Suspension 101 - Camber, Caster, and Toe

IGOTASTi

System Operator
Staff member
Suspension 101 - Camber, Caster, and Toe

by Alan LaFrance, find him on Google or read more articles at www.subaruwrxsti.org
http://www.vikingspeedshop.com/suspension-101-camber-caster-and-toe/

There's a lot of lingo and settings when in comes to suspension on a car. We're going to start by describing a handful of the more common terms and concepts and how they apply to you as an enthusiast.

camber.jpg


Camber:

Camber is a measurement of the centerline of your wheel/tire relative to the road surface. It is expressed in degrees and greatly affects the dynamics of the car.

Negative Camber is when the top of the tire tucks inwards. For a road going car you typically want to maintain a slight amount of negative camber (1 - 2 Degrees) to improve road handling. Camber improves handling by allowing the tire to apply even loading when the body rolls going into a corner. Without negative camber the tire would load the outer portion of the tire which would reduce overall grip.

Downsides to negative camber are increased inner tire wear since during normal driving conditions the tire will apply more load to that portion of the tire. Large amounts of static camber will also generally reduce overall grip during braking and straight-line acceleration.

Race teams will know how much camber to dial into their car from thermal tire data and driver feedback. At proper camber settings the tire will exhibit stable and symmetrical temperatures across the tire surface during cornering. Excessive heating on the inner or outer third of the tire can be indicative of improper camber angle.

Positive Camber is when the top of the tire extends outward, and the base of the tire tucks inwards. This is rarely ever seen on a road car since it will reduce road handling capability. In special situations, such as NASCAR, positive camber will be applied to handle heavy amounts of track embankment. If you are running a positive camber figure on your street car then it's highly recommended that you inspect your suspension for damage and/or adjust the camber to a slight negative figure.

caster.jpg


Caster:

Caster is the measure of how far forward or behind the steering axis is to the verticle axis, viewed from the side. An example of caster in action is the front wheels on a shopping cart. They run a large amount of positive caster to make the cart track straight without wandering. However, the method that the cart uses (displacement caster) is different than how your car develops it's caster angle (angled pivot), but the effect is the same.

Positive Caster is when the steering axis is "in front of" the verticle. In a road car, this would mean that the top of the coilover would be pushed towards the rear of the car. Positive caster creates a lot of align torque (the force that straightens the steering wheel when you go forward) which improves straight line stability of the car. Due to the geometry of positive caster it also will increase negative camber gain (a good thing) when turning. As you increase positive caster the steering will get heavier also, but with modern power steering systems this is rarely a problem. Generally you want as much positive caster as you can reasonably get so long as the car is equipped with power steering.

Negative Caster is when the steering axis is "behind" the verticle. This is generally only found on older vehicles due to tire technology, chassis dynamics, and other reasons. Modern vehicles do not use negative caster. It will lighten the steering effort but also increases the tendency for the car to wander down the road.

Special Note - Regardless of what caster setting you use, make sure that your caster is symmetrical. Running a different amount of caster on one side will cause the car to pull towards the side with less caster.

toe.jpg


Toe:

Toe is the measure of how far inward or outward the leading edge of the tire is facing, when viewed from the top. Toe is measured in degrees and is generally a fraction of a whole degree. It has a large effect on how the car reacts to steering inputs as well as on tire wear. Aggressive toe angle will cause the tire to develop "feathering" across it's surface.
Toe-in is when the leading part of the tire is turned inwards towards the center of the car. This makes the tires want to push inward, which acts to improve straight line stability of the car as it's traveling down the road, particularly at high speed (highway).

Toe-out is when the leading part of the tire is turned outwards away from the center of the car. This makes the tires want to separate from each other. This improves "turn-in" response considerably but again, at the cost of tire wear. Running toe-out in the rear is generally not recommended since it will make the car want to pivot (oversteer) at all steering angles, but in the right setup it can help (auto-x / technical tracks).

Special Note - Due to the small amount of angle ran you have to take into account the dynamics of your particular car. For instance, under acceleration and steady state driving a RWD car will be pushing the front along, this means that when the bushings deflect in the control arms that the tire will want to toe-out. A small amount of toe-in in the front will allow the tire to zero out. For a FWD car, the front wheels are pulling and the opposite occurs. The bushings are compressed in the opposite direction and the tires will tend to toe-in, so a small amount of toe-out is necessary to zero the angle. In an AWD car this gets complicated since the front is pulling and the rear is pushing. Generally speaking a much milder toe deflection should be realized in this situation, requiring virtually no change.

Alignment Settings:

So, what should I run on my STI / EVO / Corvette / 911 / etc.?! Well, that's a very long subject, but here's some general guidelines to go off of for most McPherson strut front / IRS (independent rear suspension) vehicles.

Front:

Caster - As positive as it will go, aim for 5 - 7 degrees, but make sure it's even side to side.

Camber - Street Tires (1.5 - 2 degrees negative) / Race Tires (2.5 - 3.5 degrees negative)

Toe - This is up to you. (See special note on toe above) Remember, a little goes a long way.

Rear:

Camber - Street Tires (0.5 - 1.0 degrees negative) / Race Tires (1.0 to 2.0 degrees negative)

Toe - Net Zero, maybe a touch of toe-in if you want a tamer straight line.

Why more camber for race tires? Race tires produce more grip which means you are going to have greater amounts of roll for a particular setup. The more the chassis rolls the greater the amount of static camber you need, especially in the front. Rear IRS setups tend to have favorable camber gain attributes, so the amount of static camber is reduced. This is vehicle specific and even setup specific if you are capable of adjusting the entire link setup. As you increase roll resistence (read: spring rates, sway bar rates, better roll center location, etc.) the requirement for static camber decreases.

If you lower your car significantly, even with stiffer springs, you can actually increase the amount of chassis roll due to the roll center migrating. So you have to think about the entirety of the suspension geometry when setting up your camber. That's why race teams will often just resort to measuring tire temperatures to help setup static camber requirements. You're looking for a nice even heating of the tire surface with maybe a touch higher temp on the inside edge (due to heating prior to pitting, post corner).

How do I Adjust these Settings?

Front camber is generally adjusted via the OEM eccentric bolt (if equipped). For a modified car, the upper strut mount (or hat / plate) can be adjustable (camber plates). This allows the user to loosen a couple of bolts and slide the upper mounting point of the shock inward or outwards to adjust camber. We sell adjustable camber plates for many vehicles. Email us with your make and model along with any existing suspension modifications and we'll get back to you with pricing and availability.

Front toe is adjusted by shortening or elongating the OEM tie rods.
 

IGOTASTi

System Operator
Staff member
Another great article from Alan [MENTION=3680]Texas1911[/MENTION]!
 
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