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A Question of Traction: 6×2 vs. 6×4 Axle Configuration

After spec’ing of the appropriate engine, rating and transmission, another important decision in the spec’ing process is selecting the axle ratio. But before doing so, it is important to ascertain the type of terrain on which the vehicle will operate, in order to determine the optimal number of drive axles.

If the vehicle will operate mostly on well-maintained, paved roads with moderate grades, where traction is less of a concern, and the customer’s goal is to maximize fuel economy, careful consideration should be given to the 6×2 axle configuration option. Fewer drive axles means lower rolling resistance, which leads to better fuel economy.

Here are the basics of how 6×4 and 6×2 axle configurations differ in the number of drive axles on a tractor.

  • A 6×4 axle configuration has two rear drive axles that provide power to all of the tires, for better control and overall vehicle operation. However, the additional drive axle lowers fuel economy due to increased efficiency losses from higher rolling resistance.
  • A 6×2 axle configuration has two rear axles, but only one of the rear axles receives power. The other axle is often called a “tag” or “dead” axle. Fuel economy benefits from using a 6×2 axle combination by an average of about 2.5 percent compared with that of a 6×4 axle configuration.

Determining which axle configuration is the best depends on your customers’ preferences, operational objectives and the equipment being specified. Here are some operating conditions in which each configuration will address better:

6×4:

  • Customers are concerned about the residual value of the tractor (6×4 tractors traditionally have higher resale value).
  • The vehicle will be operating a significant amount of time in off-highway or inclement weather conditions.
  • Drivers prefer a vehicle that provides better traction.

6×2:

  • The vehicle will be operating in a consistent line-haul operation with few slippery/icy road conditions.
  • Customers want to maximize fuel economy.
  • Customers need an increased payload (6×2 is about 300-400 lbs lighter than 6×4).
  • The vehicle has a typical direct-drive transmission spec.
  • The customer is spec’ing a faster axle ratio for downspeeding.
  • The vehicle will use single wide-base tires on the drive axle.

The biggest concern expressed by customers considering a 6×2 configuration is reduced traction. However, most potential traction issues can be overcome by the following actions:

  • Using load shifting techniques to increase weight on the drive axles at low speeds
  • Using traction control and/or automated load transfer systems to assist drivers with proper orientation

For more information on the difference between 6×4 and 6×2 axle configurations, please consult the following web pages:

 

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