BLACK OPS BUSH TRUCK BUILD
Wheels and tyres play an important part in the suspension system of a vehicle and have a direct effect on ride quality. We’re going to look at some of the do’s and don’ts when it comes to selecting wheels and tyres for your 4WD.
The standard tyres on the Ford Ranger Wildtrak are Continental Cross Contact LX highway-terrain type tyres, and the size is 265/60 R18. Just to recap the tyre-size thing, 265 is the tyre width in mm, 60 is the profile height of the tyre as a percentage of the width, and 18 is the rim size in inches. (I don’t get the metric/ imperial marriage either.)
The standard rims are 8 inches wide with an offset of +55mm. This offset is the distance from the inner mounting surface of the wheel where it fits against the brake disc to the vertical centre line of the wheel. If the offset is positive, it means that the mounting surface of the wheel is closer to the outside of the wheel. When the offset is negative, the mounting surface is closer to the inside of the wheel. In the attached “Off-Set” diagram, the red line represents the vertical centre line of the tyre. The purple line represents the inner mounting surface of the wheel rim.The red rim has Positive Off-Set, where the mounting surface sits outboard of the wheel centre line. The green rim has zero Off-Set and the blue rim has negative Off-Set with the mounting surface inboard of the wheel centre line. (See Off-Set Diagram.)
When fitting new rims and tyres, it is important, for a number of reasons, to get the off-set right. Many people stick to the original tyre circumference but go slightly wider on the tread width. Some folks go up in circumference to give a little more under-body clearance. This is all fine, if you stick to reasonably small increases. The problem is that most aftermarket rims do not cater for large positive offsets, such as the +55 on the Ford Ranger Wildtrak. The best that I’ve been able to locate have an offset of +30. This means that this aftermarket wheel and tyre, given that they are the same width as the originals, will stick out about an inch (25mm) more than the standard wheels and tyres. The tyre thus turns through a wider arc and may foul the inner wheel arch liner at full lock.
Rims and tyres that have a larger margin of negative offset can also experience wheel-alignment issues, especially pertaining to Scrub Radius. In simple terms, Scrub Radius is the distance between the kingpin axis and the centre of theswivel points. (In our attached Scrub Radius diagram, this is the diagonal blue line.) On most 4WDs with independent front suspension, the swivel points are the upper and lower ball joints. (See Scrub Radius Diagram.) important negative and positive Scrub Radius have a perceptible effect on the handling of your vehicle, especially if it is out of spec. This is most noticeable when there is increased road force on the front wheels, such as under braking. We assume that the centre of the contact patch of the tyre is where any road force is applied. (In our attached Scrub Radius diagram, this is the red line.) For this to be true, the wheel in question has to be perfectly upright, with zero camber and an even tread pattern. With negative Scrub Radius, the contact patch of the tyre sits inboard of the kingpin axis to road-contact point. Under braking, the wheels are forced backwards and will rotate slightly around the kingpin axis to road-contact point, and cause the front wheels to toe-in. This creates stability, and feels fine. With positive Scrub Radius, the contact patch of the tyre sits outboard of the kingpin axis to road contact point. Under braking, the wheels are again forced backwards, but they will now be forced to toe-out. This creates instability and can feel as if there is less control.