Tech Corner

Cert ified Porsche Technician Gold Meist er

Wheel Alignm ent Dem yst ified

Your Porsche and a grocery shopping cart have something in common!...but we'll get to that in a moment. I'm here this issue, as it is Horizontally Opposed's 'Tire Issue' , to discuss often overlooked wheel alignment. "But Joe! I park 2 feet away from any curb for fear of damaging my alloys, and pot holes these days are few and far between" .

The industry convention is to check alignment, depending on mileage driven, every 6 months or at least once a year. It's more important with your Porsche as handling is a hallmark element in the Porsche experience. Optimized alignment will pay-off in a better handling Porsche.

Factory specs for alignment are established as a trade-off in tire wear and better handling. This is because of some basic geometric principals like camber, which can improve how your car ?feels? can also cause more than normal tire wear. It all translates to the kind of driving you do. If the time spent in your Porsche are drives without really 'exerting' the car, then factory settings are close to optimum. However, with tire width and construction continuing an inexorable march forward, you might want to revisit the factory settings.

CASTER

Back to that grocery cart commonality: simply, caster is what makes shopping carts (and your Porsche's) front wheels run straight. The steering pivot is in front of where the wheel contacts the ground in the direction of travel. This is positive caster. This forces the wheel to always track straight when the cart is pushed forward or backward without wandering. The same principal applies to the front end of your Porsche.

Positive caster creates two primary benefits:

- 1) Improved st raight -line st abilit y at speed. The front wheels positioned forward of the upper ball joint/strut mount produces a self-aligning torque on the front wheels and tires. This is a fairly complicated engineering concept, but the idea is that positive caster creates a tension that wants to return/keep the front wheels pointed straight ahead while at speed. You experience the benefits of positive caster and self-aligning torque every time the steering wheel naturally ?unwinds? back to 12 o?clock when exiting a corner. And without positive caster and the resulting self-aligning torque, the vehicle would be very inclined to wander at highway speeds. - Improved handling and cornering performance. Caster has a direct relationship to negative camber, which is fundamental to maximizing the tire?s contact patch when cornering, and therefore key to maximizing the available grip. Specifically, positive caster helps to create increased negative camber on the outside wheel during cornering. When cornering, the vehicle weight shift is met with a maximization of the tire contact patch on the outside front wheel that?s under maximum cornering load. In performance driving circles a caster increase ? sometimes to the maximum available setting? is credited with improvements in vehicle handling, maximum traction, and steering responsiveness.

CAMBER When viewed from ahead, camber the relation of the top of the tire to the bottom expressed in degrees. Positive camber is the outward tipping of the top of the tire. Negative camber is where the top of the tire is tipped inward and is preferred since it will counteract the geometric tendency for the suspension to lift the contact patch as the body rolls during cornering. This setting can have the most dramatic impact on the cars ability to hold front end traction in a corner. It also can cause the most uneven tire wear over time. That is because the inside edge of both tires are loaded higher than the outside causing them to

wear more.

Often, factory specifications opt for zero to positive camber to keep the car in an understeer condtion, making the front end tend to ?push? in a corner, which is an inherently safer condition for most drivers.T

Negative camber is the characteristic generally desired in performance driving. When we use the term, we? re referring to static negative camber. That is, the wheel and tire should exhibit some negative camber while sitting. When you barrel OE

into a corner, the laws of physics have you, the wheels, and the tires pushed toward the outside of the corner.

When the wheels and tires are pushed outwards and the car?s body rolls, the contact patch, or the area of the tire which comes into contact with the surface of the road, diminishes significantly as it rolls over onto its outer shoulder.

Negative camber is implemented so that when the car is cornering and the wheel is rolling over and gaining positive camber (from body roll, not suspension travel), the static negative camber should correct whatever effect leaning and lateral load have on the wheel and tire; resulting in a near-upright tire and the greatest possible contact patch.

Simply put, negative camber helps counteract the natural tendency for a tire to roll onto its outer shoulder while cornering, keeping the contact patch squarely on the road while cornering allows for more grip and higher cornering speeds. There are some reasons that your Porsche likely did not roll off of the showroom floor with optimum camber for the track. Too much negative camber causes the car to tramline or follow cracks or imperfections in the road, and also have an excessive sensitivity to the road?s crown (an engineered curve that promotes water drainage on roads). And there's the tire wear issue mentioned earlier. As a general rule, a car that?s sole purpose is commuting should have less negative camber ? unless you don? t mind replacing tires regularly. TOE relation to the longitudinal axis of the vehicle. It is usually measured when the car is static and is measured either in degrees or in millimetres. If it is measured by distance then it is usually the difference in distance between the two rear edges of the tires and the two front edges of the tires. Toe In

On the front of the car, toe in increases straight line stability. This is because if one of the wheels is disturbed during straight line driving and is pulled rearward of the steering axis then the wheel will steer outwards.

This minimal outward steering would only straighten the wheel and develop zero toe so would help the car stay straight. However, upon turn in, the inside wheel is more hesitant to turn making for a less responsive turn in. This can be compensated for by using high Ackermann angle steering geometry. Toe in on the rear also improves the straight line stability of the car for the same reasons as above. Toe in is also used on the rear to induce a slip angle on the tire to increase the straight line grip and performance of the tire to transmit as much torque as possible to the ground. Toe Out On the front of the car, toe out decreases straight line stability. When a wheel is disturbed in a straight line with toe out, pulling the wheel rearward of the steering axis, it increases the toe out and effectively turns the inside wheel more outward, pulling the car into yaw. This decreases straight line stability as it makes the car develop a twitchy nature which can be uncomfortable at high speeds. However, running toe out makes for faster turn in on the front end. Toe out on the rear will induce a slip angle on the tire to increase rear end grip for acceleration. However, during cornering the toe out increases the tendency for the rear of the car to oversteer which is especially bad for rear wheel drive cars.

Zero Toe

Running zero toe at the front and/or rear of the car results in minimum tire wear and power loss. This is because if any toe is run, it develops scrub of the tire in a straight line due to the incurred slip angle, decreasing the rolling efficiency of the tire.

THRUST ANGLE

Our Porsches all have adjustable independent rear suspensions requiring checking. Thrust angle is the direction that the rear wheels are pointing in relation to the center line of the vehicle. If the thrust angle is not zero, then the vehicle will ?dog track?. As a result, a 'push' in one direction will be pronounced. Now, if you're running a NASCAR Cup Car, that bias would be beneficial to helping the car turn into a corner, but not for our Porsches. The best solution is to first adjust the rear toe to the center line and then adjust the front toe. This is normally done during a 4-wheel alignment.

FINAL THOUGHTS

Alignment is all about aligning the suspension/tires to ensure that the maximum possible grip from the tires is being achieved. Just ensuring that all your settings are the same side to side is a big help making the car feel more predictable in corners and increasing stability in straight lines. From fitting new shocks or even a new set of Michelins, any addition is a good reason to check your alignment settings, to ensure that your Porsche is performing at its highest level.