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Audi S3 Geo Disection


Steve-O
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Come on then tone, some before and after geo numbers on my S3 as it was in for a MOT and needed a new track rod end, 2 new rear springs and various other stuff that lightened my wallet considerably :(

 

Was there anything bad with the initial values, and what are the final values like now? :)

 

 

74C8B7EE-A4E9-4B2E-AD7F-19BE342C64B9.jpg

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No idea. There's something about these numbers that don't sit right with me but don't understand them enough to know what I am looking at.

 

Basically - are these numbers any good and inLine with other s3's and are they dangerous? It felt ok on the motorway but didn't push the handling.

 

I felt obliged to get the geo done there due to the MOT and the work on the steering and shocks but if it's not up to scratch I will pull them up!

 

The garage is an Audi specialist and seems to be winning awards every year....

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Right young Steve...... There's no target data for the rear camber, reason being........... The machine wanted the rear trim height input but that wasn't done so the target datum was withheld..... The front camber is to deep on the NSF and to light OSF, what should have happened is the subframe should have been moved to optimize the positions. The castor balance is ok but it's very fuel dependant meaning if you had to much or to little it would effect the results. In truth it's not bad but could have been better.

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Thanks for the break down mate. Question is should I complain or is this a example of what is the norm with average geometry services? Sounds like it needs to come over to joe and the boys.

 

Any idea of what negative handling traits might be experienced as it is?

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The only concern is the NSF camber "push" is stopping the castor disparity pulling the car left. The castor is not adjustable so if the camber is corrected then you might end up with a pulling car. Best advice for now is to monitor the front tyre wear and we can take it from there. Also note the cross camber is in red meaning it needs attention so i would question why this wasn't addressed.

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The left SAI (steering axis inclination) or KPI (king pin inclination) as it's noted on your printout, is sitting quite a lot lower than the off-side. These figures can move higher or lower within reason, as long as they move together. These values haven't, which has shown through in the caster angle.

I imagine there's a slight bend in one of the lower suspension components on the nearside, and rectifying that will bring your SAI values even, in turn, bringing your caster angles in line with one another.

At a guess I'd say nearside front lower arm and ball joint are to blame. but there's an outside chance its the hub itself, there's a chance it's the subframe, there's even a chance the chap measuring the car knocked one of the gauges!

Proof is in the pudding, if its something you'd want fixing, the most logical and cheapest place to start is the nearside front lower arm.

I'd like to measure it before any of this goes ahead though, evening out the front cambers will give a more definitive reading and just incase it was a spoof measurement. Don't want to be changing parts unnecessarily,

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The measurement of the SAI/ KPI ( same angle ) is the inclination/ declination of the joints pin against the vertical in the perpendicular plane towards the cars centre line not the longitudinal plane. Historically it's a diagnostic measurement so normally there's no target data or over axil disparity value. I set my disparity alarm at two degrees.

 

The hard thing to grasp with the SAI/ KPI measurement is the lower the number the higher the pin since a vertical pin = zero SAI/ KPI. The SAI/ KPI cannot be measured directly or as a single angle meaning it's position has to be acquired. This is done during the front castor swing ( normally 20 degrees )  by the camber migration lock-on-lock @20 degrees.

 

In Steve's case the low SAI/ KPI doesn't correlate with the low NSF camber because the pin is more vertical than the OSF but it does suggest the lower NSF castor is withholding the cambers migration during the 20 degree swing measurement.

 

Just to complicate things i believe on this car the upper ( sprung chassis ) also has a ball joint. The upper ball joint ( called a pick up point ) also has a pin, so the SAI/ KPI measurement is dependent on it's inclination/ declination forming a single measurement at the lower ball joints pin.

 

So as a diagnotic tool if the NSF camber was to low and the SAI/ KPI number was high compared to the OSF ( remember there's no target data normally ) then is a fair bet the lower arm is bent. However another factor is build quality? The X, Y, Z axis between the sprung and unsprung chassis cannot be perfect ( hence geometric tolerances ) so with this understanding the SAI/ KPI's position cannot be regarded as law.

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Thanks :blush: ...... Without being rude and yes i know my grammar construction is poor but that explanation was dumbed down nevertheless you can understand now why most places don't measure the SAI/ KPI.

 

Just to add, this SAI/ KPI what does it mean some might say?

KPI: King Pin Inclination was born from commercial vehicle construction where the pin holding the hub-wheel to the cross member. This allowed the steering to turn obviously but didn't allow the camber to migrate. KPI worked fine on commercial vehicles for many years and was adopted into the domestic range but was far from perfect.

 

SAI: Steering Axis Inclination is as described, a pick-up point that allows the wheels axis to orbit rather than the KPI just rotate. The SAI's true evolution evolved with suspension advances namely independent suspension.  

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What an explanation :) I still dont think i understand it completely but it sounds like there is room for improvement on the S3 :)

 

I am not sure if this is a trait of my model but the steering is quite heavy compared to other cars at low speed - to the point if feels almost a manual rack. The turning circle is dire also. Would the geo improvements have any positive effect on these observations in any way?

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Yes assuming the NSF camber can be reduced and the castor increased. For the life of me i can't remember if your car has twin upper alloy wishbones, if it does then it uses an Ackerman turn called a Delta curve which almost over rides the castor sweep, in truth i've never liked the Delta curve and to my knowledge no other manufacturer has adopted the principle.

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