When determining the right axle configuration, I think that a good starting point is to look at the gross-combination weight of your vehicle, the topography of the route and the truck’s mission. Alongside these three key factors, consider how your axle choices will affect fuel consumption.
As a general rule, the heavier a truck is and the steeper the hills are along the route, the more torque is needed. The extra payload also means that you’ll need a stronger and more stable structure with more axles to distribute the added weight.
So, how do you increase torque? With a high axle ratio. The axle ratio tells you how many turns the propshaft needs to make to spin the wheels one turn. An axle ratio of 2.64 axle, for example, means that the propshaft needs to turn 2.64 times for each wheel spin. It also means the truck can pull more weight and start on steeper hills than an equivalent truck with a ratio of 2.47, for example. But to spin the propshaft more, the engine revs need to be higher too. This means as you use more power, you’ll also pay a bit more at the pump.
It’s also possible to increase traction by upping the number of driven axles – but this is not always practical. In general, the number of axles on a truck unit depends on the local load legislation and varies between countries and applications. While 4x2 or 6x2 tractors are common for trucks with trailers in Europe, for example, 6x4 units are more commonplace in countries such as the US, for trucks with a trailer. And the number of driven axles varies between applications. While a 4x2 unit has one driven axle, a powerful 8x6 unit (sometimes used in mining applications) has three.
Another key aspect to consider: What ground clearance do you need? To gain the ground clearance needed for rough terrain, such as in construction, mining or military applications, the driven axle can include a hub reduction. With a hub reduction, the driving power is transferred to each wheel by an additional reduction gear, built into the hub. Through this mechanism, the center gear can be made smaller, which increases ground clearance and durability, while also resulting in a less jerky drive when starting on soft ground.
The hub reduction is often matched with larger tyres to raise the truck even further off the ground. But sometimes, the opposite is needed: You might be more concerned with limiting the height of the truck, in order to travel under certain bridges, for example. In this case, you may need to avoid hub reductions entirely, limit the size of the tyres and select a lower chassis height and maximise loading weight.
The ultimate axle configuration specs for your vehicle will depend on a variety of factors including the gross combination weight, axle load, topography, road conditions and the application.
All these factors are best discussed with your sales rep. Alongside meeting your transport requirements, it’s also critical to consider fuel consumption, since axle specifications play a decisive role for fuel economy.
Pär is a Manager for Heavy Duty Transmission & Rear Axles at Volvo Trucks