Build A Faster Car - helping drivers engineer

Some cars just never seem to have enough camber to get even tire wear across the tire's tread when driven at its limit. Why is it that a car with only 3 degrees of body roll can sometimes need 4 degrees of negative camber just to keep the tire perpendicular to the ground while cornering? This is counter-intuitive, as 3 degrees of body roll only should need 3 degrees of correction- or less, if the car's suspension geometry is good. This article explores that "if" and shows how to improve your dynamic camber curve on a car with a strut-based front suspension.

Let's explore the Subaru WRX STI. The car is quite stiff and has substantially less body roll than many other cars on the road. Yet, it suffers from loss of grip during body roll due to the tire going off-camber in turns. The tire's outer edge ends up doing all the work. I've measured a slightly-modified STI at exactly three degrees of body roll, yet many autocrossers have found themselves needing 4 degrees of camber or more. They tend to buy camber plates and slant them inward towards their maximum camber setting to achieve this.

The reason such STIs need a lot of camber is because they run a very high kingpin inclination without nearly enough caster to compensate. The Kingpin Inclination, also called the steering axis inclination or SAI, is the pivot line for your steering as viewed from the front of the vehicle. Here is a diagram:

A factory STI runs just shy of 15 degrees of kingpin inclination. This is as much as other strut-equipped cars, but far less than what cars with superior suspensions run. For comparison, a Nissan 350Z/G35 runs less than 5 degrees. Nissan uses a special upper wishbone for the front suspension which mounts above the tire to achieve such a low angle.

Why do we need a steering axis inclination?

The purpose of the kingpin inclination is to define the scrub radius. The scrub radius is the distance between the steering axis and the actual center of the tire. If the steering axis does not go through the center of the wheel, a lever arm is created between the two points. This means braking and acceleration forces will try and cause the car to toe-in or toe-out, which may cause the car to steer. Yuck.

Your are directly altering your scrub radius if you change either your kingpin inclination or your wheel offset. I believe, but am not certain (I don't trust my measurements) that the stock scrub radius is about 0.6 inchs negative. Raising your kingpin inclination makes your scrub radius larger; reducing your kingpin inclination or running lower-offset wheels brings it closer to zero.

The other benefit to having a kingpin inclination applies only to strut suspensions, and that is a high kingpin inclination helps raise the vehicle's roll center. It helps the camber curve a little bit too. These effects are somewhat minimal when talking about making minor adjustments because with struts, your choices are a high kingpin inclination or very high kingpin inclination. A strut setup doesn't work otherwise.

Why a high kingpin inclination is bad:

However, there is a huge drawback to a high kingpin inclination. To keep your wheel upright despite your steering axis being angled, the wheel hub must compensate for this. On an STI with a 15 degree kingpin inclination, a wheel parallel to the kingpin inclination would result in 15 degrees of negative camber- way, way too much!!!

So, the hub of your wheel is angled to compensate for this. The angle of your hub (and thus wheel) relative to the kingpin inclination is your included angle, pictured below:

So, an STI with a kingpin inclination of 15 degrees and -1 degrees of camber would have an included angle of 14 degrees. The problem with this is that as you turn your wheels, 14 degrees of correction is not needed anymore, but you're stuck with it. In fact, if you could turn your wheels a full 90 degrees, you'd need no correction at all, giving you 14 degrees of positive camber. Yes, positive. The "correction" angles the wheels towards positive camber as you steer.

For a steering input of 30 degrees, 0 camber, and 15 degree kingpin inclination, (15 - (15 * COS 30)) = 2 is the answer we want, which would tell us that a steering input of 30 degrees causes us to get 2 degrees of positive camber. A graph showing this is below:

(In this and the following graphs, a negative steering angle means the driver's side wheel is turned right- that wheel is the "outside", and thus more important, tire in a turn.)

Caster: a steering axis in the other direction

Like your kingpin inclination, caster is a measure of your steering axis, except viewed from the side. Caster requires no correction in the hub when your tires are pointed straight, so caster simply leans your tires into a turn. This means positive camber for the inside, but more importantly, negative camber for the outside tire. These are both changes we want, as both tires are now angled in such a way that they'll create thrust in the direction you are trying to steer the car. Caster is how we cancel out the bad effects of a high kingpin inclination.

A stock 2005 or 2006 STI has 4.5 degrees of caster. 2007 models have even less. While 4.5 degrees is far less than the amount of kingpin inclination in the car, the changes in dynamic camber due to caster take effect much more quickly than do the kingpin inclination angle change. This is due to where those dynamic camber changes are along their arc (again, think of a sin wave). This means your dynamic camber from caster has a larger effect than your kingpin inclination at small steering inputs.

For example, dynamic camber from 4.5 degrees of caster on a 30 degree steering input is 4.5 * SIN 30 = 2.25 degrees. Below is a plot of dynamic camber due to 4.5 degrees of caster assuming a kingpin inclination of zero:

Can caster cancel out a high kingpin inclination?

Even though our 15 degree included angle lost us 2 degrees of camber, that tiny bit of caster got us 2.25 degrees back, for a total net gain of 0.25 on the outside tire. That's not nearly enough for a car with three or four degrees of body roll! This is why folks generally find themselves running three degrees of negative camber before they get even tire wear. The inside tire in a turn would now have 4.25 degrees of positive camber, which means it'd be pretty vertical in a turn. Unfortunately, the inside tire does very little work in a turn, so this silver lining does not save the steering.

Below is a graph showing the dynamic camber curve of a stock STI.

What this means is that the inside of a tire gets lots of positive camber, like we'd want, but the outside tire doesn't really gain any negative camber. Thus the outside tire, which much more weight on it, is not leaning the direction we'd want to compensate for body roll. It will lose grip as a result.

Folks try and get around this by buying camber plates and slanting them inwards as far as they'll go, which gives them -4 degrees of camber or more. This means the tires are permanently locked into compensating for 4+ degrees of body roll whether the car is turning or not, and doing so sacrifices some amount braking grip and straight line stability.

For some folks, this is the only way they can get even tire wear in a motorsports situation. I started exploring this topic because I wanted to find a better way to go about getting even tire wear. I looked at alignments from other cars to see where our car differed. Here is one such example- the Infiniti G35, which has 8 degrees of caster and a steering axis inclination of just 5 degrees:

Wow, what a huge difference! The added caster steepens the dynamic camber curve quite a bit while the decreased kingpin inclination no longer prevents the outside tire (negative steering angle) from seeing good camber gain too. This is good suspension geometry (shared with the 350Z, too). If the car pictured at the top of this thread had such good suspension geometry, it would not need any static camber whatsoever to compensate for 3 degrees of body roll. Can we mimic this geometry on the STI?

Unfortunately, no- we can't get a kingpin inclination of just 5 degrees with struts. However, we can improve it from the stock level of 15. Remember that it's the included angle that matters, which is your kingpin inclination + camber. Therein lies a secret: you can decrease your included angle by adding camber via camber bolts, not plates. Camber plates instead give you camber by increasing your kingpin inclination, which is bad.

Incidentally a few Google searches say the Miata and Corvette both seem to run somewhere inbetween, with an kingpin inclination around 10 degrees and caster around 6-7 degrees. Only strut-equipped cars run a kingpin inclination of 15 degrees (BMW does too), but the better-driving ones do so while also running more caster.

How to adjust Camber / Caster Plates

There are typically two ways to mount a camber plate. Mounted the traditional way, you have a plate that adjusts camber. However, rotated, the plate can adjust caster only. Some plates can do both simultaneously.

Configuration one (red line):

• Camber plates add -2.5 degrees camber
• Camber bolts at factory position
• -2.5 degrees total camber
• 4.5 degrees (stock) caster

Configuration two (blue line):

• Camber plates add two degrees of caster, no camber
• Camber bolts add -2.5 degrees of camber
• -2.5 degrees total camber
• 6.0 degrees total caster

Let's break down where those improvements come from. The graph below shows the effects from only adding two degrees of caster. All else is equal. The blue line has 2 degrees more than stock, for 6.5 total:

Adding caster makes quite a bit of improvement even at 15 degrees. The largest difference may be beyond 30 degrees, but that's only for making U-turns.

And here is where we get a bit unorthodox. The graph below shows a camber plate adjusted for full negative camber versus a camber plate adjusted to full positive camber, but with camber bolts used to once again get zero camber. Thus, we've added or subtracted 2.5 degrees from the kingpin inclination versus stock, for totals of 17.5 (red) and 12.5 (blue).

You can see that decreasing your kingpin inclination (or more specifically, the included angle) has effects similar to adding caster, but the camber change doesn't take effect as quickly (the difference at 15 degrees of steering input is quite small). So while beneficial, it's not as beneficial as caster. Still, with too high of an included angle, you may find yourself losing grip in the tightest turns. Folks who try max their camber plates for both camber and caster may actually be better off focusing on only caster (which you can get more of if you move the strut mount in just that direction) and concentrating on getting camber with bolts instead (even if it's less camber overall).

Note that you may need special camber bolts to be able to compensate for that much camber with bolts.

Is there such a thing as too much caster?

Some will tell you no. They're mostly right. Some OE cars nowadays are coming with 8 or even 9 degrees. Lots of caster gets you heavy steering and darty turn-in that some folks might not like, but there's something more important to be concerned about.

Remember what a scrub radius is? Well, there's also a radius along which the tire changes camber. I'll call it the jacking radius, since that's what it causes. As your wheels cambers in and out, it is also changes your ride height a small amount. As a tire gets negative camber, it gets sucked into the fender a bit and vice-versa on the other side. The disparity in ride height between the two sides causes load transfer. This will manifest itself as both increased body roll and a tendency for the car to oversteer. Both of these effects, like your dynamic camber, will become more prominent as your steering angle increases. Is this a downside? It could be- perhaps it is moreso for those with stiff suspensions (which would mean lots more load transfer).

The other downside is that with a high amount of dynamic camber, your camber angle depends greatly on your steering angle. This means your tire might be happy at low-speed, sharp turns but unhappy with high-speed, shallow turns. Perhaps maximum caster is a trick best reserved for autocrossers, but some circuit racers run as much as 10 degrees.

Experiment, an see what you like.