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A few weeks ago, I treated my VW Touareg to a fresh set of tyres and decided to get a wheel alignment done since it was pulling ever so slightly to the right after the front suspension bushings had been swapped out by my local dealership.

The tire shop technician fed the make and model (2006) to the Hunter system, which prompted that the vehicle needed 8.25 deg of caster on the front wheels. The tech then set the right tire to within spec but kept the left tire at 7.54, which put it out of range.

Despite my protests at what I perceived to be a shoddy job ("The left wheel's caster out of spec!") the tech was adamant that if he didn't the car would pull to one side because of the crown of the road. This was later corroborated by another tech who looked at the vehicle on a second visit (turns out they hadn't done a stellar job with the alignment, no surprises there).

Before I take further action, I wanted to understand if there is a genuine use-case for offsetting caster angles on a vehicle that hasn't been involved in any front-end collision.

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    What side of the road to you drive on? – dlu Dec 17 '16 at 21:22
  • @dlu similar to the US, we drive on the right side of the road – Zaid Dec 17 '16 at 22:18
  • Hmm, well at least the cross caster is biased in the right way (to pull towards the side with the smaller amount of caster), but it still seems like way too much. – dlu Dec 17 '16 at 22:23
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I don't think the alignment techs' position is defensible. Both because they aren't setting the alignment to spec and because they seem to be adjustmenting the wrong variable.

Caster's primary effect is straight line stability. Increasing positive caster (making the line between the upper and lower ball joints or the lower joint and the strut mount intersect the road farther ahead of the vehicle), will take more effort to steer and there will be an increased tendency to return to center. Increasing caster also increases the rate at which negative camber increases as you turn the wheels.

Camber tries to optimize tire contact, but it also has a turning tendency much like leaning a bicycle or motorcycle. Asymmetrical negative camber (the degree to which the top of the tire pitches in towards the center of the car), called cross camber, can be used control "downhill drift" on crowned roads. As a result cars are often set up with some small (around 1/4°) amount of cross camber. If you drive on the right hand side of the road, the right wheel would typically be a bit more negative than the left.

On Tire Rack's page on alignment, they have this to say in their Recommendations section:

If you are a reserved driver, aligning your vehicle to the vehicle manufacturer's preferred settings is appropriate.

If you are an assertive driver who enjoys driving hard through the corners and expressway ramps, a performance alignment is appropriate for your car. A performance alignment consists of using the vehicle manufacturer's range of alignment specifications to maximize the tires' performance. A performance alignment calls for the manufacturer's maximum negative camber, maximum positive caster, and preferred toe settings. While remaining within the vehicle manufacturer's recommendations, these alignment settings will maximize tire performance.

If you are a competition driver who frequently runs autocross, track or road race events, you'll typically want the maximum negative camber, maximum positive caster and most aggressive toe settings available from the car and permitted by the competition rules. If the rules permit, aftermarket camber plates and caster adjustments are good investments.

My takeaway from Tire Rack's recommendations is that even if you're looking to optimize performance through your alignment settings you should still be within spec.

Yospeed also has a concise post describing the alignment variables.

Well, on top of all this, I'd expect VW to have a better idea than the tire shop for what the alignment specs should be. The have a vested interest in getting it right, both from the point of view of the handling and drivability of the vehicle as well as to avoid legal liability for a flawed design. So of the face of it, it seems rather presumptuous of the alignment tech to suggest that he has a better idea of how the alignment should be set up than the engineers at VW… Especially in the absence of you telling them something which would suggest altering the specs.

  • Thanks for the insights. Since you've mentioned it, I should add that the tech really struggled to get the camber in spec for the left wheel. It was as if he had run out of adjustment. I wanted to tell him to sort out the caster first but decided to not micromanage him since this is what he does for a living. My theory is that he could have obtained extra camber had he sorted out the caster first, but it's a bit late to test that theory I guess – Zaid Dec 17 '16 at 22:25
  • If the Touareg suspension design is anything like the Jetta then I think most of the adjustment is at the lower ball joint. Could be that as you max out one the other becomes more limited. – dlu Dec 17 '16 at 22:28

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