Tag axles – why do people want tag axles on today’s diesel pushers? Well, first I guess we should define what exactly is a tag axle. It is a trailing axle that normally is mounted directly behind the drive axle. It is a non-driven axle that generally is equipped with two wheels, one on each side. In today’s coaches it follows a dual-tired axle that has two wheels on each side.
What benefit does a tag deliver anyway? Well, it provides an increase in carrying capacity, usually an additional 10,000 lbs. to 20,000 lbs. It also may decrease the distance from the rear most axle center to the further rear-most portion on the vehicle, often termed the drawbar. This decreased distance increases the departure angle of the rear overhang of the coach body and frame, making the rear of the coach frame less liable to contact the roadway on uneven ramps or sloped surfaces. Further more, the extra axle today is equipped with an additional set of air brake drums, or in some cases, air disc brakes.
So, is that all? Not really. There are less subtle advantages, like the length of the drive shaft. In as much as the engine is located in the rear most portion of the vehicle, the driving axle is positioned somewhat further than a single rear axle coach, as the tag is between the engine and the drive axle. This substantially reduces the acute angle of the drive shaft universal joints at the transmission output and the differential input during high chassis height maneuvers or low bottom-out events. This nearly eliminates premature “U” joint failure or shaft seal damage caused by excessive drive line angle.
Okay, I see it on your face. How can the transmission’s drive shaft power an axle ahead of the one directly in front of it? Easy really, the tag axle is made up of a “U” like axle frame allowing plenty of room for the drive shaft to pass above it. So, now the gear heads are happy anyway.
Does it drive and handle better than that of a single axle? In my opinion, no! I feel that the stories of it being far more stable are grossly over stated. If your weight balance, alignment and running gear are set properly, a single rear axle will perform equal to that of a tag equipped coach.
What about the extra cost of tire replacement? Well, you are hauling more weight given the additional size and weight of your coach, so……… Okay, think of it this way, your rear tires will last longer than that of a single rear axle coach. This is simply because less weight per tire may well be the result. As far as turning sharp corners, all tags offered today have a selectable lifting, unloadable or steering type tag actuator. This may add up to only a slightly higher per mile tire cost that may be attributed to the extra set of wheels.
So, how about the “Cool” factor? You know. that macho thing that some of us, not me of course, suffer from. I notice this rarely affects females. Most wives don’t know what a tag axle is, nor care to have one. This may in fact work in our favor, ….. Shhhhhhh!! Next week, we will take a look at the different tag axle configurations.
Okay! Let’s hear from those opposed and those in favor of the right to have a tag axle!
Tag, You’re It! – Lug_Nut – Peter Mercer