Difference between 2 & 3 rear Axles?????

Mostly for legal weight limits aka permit loads.

 

More axles don't help with hill climbs per se, more axles can haul more weight.

 

Heavyhaul to trucking is like an 18 wheeler is to a car. Cars are called 4 wheelers, remember? Instead of counting the wheels, heavyhaul counts the axles. The tractor you are referring to is called a 4 axle. Your normal everyday freight truck is called a 5 axle rig. And the general consensus is 12000+34000+34000 for the interstate unless you have a spread axle trailer (and that depends on your axle spacing). The heavyhaul 4axle tractor goes 20000+20000+20000+20000 (less in some areas, more in others (and that depends on your axle spacing)).

Generally, the 4 axle tractors will have 2 drive axles just like the ordinary trucks, but a dummy axle. So it's not necessarily about the traction as it is about the weight (refer to @Ruthless's post). However there are some 4 axles that have 3 interlocking drive axles. This type truck is called a "tri-drive" and it is beastly. It's not easy to drive...not easy at all, because the extra drive axle makes it difficult to get weight on the steer axle and steer unless you are loaded freakin heavy. So, instead of turning, it will want to slide around c

And a tri-drive is H E A V Y. Bobtail, full of gear will be around 40000 lbs. Tri-drives usually pull the biggest of loads. Normally the 13 and 19 axles or bigger.

 

You see these rigs out west and northern plains a lot. They can license for and legally manage more gross weight and theoretically charge more per load. They add more square inches to spread the extra weight to reduce road wear and tear. Also, the extra axles, add additional braking ability, needed for the additional weight.

In Michigan, Ohio etc??? those bazillion axle rigs are beyond my comprehension what they are doing gross weight wise, or how much has to do with frost laws and heavily weight-regulated roads

 

When you get into trucks with 3 live rear axles (aka tri drives), you also get the benefits of better traction. Lock everything in and you have all 12 rear tires putting power to the ground. Just good luck with the steering lol.

 

The key with those tri drives is to set them up for proper weight distribution. My old man had to bolt his logging bunk on to the frame 22 inches ahead of the center diff in order to maximize the payload on all axles. If you're pulling low beds you need one heck of a long neck to reach up there lol.

 

Mad Monkey, you see 4 axles a lot in the PNW. Up in that area you see a lot of what is referred to as maxi trucks, or some call them heavy haul, but it's not the same as hauling heavy equipment. Depending on state and axle spacing, I believe that they are set up to legally gross up to 105,000 or so. 65,000 or so loads. Generally it's a normal tandem drive, with a drop axle in front of those. The trailers are 4 axle, or 2 or 3 axle with 1 or 2 drop axles.

 

Unless you're pulling a 19 axle or way up in the Territories, a tri-drive is wayyyyyyyyy overkill. In Canada, where the dummy lift axle isn't kosher, and in the states bordering Canada, they run these really beefy 3 axles with full locking diffs. These are called Heavy Three's. If youre going to Hell, its the truck that you want to take you there. Before you throw down your money, keep this in mind: tbose locking diffs, beefy axles and thock frame rails adds alot of weight. Where your mega fleet trucks starts in at 17000 to 18000, the heavy 3 is going to start in at 25000 lbs.

"But Six, it will bring me bck from Hell...isnt that worth the extra weight?"

Honestly, the places where you want the lightest truck, and come in with the bare minimum fuel level, so they can load you with as much product as legally possible DON'T PAY SQUAT. Alas, most people just stick with what they know.

If you really want an education, take a job with a company that runs the US, Canada, Alaska and the Territories. Even the grocery haulers are rugged. You will see doorslammer rigs with 4-5 axles under the trailer. You will be sitting in a mega carrier box truck backed up to a dock, and the toughest van truck you've ever seen backs in next to you.