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 the contact patch of the wheel, where both would theoretically touch the road when viewed from the front. The kingpin axis is the line between the upper and lower suspension swivel 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.)
What is important to note is that 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.
Another problem induced by fitting wheels and tyres that have a heavy negative offset, is an increase in wheel-bearing fatigue. We again refer to the centre line of the wheel contact patch. The design and placement of the wheel bearings during the design phase of the vehicle take into account the forces applied to the bearings by the weight of the vehicle on the road wheels. Modern wheel bearings on these types of vehicles are large composite bearings that have a pair of bearing races per bearing assembly. One race is inboard, and the other race is outboard. Should you fit a nice set of deep dish mags to your vehicle, and the offset and tyre-contact patch are now moved significantly outwards, you are effectively placing more stress on the outer race of the bearing − which will lead to premature bearing-failure. Anybody who owned a Golf Mk1 with wide mags will know what I’m talking about here in regard to the rear wheel bearings.
It is therefore important to make your tyre and rim choices carefully and take all of the pros and cons into consideration. There is almost always some type of trade-off, as with any modification, and you need to be sure that you are happy to live with the consequences.
There are some compelling reasons to upgrade your vehicle’s wheels and tyres. I’d have to say that wheels are certainly a fashion item. A nice set of black mag wheels have been very popular on 4WDs for some time now, and the fad just does not seem to be waning. When it comes to tyres, though, the reasons to upgrade and change are far more compelling.
Most 4WD vehicles are sold with highway-biased tyres, or, at best, an all-terrain tyre with a 50% road and 50% off-road rating. These tyres are fine for town and country, as long as the going doesn’t get rough. They also have a lower tolerance to punctures; but, on the positive side, they are lighter, quieter and often easier to balance than proper all-terrain or off-road biased tyres. Tough ATs tend to be a lot more rugged, with many sidewall layers or “plies”. They are firmer to ride on, and somewhat heavier, which manifests itself as an increase in feedback through the suspension.
Tyre choice is a very tough decision unless you have a tried-and-tested favourite that you stick to and that works for you. I have been driving 4WD vehicles for a number of years, and am very sensitive to ride-quality and safety because of my suspension background. The first 4WD that I owned was a Hilux double cab with solid front axle, manufactured in Feb of 1998. Once I’d done with it, it had a 4 inch suspension lift, 50mm body lift, custom Old Man Emu leaf springs, custom-made Bilstein long-travel shock absorbers and a motor swap from Alpine Turbo Developments – a 2.7-litre fuel-injected petrol from a new-generation Hilux.
It ran on 32-inch BF Goodrich All-Terrain tyres which lasted forever and never got a puncture. I appreciated these features, as I was paying for my own tyres back then. What I did not enjoy was the abhorrent road-holding when the road was wet. The brakes on the Hilux did not have any work done to them as the truck was not that fast, but the braking in the wet was hugely let down by the tyres. I ran two sets of wheels and tyres, and for serious off-road work, I ran a set of 33-inch Bridgestone mud-terrain tyres. They were big and noisy and worked hard off-road, where they performed well. They were, of course, not so great on a wet road.
While in Australia, my Prado 120 4.0 V6 ran on Goodyear MTR mud-terrain tyres. Again, they worked well off-road, especially on muddy roads, and certainly looked the part. They were also not that bad in the wet on tar. I have, however, always had a soft spot for the Bridgestone Dueler All-Terrain tyres. I have always known them to be quite soft off-road, and there is a constant need to keep a lookout for sharp rocks and sekelbos when off the beaten track. I got three punctures down one stretch of road in the Biedouw Valley while visiting the Tankwa, and I will certainly not be taking Bridgestones back there again. I do most of my driving in Gauteng between home and the office, and I find that the Bridgestones are superb in the wet, with massive water-displacement and confident braking. Mileage-wise, they’re not great; but with a company vehicle, this is not a consideration for me. I ran these on my FJ Cruiser for four years and they were sublime.
However, I have yet to come across an all-round tyre that does everything very well. Tyres that are good off-road, are not the best on tar in wet conditions. Tough off-road tyres tend to be noisier and less fuel-efficient. There are, however, some tyres that are good off-road and okay on-road, if that is your requirement. It all depends on how much you’re going to be off the beaten track and taking advantage of those features. Vehicles that almost never see tar don’t really have to worry about the tyre’s “wet-weather road manners”. I know a few farmers who just want a tyre that will last on that bad gravel road that they have to travel every day – tough and hard.
And so, on to new takkies…
For the Ironman 4×4 Bush Truck, tyre choice was not as cut and dried as I had expected. Once we had lifted our truck and fitted all of the gear – including some plastic fender-extension flares – it became glaringly obvious that we would need to address the problem of skinny standard wheels and tyres.
Also, the 18-inch rim size does not work for me. It is standard procedure to deflate one’s tyres in many off-road situations. Sand driving requires soft tyres to promote flotation. Even a 0.1 bar drop in tyre pressure on sand can make the difference between forward motion and getting stuck. In very rocky conditions, lowering the tyre pressure can help prevent tyre damage and punctures.
Older bakkies and SUVs rode on 15-inch rims and tyres for a long time. Over time, rim and tyre sizes have increased. Somehow, 16-inch was mostly skipped, and 17-inch became very popular. Now, more and more top end double-cabs and SUVs have 18-inch rims and bigger. I have found that on bakkies and mid-size SUV wagons, 17-inch rims and tyres air down enough to create good flotation and good resistance to punctures, and still maintain a healthy degree of sidewall height between the rim and the terrain.
I have found that 18-inch rims and tyres do not deflate enough in some situations, and when you air them down more, there is a real risk of an impact puncture or rim-damage due to diminished sidewall height when deflated.
Another important consideration is that 17-inch tyres still represent the best value for money because of their popularity.
So, a new set of A-Line Battle rims in 17-inch x 9-inch wide were ordered for our Bush Truck. The +10 off-set of the A-Line rims is not as positive as the +55 of the original rims, so our Battle rims do stick out further – and, with the right size of tyre, fill the extended wheel arches nicely.
Choosing the correct tyre size is an interesting exercise. Initially, I wanted to go up in diameter by as much as the wheel arches would allow without the tyres fouling any bodywork when off-roading or steering full lock-to-lock. I figured that the power upgrade would compensate for the extra work required to turn those bigger wheels. I ended up fitting a 285/70 R17 tyre, in place of the original 265/60 R18. I wanted a 285/65 R17, but could not find a tyre in this size anywhere.
The 285/70 R17 size represents an increase in tyre-width of around 20mm. More telling, though, is the increase in diameter and circumference of just over 7%. It does not sound like a lot, but it is huge. The sidewall-height gain was significant at 25%. This directly impacts on the handling and ride comfort of the vehicle. The taller the tyres, the more forgiving the handling and ride comfort becomes.
On the recommendation of a good friend who is a tyre expert, I opted to try out a set of Cooper tyres for a change. I have noticed that Cooper tyres have built a solid following in recent years, and are almost always compared to BF Goodrich as being tough tyres with perhaps a wider spread of applications. I opted for the AT3 All Terrains. I cannot comment on how they perform in comparison to other tyres, as I have no direct comparison available. I also do not give any credence to magazine tyre-shootouts. The attempts are valiant, but a totally objective comparison is near-impossible to achieve.
The Coopers are not bad in the wet, until the 650Nm on the back wheels of the go-faster Ranger is given a poke. The Ford Traction Control system is lethargic, to say the least, with a high level of entertainment value. A recent off-road excursion indicated that the Coopers have a substantial sidewall composition. I had dropped the tyre pressures down to 1.2 Bar with only a subtle bulging of the sidewalls. By comparison, the Bridgestones on the FJ were nicely bulged at 1.4 Bar in the sand. I think that the Coopers may need to go down further to get the same amount of flotation.
However, after having spent some time driving with this wheel-and-tyre configuration, my conclusion is that the tyres are too big, even for this 200kW/650Nm truck. It has blunted the performance somewhat. The ride quality from the tyres is not bad, given the ample sidewall height. However, they do not balance very well, and (after three attempts) I am going to relegate these tyres to off-road trips only.
For driving in and around town, I have decided to go for a more road-oriented tyre-and-rim combo. I’ll stick to 17-inch rims, as they have a high comfort level. My ideal tyre-size would be 285/60 R17. This gives you the same diameter/circumference as the original tyres, but with a bit of added width to fill the wider fenders better. Sadly, this size is not readily available. The closest I can get is 275/65 R17 in a Yokohama Geolander AT tyre. This represents an increase of around 2% in diameter, which should be a lot better for the performance of the vehicle. I’ve used these tyres before, and they are quiet, with great wet-weather manners. I’ve taken them with care over several Southern Cape off-road mountain passes, without any dramas. They’re very similar to the Bridgestones for me; so we’ll see how we go.
Since I’ve been messing around with tyres and tyre-sizes on our Bush Truck, I’ve become more aware of what other people are doing with tyres and tyre-sizes. It is not my area of expertise, and I’m learning a lot. What I have noticed is that many people go up in tyre size. This increase in tyre size has an important effect on your vehicle which is not always obvious, and which does not seem to enjoy much discussion or interest. Fitting wheels that are bigger in diameter/circumference compared to the standard size, places more strain on your braking system. In doing research on this subject on the internet, I came across a plethora of opinions and very little of what seemed to be fact.
There were many people singing the praises of a brake upgrade or transplant, and then there were others who stated that “brakes do not stop cars, tyres do!” What I can say is that my Ranger is now substantially heavier, and the wheels and tyres are almost 8% bigger than standard. I drove around quite a bit after the performance upgrade was done and before any other work was carried out. The brakes performed very well and the performance was at its best. After the build, and with the bigger wheels and tyres, the front brakes constantly overheated when I braked from high speed. When I came off the highway from 120km/h, the brakes would start to shudder slightly and then start fading.
In my search for a solution, I was referred to the local brake specialist company, Powerbrake, which has been making steady inroads into the performance braking market in South Africa and abroad. Powerbrake was established in 2004 and is based in Pretoria. They manufacture a range of high-performance and race-brake components as well as complete big brake kits. All the design, FEA and manufacturing is carried out in-house, and exported to more than 25 countries world-wide. They have sales offices in the USA and in Switzerland. Their brake systems have been used to win a number of national, as well as world, championships. Their air-cooled and liquid-cooled off-road race calipers have achieved four FIA world championships and five Dakar Rally podiums. Five of the top 10, and nine of the top 20 finishing cars in the 2018 Dakar Rally were equipped with Powerbrake race calipers. I think they know a thing or two about performance brakes.
Powerbrake AT-Series brake discs
The product that interested me most was their Powerbrake AT-Series brake discs. Powerbrake originally launched the first-generation high-performance brake discs for 4×4 vehicles (named All-Terrain Series) back in 2005. This was in response to requests from 4×4 owners who were complaining about brake fade, insufficient stopping power, brake judder and insufficient disc and pad wear life. This was especially true of 4×4 owners who had either added significant weight to their vehicles in the form of accessories, or were running larger wheel and tyre packages, much like our Black Ops Build vehicle.
The All-Terrain Series discs were the result of an intensive test-and-development program in which a number of 4×4 vehicles were fitted with test equipment that could accurately measure braking distances and braking G-forces, as well as the maximum operating temperatures of the discs, pads and calipers. It became apparent early on in the testing that the significant extra weight that 4×4 enthusiasts were adding to their vehicles was elevating maximum operating temperatures massively, resulting in brake fade, increased stopping distances, and high brake-component wear rates. Disc temperatures of over 600 deg C were recorded on some of the heavily accessorised test vehicles when they were descending mounting passes. These are disc temperatures normally associated with race cars, and not civilian 4×4 vehicles!
At the time, Powerbrake was heavily involved in manufacturing and supplying brake discs for circuit-race applications. A lot of the material technology that was being used in their race discs was employed in deciding on the cast-iron alloy selected for the original All Terrain Series disc range. The Powerbrake discs were cast from a high-grade cast-iron alloy that offered increased durability and stability at high operating temperatures, and then machined to tolerances up to three times closer than the industry norm. Over a year’s worth of testing went into the slot pattern to be employed on the final-production Powerbrake All Terrain Series discs. The slots that are machined into the friction surface of high-performance brake discs perform a number of functions:
- They channel away gases that result from burning resins found in certain brake pad materials, thereby reducing brake fade.
- They channel away water, mud and dust when driving off-road, thereby improving initial brake response.
- They lightly scrub the surface of the brake-pad friction material, ensuring that burnt/carbonised material (often referred to as glazed friction material) is removed, and ‘fresh’ friction material is always exposed to the disc surface, ensuring optimal pad performance.
- Finally, the slots create additional ‘biting’ edges (in layman’s terms) which increase initial brake-response or ‘bite’, as the brakes are engaged.
Unfortunately, there are several inferior “performance” disc products available on the market. These are imported from low-cost-manufacturing countries. Often the slot patterns on these discs include a mixture of cross-drilled holes and slots that are applied purely for cosmetic reasons. It is apparent that zero engineering-consideration has been given to these. While they look fancy, they will, in fact, often reduce brake performance because of the removal of too much disc surface area, as well as reducing disc life (especially in the case of cross-drilled discs). The slot pattern employed on Powerbrake’s All Terrain Series discs was designed and tested to provide moderate improvements in brake ‘bite’ but not so much as to overwhelm off-road tyres and result in premature wheel lockup under emergency braking on road. In addition to the on-road benefits, the slot geometry, width and depth was specifically designed to clear the sand, dust, mud and water often encountered under serious off-road driving conditions.
In 2015, Powerbrake launched their 2nd generation 4×4 discs (simply named AT-Series) which remain in current production. The second-generation AT-Series discs feature further improvements in the disc metallurgy, and are cast from a proprietary high-carbon cast iron alloy. The new alloy further improves the wear life and high-temperature stability of the discs. The proven disc slot pattern has remained unchanged. The 2nd generation AT-Series discs also have Powerbrake’s latest disc coating, which is black − the first generation featured a silver coating. The current generation Powerbrake AT-Series discs also feature Powerbrake’s Max Temperature Recording (MTR) system, which provides a permanent record of the maximum operating temperature that has been reached on the disc. This can be very valuable data when selecting the most suitable brake-pad compound or diagnosing a wide range of brake issues.
Many 4WD owners drive long distances in order to get to their off-road destinations. Often these vast distances are covered at considerable speed, in vehicles that are loaded to the limit with passengers, luggage, accessories, fuel and water. It is also common for 4×4 owners to substantially increase their vehicle’s power outputs. One of the dominant causes of brake fade on a heavy 4×4 vehicle is friction gases which form when pad-bonding agents burn up and turn to gas at high disc-temperatures. These gases can form a barrier layer between the disc and pad, thereby reducing brake efficiency. The slots machined into the face of Powerbrake’s AT-Series discs act as gas-relief pathways that efficiently channel away friction gases, resulting in substantially reduced brake fade.
Selecting the correct brake pads to run in combination with high-performance discs
There is a real misconception in the market that you can simply take any high-friction brake pad, run it on any aftermarket brake disc and (in doing so) upgrade your brakes. Likewise, a lot of people believe that simply cutting slots or drilling holes into any OEM or aftermarket disc will improve braking performance. However, it simply doesn’t work that way. In order to stop a car more quickly, you need more friction. More friction equates to more heat, and more heat can lead to a number of negatives if the components being used are not designed to operate together at these higher temperatures. The higher disc temperatures generated by high-friction brake pads can damage standard aftermarket or original equipment (OE) discs, leading to disc distortion, surface crazing or cracking. The cast-iron alloy from which a disc is cast, as well as the manner in which it is stress-relieved and machined, are what give a high performance disc the ability to operate for sustained periods at elevated temperatures.
According to Powerbrake, the secret to outstanding brake performance is matching a high-quality alloy disc design to ultra-tight machining tolerances, and a high-performance pad compound that has been selected and tested to perform optimally at the disc temperatures generated by a particular driving style and application.
Powerbrake have invested a significant amount of time and resources in testing different disc and pad combinations using state-of-the-art brake dynos, as well as conducting continuous on-car road and track testing using sophisticated data-logging equipment. They then select the most suitable pad compounds from the world’s leading manufacturers and make qualified and tested recommendations on the correct brake pad to use in combination with their AT-Series discs.
The brake system on your 4×4 converts the momentum of the vehicle into heat during the braking process. There are two variables which influence how much heat is generated under braking: the speed at which the vehicle is travelling when you start braking, and the weight of the vehicle in question. According to Powerbrake, if the weight of a vehicle is doubled, twice as much heat will be generated under braking. If the speed is doubled, four times as much heat will be generated. Therefore, if weight and speed are doubled, eight times the heat is produced under braking.
This explains why Powerbrake have recorded front disc temperatures of up to 600° C on highly accessorised 4×4 vehicles during testing. These extremely high temperatures were recorded on heavily loaded 4x4s, typically while descending mountain passes or towing heavy caravans, off-road trailers or boats. Brake temperatures this high would normally be associated with high-performance cars under fast-road and even light-track use, and immediately explains the brake fade, disc distortion and high disc/pad wear that many 4×4 owners are reporting with standard brake components. The accompanying table shows some examples of the rise in front-disc temperatures measured on a common double-cab 4×4 vehicle when braking from various speeds with differing loads during Powerbrake’s testing. Most factory (OEM) or standard aftermarket brake pads tend to start exhibiting fade at disc temperatures of around 350-400 deg C.
Once I had fitted the Powerbrake AT Series discs and accompanying pads, and bedded the brakes properly, I could feel a massive improvement in the braking ability of my truck. They just do not fade. They do produce a fair amount of brake dust and a slight squeal every now and then. These are, however, performance brake trade-offs that I am happy to live with. Once I fit the smaller tyres, things are bound to be even better. Tyres and brakes are what I call “Life and Limb” parts. When they fail, you can lose Life or Limb. These items are not cheap, but they are certainly a non-negotiable upgrade.
By Mic van Zyl