Steering Systems: Understanding The Forces Of 4x4 Turning

  • Author: OPONEO.IE

Previously, we looked at how front wheel steering systems influence the driving experience. Of course, not every car maneuvers with the front wheels so, in this article, we will look at what occurs when using a 4x4 vehicle.

How does turning change when using all 4 wheels? Are the specific forces applied any differently and will this change how you drive? Learning how this works can explain a great deal about your experience with 4x4 cars, as well as what forces are applied to your 4x4 tyres.

Before we begin, however, it’s worth noting that we’ve previously discussed the difference in 4x4 steering systems in true 4x4 vehicles and SUVs. For the sake of this article, we’re focusing on any car that turns with all 4 wheels, regardless of whether or not it has an inter-axle lock, reduction gearbox or transverse engine. Similarly, there are models which use 4 wheels all the time (commonly referred to as “all wheel drive”) and various 4x4 and SUVs that switch between 4 and 2 wheel drive systems. Again, for this article, we’re just looking at vehicles that can turn with all four wheels.

Here are a few examples of cars that feature four wheel steering systems:
 
●     BMW 5 and 6 series, starting from 2011
●     Renault Megane GT (4th Generation)
●     Porsche 918 Spyder
●     Honda Prelude, Accord (1991) and Ascot Innova (1992)

Of course, there are many more vehicles available. 4 wheel steering was very popular in the 1980s and it’s seen a recent comeback thanks to modern technology and power steering.

The Basics

mechanical trails and steering system

Steering system basics: visualising
the mechanical trail

In our previous article, it was discussed that the mechanical trail should be ‘positive’ and in front of the tyre. The trail is the difference between the steering accepts projected line and the tyre’s actual point of contact.
 
We also looked into the pneumatic trail, which is created from the tyre’s contact with the road. This generates self aligning torque which pushes the wheel out of its intended direction. Fortunately, the self-righting forces generated from the mechanical trail counter this when using front-wheel steering. Rear-wheel steering does not have this, thanks to more powerful torque. A 4x4 steering system, on the other hand, has both.

Wheel Balance

In most 4x4 steering systems, the majority of the turning is done by the front pair of wheels. The rear wheels still turn, but they often turn around half as much as the front wheels. This is to help resolve the above mentioned disbalance of torque. On their own, the rear wheels would push the car in the wrong direction.
 
At this ratio, they offer a sharper turn. Because all 4 wheels are turning inwards, the turning ratio is greatly reduced. The rear wheels often act as counter steering, offering greater control during these turns as they produce torque counter to the self-righting effect in the front wheels.

4 wheel steering systems have sharper turning angles, making them popular in off-road racing

This is also one of the reasons many models switch between four and two wheel drive. Cars such as the Volkswagen Golf R only use the rear wheels when a slip is detected. This allows greater control when needed, while also leaving the greater flexibility of front wheel steering for most of the drive.

Off-Road Terrain Factors

True off-road vehicles use 4 wheels as much as possible. This is because, on an uneven surface, you cannot always determine which wheels will have the best contact area with the surface.

Of course, this means off-road steering systems need to be supported by strong, durable 4x4 tyres. Because of the uneven surface, a bigger contact area is prefered, which is why 4x4s tend to favour larger wheels. There are also other additional factors that off-road vehicles consider, such as greater suspension and a heavier weight, all of which put pressure on the tyres.

What Does This Mean For 4x4 Tyres?

While off-road vehicles need very durable tyres, preferably with a thick tread that isn’t too worn down, the rate of which tyres deteriorate depends on how much each wheel is steering. For example, most models steer at wider angles with the front wheels than they do with the rear pair, while some models don’t even always use the wheel pair unless they need the extra control. 

4x4 tyres

Off-road and 4x4 tyres often tend to be bigger and tougher than road tyres

This means that, just like front wheel steering systems, the front tyres will receive a greater level of wear. However, the back pair will not last as long as those of a front wheel system, as they still see some use. As a result, while you can rearrange the placement of tyres on front and rear wheels, the overall quality of tyres will depreciate quicker.

Many other factors influence the conditions of your tyres, including your speed, road surface and individual driving style. Of course, 4x4 steering often causes more wear and tear than you might expect, which is why some spare tyres are always recommended, and to inspect your existing set more regularly.

 

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