Many commercial fleets are evaluating the benefits of specifying new tractors with a 6×2 configuration versus the more popular 6×4 configuration. The standard 6×4 consists of three axles and six wheel-ends; both axles have driven axles. The new 6×2 features three axles and six wheel-ends on which only one of the axles is driven.
The proper nomenclature for the non-driven (or “dead axle”) in a 6×2 in which the first axle is driven is known as a tag axle. If the second axle is the driven axle, then it is called a 6×2 pusher.
Pusher axles are not as common as the 6×2 tags. However, the option for automatic lift axles is only available for a 6×2 pusher configuration. If loads are light, having the ability to automatically lift an axle will typically reduce irregular wear from developing. If the vehicle is running on a toll road, the toll charge will be lower if the axle is in the up position. Most tolls are based on number of axles on the ground and in use.
So what is the primary advantage for spec’ing a 6×2 configuration? It’s all about reduced vehicle weight (about 400 lbs.), reduced energy losses through friction and improved fuel economy. Fleets are showing a 2 to 4% improvement in fuel economy using 6×2 configurations, which is a significant improvement. There is also a cost advantage of lower maintenance since there is only one drive axle. A reduced number of driveline and axle components will reduce maintenance costs.
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Fleets have reported a drop in drive tire removal mileage on 6x2s— as high as 50%. On the non-driven, free-rolling axle, spec’ing a trailer rib tire versus a typical drive tire lug design is appropriate.
Treadwear is accelerated on the drive axle with 6x2s since all of the engine driving and engine braking torque passes through the single axle drive tires. With the average price of a dual drive tire in the $500 range, losing up to half of its tire life is a concern. It is the same issue for widebase tires that cost in excess of $900 per tire.
Prior to spec’ing 6x2s, fleets need to determine the overall cost advantage for their operation by increasing fuel economy in the 2 to 4% range but replacing tires at a much more rapid clip.
Vehicle traction maybe a concern with 6x2s. If your vehicles travel in conditions such as deep snow, ice and loose gravel, be aware that reduced traction may occur, but it can usually be mitigated by selecting the proper drive tire tread design and using available load shifting technologies.
It is not always easy to determine which is the best new tire and retread to select when running a 6×2. Keep in mind that the torque on the 6×2 configuration will be twice that of a similarly specified 6×4. The torque is concentrated on two wheel-ends versus four wheel-ends with a 6×4 setup. Fleets should work closely with their local tire professional when determining what tire to spec. Both duals and widebase tires are viable options with 6×2 configurations. A big advantage of running widebase tires with 6x2s is the additional weight savings which will add to the overall vehicle fuel economy. A potential concern with widebase tires on 6x2s is that only two tires will have all the torque applied to them as opposed to four tires with duals.
A 6×2 with the one free-rolling axle will pivot off the drive axle during turning, which leads to rapid tire tread scrubbing. With a 6×4 setup, the pivot point is the center of the tandems. The solution for the free-rolling axle is to spec a high-scrub resistant trailer tire. The same tire designed primarily for fleets with a 10-ft. spread axle trailer configuration. Choosing a deep tread lug design for the drive axle on a 6×2 will help increase traction and also be better for removal mileage.
Since there are many tire options available for both duals and widebase, it is always a good idea to conduct your own internal evaluation to see what the best combination of tires is when running a 6×2 tag or pusher configuration. It is not always a straightforward decision when it comes to tires and your 6×2 configuration.
An excellent source of additional information on this subject is a recent report published by North American Council for Freight Efficiency. The TMC of the American Trucking Associations also has addressed the subject of tires and 6X2s.