When it comes to car steering systems, with a few obvious exceptions - such as 4x4 vehicles - most cars operate with front wheel turning. It doesn’t matter if the vehicle is front or rear wheel drive, steering via the front pair of wheels is the industry standard.
How does your car steering system influence your driving performance? What turning forces are applied when steering from the front? More specifically, there are a lot of interesting facts worth knowing about various forces vehicles experience when being steered as well as about the effect the steering has on car wheels and tyres.
Steering system basics and mechanical trail
To understand how any vehicle interacts with the road, you should look at the steering axis. It refers to the deviation the steering has compared to a straight vertical line with the road.
While it is often used to help with the vehicle’s camber and lift, the relevant aspect here is caster angle, the angle between the steering axis and of a steered wheel and a vertical line viewed from the side. Imagine the point at which this imaginary axis meets the road. The distance between this point and the tyre’s actual point of contact is known as mechanical trail. It is typically positive, meaning it takes place in front of the tyre.
Positive mechanical trail
Mechanical trail is one of the two important forces your wheels encounter. A long trail may make the driving experience easier, but a shorter trail typically allows for greater control.
Positive and negative mechanical trails in relation to the steering system
The reasons why this trail is positive are because:
- It ensures the wheel is essentially pulled.
- It helps create a self-righting effect that keeps the vehicle in line.
Negative mechanical trail
A negative trail, on the other hand, has the point at which the steering axis meets the road behind the contact patch of the tyre. Because of that, a vehicle with negative trail would not have such self-righting support. Steering it would be very susceptible to lateral movements, making the wheels lack directional stability and, consequently, such a car would get much more easily unstable.
Think of the experience of pushing a shopping trolley. Shopping trolleys are stable but if you apply even minimal force, they change the direction and -- unlike a car with front wheel turning -- their wheels do not have the tendency to return to a straight-oriented position. This is because they have a relatively big trail and no caster.
Alongside this, there is also the pneumatic trail, which is created when your car tyres make contact with the road surface. Unlike the mechanical trail, the pneumatic trail typically occurs behind the tyre’s centerline. One of the most important aspects of the pneumatic trail is that it causes self-aligning torque, along the side of the wheel. When turning, this can create torque to turn the wheel away from the turn.
For most drivers, a basic understanding of how steering mechanical trails should be enough. However, if you are interested in a more advanced analysis of a car steering system, you should also consider the camber angle and how it influences tyres.
Front wheel vs rear wheel steering systems
So how do these forces work when considering different parts of a car steering system, namely both front wheel and rear wheel options? On a front wheel drive vehicle, the self-righting effect created by the mechanical trail works with the self-aligning torque of the mechanical trail.
These two forces work together in balance to keep the wheel in its intended position. This effortlessly turns the vehicle, with the remaining part of the car adapting to the new incline.
Front wheel steering systems offer better control to the driver.
On rear wheel steering systems, however, there is a greater imbalance of forces. This is because the wheel actually faces away from the direction you wish to go. For example: if you wish to turn right, your rear wheels rotate to face the left. This causes problems with the pneumatic trail, which is behind the contact patch of your tyres.
Since the torque is effectively pushing your tyre in the wrong direction, this can cause an unstable driving experience. It is also rather uncomfortable to experience, as the car moves in a direction that its directional wheels are not aligned to.
Additionally, this would not be good for the tyres. In such a situation, tyres would be moving against momentum, rather than rolling with it, which could lead to excessive wear and tear or tyre damage.
Rear wheel vehicles do exist, of course, but it’s a niche category. Depending on the speed and other factors, the forces applied to the rear wheel could cause the torque to flip and allow more control, but this isn’t suitable for regular everyday driving. As such, it’s often reserved for sports cars and high-performance vehicles.
How your steering system affects tyres
Even with front wheel steering, however, the front tyres are affected more than the other pair. This will be true regardless of which steering setup you have. Because the wheels rotate and push against the surface to steer, the treads on these tyres will be worn down more quickly.
Tyres of the wheels that turn have will often wear down more than the others.
This is why it is often recommended to swap your tyres around after a certain amount of time (depending on how much you drive). Half way through a tyre’s typical lifespan, for instance, the front wheels will typically be in a worse condition than the rear pair. Swapping them around returns some better control into your steering, in addition to allowing the whole tyre set to last longer.
That said, having better tyres on either the front or rear wheels, depending on the type of steering your vehicle has, brings additional benefits and drawbacks. With that in mind, also consider how to fit your car tyres on the steering system as different ways of doing this will also change your performance.
Of course, there are other factors that contribute to the handling and performance of your vehicle on the road. Steering forces discussed above are crucial, though. They are the reason why car manufacturers prefer to design front wheel drive vehicles with front wheel steering. Additionally, this is just another circumstance which proves how important it is to ensure that the tyres you use are of high quality.