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This is probably a naive question but I've always wondered about single axle tractors.  Maybe my memory is failing me but at a lot of truck shows it seems I'll see single axle tractors pulling full size tandem step deck trailers with a couple of big trucks on board.  It seems to me that these single axle tractors are often pulling the same size/weight tandem trailers that the twin screw tractors are.  I know back in the pre-war days you could get massive single rear axle trucks with high rear end weight ratings (like the big ol' FKs and so forth) but I thought modern single axle tractors generally came with approx. 23k lb rear ends.  Is this not the case?  Wouldn't a 45' drop deck with two tractors on it be quite a load for that single axle?  They sure don't seem to struggle with it and it seems like I see more than a few of them at shows which is why I ask.   Or am I just remembering wrong? 

I never drove a single axle tractor but I remember driving single axle straight trucks with trailers behind and a lot of times the tail would wag the dog.  Is this much of a problem on single axle tractors, too?  I never much liked that squirmy rear end feeling.  just curious.  Thanks.

Edited by sodly

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I'm not going to answer your question....exactly.

In the U.S. market, most 4x2 tractors always ran 23,000lb drive axles. Rockwell sold thousands of (single reduction) R-170s.

But you will of course break leaves and damage axles by attempting to carry a 6x4's load.

In the European Union, most single drive axles and suspensions (e.g. Scania ADA1300) on 4x2 tractors are rated at 28,660lb (13 metric tons), and they pull tri-axle trailers.

(The U.S. should allow 97,000lb GCWs like the EU, with 6x4 tractor/3-axle trailer combinations equipped with "road friendly" air suspensions)

The former Mack Trucks used to offer the superb 29,000lb-rated RAD29 single drive axle equipped with the dual reduction CRD118 carrier assembly. 

For those wondering, the RAD29 weighed 671 pounds more than the standard Mack RAD23 equipped with the CRD117 dual reduction carrier.

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A single axle box truck with a pintle hitch(or ball) would/could be a handful if the weight is improperly set on the trailer.  Just like a pickup with a heavy trailer.

A single axle tractor isn't very heavy on the rear by itself, maybe 4-5K lbs at the most.  That gives you 18K more to put on that axle itself.   A 45ft trailer, even with two trucks isn't putting 18K on the tractor.  Two trucks might be 20-sumthin thousand pounds.  Split that between the trailer and truck and that is only 10K sum pounds on the rear axle of the truck.

Since the weight of the trailer is directly OVER the axle,  there is no squirm like a ball hitch.

Edited by Freightrain
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I ran a single axle mack for around 12 years hauling bread grossing around  45,000 with a 48' tandem trl.    Through corporate take overs I do the same thing now except with a new twin screw freightliner and super singles.   the single axle had  more road wonder then the screw, but I  Didn't really notice it at the time.     In PA a single axle tractor with a 2 axle trailer is aloud 73,280.   With 3 axles either on a single trailer or a set of doubles 80,000.    When I went through the process of getting the title for my b-67 we wrote it up for a combination weight of 73280 wich it will never see.   

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As usual I'm impressed by the cumulative knowledge of youse guys lol with a 10 ft spread on the trailer you're allowed 73 280 on a single axle combo.on the interstate system.the average sleeper road horse weighs 18000lbs  so you're not stressing that single axle.Also you've got rid of the extra suspension system and differential,power divider,etc that's a whole bunch of parasitic drag!most single axles gain up to 1mpg over the identical full screw.That's a big number.As usual I have an anecdote to go with the subject! I was driving an 85 mh cabover single screw with a 50 ft aluminum drop deck 10 ft spread on air. It looked like a big truck trailer but had 12k rated axles, and the rig was licensed for 50000.We had two deuce and halfs going from Ft Gorden Ga to March Air force base in Riverside cal. Nice load! Only problem you can only put 1 gbl (govt bill of lading) on a truck! They were estimated @14000lb apiece so we figured if I watched my fuel I could stay under 50000.well the army loaded them both to the top of the stakes with heavy electronics equip. I told the military expediter I would take them to our terminal,and put one on another truck! Not kosher,but she bought it.I noticed my trailer tires were squished down,and it pulled a little heavy.Well what should have weighed 28000 weighed over 40000! So I was forced to do exactly what I told her I was gonna do! "The best laid plans" right?

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Of course now that I go through my photos I can't seem to find example pics of the kinds of single axle trucks I'm talking about hauling tandem trailers.  If anyone has good representative photos please post them here.  I think it's interesting to see what kinds of loads single axle trucks can haul.  Thanks.

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Tandem axle trailer or tandem trailers(like california doubles?).

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Single-axle tractor, tandem-axle trailer, please. 

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That's the combination I did at least 80% of my driving career in! When I worked for Continental Baking (Hostess), even with a 45' trailer stacked to the roof with Hostess pies (heaviest product we hauled), we only scaled a hair over 17,000 pounds on the single rear axle with the trailer tandems welded 10 1/2' forward of the rear of the trailer. At that time 90% of the fleet was single rear axles, only thing we needed a few tandems for were a few routes with seasonal 7 ton axle weight restrictions and hauling the occasional loads of bagged flour and sugar. With the wider and longer 48' and 53' trailers they could get 20% and even 30% more on a trailer, and with a few clowns that wanted the "big rig" look sliding the trailer axles back, we started having occasional overloading problems even with the heavier 20,000 axle weight the STAA allowed. So from the late 80s on, almost every tractor Continental bought was tandem drive, except for some northeast locations where the old 22,400 pound axle weight was allowed or tractors specced to pull short doubles on the west coast.

Same with UPS and the Postal Service- Very few loads required tandems. Our UPS hub needed a couple tandems because they had a paper company that would fill whole trailers with heavy copy paper. And the new UPS CNG tractors all seem to be tandems, probably to handle the extra weight of the fuel tanks. The Main Post Office I worked out of weighted their trailer loads and found that only 5% required a tandem tractor, so our manager ordered the last bunch of new tractors with 95% of them single rear axles. Oops... Problem was most of those heavy loads all came in about the same time every evening, and they had to order up a few more tandems!

This is pretty typical of american trucking- I ran across a study a while back that showed that the average tractor trailer rig on the highways is only loaded to about 40,000 pounds total weight! That means that most loads could be hauled with only 3 or 4 axles down, and a lift axle could be dropped down to stay legal with the occasional maximum legal weight load. This is why tandem drives for on highway use are mostly obsolete.

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Here's the only pic I could find handy.  I know, not much of a load on that trailer.

14086238_896383207163093_8597623295730960220_o.jpg

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I pulled a 27000 ish pound rough terrain fork lift for about 6 years on a tandem axel trailer and a single axle baby freightliner 275 horse 9 speed fuller trans my trailer would way around 29000 lbs and the tracker rear axle would way between 14500 and 15200 depending on what I had on the upper deck. But I much prefer my ch613 twin screw in for nothing other than the stability.

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Here's another pic of an old Autocar.  Seems odd to me to have a tandem trailer but a single axle tractor.  The load seems pretty well evenly distributed between the truck and trailer.  So why is one tandem and one not?  Just the discrepant tire sizes?13730999_1158645290845080_6537374984342745381_o.jpg

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Perhaps running on "contractor plates" in the northeast, and thus exempt from axle weight limits?

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Original owner of my single axle Autocar told me he pulled a triaxle tank trailer with it.

Later on he added a pusher though

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image.jpghere's my outfit hooked up for the first time.   

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Sodly,that single screw L model fire truck makes a beautiful double bunk road tractor! Hobert was that B model a twin screw originally? It's really coming along great! Freightrain you reminded me of the days before deregulation when Pa didn't permit doubles and there were "drop yards" in Ohio and New Jersey on the tpke and I 80 where the freight companies left behind their second pup going west in Jersey and going east in Ohio. Consolidated Freightways had a long wheelbase Freightliner day cab that backed under a standard length pup trailer and locked into place creating a long straight job.I've never seen them hook up but the dolly legs must have been on the outside of the trailer and removable! Anyway the thing had a pintle that hooked up to a converter dolly and second trailer,therefore creating a de facto set of doubles! The things were all over the road,nobody wanted to be behind one! And the drivers had their hands full but the things could be "cross docked" just like a standard set of doubles!

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Billy

I believe that the CF lead trailers had dolly legs which folded up under the trailer as it was hooked up. These lead trailers and long tractors were kept in the Akron Ohio / York PA turn around while thry could use any kite trailer. Their nickname was "Suicide Doubles" for their interesting handling.

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The ones I've seen had a separate dolly leg, gearbox, and crank on each side as the truck's frame got in the way of the usual cross shaft. Didn't see any stability problems with them, but I only ran that stretch of I-80 for a few months back in '78.

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Brocky and T Grrrl, thanks for the response! I can picture either scenario for the dolly legs! The fifth wheel  was right up behind the cab,so once the driver got under the trailer to support it he could fold the legs up under,and continue all the way back,or have a gear box for each side and remove it.Sounds like a real pain in the butt! Of course in a union operation a yard jockey sets up and breaks down a set of pups on the clock and the driver is paid per mile. Brocky the last time I heard a rear pup referred to as a "kite" was when I applied at Conway Central in Irwin Pa. They put you thru a two week training period with one of their drivers.You  drove a straight truck,a city pup,a 45ft regional trailer, and two round trips with a set of doubles.One run to Columbus Oh. And one to Cleveland Oh. The terminal manager says you passed all our tests an you're going to be working a split shift on the dock! I lived 45 mi away in Washington pa, so unless I slept in my Car between shifts I'd be driving 180 mile per day to work! One guy actually parked a camper at the terminal,and went thru all that b.s.!  Of course you were given the impression you would start driving as soon as you passed their tests! Needless to say that job didn't work out!

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