When it comes to steering systems, with a few obvious exceptions - such as 4x4 vehicles - most vehicles 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 this influence your driving performance? What specific forces are applied when steering from the front? More specifically, there is a great deal we can learn here about both the various forces vehicles experience as well as the effect this has on the tyres.
Steering System Basics & Mechanical Trails
Positive and negative mechanical trails
in relation to the steering system
To understand how any vehicle interacts with the road, you need to look at the “steering axis”. This is deviation the steering has compared to a straight vertical line with the road. While this is often used to help with the vehicle’s camber and lift, the relevant aspect here is the point that this imaginary axis meets the road. The distance between this point and the tyre’s actual point of contact is known as the “mechanical trail”. This is typically positive, meaning it takes place in-front of the tyre, and 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.
The reason this trail is positive is because:
A positive trail ensures the wheel is essentially pulled. This is not unlike the experience of moving a shopping trolley.
This helps create a self-righting effect that keeps the vehicle in-line
A negative trail, on the other hand, would not have this self-righting support. The steering would be very suspicable to lateral movements, causing it become unstable much more easily.
Alongside this, there is also the pneumatic trail, which is created when your tyres make contact with the road surface. Unlike the mechanical trail, the pneumatic trail typically occurs behind the tyre’s centerline, as well as the previously mentioned mechanical trail. One of the more 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.
In a more advanced analysis, you could also consider how the steering system’s camber angle of the wheel influences the tyre. For drivers, however, a basic understanding of these factors should be enough.
Front Wheel vs Rear Wheel Steering Systems
Front wheel steering systems
offer better control to the driver
So how do these forces work when considering 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 mechanic trail. These two forces work together in balance to keep the wheel in its intended position. This effortlessly turns the vehicle, with the majority of the car adapting to the new incline.
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 wheel rears 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. Needless to say, this would not be good on the tyres, which would be moving against momentum, rather than rolling with it, creating excessive wear and tear.
Rear wheel vehicles do exist, of course, but they are often 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 typical 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 tyres are affected more than the other pair. This will be true regardless of which steering set-up you have. Because the wheels rotate and push against the surface to steer, the treads on these tyres will be worn down more quickly.
the tyres that turn have will often wear down more than the others
This is why it is always 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 these around returns some better control into your steering, as well as allowing the whole tyre set to last longer.Of course, having the better tyres on either the front or rear wheels, in relation to the type of steering your vehicle has, brings additional benefits and drawbacks.
We previously discussed how to to choose the right car tyre fitting on steering system in this manner will change your performance.
Of course, there are other factors that contribute to the handling and performance of your vehicle on the road. Yet the forces presented here are why car manufacturers prefer to design front wheel drive vehicles and demonstrate just how important it is to ensure high quality tyres are in place.