Off-road Wheels and Tyres

Off-road Wheels and Tyres for the 4×4

From the off-road driving perspective, fitting the right rim/tyre combination can make all the difference between successfully completing an overland expedition, or a cancelled trip, or perhaps worse, suffering serious injury because either a rim broke against an obstacle, or several tyres peeled off their rims while driving through thick sand with reduced tyre inflations.

4WD vehicles that are used in serious off-road driving do not have to look pretty or project a macho image; while tyre/rim combinations are the easiest way of making a 4×4 vehicle look pretty, if not macho, the success or failure of an expedition depends on the correct and sensible selection of tyre/rim combinations as much as on anything else. Tyres and wheels are arguably the hardest working components of any 4WD vehicle and it makes good sense to use some sound judgement when selecting wheels and tyres.

While wheel and tyre failures on off-road trips in Southern Africa can be relatively easily resolved, the same issues in the rest of Africa can be vastly more problematic. It is therefore vital that the correct choices are made at the outset, with prettiness or a macho image the very last thing that should figure into the equation. So, which is best where rims are concerned? The choices are rather limited but they include the following:

Wheel rim measurements

To the uninitiated, the many measurements that describe a wheel rim can be more than confusing; however, these measurements are of critical importance in relation to fitting the correct tires. Standardised measurements with regards to features such as the rim width, bead diameter, offset, pitch circle diameter, and others make it possible for everyone dealing in the subject of rims and tyres to talk the same language, as it were. Below is a diagram that shows the most important dimensions of a modern rim that is suitable for off-road usage.
There is no real difference between how measurements are taken on steel and alloy rims, however, it is of critical importance that the rim widths given on the sidewalls of tyres conform to the relevant sizes of the rim.

Types of wheels used on a 4WD

• Steel Wheels:

Of all types of off-road wheels, stamped, one-piece steel wheels are unsurpassed for sheer strength, uniformity, durability, and resistance to all kinds of damage deriving from off-road use. Although they are heavier than alloy wheels because of their rugged construction, it must be borne in mind that driver assist systems, as well as suspension systems themselves, fitted to newer off-road vehicles are designed to work in conjunction with wheels of a specific weight. This weight is commonly referred to as the “unsprung” weight and it has a direct bearing on the way mainly IFS suspension systems react to obstacles, particularly at high speeds.

Replacing a standard steel wheel with a lightweight alloy wheel could mean that several driver assist systems could potentially be affected because of the altered rates of up and down movements of the wheels. Additionally, if the track width is changed by fitting alloy wheels with bigger offsets, systems such as Stability Control could be affected because a wider wheel track means that suspension-mounted sensors could give false readings during dangerous situations, severely interfering with a system’s ability to correct a dangerous situation.

• Alloy Wheels:

Although forged alloy wheels are heavier and marginally stronger than cast alloy rims, neither type is suitable for serious off-road driving conditions. Being soft and not very resistant to impact damage, their use should be avoided, even if multiple spares are carried.

Apart from the fact that a sudden impact can shatter an alloy rim, these rims are extremely susceptible to deformation, causing wheel imbalance and worse, the ingress of sand and other foreign matter between the tire bead and the bead seat on the rim, causing tyre pressure losses which can at times be of an explosive nature.

• Split rims:

The issue of split rims vs. normal, solid tubeless wheels have as many proponents as detractors. However, the fact that split rims are best suited to bias-ply tyres, which are tires that have reinforced sidewalls, and are therefore extremely tough; make split rims an excellent choice in extreme off-road conditions. The tyres that work best with these rims are very resistant to punctures but if and when punctures do occur, a simple tube patch will usually resolve the problem.

Nonetheless, these tyres are usually narrow and because of their tough sidewall construction, usually give a very rough and bumpy ride, but this must be balanced against the facts that these wheels are as tough as any other steel wheel on the one hand, and the ease of puncture repair on this type of wheel on the other.

The choice of whether to use split rims or the more comfortable solid type largely depends on the driving conditions in the area that is most likely to be encountered. While it is true that split rims do not do anything better than solid rims, their clear advantage lays in the fact that they accommodate tougher tyres, which is a good thing in rocky areas where sharp stones can cut a softer tyre to shreds in very short order. On the other hand, these tyres do not work well in thick sand or mud: in the case of sand, tyre pressure cannot be lowered much below 20 psi, which is very often not enough to significantly increase the footprint of the tyre, and in the case of mud, these tyres generally do not provide sufficient contact area to cross mud obstacles reliably.
In the off-road context, the only viable choices regarding wheel rims are solid steel rims, or steel split rims, with the terrain most likely to be encountered being the single most relevant factor. The terrain in Southern Africa where the use of split rims might be justified falls mainly in National Parks, such as the Karoo National Park, or the Etosha and Okavango areas of Namibia and Botswana, where it is a serious offence to leave established roads. The rest of Africa, where there are few roads, consists mainly of sand, mud, more sand, and mud interspersed with deserts and savannah, and with this in mind, very serious consideration should be given to the choice of rim/tyre combinations during the planning phase of any excursion into Africa.

Off-road tyres for the 4×4

Once the matter of which rims to use has been decided, the next issue becomes one of tyres: while some tyres are designed to be effective in certain conditions, a reasonably experienced off-road driver could successfully cope with most off-road driving conditions with any tyre. However, this is not always the case and space limitations usually preclude the carrying of tyres suitable for any and all conditions. Since there is no perfect off-road tyre, it therefore makes good sense to decide on the type of tyre that is able to cope with most conditions most of the time during the planning stages of any off-road excursion. So, what do the numbers and symbols on the sidewalls of tyres mean and how do they relate to off-road effectiveness?

• AT, or All Terrain Tyres:

Although a wide range of all-terrain tyres are available, these tyres are essentially a combination of tyres that make it possible to use them in off-road conditions as well as on paved surfaces. However, since they are basically off-road road tyres that are in some respects modified to enable them to be used on paved surfaces; they never perform as well in extreme off-road conditions as specialised off-road tyres. Similarly, these tyres do not perform as well on paved surfaces as true on-road tyres, which have led some 4×4 drivers to comment that they are “jacks of some terrains, but master of none”, even though their longevity and durability in on-road conditions approach that of true on-road tyres.

• MT, or Mud Terrain Tires: (Mud Terrain/Deep Sand/Deep, Powdery Snow)

The main task of the large tread blocks and deep grooves on mud tyres that extend to the sidewalls is to clean themselves of the mud that clings to the tyre tread during mud crossings. As the tyre rotates, mud is flung off, leaving the edges of the tread blocks free to grip onto any available solidity in the mud.

Many brands of mud tyres come in bias-ply as well as radial variants, which makes bias-ply mud tyres ideal for rock crawling, particularly at low pressures, which allows the tyre to more easily conform to uneven surfaces, with the large edges of the tread blocks providing extra traction. The reinforced sidewalls also ensures longer tyre life and this type of tyre should be considered for use during off-road excursions where a variety of driving conditions are likely to be encountered.

Since it is often, if not invariably, impossible to carry tyres for all conditions, mud terrain tyres are the next best thing to negotiate deep sand obstacles with. In fact, many serious off-road drivers will use nothing else because of mud terrain tyres’ ability to cope with a wide range of conditions, including sand with reduced pressures, and even deep, powdery snow, where the aggressive tread pattern provides sufficient traction.

• HT, or High Terrain Tyres:

Although many new 4WD vehicles are sold with HT tyres, these tyres have severely limited off-road capabilities due to their closely patterned tread designs, shallow tread depths, and relatively soft sidewalls.

The main purpose of these design considerations is to provide less noise on paved roads, a more comfortable ride in urban situations, and slightly better handling characteristics on wet paved roads than standard on-road tyres. From the off-road perspective, these tyres are less than useless in serious off-road driving conditions and should be avoided altogether where deep mud or sand is likely to be encountered.

Are you ready for taller rather than wider off road tires?

Neither taller nor wider tyres necessarily provide more traction or power: much of the effectiveness of any given tyre depends on driving skill and technique; however, there is something to be said for taller, rather than wider tyres although there are several serious negatives that should be mentioned first.

Although taller tyres provide more ground clearance, they can do so only up to a point: generally speaking, 4WD vehicles have suspensions that are designed to provide maximum travel with wheels of a particular size, and the suspension will travel its full distance without the wheel contacting anything. Increasing wheel size reduces the distance the wheel can travel in any direction before hitting or scraping against bodywork or other obstructions.

Considering the small gains in ground clearance by fitting wheels that will not scrape against anything, and the huge cost and effort in increasing the body height to the point where large enough wheels can be used to significantly increase ground clearance, the question should be asked if the changes in handling, braking performance and torque characteristics are worth the hassle.

• Torque characteristics:

While larger wheels will not increase torque, the larger wheels will mean that the engine can run at lower revs to cover the same distance in the same time, because of the larger circumference of the wheels, which in the case of a turbo diesel could mean increased turbo lag. It also means that mileages might not be recorded accurately on the odometer. While relatively small changes in wheel size may not produce any noticeable effect, large drastic changes may very well make the vehicle difficult to drive.

• Braking performance:

Brakes are designed to work at maximum efficiency with standard wheels. Since the radius of a wheel acts like a lever, the longer this lever is, because the wheel size has increased significantly, the more braking force will be needed to stop the vehicle. Again, small changes might not be noticeable but large drastic changes certainly will.

• Increased fuel consumption:

Bigger wheels mean more weight and more rolling resistance: depending on the size of the larger wheels, the small gain in ground clearance might not be enough to offset the potentially huge increase in fuel consumption.

• Handling characteristics:

It is sometimes difficult enough handling a heavily laden off-road vehicle with standard wheels as it is. Large, oversized tyres have the effect of raising the vehicles’ centre of gravity, which means that the rollover point will be reached much sooner since a tall object falls over much more easily than a short one.

Although all of the above applies to vehicles without electronic driver assist systems like ABS, Traction, and Stability Control, and Rollover warning systems, the effects of drastically larger wheels has major impacts on these systems where they are fitted. Larger wheels have a direct bearing on suspension travel, which could mean that a stability control system might not recognize a dangerous situation; similarly, the increased centre of gravity might cause a degree of body roll that could trigger a simultaneous response from several systems such as the ABS, rollover warning, and traction and stability control systems in an attempt to correct a perceived dangerous situation where none existed.

The only real advantage in having significantly larger wheels is the fact that the larger the wheel, the easier it is in most cases to mount and overcome an obstacle. Nevertheless, this ability should be balanced against the huge cost and effort of the modifications to accommodate the larger wheels, as well as the reprogramming of all driver assist systems, which may or may not be possible.

Which off-road tyre then?

This issue is best decided by experience and a large measure of common sense: there are no clear answers because of the large number of variables involved, ranging from suspension set-ups, a multitude of possible tyre choices from dozens of manufacturers and the personal (and sometimes conflicting) choices of thousands of off-road drivers, most of whom have very strong opinions on which off-road tyre is best in any given situation.

Off-road drivers contemplating a 4×4 trip through Africa, or even just Southern Africa, will be well advised to spend as much time as possible researching the tons of information available on the Internet with regard to off-road tyre choices for their chosen vehicles; mistakes in tyre choices cannot easily be rectified in the hinterland of Africa, so, choose wisely.