In a previous article, we looked at the physical forces of both front wheel steering systems and 4x4 cars. A basic understanding of these forces suggests that rear wheel turning systems aren’t beneficial, yet there are still a number of vehicles that use them. So, how does rear wheel turning work and how does this benefit the car, the driver and the tyres?
Specifically, rear wheel driving is often found in larger vehicles, such as trucks.So there must be some benefit to using rear wheel steering systems. Racing cars, such as the Porsche 911 GT3, also use rear wheel steering to assist in certain turns, similar to a 4x4 drive vehicle. Here we will look at the forces that take place when turning in this fashion and how some vehicles can use this to their benefit.
A Summary Of Forces
When turning, there are two major forces that influence the wheel. The mechanic trail - the difference between the steering axis and the actual contact patch - provides a self-righting force while the pneumatic trail - which is related to the tyre’s contact area with the road - creates additional torque in the opposite direction.
So, why is this a problem with rear wheel drive?
When turning, front wheel systems have these forces in balance. The self-righting force is balanced with the additional torque pushing the car out. This is why the wheels naturally return to the right orientation when you let go of the wheel.
There are many forces at work when considering steering systems
With rear wheel drive, these balances do not work. The ‘self-aligning’ force is, in this case, now pushing the tyre the wrong way. This causes the car to drastically oversteer, since the force multiplies even the smallest of movements.
On top of all this, there is also the simple fact that turning in rear wheel drive often means rotating the rear wheels against the direction you wish to rotate. This will cause more wear and tear on the car tyres. This is why, even when using 4x4 drive, the rear wheels do not rotate as much as the front wheels do.
How Rear Wheel Turning Affects Driving
The fact that there are still some vehicles with rear wheel drive steering systems suggests that there must be some advantage or use. One of the biggest advantages is the reduced turning arc offered by only using rear wheel drive. This is due to the wheels rotating in the opposite direction, reducing the turning ratio. This can be good for tight corners or parking - it is best done at low speeds.
Due to the nature of rear wheel steering, oversteering is a common problem. Even a minute shift (on a purely rear-wheel drive vehicle) at high speeds scales into a noticeable change in direction. This can often panic drivers, since the car feels as if it is going in the wrong direction, an effect created by the position of the rear wheels.
Needless to say, even if this oversteering is corrected, rear wheel steering systems cause a lot of wear and tear on the tyres. This quickens the rate of deterioration, causing grip and other parameters to become less and less effective.
Uses Of Rear Wheel Steering Systems
It also used for the likes of forklift trucks, as it can help with more precise maneuvering. As mentioned earlier, rear wheel steering can offer a smaller, sharper angle of rotation compared to front wheel steering systems. Since forklift trucks do not require to go at high speeds, they can make the most of rear wheel drive options.
Similarly, rear wheel drive can often be found in motor sports. The ability to oversteer is actively encouraged in drifting. When the rear end of the vehicle loses its grip and starts to swing or veer out, steering in the opposite direction can often reduce this effect and redirect the car in the desired direction. However, power-sliding like this causes a great strain on the rear tyres, causing them to smoke. While this is useful in demonstration driving or drifting sports, it’s certainly not useful for typical drivers on public roads.
One of the more unique examples of rear wheel steering can be found in the Thrust SSC, one of the fastest land vehicles of all time. Capable of supersonic speeds, the rear wheel system was used for various design issues, rather than performance. Since the vehicle’s objective was to go as fast as possible, turning was not a high priority in the vehicle’s design.
Rally cars incorporate rear wheel turning as part of their all-wheel steering systems.
Rear wheel designs can be found in various long goods vehicles (LGV), such as articulated trucks and buses. Because the rear wheel steering offers tighter cornering abilities, this helps counter one of the biggest problems faced by LGVs - their length. Of course, such vehicles seldom go at high speeds and many designs that need this feature may even use all wheel drive systems, only using the rear wheels when necessary. These designs are also often influenced by whether or not the vehicle is front or rear wheel driven and the types of axles used in the carriage.
Summary
In conclusion, there are a few uses and areas where rear wheel turning can prove useful. However, when it comes to normal roads, few cars will utilise this. That being said, many 4x4 and all wheel drive systems rely on the rear wheels.
If you drive such a vehicle, it is worth inspecting the quality of your tyres, in case the rear and front wheels wear down at different rates. In these instances, you can consider swapping the tyres over or replacing tyres with a new set for better grip and performance.