THE DEFINITIVE TRUCKING SITE

May 2009

Choosing the correct tyres, axles, suspension and braking systems for a specific trucking application is something of a specialised task, requiring a solid understanding of what demands the given operating environment will present to truck, load and driver. It goes without saying that lowest CPK is a primary concern of any operator as is road safety and the prolonged life of all brake/driveline components. With these considerations in mind, Paul Collings asks leading South African traction and friction exponents to advise on what to bear in mind when selecting brake systems and tyres for specific trucking applications. 

Trucking is an outdoor game, an extreme sport; some might say, and subject to the unpredictability of the elements, adverse road conditions and the often questionable behaviour of both truck drivers and the motoring public. Whichever way one chooses to describe the business of trucking, there can be no argument that no two trucking environments are the same, which makes it very difficult for tyre and brake suppliers to offer generic advice on optimal selection of their components. 

However, there are ‘general rules of thumb’ within specific trucking applications, which, over the years, have grown to constitute a knowledge-base, a repository of ‘wheel-wisdom’, so to speak, available to all truck owners who want to get the best out of their brakes and tyres. This includes insights into the optimal selection of axle types (hub reduction or hypoid) and suspension systems (steel or air). 

At the risk of being too generic, here are four typical applications with tyre and brake selection advice (or ‘wheel wisdom’) from Bridgestone’s Marcus Haw, Bandag’s product manager, Monal Naik and Wabco’s Enoch Silcock.

APPLICATION ONE: On/off-road tipper/construction 6x4 truck tractor with tridem sloper trailer [steer, drive and trailer axles]. Payload = 34 tons. 

CONSTRUCTION AND coal tippers operate in extremely diverse conditions and operators need to choose tyres that are designed to function optimally in the specific conditions applicable to the job at hand.

Bridgestone: Use on/off road tyres on all axles for all on/off-road vehicles. These tyres are designed to be used on steer, drive and trailing axles in a wide variety of conditions. All tyre manufacturers have these tyre types in their ranges. Choosing the correct tyre type in these instances is relatively easy, but one needs to be careful of the load and speed specifications. 

In Bridgestone’s range, there are the Bridgestone M840s, and the Firestone UT 3000 Supers. These are both on/off road designs and come in a few sizes, each with its own load and speed index. 

For a 6x4 truck carrying 34 tons, presuming the drive and trailing axles are using dual tyres, I would suggest that the size to use would be the 315/80R22.5. These have a load index of 156/150 and a speed rating of K (110 km/h) on the Bridgestones, and 154/150K on the Firestones. 

The first number of the load index relates to single tyre fitment i.e. the steer tyres. The ‘156’ refers to 4000kgs per tyre, or 8000kgs across the axle. The ‘154’ refers to 3750kgs per tyre and double across the axle. The ‘150’ refers to 3350kgs per tyre, or four times that across an axle with dual fitted tyres. 

So, in the case of a vehicle fitted with Bridgestone M840 tyres to all axles, the total carrying capacity of the tyres would be 8000kgs on the steer axle, and 13 400kgs on each of the other axles, equaling 34.8 tons. In the case of the Firestones, this would be reduced on the steer axle to 7500kgs. The ‘K’ speed rating is 110km/h. Note though that this covers the “payload” mentioned but not the GVM. 

It is also above the legal 9-ton limit on the drive and trailing axles. If one uses 385 or 425/65R22.5 tyres you are looking at 160 and 165K respectively. Bridgestone does not have the 425 in an on/off road pattern though. 

Remember too that by reducing speed you can increase the load slightly, but your calculations must be very accurate, and driver discipline absolute. 

Bandag: The Bandag BMS tread design, while being application-specific, is ideal for both drive and trailer axles and for mixed service applications. It has stone penetration resistance which will ensure minimal casing damage, reducing fleet downtime and in turn improve cpk. 

BMS has a low void ratio to put more rubber on the road for long, even wear and improved traction. Being lighter than a lug design, fuel efficiency is also improved while reducing stress on fleet casings. 

Wabco: Braking recommendations for the truck tractor would be an EBS brake system and air suspension and preferably drum brakes. Pad wear indicators are optional but recommended. 

For the trailer, EBS brakes, air suspension and drum brakes are recommended. With this set-up, a double axle lift system can be installed (to reduce tyre wear in unladen conditions) with an automatic suspension drop during dumping to ensure stability. 

Drum brakes have proved to be excellent for off road conditions, although mud collection and prevention of heat dissipation can still be a problem. Disc brake lining wear tends to be aggravated by dust and mud collection on the linings in off road operations. The latest trailer EBS products are equipped with warning systems should the vehicle become unstable during tipping. 

APPLICATION TWO: On/off-road timber/forestry 6x4 rigid-drawbar combination [steer, drive and trailer axles]. Payload = 34 tons. 

TIMBER TRUCKS operate both off and on road and therefore require tyres and brakes that are robust enough to handle dust, mud, loose road surfaces and steep inclines.

Bridgestone: Exactly the same as in the tipper application 

Bandag: Select Bandag’s LGT tread for the drive axle as it is ideal for severe service applications. It has a deep, aggressive tread design, which provides optimum traction in logging and quarry applications. With stone penetration resistance, LGT is yet another Bandag tread design that offers minimal casing damage in severe conditions, which in turn, reduces downtime and improves cpk. Opt for the Bandag BMS tread design for trailer axles. 

Wabco: The rigid truck recommendation would be an ABS or EBS brake system and mechanical suspension and preferably, drum brakes. Pad wear indicators are optional but recommended. 

The trailer should have EBS brakes, a mechanical suspension system and drum brakes if the truck tractor is fitted with EBS. Air suspension air bags are easily damaged in logging operations due to piercing by sharp objects hence the choice of mechanical suspension. 

APPLICATION THREE: On-road long haul freight distribution 6x4 interlink [steer, drive and trailer axles]. Payload = 34 tons. 

Bridgestone: I never recommend tyre fitment to this type of operation without getting a lot of info beforehand. Correct tyre fitment is too important for random recommendations. In such an operation, one would generally suggest steer-type tyres for the steer axle, drive axle tyres for the drive axles and trailing/steer tyres for the trailing axles. There are many to choose from, though all with differing wear characteristics and load speed ratings etc. Also, the operations can vary greatly from day to day with different routes and different loads to and from the main yards.

One has to get to know the operations very well before making recommendations, and if possible, I like to get some history from their records if they have any. I also like to do a scrap tyre analysis. This way, one can really be of service to the transporter. Failing to do this can end up costing the operator a lot of money. 

Bandag: For drive axles, Bandag’s BDR-HT2 is ideal for drive axles. This ‘next-generation’ over-the-road drive application tread is designed to operate on vehicles with high torque specifications. The design of the tread helps minimise tread movement which improves tyre wear.

Trailers for this application can be fitted with tyres using Bandag’s BTR-SA, a trailer- specific tread design which cuts at 12mm. Designed for over the road applications, BTR-SA also features Bandag’s mini wing technology. Mini wings are a feature that provide the retreaded tyre with a new tyre-like appearance and also greatly assists with scrubbing when cornering. 

BANDAG BDR-HT2 is ideal for drive axles - designed to operate on vehicles with high torque specifications.

Wabco: The truck tractor should be fitted with an EBS brake system and air suspension and preferably disc brakes. Pad wear indicators are optional but recommended. 

The trailer should have EBS brakes, air suspension and disc brakes. With this set-up, a double axle lift system can be installed for driving in unladen conditions to save tyres. 

Disc brakes systems with full EBS on truck and trailer have proved to be excellent for on- road conditions. With this combination, the best vehicle harmonisation can be achieved, optimising braking efficiency, stability, lining wear and foundation life throughout the rig. 

Application Four: On-road short haul freight distribution 6x2 rigid volume van [steer, tag and drive axles]. Payload = 16 tons. 

Bridgestone: These are arguably the hardest vehicles on tyres and the most difficult to make recommendations for simply because they are usually working very close to the limits of their (and their tyres) carrying capacities. Their load bodies are often so heavy they can’t take much more in them. But they do never the less. 

Another problem is that they are very often high bodies, and the consequent load sway plays hell with the tyres. They are constantly going from an under-load to an over-load situation, creating huge heat build up. Then there is the fact that they are short distance vehicles and spend a lot of time in and around the towns. Stop/start driving is hard on tyre tread life and increases the heat build up as well. 

There is the added fact that they are often overloaded on the out trip and totally empty on the return trip. The tyres run hot and build pressure on the loaded run and then have to run at the higher pressure with an empty vehicle, which pounds the life out of them. 

The highest percentage of cross-ply tyres are still found in this sector, and the sizes vary from 750-16 crossply ply tyres through to 12R22.5 radial tyres. Sixteen tons is at the top end of this sector while the lower end is around four tons, but they all have the  same problems. 

The short haul rigid volume van requires more operational studying before selecting tyres than any other in the transport industry. Correct tyre selection is extremely important with these vehicles as they are so expensive to run when compared to other sectors.

Bandag: Bandag’s BDV is an urban drive wheel position tread that offers much versatility. Ideal for the stop/start operation, BDV has earned enormous success in both regional delivery operations and bus fleets. BDV cuts at 17.5mm and offers excellent mileage in the urban environment by offering great traction and even wear. It is an ‘applicationspecific’ product that is re-defining notions of tyre performance and reliability. 

BANDAG BMS tread design is ideal for trailer axles.

Wabco: An EBS brake system, air suspension and disc brakes are highly recommended for this type of application. Pad wear indicators are also recommended.