'vague'.

To be fair, the alignment has tuned out a lot of left pull so it's done something halfway decent. As Britishtrident mentioned, 12 degrees should be extremely noticeable, visually and handling-wise. But at the moment it feels pretty good, hence me wondering how they could have measured 12 degrees and my car still having the ability to go straight.

The car is a SEAT Toledo, it uses the Mark 4 Golf chassis. Standard toe measurements are 0 degrees up to minus 10 minutes, for both front and back.

adithorp - 22/9/12 at 01:55 PM

Did they measure 12 degrees which would be extreme, or 12 Minutes (0.2degrees) which might be about what you might expect?

As BT says, it's all done with mirrors.

Slimy38 - 22/9/12 at 02:16 PM

They showed me the scale at the end, the lasers pointed to '6' on both sides. I thought I remembered him saying degrees, but 6 minutes would actually be more likely and within tolerance. And the way it drives would also suggest the same.

thunderace - 22/9/12 at 02:58 PM

this is how i do it but i put the front end on axle stands . works evry time and im 100% happy with it


http://www.youtube.com/watch?v=0ziovI1651o

Slimy38 - 22/9/12 at 03:05 PM

quote:
Originally posted by thunderace
this is how i do it but i put the front end on axle stands . works evry time and im 100% happy with it


http://www.youtube.com/watch?v=0ziovI1651o


Mmm, I'll give that a go when I have a few minutes spare. Silly question though, why the axle stands? Won't the toe setting be different if the wheels aren't on the ground?

froggy - 22/9/12 at 03:30 PM

im assuming that they use a similar system to my supertracker which is the most popular kit .the rear flags have nothing to do with the toe reading at the front it merely shows that the toe is equally spread on both sides with the steering wheel straight .

the normal procedure is to clamp the wheel in the straight ahead position them any adjustment must be equally done to keep the wheel straight . when the rear flags show the same figure and the toe is right at the front it means the wheel will be straight as you drive ,the figure means nothing as a car with slightly wider track at the rear will give a different reading but for the majority of cars running a touch of toe in or out the flags will read between 5-9 .if the front was 100% parallel the flags would be close to zero and say 2mm of toe in over the 5-6ft between front and rear wheels would show around 5 .

the front measuring system gives a numerical value rather then degrees and minutes and the usual reading are oppsite each other i.e one side says toe in 5 the other toe out 5 which cancel each other out so in fact they are parallel . if you have one side saying +5 the othe side says -6 and the rear flags are both reading the same you have toe in of 1 if you get my drift

Slimy38 - 22/9/12 at 04:17 PM

Yep, having gone on the supertracker website it looked like they were using the laser wheel aligner. Thanks for the explanation, so it sounds like they've done a 2 wheel alignment and the rear flags were just there to make sure that the front wheels were straight ahead at the same time as the steering wheel.

I guess I'll have to find a more competent garage to determine whether the rear wheels are actually out of alignment.

bi22le - 22/9/12 at 04:45 PM

quote:
Originally posted by thunderace
this is how i do it but i put the front end on axle stands . works evry time and im 100% happy with it


http://www.youtube.com/watch?v=0ziovI1651o


This seems like quite a clever wsy of checking the tracking and toe. It does not give measurements but allows you to set it up a little.

Are there any major problems with going this on a kit car? I suppose that if the rear axle is shifted literally over then you would be tempted to make the car crab by following the string. Also if The years are wider then the fronts your in trouble.