Pelican Technical Article: Alignment Principles Wayne R. Dempsey
Time: 2 hours
Tab: $100
Talent:
Tools:
Alignment Rack
Applicable Models:
986 Boxster (1997-04) 987 Boxster (2005-08)
Parts Required:
-
Hot Tip:
Have a professional perform this job
Performance Gain:
Better handling and tracking of your suspension
Complementary Modification:
Replace shocks, wheel bearings, tie rod ends, ball joints, tires
This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your Porsche Boxster. The book contains 312 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 950+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Boxster owner's collection. The book is currently available and in stock now. See The Official Book Website for more details.
Check out some other sample projects from the book:
The Porsche Boxster is known for its good handling and excellent suspension system. Of course, precise handling and cornering are nonexistent if the car is not aligned properly. There are five different specifications that must be within spec to properly align the chassis. These are front-end caster, camber, toe, and rear-end camber and toe. On the stock Boxster, the settings that you can easily change are for camber and toe on the front and rear suspension. Caster is adjustable if you replace your suspension control arms with ones used on some of the GT racing cars. Also, unlike the earlier Porsche 911, the ride height is not adjustable using the factory suspension components. To change the ride height you need to upgrade your front and rear suspension to a fully adjustable coil-over system, or install lowering springs (see Pelican Technical Article: Performance Suspension / Lowering / PSS10 Installation).
If the alignment of the suspension is slightly off, then you might get some significant tire wear and a loss of power and fuel economy. The most common sign of a misaligned front suspension is the car pulling to one side of the road when driving straight. Although the home mechanic can perform the basic front-end toe-in setting, I suggest that you allow a trained professional with an alignment rack make the other adjustments. It's nearly impossible to determine the correct angles and settings for your car without the use of an alignment rack.
Camber refers to the tilt of the wheel, as measured in degrees of variation between the tire centerline and the vertical plane of the car. If the top of the wheel tilts inwards, the camber is negative. If the top of the wheel tilts outwards, the camber is positive. On the Porsche Boxster, the camber should be slightly negative for Euro-spec cars, and slightly positive for the USA-spec cars. On some older cars, the chassis deformation due to rust and age can sometimes lead to the camber adjustments and measurements being slightly off. If the car has been in an accident, then often times the resulting chassis damage will show up in an alignment that indicates values not within spec.
The front stock Boxster suspension has a limited range the camber setting can be adjusted to. If you wish to run more negative camber (useful for racing purposes), then you can install aftermarket upper strut mounts that allow you to dial in more than one whole degree of additional negative camber (see Project 60 for more details). Installing aftermarket strut mounts can also be used to correct the chassis camber when it falls out of factory specifications. In addition, you can install the 996 GT3 lower control arms, which are a two-piece unit and allow you to shim the arm for additional camber adjustments (see Figure 8).
Also adding to odd alignment measurements may be worn suspension bushings. As the bushings and suspension mounts age, they have a tendency to introduce some slop into the suspension system, which can result in poor alignment readings. Lowering your Boxster (through an aftermarket strut swap) will also change your alignment specifications from the factory defaults. If your alignment specialist tells you that your fixed specifications are outside the factory ranges, and your car has not been in any accidents, then it's likely that some of your suspension components are worn and need replacement (see Pelican Technical Article: Front/Rear Suspension Overhaul). Lowering your Boxster beyond about 1” or so may result in an inability to set the proper toe on the rear suspension without the use of adjustable rear toe links. The stock adjustment does not have enough adjustment flexibility to accommodate the proper toe and correct camber with a ride height change greater than one inch or so.
The rear wheels should be set from the factory for a slight negative camber (about –1.5 degrees), as the trailing arms tend to bend slightly outward as the car accelerates under power. Since one half of the wheel is mounted firmly on the ground, the top of the wheel has a tendency to twist outward when power is applied. Setting the rear wheels to have a slight negative camber means that under power they will be mostly neutral.
Caster is the angle that the steering axis is offset from the vertical plane. On the Boxster, the strut points toward the rear of the car, resulting in a positive caster angle. From the factory, the default caster angle should be about 5 degrees. The amount of caster in the suspension directly influences the control and stability of the wheels when traveling in a straight line. Since the Porsche rear suspension utilizes a trailing arm design, which has a tremendous amount of built-in caster, there is no specification for the rear caster. Front suspension caster is very good for high-speed stability: it helps to keep the wheels aligned and straight. If you wish to reduce the amount of front caster, you will need to install the GT3-style front control arms that allow you to vary the caster angle for the car (see Figure 8).
Toe refers to the angle of the two wheels with respect to each other. If a car has toe-in, it means that the front edges of the wheels are closer to each other than the rear edges. Toe-in is adjustable by changing the length of the tie rods (see Pelican Technical Article: Front/Rear Suspension Overhaul). With rear-wheel-drive cars like the Boxster, sometimes the front wheels try to move toward a toe-out position under power. Setting the wheels to have very slight toe-in can help neutralize this effect. Toe-out occurs when the front edges of the wheels are farther apart than the inner edges. Some toe-out is necessary when turning, since the angle of inclination of the inner wheel must be tighter than the outer wheel. The rear toe should be set as close to neutral as possible.
So how should your Porsche be set up? If you are planning to race your car, then you need non-stock suspension components and you will probably want as much negative camber as allowed by the racing rules. This is because the car will have a tendency to straighten out in turns, and you want the maximum tire patch on the road when you are cornering. Setting the camber to a negative value means that when the camber starts to change to slightly positive through turns, the negative setting will help neutralize this effect. There's also a misnomer that a lot of caster is good for racing. While adding more caster to your suspension can indeed make it handle better, the reality is that introducing too much caster into the suspension can reduce your track times. On a perfectly balanced rear-wheel drive car, adding too much caster can have a tendency to transfer loading from the outside front and inside rear tires to the opposite corners. This can upset the balance, and cause a corner entry push. The bottom-line here is to seek professional help for alignment specifications and any answers to questions that you might have, and don't accept the common misnomers about suspension upgrades: do your own independent research. Two books that I refer to on these topics are Race Car Engineering by Paul Van Valkenburg, and How to Make Your Car Handle by Fred Puhn.
Figure 1
Zero Camber. When the car is aligned with zero camber, it means that the wheels are directly perpendicular to the ground. The tires make even contact with the road, and exhibit a minimal amount of wear and friction when turning. The weight of the car is distributed evenly across the tire tread, but the steering control can be a bit heavy. Tire sizes are shown smaller than scale and camber angles are exaggerated for ease of illustration in these diagrams.
Negative Camber. The lower parts of the tires are angled outward, causing tires to wear more on the inside edges. The Boxster has an independent front suspension, which creates a slight negative camber when traveling over bumps. As the suspension compresses upward, the wheel tilts in slightly to avoid changing the track (distance between left and right wheels). Although this momentarily changes the camber of the wheel, it prevents the tires from scrubbing and wearing every time that the car travels over a bump.
Positive Camber. This can cause the outer edges of the tires to wear more quickly than the inside. Positive camber is sometimes designed into the suspension to provide increased stability when traveling over bumpy roads, or through turns on the typical high-crowned roads.
Positive Caster. The concept of positive caster is best demonstrated by the wheels of a shopping cart. The steering axis of each wheel is located in front of the point where the wheel touches the ground. The load of the cart is in front of the wheels, and as the cart moves forward, the wheels rotate on their axis to follow the cart's direction. This creates an inherent stability that tends to keep the wheels straight, unless they are forcibly steered in a different direction.
Positive Caster. All Boxsters have slight positive caster, which creates an inherent stability when the car is moving in a straight line. With the angle of the strut tilted back, it places the steering axis and the load in front of the contact patch where the tire meets the pavement. Like the shopping cart example in the previous illustration, the car tends to move forward in a stable, straight line until the wheels are turned in a different direction. The rear trailing arms of the Boxster, by their design, have extensive positive caster built in.
Toe-In and Toe-out. The toe of the front suspension refers to the angle of the two wheels with respect to each other. Significant toe-in or toe-out will cause extreme tire wear, as the wheels constantly try to move toward each other (toe-in), or move away from each other (toe-out). The result is that severe friction is created on the tires, and at highway speeds, the tires will wear significantly and power/fuel economy will suffer.
Toe-out through Turns. When going around a turn, the inner wheels will turn at a tighter radius than the outer ones. This is so that both wheels will be able to turn around the same point without any tire wear. The outer wheel turns at an angle less sharp than the inner wheel. This minimizes the amount of “scrub” of the tires on the pavement as the car turns.
The only way to get the proper measurements for aligning your car is to have it professionally done on an alignment rack. The proper alignment of your Porsche is not something the home mechanic can reliably perform. Don't cheap out either: the Boxster has a lot of adjustment that needs to be set and measured, so be sure you take your car to an expert who has done plenty of them previously. The inset photo shows our project Boxster being corner balanced on top of four scales. Corning balancing is the process of shifting weight from one corner of the car to another in order to achieve an optimum balance. Changes to the balance is typically achieved by raising and/or lowering the suspension spring height by very slight amounts.
This photo shows the underside of a Porsche Cup racing car with fully adjustable suspension. Specifically, this photo shows the GT3 adjustable control arms. The caster can be adjusted using an eccentric bolt that mates with the control arm (yellow arrow). The inset photo shows the opposite side of the control arm, which is a two-piece design that can be shortened or lengthened depending using shims, based upon the amount of additional camber required (blue arrow). The green arrow shows the smaller inside part of the control arm: the red arrow shows the studs that mount into the outer part of the control arm. Add or subtract shims between the two in order to increase or decrease the camber.
Loading Customer Comments...
