Wheel Alignment Accuracy - any experts here?

[Rob]

Just returned from having my <cough> Audi aligned at a well-known alignment specialist.  I'm interested in the precision/reproducibility of the equipment used, having a scientific background.  What has piqued my interest is that I had an initial measurement F&R done on Friday, before fitting PU bushes to the FRONT only.  Today I went back after fitting the bushes and had the full alignment done. 

To be clear, the car was only driven to and from the alignment place and not driven between visits.  I should also add that the suspension is in extremely good condition.  I would expect the rear measurement done on Friday to be very close to the initial measurement today...

Here's Friday's rear measurements:                                                               Camber: (L) -1.19^o (R) -1.76^o                Toe: (L) -0.5mm (R) 5.3mm

Here's Monday's rear measurements (not adjusted, same as Friday's) : Camber: (L) -1.59^o (R) -1.44^o                 Toe: (L) 1.0mm (R) 2.8mm

I would expect these readings to be much closer, given that they are essentially replicate readings.  Are my expectations too high?  I'd like to see +/- 0.1...

 

[Redracn]

Did the thrust axis vary? Changes to the front can affect the individual rear toe. It is the total numbers that matter. With the camber the total looks similar but the vehicle apears to have a left to right load change like a side fuel tank or a different stop height on the hoist resulting in a slightly different hoist leveling. 

In general you can drive a car off then back on and it will be different. How the driver gets out matters, how they settle it matters. A lot of operators do not do the wheel runout to save time so another error that depends on the wheel position on the hoist. There is a lot that matters and it is hard to control it all from the individual test setup errors to equipment calibration.  

I have yet to get a spot on alignment and I have had many that were complete rubbish. 

[Rob]

So - the variability in a number of parameters outweighs the number of significant figures quoted...

Thrust angle was -0.23^o Friday, -0.07^o today.  After the alignment, thrust angle was 0.00^o (+/- ?)

I should add that I've been using this shop for more than 20 years and have always been happy with the on-road result of alignments.  Just doing the "back to back" measurement gave me cause to query the repeatability of the measurements and how much importance we should place on the final figures.

 

[Redracn]

31 minutes ago, Rob said:

So - the variability in a number of parameters outweighs the number of significant figures quoted...

 

Yep by quite a margin. So it takes a good operator with the best equipment to reduce the possible setup error sources. Luckily there is a reasonable manufacture tolerance that is big enough to cover most of the issues. 

Looks like the front moved a bit so it will affect a 4 wheel alingment as the rear is not independant of the front and vise versa. 

If your guy has been doing good work I would just go with it. 

[OzJustin]

Interesting read Rob. I have often wondered how much variation there should be but have never done back to back like you to know for sure. How does the RS drive now with the poly bushes?  

[Rob]

2 minutes ago, OzJustin said:

Interesting read Rob. I have often wondered how much variation there should be but have never done back to back like you to know for sure. How does the RS drive now with the poly bushes?  

Hi Justin

I've only driven from the alignment shop to home, but can feel the improved immediacy of response from straight-ahead, so pleased with the result.  Next is rear bar, then PU in the rear (lots of bushes there!)

Cheers

Rob

[bear924]

Great discussion. This is something I have also pondered.

Once I had all my wheels balanced as I had a strange steering shake. On return, my car wasn't any better so I decided it must have been something else.

Few years later I replaced the tyres and had it balanced and car had no more shake. Could have been the tyres I suppose however wondered if perhaps I'd received a dodge balance.

Not quite the same as above, however my story kind of rhymes.

[carl888]

On 18/06/2018 at 16:40, Rob said:

Just returned from having my <cough> Audi aligned at a well-known alignment specialist.  I'm interested in the precision/reproducibility of the equipment used, having a scientific background.  What has piqued my interest is that I had an initial measurement F&R done on Friday, before fitting PU bushes to the FRONT only.  Today I went back after fitting the bushes and had the full alignment done. 

To be clear, the car was only driven to and from the alignment place and not driven between visits.  I should also add that the suspension is in extremely good condition.  I would expect the rear measurement done on Friday to be very close to the initial measurement today...

Here's Friday's rear measurements:                                                               Camber: (L) -1.19^o (R) -1.76^o                Toe: (L) -0.5mm (R) 5.3mm

Here's Monday's rear measurements (not adjusted, same as Friday's) : Camber: (L) -1.59^o (R) -1.44^o                 Toe: (L) 1.0mm (R) 2.8mm

I would expect these readings to be much closer, given that they are essentially replicate readings.  Are my expectations too high?  I'd like to see +/- 0.1...

 

Rob, I can help you here however re: camber can you confirm firstly what the 1.19.....1.76 actually is?  I am assuming the 0.19 is actually 0.19 of a degree, so 11'.  You can't have 0.76 minutes (') since there's 60 minutes to a degree.  Mixing degrees and then fractions of a degree using the decimal system is odd.

I would suggest the alignment ramps or platform has moved between the two readings and the car has settled downwards longitudianlly on the drivers side for Monday's reading.  If you add the differences between the two readings from Friday and Monday, you get about the same reading overall.

I suspect your aligner uses a hoist type system raised hydraulically.  These are notoriously inaccurate as they settle in a different position as the locking pins wear every time they're raised.  Or a funny bugger let 20 PSI out of the drivers side tyres.

The toe reading BTW makes no sense.  Either the clamps were not applied properly, a reading was taken immediately after the car was raised, there was run out in the rims or an alignment head or mirror was dropped/damaged in the meantime.

With regards to to repeatability, with my own machine I find results easily repeatable within usually 7 minutes to a degree and 0.2mm.  Assuming wheels that have no run out of course.  A good aligner will always perform run out compensation if there's any doubt.  A simple test to check an aligner if you don't have access to a jig is to put the car on the ramps backwards.  You won't be able to measure caster, however toe and camber should mirror the forward reading 180 degrees.

With my own machine, I calibrate it every 10 alignments and run the spirit level over the ramps (fixed) at the same time just to keep on top of any possible errors.

[Rob]

3 hours ago, carl888 said:

Rob, I can help you here however re: camber can you confirm firstly what the 1.19.....1.76 actually is?  I am assuming the 0.19 is actually 0.19 of a degree, so 11'.  You can't have 0.76 minutes (') since there's 60 minutes to a degree.  Mixing degrees and then fractions of a degree using the decimal system is odd.

I would suggest the alignment ramps or platform has moved between the two readings and the car has settled downwards longitudianlly on the drivers side for Monday's reading.  If you add the differences between the two readings from Friday and Monday, you get about the same reading overall.

I suspect your aligner uses a hoist type system raised hydraulically.  These are notoriously inaccurate as they settle in a different position as the locking pins wear every time they're raised.  Or a funny bugger let 20 PSI out of the drivers side tyres.

The toe reading BTW makes no sense.  Either the clamps were not applied properly, a reading was taken immediately after the car was raised, there was run out in the rims or an alignment head or mirror was dropped/damaged in the meantime.

With regards to to repeatability, with my own machine I find results easily repeatable within usually 7 minutes to a degree and 0.2mm.  Assuming wheels that have no run out of course.  A good aligner will always perform run out compensation if there's any doubt.  A simple test to check an aligner if you don't have access to a jig is to put the car on the ramps backwards.  You won't be able to measure caster, however toe and camber should mirror the forward reading 180 degrees.

With my own machine, I calibrate it every 10 alignments and run the spirit level over the ramps (fixed) at the same time just to keep on top of any possible errors.

Hi Carl, thanks for the response.  The camber measurements are (negative)1.19degrees, (negative)1.76 degrees.  The ^o is scientific notation for degrees.  I'm in Brisbane and would like to be able to access an aligner with very good equipment.  what brand/s should I look for?

thanks

Rob

[carl888]

19 hours ago, Rob said:

Hi Carl, thanks for the response.  The camber measurements are (negative)1.19degrees, (negative)1.76 degrees.  The ^o is scientific notation for degrees.  I'm in Brisbane and would like to be able to access an aligner with very good equipment.  what brand/s should I look for?

thanks

Rob

Rob I didn't explain myself properly, the first numeral to which we refer is degrees and that is not in question,  I was not referring to that, I was referring to the numerals after the "."   It's a technicality really, but it should be read in minutes of a degree, which it isn't.  Bit hard to have the minutes of degree up to 0.99 when there's 60 minutes to a degree.  It's what you're used to I suppose.

But back to your issue.  The aligner is not the problem, it's the maintenance and operation of it coupled with the skill of the operator.  No amount of decent equipment will compensate for poor calibration and operator error.  I use a Corghi which is V.A.G. approved and has Audi figures and procedures in the database.  So does the Beissbath.  I am not sure about others like the Hunter. 

Did you get a print out with your alignment showing the factory figures the operator is trying to achieve?  And when was it last calibrated? 

An accurate alignment can be done with a piece of string and a caster gauge.  There are even some remarkably accurate apps that measure wheel camber with your phone too.  (Assuming you're on level ground).

 

[Rob]

3 hours ago, carl888 said:

Rob I didn't explain myself properly, the first numeral to which we refer is degrees and that is not in question,  I was not referring to that, I was referring to the numerals after the "."   It's a technicality really, but it should be read in minutes of a degree, which it isn't.  Bit hard to have the minutes of degree up to 0.99 when there's 60 minutes to a degree.  It's what you're used to I suppose.

But back to your issue.  The aligner is not the problem, it's the maintenance and operation of it coupled with the skill of the operator.  No amount of decent equipment will compensate for poor calibration and operator error.  I use a Corghi which is V.A.G. approved and has Audi figures and procedures in the database.  So does the Beissbath.  I am not sure about others like the Hunter. 

Did you get a print out with your alignment showing the factory figures the operator is trying to achieve?  And when was it last calibrated? 

An accurate alignment can be done with a piece of string and a caster gauge.  There are even some remarkably accurate apps that measure wheel camber with your phone too.  (Assuming you're on level ground).

 

Hi Carl

OK, we've got that cleared up now.  Don't know what the brand of aligner they use, but it reads out the values to the right of the decimal point in degrees.  For example, one of the other readings is -0.68degrees, which means that the readings are all in decimal degrees, not degrees and minutes.  My alignment printouts do have the factory specifications for the Audi included, but do not report the calibration state, nor the brand of the aligner.  After a full re-alignment, all readings, bar front camber, were within factory specs.  Font camber was set at 1.3-1.4 degrees, at my request.

Alignments done on my previous vehicles have had a Beissbath logo, but later readouts have a different format and no logo.  Must have a peek at the branding next time I'm near the shop.

Cheers, Rob