TWIN SCREW/DRIVE AXLES VS. SINGLE DRIVE AND TAG AXLE?

Discussion in 'Truck Maintenance and Repair' started by butter, Aug 18, 2012.

  1. butter

    butter Member

    If you are driving in mostly dry weather (with occasional rain), then what is the advantage, if any, with having 2 drive axles vs. having the front rear axle driven and the rear most axle a tag?

    If seems to me that most trucks do not need two drive axles?
  2. Duck

    Duck . Staff Member

    They don't really. If you're only driving the truck on pavement.

    Actually in that case you're better off with a tag axle instead of spinning differential gears all the time for no reason. You might get 0.000000001 extra MPG with a tag axle instead of an extra drive axle.

    Just keep it south in the winter.
  3. Micabay

    Micabay Land Dweller

    Also keep out of loose gravel. Got stuck at a shipper a couple of months ago and barely made it out with out a tow.
  4. Tim

    Tim U.S.Constitution Supporter

    The main reason is resale value. Just because you may not need a twin screw the next owner might. But your right, most over the road tucks dont need them.
    If I can find a good used tag axle I'm gonna do away with my power divider rear end.
  5. Duck

    Duck . Staff Member

    Lots of times I've had to engage the other axle on dry pavement too.

    Some trucks have the front axle full time, and when you hit the switch it engages the rear. On others, the rear is full time and the switch engages the front.

    You know those docks where the warehouse floor is at ground level, so the dock is a down-hill grade? Right at the top of those, where there's a sharp transition between level ground and the incline, I've had my rear drive axle end up sitting 2" off the ground even with the air bags deflated. The truck's rear axle was the full time axle, so I had to hit the power divider to engage the front drive axle to get out of the dock.
  6. That guy

    That guy Active Member

    Um......I know a guy! Sell the Pete and get a K-Dub and I've got a factory drive/tag. Direct bolt on for KW Airglide suspension.
    View attachment 17527
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  7. Gus

    Gus What?

    Tag axles suck. Unless you can pull the tag at least 6 inches off the ground they just get you stuck. Ask a Fedex freight guy about the International tag screws. Ask the Conway truckload folks about them. You won't find many that disagree with me.
  8. That guy

    That guy Active Member

    True you either have to be able to lift it or dump it. We just dump all the air off the tag (which also kills the brakes) and then it's 100% better when empty or slick.
  9. Gus

    Gus What?

    The ones at Fredex would only dump until 15 mph. After 15, they'd air back up.
  10. Duck

    Duck . Staff Member

    I forgot I had these pictures.

    This is the process for taking the dolly that pulls the 2nd (or 3rd) trailer on a Roadway rig and literally shoving it up the tractor's butt to convert it to a 3-axle tractor. The dolly becomes a tag axle. You have to slide the tractor's 5th wheel up against the cab, then slide the dolly's 5th wheel up onto the tractor frame though, because the pintle hook inside the tractor's frame isn't designed for pulling loads.

    Two axle tractor:

    View attachment 17528

    Pintle hook inside tractor frame, just ahead of the single drive axle.

    View attachment 17529

    You back up to the dolly. Making sure the brakes are set on the dolly and that the pin for the retractable nose wheel is unlocked so it doesn't break when you ram it. And yes, you have to ram the fuggin' thing. If you try doing it slow, you'll just push the dolly backwards across the parking lot.

    View attachment 17530

    This is the tail edge of the tractor frame, with the dolly' two little wieners about to go into the holes that keep it aligned.

    View attachment 17531

    Here is the dolly's tongue locked into the pintle hook inside the tractor frame. At this point you twist up the safety chains so they don't drag on the drive shaft, and hook them to some hooks in there, hook up the air lines for the brakes on the dolly, and relocate the two 5th wheels.

    View attachment 17532

    Done. Now it's a 3-axle tractor & you can pull a "long box" and have up to 34,000 lbs across those 2 axles like any other truck. Except you will get stuck in mud, snow, or uneven ground easily. And when you're bobtailing in this configuration, with air ride on the tractor axle and just rubber blocks on the dolly, it's noisy & rattles and drives you bat-**** crazy.

    If you ever see a Roadway truck bobtailing around and it has two 5th wheels on it, it's one of these setups. Since Yellow bought 'em out and they became YRC, they quit buying this type of equipment.

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  11. Tim

    Tim U.S.Constitution Supporter

    Why would I wanna down grade to a KW for?:eek2:
    How much you wantin' for all that? I believe I could cut all the brackets off and set it up for a Flexair suspension. Flexair parts clamp on. Oops never mind the tag axle is round, I'd need a square one.

    I drove a '68 Diamond Rio with a air tag and never had a problem with it till some smart *** would raise it up till the drives were just off the ground. Talk about doing a burn out.
    View attachment 17534
  12. Copperhead

    Copperhead Active Member

    If the operational area allows for it with no problems of getting hung up or stuck, I would have no problem running a single drive and a tag. That is, ballpark, 500 lb of weight savings right there alone. And it is more efficient to not be turning a bunch of gears in the second axle all the time. As opposed to .0000001 mpg improvement, there are many who have done this and pull off a full tenth or two better mpg. I would advise that if someone is going to do this, that they have lockers in the drive axle and the ability to dump the air to the tag, both for extra traction in a tough spot.
  13. Tim

    Tim U.S.Constitution Supporter

    Beings most if not all air leveling valves are setup on the rear axle all one would have to is measure ride heights on the front axle as it is now and move the leveling valve to the front axle and install a dump valve on the rear axle like on spread axle trailers but in reverse. Or you could do it like it was set up in that Rio I drove. Had a hand value with a air gauge on it and once you got use to it you know were to set it. Oh and the tag was on air the drive was on Rayco springs, never once did I get hung up with that smoooth ridin' *****in' setup.
  14. PBaxter

    PBaxter Guest

    Well I do personally think that FWD drive axles would do fine on any weather and yes I also know that 2 drive axles on a truck don't really give it a bigger power boost or anything regarding it's steering. I think it's just more on a matter of being safe.
  15. blackw900

    blackw900 Flatbedder For Life!


    What are you talking about? That doesn't make any sense.
  16. Racer X 69

    Racer X 69 Member

    Sounds kinda Spammy to me.

    He links to this:

    View attachment 17535
    View attachment 17536

    And his IP Addy traces to the Philippines. Smells real Spammy, Mammy!
    View attachment 17535 View attachment 17536
  17. That guy

    That guy Active Member

    I know there are people that just love the idea of a dead axle, we wanted to try one but was too chicken to order a new truck with one. Found a used W9 daycab with a factory tag and bought it. Had a local dealer with another KW with the same setup but had it switched to a twin screw to help with resale so I bought both rear axles cheap.

    Now the truth about them. They're exactly like super singles, good if used in the proper application. They're not the magic bullet some like to claim with added fuel mileage, yes it seems logical that less gears = better mileage but in the application we use it there is no measurable difference. The W9 we have runs the same routes as a twin screw T800 we have pulling the same type trailer. The W9 has a C12 10spd and super singles all the way around. The T8 is identical but has a C13 and is a twin screw.....same mileage. The only difference is the W9 burns through tires faster because all the power is transferred through one drive axle. If they were as great as some like to say they are I would have had the complete rear rear axle setup swapped in the T800 I ALREADY own. Only took a day to have it swapped from the donor truck. The W9 I have pulled a gasoline tanker originally and now pulls a grain hopper. One other thing is if ordered from the factory the drive axle is larger, just moving the small rear drive axle fwd on a twin screw and putting a tag behind it won't last long with high HP. (in other words as long as your motor isn't yellow you shouldn't have a problem)
  18. Duck

    Duck . Staff Member

    How does that make a difference? Isn't a twin screw normally operated with the power only sent to one axle, unless you engage the power divider? I've never driven a truck more than a few hundred yards at a time with the power divider locked. I've only needed them when I was stuck, or about to go through some **** that looked like I was going to get stuck in, .. like my own friggin' driveway at home.
    DubbleD likes this.
  19. That guy

    That guy Active Member

    Normally the right rear does most of the driving on a twin screw but not 100%. The power divider just forces one side of each axle to drive. The W9 we have will slip the tires loaded taking off up hill in gravel. You can't always dump the dead axle because the suspension of the remaining axle won't carry the extra load and will just bottom out so your back on two axles. This is what I mean when I say it's not the magic bullet. They have drawbacks, some of which are a PITA and not worth the added headaches.
  20. Les2

    Les2 Hobbie Trucker

    My dad had this setup on an old Manfredi cabover except the drive axle was on springs instead of air. The problem was it acted like a tetertoter. He hauled steel pulled a van and even hauled coal out of coal pits with it. The nice thing was he could transfer alot of weight up onto the steering.

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