What is Castor Correction?

It’s one of the most common necessities when lifting a suspension system; adding some sort of castor correction. But what is castor correction, and how does it affect your 4WD’s driveability, and is it even really necessary?

In basic terms, castor refers to one of your vehicle’s steering angles. When the ride height is raised by 50mm or more, the castor angle is changed, and not for the better. So, you need some means of adjusting the steering geometry of the front end, bringing it back to or better than stock measurements. This is where castor correction comes in. It prevents dangerous steering operation and allows you to get the full benefit of a suspension lift without any of the negative handling outcomes.

Castor correction isn’t an option so much as it is a necessity for the majority of lifts. The safety and handling side-effects experienced without it are simply too dangerous to quantify.

Understanding Wheel Alignment & Castor

In a nutshell, there are three “main” factors when it comes to a wheel alignment: Castor, camber and toe.

Toe is the direction the front wheels are pointing when positioned in a straight line and looked at from directly above. If the leading edge of the wheels are pointed away from each other, it’s called “toe-out” and if they’re pointed towards each other it’s called “toe-in.” When aligned toe-in, it can help with understeer and a toe-out alignment can offset oversteer. Keep in mind that these measurements are super small, usually just fractions of a degree.

Camber is the angle of the wheels positioned in a straight line and viewed directly from the front. If the tops of the wheels are slanted in it’s called negative camber and if they’re slanted slightly out it’s called positive camber. A small amount of negative camber is generally favoured as it helps maintain the tyre’s contact patch during hard cornering. Positive camber is never desirable and should be avoided at all costs. Keep in mind that camber is only adjustable on independent front ends. Solid axles have the camber set by the position of the steering knuckles and is difficult to change without some pretty major fabrication (and cost).

Castor is the angle of the steering axis when the wheel is looked at directly from the side. On solid axles it’s the line between the kingpins on the swivel hub, and on IFS vehicles it’s generally the line between the upper and lower control arm mounts on the steering knuckle. When this line leans back towards the rear of the car, it’s called positive castor, and when it leans towards the front, it’s called negative castor. On 4X4s, negative castor is never a good thing, most vehicles have between 2º and 5º positive castor as stock. It enables straight-line stability and for the wheels to return to centre after cornering, both of which are surprisingly important attributes for a well set-up suspension system.

Think of castor like pushing a wheelbarrow. It’s fairly easy to steer it and keep it on track when you are positioned behind the leading wheel and the handles are being held by your sides, right? But if you were to lift your arms up so that they were directly out in front of you, and the wheelbarrow’s handles were directly above the wheel itself, it all of a sudden becomes incredibly difficult to maintain a direction, let alone steer it effectively around any obstacles. This is an example of negative and positive (respectively) castor at work.

Why Castor Correction Is Needed

When you lift your 4WD it’s just like in our wheelbarrow example. You’re raising your hands up and the angle of the handles relative to the wheel (the steering axis) is getting steeper. Keep in mind that vehicles like LandCruisers only have a couple degrees of castor to begin with, so you really don’t need heaps of lift to reduce that in-built stability significantly.

Without castor correction, you’re likely to experience any number of issues. Best case is that you start getting uneven tyre wear, but quite often symptoms like shaking through the steering wheel, vague steering and handling – even wandering between lanes on the highway can occur.

It doesn’t have to be that way though. For much better on-road performance, highway safety and off-road comfort, you need some form of castor correction, which not only gives you as-good-as-factory handling, but often improves it.

Methods Of Castor Correction

There are three main types of castor correction available. Which one you use should be decided on after a chat with your suspension specialist, as some will suit certain lifts and vehicles much better than others.

·       Correction Bushes
Castor bushes replace the stock bushes in your radius arms with items with an offset hole. They work to slightly tweak the radius arm mount locations, correcting the steering angle for lifts up to around the 2” mark.

·       Plates/Brackets
Castor plates are either bolted or welded to the radius arms’ axle housing mounts and tweak the standard arms’ position to allow for the correct castor angle to be maintained. Designed for 4in to 7in lifts, they can correct even the angles found in larger lifts.
Drop boxes, which lower the mounting point of the radius arms at the chassis end, can also be used to correct a small amount of castor, although they’re more commonly used in conjunction with other methods of castor correction to optimise driveline angles.

·       Replacement Radius Arms
The best method for castor correction thanks to the fact replacement arms retain the use of factory mounting points and bushes. Replacement arms have the castor built in to the mounting points and will often include more drop to offset the lift amount. Available as Superflex or Hyperflex arms too, so articulation can be improved out of sight, and can be made to suit 2in all the way up to 6in lifts.

Benefits of Correcting Castor Angle

As you have almost certainly worked out by now, there are no downsides to correcting your castor angle after installing a suspension lift, and it’ll be appreciated every time you get behind the wheel.

Your vehicle will be stable and exhibit predictable steering characteristics under all conditions and your highway driving will be just as it was when your rig was stock, if not better. On top of the generally improved handling, your suspension, particularly the bushes and steering components, and your tyres will have much longer and happier lives.

Signs You Might Need Castor Correction

As mentioned, even a small lift may need some form of castor correction. Here are a couple things to look out for if you’ve fitted up some new suspension recently and things aren’t feeling quite right.

The first sign is that your steering doesn’t self-centre. That is, you take a corner, and your steering wheel doesn’t want to straighten back up on its own without some input from you. Your vehicle wandering on the road is another sign. Think of our wheelbarrow analogy. With your hands out right in front of you and the wheelbarrow’s handles right on top of the wheel it’d be difficult to keep it from erratically changing direction, even while walking a straight line. Your vehicle feels the same way with its newfound loss of negative castor angle.

Your tyres are also a giveaway. If they’ve started wearing unevenly since you installed the lift, then too much positive castor is a likely culprit. Same goes for steering shake at any speed or any sense of steering vagueness or less-than-ideal feedback through the wheel.

Small angle, big differences.

Castor Correction Conclusion

To recap, castor is a steering angle that helps with return to centre and overall vehicle stability, so is an important factor when considering a lift kit. By including it in your chosen kit, you’re ensuring that not only do you get the usual benefits of a suspension upgrade – more wheel travel, tyre clearance and off-road capability – but you will also retain factory handling, if not improve it.

For our money, even with a 2in lift, castor correction should be viewed as an essential part of every complete suspension makeover.

Take a browse through the Superior engineering website to find the right castor correction for your vehicle today!

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