What is Caster and How Does it Affect Driving?

If you are unfamiliar with car terminology, you would be forgiven for not knowing what caster is and how it can affect your driving. However, it is one of the most critical vehicle adjustments. Without caster, vehicles would be unstable and more difficult to drive. So here we look at what caster is and how it makes a difference to the handling of the vehicle.

Here are the topics covered in this article:

• Caster: what is it?
• Positive caster
• Why positive caster is a good thing
• Negative caster
• How caster can influence tyre wear
• Toe – another important adjustment

Caster: what is it?

When looking at the side of your vehicle, imagine a vertical line going straight down through the centre of the front wheels. Caster is the angle from the top of the steering axis down through the wheel. If your vehicle were to have a neutral/zero caster setting, this line would be perpendicular, meaning the upright mounting points are directly above the centre of the wheel.

Caster should not be confused with camber, which is the angle at which the tyres vertically lean in or out to the side. However, as we will explain, the two adjustments work together to improve the vehicle’s handling.

Positive caster

If your vehicle has a positive caster position, the lower ball joint is positioned in front of the strut mount or upper ball joint. This means the tyres touch the ground ahead of the suspension components and steering system.

Why positive caster is a good thing

There are two main benefits of positive caster:

1. More stable straight-line driving at speed. Because the front wheels are ahead of the strut mount/upper ball, this causes there to be a self-aligning torque placed on the front wheels and tyres. It creates a tension that causes the front wheels to want to point straight ahead when driving at speed. You will notice this for yourself if you find the steering automatically unwinding back to the centre after you have exited a corner. Without positive caster, the vehicle would have a much more difficult time going in a straight line when driven fast. You can notice this effect for yourself if you push a shopping cart forwards. Just make sure there are no people or cars nearby if you do that!
2. Better for handling corners. Negative camber is vital for maximising the tyre’s grip during cornering, and caster plays an important role in increasing the negative camber on the outer wheel when driving round a corner. The associated shift in weight maximises tyre contact on the outer front wheel. In competitive driving, maximum positive caster is sometimes used to optimise the handling and responsiveness of the vehicle at high speeds.

Negative caster

As you can imagine, negative caster is the opposite of positive caster. The angle is reversed so that the lower ball joint is behind the strut mount or upper ball joint. This means the front tyres touch the ground behind the suspension components and steering system.

Modern road cars do not use a negative caster angle as it doesn’t have any of the stability of positive caster. In fact, negative caster makes steering much more difficult and unstable.

How caster can influence tyre wear

Although caster settings are not the main reason behind tyre wear, an overly positive caster setting can make tyre wear worse when combined with wheel alignment maladjustments. One example is a type of tyre wear pattern known as ‘feathering’. This can happen when incorrect toe settings are combined with an excessively positive caster.

However, your caster setting alone will be unlikely to cause uneven wear to your tyres. Reducing tyre wear involves a holistic approach of optimising various settings, including caster, to the specification of the vehicle.

So, although caster is not the main vehicle setting that people tend to rant and rave about, it is still a vital part of optimising your vehicle safety and performance. Just be thankful that you don’t have to drive around in a motorised shopping trolley!

Toe – another important adjustment

So, we have explained how camber and caster can significantly affect your car’s handling. But you may have noticed we also mentioned something called ‘toe’. So, what is toe, and how can it affect your driving?

Toe refers to whether the front of the wheels are pointing outwards or inwards and by how much. ‘Toe out’ is when the front of the wheels are pointing outwards, and ‘toe in’ is when they are pointing inwards. Makes sense, right? But which is best, ‘toe in’ or ‘toe out’? Well, it depends on the type of car you are driving.

Regular street cars use ‘toe in’ because it provides better stability when driving straight, and street cars don’t usually need to drive round corners quickly. Racing cars, on the other hand, use ‘toe out’ because it enhances turning ability.

Summary

So, to recap, here are the main points we have covered in this article:

• Camber is the angle at which the tyres vertically lean in or out to the side.
• Caster is the angle from the top of the steering axis down through the wheel when viewed from the side.
  • Positive caster is when the lower ball joint is positioned in front of the strut mount or upper ball joint. It makes the vehicle more stable when driven at high speeds and, when combined with negative caster, gives better handling around corners.
  • Negative caster is when the lower ball joint is behind the strut mount or upper ball joint. It makes steering much more difficult.
• Toe is the angle at which the front of the wheels point outwards or inwards.
  • Regular street cars use ‘toe in’ for better stability.
  • Racing cars use ‘toe out’ for better turning.

We hope you found this article on caster informative. Hopefully, you now know what caster is and how it can affect your driving. If you have any questions, please don’t hesitate to get in touch with the SUPALIGN team today.