Suspension 101 for novice drivers

[Kim Crosser]

I have discussed the following with some of my recent students and became aware that many drivers do not know how much some basic (and zero points!) suspension adjustments can help make their car handle better. Just from looking at a couple of recent Boxsters, I could tell that some suspension changes would be likely to improve their handling.

First, I am NOT a professional auto specialist. There are a number of them in the club (including Steve Grosekemper, John Rickard, and Mark Kinninger). I hope they (and others) will feel free to correct any misstatements below.


A couple of years ago, after talking with some of the more experienced drivers, I took my car into Pioneer (it was still under warranty, so I had the work done as part of a regular service) and had them set up maximum negative camber on all 4 wheels, and also take out virtually all the toe-in on all 4 wheels. I noticed a significant difference in handling immediately, and yet there are ZERO points assessed for these improvements!

What does this mean? It means that if you want to maximize your car's handling, you may also want to have your suspension adjusted for maximum negative camber and zero (or near-zero) toe-in.


Camber

Camber is the measure of how much a wheel "leans" relative to the vertical. If a tire leans inwards (top of the tire leans into the car), that is negative camber. If a tire leans outwards, that is positive camber.

As a car rounds a corner and the suspension compresses, camber can radically change - for most cars, the outside tire will "gain" positive camber during the turn (the suspension will cause the tire to lean more outwards). Since you want as much tread contact as possible, you therefore want to have some negative camber before entering the turn. If your suspension already has positive camber before entering the turn, your tire is just rolling over onto the outside edge and you are losing traction. One article claims a tire achieves its maximum cornering force at a -0.5 degree angle of camber. Obviously, this means you need quite a bit more than that before going into the turn.

With just standard adjustments (no changes to the suspension parts), I was able to get about -2 degrees on the rear wheels and about -0.5 degrees on the front wheels on my 2000 Boxster. To get more would require changing suspension components.


Toe-in (or toe-out)

Toe-in is a measure of how much the front of a tire is pointed inwards (towards the nose of the car) when rolling straight ahead. A wheel with zero toe-in would be rolling directly in line with the vehicle's direction of travel when going straight ahead. A wheel with positive toe-in is pointing inwards, and is thus "scrubbing" a bit while going straight ahead.

Pretend your hands are your car's front wheels. Hold them out in front of you pointing straight forwards (zero toe-in). Now, point your fingers on each hand a little towards the other hand (positive toe-in). Now, pretend you are turning left, and rotate both hands by the same amount towards the left. Notice that the left hand is now pointing straight ahead, while the right hand is pointing more to the left.

When this happens in your car, your left wheel isn't helping you turn left until you rotate it much further. Meanwhile, your right wheel is providing all the turning force (or simply scrubbing, for those of you who encounter a lot of understeer).

Now, try the same thing, but start with both hands pointing straight ahead (zero toe-in). When you rotate both hands to the left, notice that BOTH wheels are now helping you turn. The result is that your car will turn in much more crisply and you will be able to sustain much higher cornering forces with less understeer.

From posts on various web forums, the factory tolerances for toe-in seem pretty large, and you may even have asymmetrical settings (both within tolerance, but at the extreme ends). Obviously, if your toe-in settings are asymmetrical, your car will turn in differently depending on the turn direction.

By the way, the rear wheels are also set up with toe-in, and can be adjusted. Forum postings suggest a decent compromise between track and street is zero toe-in for the front wheels and around 1/4 degree (15') for the rear wheels. A zero (or toe-out) setting for the rear wheels isn't recommended, as the car may be excessively "twitchy" on the highways.


Why isn't my car already set up like this?

Cars (including Porsches) are mass-produced. That means that they are manufactured with a "tolerance" that is pretty broad. Your car may be well within manufacturing tolerances and still have positive camber, asymmetrical toe-in, etc.

For normal street use, most people would never notice or care if they had positive or negative camber or excessive toe-in (although the latter could cause excessive tire wear even in normal use).

Setting up with maximum camber "might" cause more wear on the insides of the tires. I haven't noticed uneven wear, but then if you are driving aggressively, maximum negative camber may make the wear more even, as you will be cornering with a flatter profile on the outside tires. If you have positive camber and you track the car (or drive aggressively on the street), you will almost certainly notice higher wear on the outside of the tires.

A bigger issue is the toe-in. If you set up for zero toe-in (or go really aggressive with positive toe-out), you car is going to "wander" a bit on the street. My car definitely requires constant attention on the freeways and is a bit "twitchy". When I first got it (with the factory toe-in), it tracked very nicely (sedately?) down the freeways. With the near zero toe-in, it turns in really nicely on the track, but it is not as friendly to drive for several hours on the highways.

Positive toe-in makes the car drive more smoothly on the highway, makes the car more prone to understeer (for safety with inexperienced drivers), and makes the car less sensitive to bumps in the road (aka "bump steering").

If this is a family car, you may want to keep a small amount of toe-in just to make it a bit easier in normal driving.

In any case, I would strongly recommend consulting your dealer or technical friend (or club member) and then getting the maximum useful camber and minimum reasonable toe-in that you can have on your car. You may then find yourself shaving another second or so off your track times.


For those looking for more detail, you might look at the following web page, or consult some of the excellent technically knowledgeable people in the club - http://www.ozebiz.com.au/racetech/theory/align.html.

[Kim Crosser]

Just bumping it up - some of the Boxster drivers asked about this at the PDS this weekend.

[Otto]

Kim:

Your write-up is very helpful to all of us. Thank you for taking the time to do it.

[Taichi Takayanagi]

Hi Kim,

Thanks for the tip.

I took my car to the Pioneer Center yesterday and had my suspension recalibrated per your suggestion.

First thing I noticed was that my original settings for Camber and Toe-In were all over the place. So it needed adjustments in any case.

Second, I had my Camber and Toe-In adjusted accordingly.

Front Left:
Camber -0.18 (-0.06 before)
Toe-In 0.02 (0.04 before)

Front Right:
Camber -0.19 (0.29 before)
Toe-In 0.02 (-0.05 before)

Rear Left:
Camber -1.47 (-1.24 before)
Toe-In 0.04 (0.00 before)

Rear Right:
Camber -1.45 (-1.29 before)
Toe-In 0.04 (0.12 before)

The PC told me that this is probably the best compromise between track performance and tire tread wear.

I drove the car this morning to work and the difference was immediately noticeable. Even during slow corners, I noticed that the car had better turn-in. This car turns more eagerly.

Only drawback was that it felt slightly less stable on the freeway as mentioned in your article.

Thanks for the great tip.

Ty

[Kim Crosser]

Thanks, Tai.

In case anyone is confused by the different numbers used in this thread, there are at least 3 different ways to measure and state camber and toe-in: degrees, minutes, and inches.

In my first posting, I used degrees, as that is a concept most people intuitively understand. Tai's numbers are stated as inches. I will try to relate the numbers:

His front right tire was measured at 0.29 inches of positive camber at the start. This means that the top "edge" of his tire was 0.29 inches further "outside" (leaning away from the car body) than the bottom "edge" of the tire. Since all Boxster tires are approximately 25 inches in diameter (between 25.03 and 25.30 inches for all standard 16", 17", and 18" rims), that means that the tire was leaning outwards by approximately 0.66 degrees. As noted above - positive camber is bad... After the camber was adjusted, his tire is now at -0.19 inches, which is approximately -0.44 degrees - i.e., a little under 1/2 degree of negative camber, which is about the same as my car.

He appears to have done better than me on the rears - after adjustment, they are -1.45 to -1.47 inches, which translates to a little over -3.3 degrees (i.e., 3.3 degrees of negative camber). This is very good.

(For the mathematically inclined, the angle (in radians) = arcsine(camber [inches]/tire height [inches]); then the angle (in degrees) = angle (radians) * 57.29578. Example - a camber of 0.29 inches means the angle (radians) = arcsine(0.29/25.0) = 0.0116; and then the angle (degrees) = 0.0116 * 57.29578 = 0.66. If the camber or toe-in is expressed in minutes, simply divide by 60 to convert to degrees.)

Note the wide variations in camber and toe-in in Tai's car before the adjustments. His left front tire was almost vertical (about -0.14 degree camber) while his right front tire actually had about 0.66 degrees of positive camber and was "toed-out" by about 0.11 degrees. The front toe-in settings weren't too bad. His rear tires had reasonable negative cambers, but the left rear had zero toe in, while the right rear had about 0.28 degrees of toe-in.

The amount of camber that you can get will vary with each car due to minor manufacturing variances and wear (stiffness of springs, shocks, etc.) - the objective is to make sure the cambers and toe-ins are matched left/right, and that you get the maximum possible negative camber on the tires, with the minimum toe-in.

Overcoming understeer is mostly driving, but optimizing your suspension setup can certainly help you by eliminating some of the built-in understeer. All other things being equal, Tai's suspension adjustments could easily make a difference of a second or more in his AX times.

[Taichi Takayanagi]

Hi Kim,

Some clarification on my numbers.

In the report that I received from PC on suspension setup, I believe the data was presented in degrees. My fault for not making this clear.

For example, the camber for the front left tire was indicated as negative 0(degree)18(hyphen) which I wrote as -0.18. I wrote the way I did because I could not use the degree sign and I also did not know that settings can be indicated in inches.

I think this changes the way we should interpret the numbers?

Sorry for the confusion everyone...I am really not good with math either.

Ty

[Kim Crosser]

If the numbers were printed by Pioneer as x (degrees) yy, then the numbers were in degrees and minutes of arc.

Soooo... quick recalculations...

For the fronts, -0.18 is really -0 degrees 18 minutes, or -(0+18/60) degrees, or -0.30 degrees. Similarly, -0.19 is -0.32 degrees.

On the rears, -1.45 is -1 degree 45 minutes, or -1.75 degrees.

These are more consistent with what I have - if I recall correctly, I have around -0.4 degrees on the fronts and around -1.8 degrees on the rears.

Everything in the posting still holds - only the magnitude of the adjustments varies a bit.