Rolloff truck locking differential question...

Discussion in 'Trucks and Trailers' started by Albemarle Lawn, Nov 18, 2006.

  1. Albemarle Lawn

    Albemarle Lawn LawnSite Bronze Member
    Messages: 1,544


    I'm looking at gettting a new rolloff unit for a new dumpster business we are starting. I will also get a flat body and use it to move my skidsteer and wide area mowers, etc, from time to time.

    I'm looking a new pre-emission Volvo VHD units.

    Option 1:
    20K front 44 rears, GVW 64K with 4.10 gears and 22.5 tires. Locking differentials both rears.

    Option 2:
    20K front 52 rears, GVW 72K with 4.30 gears and 24.5 tires. Locking diff last axle only.

    For some reason, on the 52 rears Volvo only offers locking on the last axle.

    Am I that much more likely to get stuck? I know, you have to use common sense about where you take your truck. But my concern is rolloffs find themselves on some of the nastiest mud on jobsites, and pulling away from a dumpster unloaded in the mud.....I'm concerned.

    Any thoughts? I really like the idea of the heavier gross and 24.5 tires but don't want to ruin half a day and pay $500 tow bills very often.
  2. The Rookie

    The Rookie LawnSite Senior Member
    Messages: 290

    If I had your guys job, and i was just starting
    in it, I would want both axles to lock just in case you had to back into a nasty subdivision under construction. These places can get real nasty.
  3. twj721

    twj721 LawnSite Senior Member
    Messages: 619

    Drive Axles
    Ratio, carrying capacity, and traction enhancers are among the considerations.
    Get two drivers together to talk about their trucks and one of the most basic questions is, "What kinda rear's it got?" They're talking rear-end ratio, which is vital because it describes how fast the rig will run. With today's high fuel prices, a "fast" ratio also lets the engine loaf and save money if the rig is driven slowly. Choosing the correct ratio for the job is among the most important spec'ing tasks.

    The ratio is among the many choices you can make in selecting a drive axle. It helps to know something about gears, shafts, housings and carriers -- some of the terminology involved in this important component -- and of course exactly what the axle does.

    A drive axle has two important functions: carry the bulk of the chassis and load weight, and transmit power from the driveline to the wheels. It does so through a clever combination of brute strength and sophisticated gearing. There's a lot that could go wrong, but engineering and manufacturing savvy has resulted in axles that can take punishment with little trouble.

    Some Basic Terms
    An axle in its simplest terms is like the one on your old coaster wagon: a rod or cross beam with wheels on each end. The beam keeps the wheels parallel to each other as weight from the vehicle and its load passes from the chassis, through the suspension to the wheels and tires, and thence to the pavement or ground.

    In addition, a drive axle has some means of transmitting power from the engine and drivetrain to its wheels. In the old days it was done with chains and sprockets. Today, power changes direction 90° as it passes through geared differentials and shafts to the wheels. Let's look at each:

    • A differential's gears transmit power, but also allow each wheel to travel at different speeds. This is necessary for turning maneuvers, because while passing through a circle the wheel on the outside of the turn must spin faster than the inner wheel. The "diff" accomplishes this through ring and pinion gears, and a set of "spider" gears that rotates in planetary fashion and lets the bodies adjacent to it act semi-independently. The gear set nestles inside a carrier -- the banjo-shaped housing toward the center of the drive axle.

    • Shafts, two per axle, extend outward from the differential. The shaft is enclosed in a housing -- the external part we see and, together with the differential, call the "axle."

    Axle gearing can be single reduction, which is most common, or double reduction. The latter can be accomplished by adding extra gear sets in the differential or in the hubs, at the axle ends. Double reduction axles split the power and torque twice, yielding a numerically higher axle ratio for extra heavy duty service.

    Two-speed axles were once common on medium-duty trucks, but transmissions with six and seven speeds are now used instead of the four- and five-speed transmissions that needed the low and high ratios in the axle. Two-speed axles are occasionally ordered on heavy trucks engaged in specialized hauling or severe off-road service.

    The 'Twin-Screw' Tandem
    What we've described above is a single-drive axle. A tandem-drive axle comprises two single-drive axles linked by a short drive shaft. Because the tandem has two sets of gears, it is often called a "twin screw." It's also called a "live tandem" because both axles drive. (A tandem using one live and one "dead," nondriving axle is called a "tag tandem" if the dead axle is behind the live one; and a pusher tandem if the dead axle is ahead of the driving axle.)

    Attached to the forward of the two driving axles in a twin-screw tandem is an interaxle differential, or "power divider." This takes incoming power from the driveshaft and splits it between the two drive axles.

    Power is therefore divided between the two axles, and between each wheel on each axle. The power splitting is almost never equal, for it is automatically and constantly adjusted according to conditions at the moment.

    Standard differentials, as clever as they are, are rather stupid in the power splitting function. They send power to the axle and wheels which are spinning the fastest. This is fine when going through corners, as more power to the outside wheel will tend to push the truck in a more stable fashion.

    But if the wheel is merely spinning on mud, snow or ice, it will get even more power because the differential doesn't know any better. This is true, too, of the interaxle differential, because it will send power to the axle whose wheels are spinning faster. If the rear axle is on ice, it will get even more power.

    However, most interaxle diffs come standard with a locking device which stops the differential action and splits power 50-50 between the two axles. Most axles do not have locking diffs, so the wheels can still spin uselessly. Locking the interaxle diff, tho, will likely result in at least one of the four drive wheels grabbing onto something and pushing the rig out of a slippery predicament.

    Nowadays, some axles can be ordered with locking differentials, either automatic or driver controlled. These are something to consider if you're interested in traction, which is priceless when you encounter slippery pavement or tough going off-road. Locking diffs are useful in single- or tandem-driving axles.

    A variation on this theme is automatic traction control, which is a cheap option with today's anti-lock braking systems. ATC is electronically engaged whenever the wheels begin spinning; the electronic brain gently applies brakes to stop spinning wheels and sometimes cuts back engine power, as well. Fleet experience shows it works as well as most mechanical locking devices.

    Beware The 'Torsionals'
    A recent phenomenon in the drive-axle business is the appearance of "torsional" vibrations. These build up when driveshaft U-joints bend too much as the axles rise and fall. Torsional vibrations are cyclical, coming and going rapidly with each twist of the joint. They are bothersome to drivers and can be very powerful and destructive.

    Torsionals can appear if U-joint angles exceed just a few degrees. Usually this happens in the short shaft between the two axles. This can sometimes be corrected by loosening the big U-bolts that hold the axles in place and twisting the two axles so their differentials face each other more squarely. Occasionally vibrations are so severe that factories have bought back trucks that could not be fixed.

    Experts say torsionals are possible, too, in a "negative-angle" shaft that rises (from front to back) between two U-joints. This arrangement is best avoided, but sometimes cannot be. If it's on a truck you're considering, ask what's been done to cancel out possible torsionals. A knowledgeable sales person will know what you're talking about, and will either answer your question or find an answer.

    By The Numbers
    A truck or tractor with a steer axle and a twin-screw tandem is known as a 6x4, since the truck or tractor has six wheel positions with four of them powered. If the rear drive axle is a single, the unit is a 4x2 with two of the four wheel positions powered. And yes, you can get a 4x4 or a 6x6 in a heavy truck, though like your pickup, these are meant for lots of off-road use.

    North American Class 8 trucks tend to have the 6x4 configuration because it's thought to have superior traction. But in Europe, where weather is as bad and roads as slick as here, the 6x2 is common. This is a three-axle, six-wheel truck or tractor with only two powered wheels. It's lighter and less costly than a 6x4, and can work as well for traction.

    The drive axle has a locking differential, usually in conjunction with a liftable "dead" axle. Raising the unpowered axle adds weight, and traction, to the powered axle, and the rig can usually get through snow and ice better than a twin-screw. Raising the dead axle completely would put too much weight on the drive axle (and the pavement), so the amount of lift should be modulated, and is, in automatic systems available in a few North American trucks. Doing so, however, reduces the traction advantage, and the system has not caught on here.

    Other reasons the 6x2 is not popular here are driver acceptance and resale. Most drivers believe the twin- screw tandem is inherently better (and with locking axle diffs or ATC it probably is). The typical second owner also wants a 6x4 for the same reasons, so will shun a 6x2. This lowers the 6x2's trade-in value. Resale is an important consideration in spec'ing, so if operators of your type of truck in your area don't use anything but twin-screws, you'd be financially wise to stick with it.

    Spec'ing For Application
    Even in Europe, a twin-screw is favored for construction trucks and others that go off the pavement. Loose soil, mud and what-all-else you run into require weight to be floated on all available wheels, and power must go to as many of those wheels as possible.

    This leads us to application -- one of the major factors in spec'ing an axle, or any other component on a Class 8 vehicle. What type of job must the truck do? The tougher the service, the stronger the components and the higher the rating the axle must have.

    What the truck will carry, how much it will weigh, and where it will travel are among the spec'ing factors. Flat terrain is less demanding than hilly or mountainous regions; travel on Interstate highways or turnpikes, with their limited grades, is easier than on older highways with steeper grades. Carrying dense cargo to maximum allowable weights -- or routinely overloading the truck -- will be harder for it to take.

    Axles are rated according to weight-carrying capacity, which refers to the actual weight placed directly upon it; and to gross combination weight, which includes total weights of the truck or tractor and any trailers it will pull. GCW is important because it will govern the strength of the gears and shafts built into the axle.

    For example, a given axle model may be used for an 80,000-pound-gcw tractor operating in general over-the-road service, but might also be approved for pulling a 110,000-pound set of turnpike doubles because it will encounter gentle grades on the turnpike.

    You must be realistic when ordering the vehicle so that the axles and all other components will have the weight-carrying and power-transmitting capacity to handle any job the truck will encounter. Saving a few bucks at buying time will result in big towing, repair and downtime expenses later -- not to mention heartache when the builder refuses to honor your warranty claim because you've abused the vehicle.

    Choosing Axle Ratios
    Here we go with a conversation on the rear-end ratio. Much depends on the transmission's gears, from its "low" to "top" gear. Top gear can be direct (1 to 1), where the engine's crankshaft spins at the same speed as the transmission's output shaft, or overdrive (usually 0.73 to 1), where the engine spins a little slower than the output shaft.

    An overdrive transmission will usually require a high-numerical axle ratio (also called a "slow" ratio). Conversely, a direct-drive transmission will often require a low-numerical, or "fast" axle ratio. Each has advantages and disadvantages, though some are more academic than anything.

    Sometimes the axle will be chosen first. This is true of severe-service trucks, such as dumpers and oil field rigs, where the stronger torque-carrying capacity of a higher numerical axle ratio is desirable. This decision being made, you might then need the overdrive transmission to get the truck up to freeway speeds.

    Wheel size, too, will affect overall gearing, and therefore the axle ratio you choose.

    Ordering Options
    If there's some doubt that a recommended axle model will hold up in your type of service, you can spec the next higher model. It should provide the beef needed to handle what you can throw at it.

    Among options to ponder are a lubrication pump, which augments the natural and engineered-in pumping forces of the gears in distributing lube grease. A pump is commonly used in higher-weight operations, but improved lube flow in recently engineered axles sometimes makes a pump unnecessary.

    Already mentioned is the locking differential or electronic ATC, which will add traction in slippery conditions and probably add to the vehicle's resale value.

    Extended warranties are usually optional at reasonable prices. These don't guarantee that failures won't occur, but they do show the builder's confidence in his product and give you peace of mind because parts and labor costs will be covered. These usually require use of more costly synthetic lubricants or a special mineral oil, but these last longer and protect the gears better.

    Considering The Source
    Axle makers today are highly competitive in features, and have spent great sums of money in time and effort to engineer long-lasting parts into their products. If an axle is approved for your application and you maintain it according to recommendations, chances are it will do an excellent job no matter who made it.

    But problems may still occur, and that's when the manufacturer's local or regional service force comes into play. How strong is it in your area? Is the regional service technician known to be conscientious and helpful, or is his reputation that of a jerk?

    Consider these and other questions when picking the axle make. Then choose the model that's right for your job.


    CLARK LAWN LawnSite Silver Member
    Messages: 2,526

    i drove roll off for a few years i only got stuck one time. that was in snow. i would stick with the lighter of the two. the costs of maintianing the 52K rears will be alot more due to different brake components also the 4.30 gear will make the engine turn faster at highway speed. when hauling boxes out of a construction site have it in writing that the box must be acessable or the contractor pays the tow and for lost time. you would be suprised were they will move them to with a backhoe or a skid. also i would look at a mack instead of a volvo for a truck of this type volvo is designed for an over the road truck not in town driving,parts are very expensive,and can be hard to get ( had to wait 6 weeks for a turbo to come from switzerland),and they are wired different from most other trucks(all switches run off complteing a ground) very hard to find wiring problems.
  5. Jpocket

    Jpocket LawnSite Silver Member
    Messages: 2,282

    Tell them to put a lift axle on "FREE" and they have a deal. It would be nice to have the 24.5s and the heavier spec, but a lift axle should put you to altleast 73,280 GVW. With the 4.10's the truck will just be more labored. Also are you getting an AUTO or a manual. The 4.10's will be fine with an 8LL or 13spd. An auto may be different.
  6. Gravel Rat

    Gravel Rat LawnSite Fanatic
    Messages: 9,544

    Myself I don't know if I would by a Volvo talk about expensive when it comes to repairs.

    As for locking diffs they are good when your empty but if you need to used them with a loaded truck you are going to break something. It doesn't matter if you have heavy rear axles you get mired in mud and spinning something is going to blow.

    For us a tandem axle is only allowed to gross 58,000lbs we don't use lift axles but we do have tridrive trucks they go anywhere and are becoming very common. They are allowed to gross around 70,000lbs they sure are hard on tires thou having 3 drive axles.

    If I was going with a brandnew rolloff it would be a T-800 Kenworth or a 4900 Western Star. I would consider a Mack or Sterling aswell.

    You should see where we have to take trucks some places the road is so steep your backing down there is a steel cable hooked to the front bumper to help decend the truck. Usually the truck is pushed back out with the excavator or pulled up with a cat.
  7. Digdug

    Digdug LawnSite Member
    Messages: 125

    abermele- I am confused a little , but it sounds like both trucks have the locking rearends . The first one have true locking rears which locks up both tires on each axle giving it maximum traction. The second option still has locking rears just limited slip. One set of tires on the front axle and one set of tires on the rear axle. The second option is what most trucks with locking rears have. Is that what your trying to ask? doug
  8. SLSNursery

    SLSNursery LawnSite Senior Member
    Messages: 442

    First I would like to comment that twj's response was very informative.

    I have a couple of different configurations, both of which were covered by his synopsis. Both have advantages and disadvantages.

    I have an international rolloff with a stinger (extendable tail), Beefy drive axle/rear end configuration (not sure of the rear end off the top of my head) and 24.5 tires. We just updated the rear springs because truthfully it gets loaded a lot all of the time. Regular roll-off companies don't usually haul much more than a few tons even in 20 yard cans. We haul stone, soil, mulch, even a flatbed with a piggyback forklift. This truck has a locking differential that we rarely use. It also has a tag axle behind the drive axle which we use all of the time. The benefit of this set up is that we rarely need both axles to drive the truck, and by lifting the rear axle, the turning radius increase tremendously. In purchasing a truck to replace this, I would be most concerned with cab to axle length, heavy duty rear end and drive train, and overall gross vehicle weight rating. To that end, I probably would sacrifice the rear tag axle and go with a standard tandem axle set up, with the future possibility of a pusher in front of the front axle. This would up the capacity of the truck, and on a new one probably be a cheap upgrade to locking axles and diffs. In summary, on the rolloff - don't back into a spot where you won't be able to get out and you should be fine. We go a lot of places with the truck, and just don't go where there might be a problem, diff lock or not!

    The triaxle we have, has it all, 2 locking diffs and axles, all air controlled. Guess what. It still gets stuck occasionally. Last summer a greasy lawn with a slight pitch tied the truck up like crazy. In about 1 inch of mud (after the grass got spun out) everything in my mirrors was turning and I was starting to sink. I had dropped a load of stone, and ended up putting a dozen or so shovelfuls in front of each drive tire. I pulled out fine after about 15 minutes of work. The next load we made was better, I brought an extra truck and the plastic mats. This worked like a charm, diff locks or not.

    I guess the bottom line is to get the truck that will hold your expected cargo. That is, are you going to overload the lighter truck every day? Here is a hint, the answer has three letters and starts with a 'y'. As soon as you figure out what the roll off can do you'll always use it. As a side note, we have boxes that go between the conventional roll-off and the smaller hook trucks. I am putting a Stellar hook on a 550 4x4 this week so that the other trucks don't need to risk going into slick spots. The upside is that the 550 will get in and out easier. The down side is that it is expensive and won't be able to grab the bigger cans. The full size trucks just cannot go everywhere. Don't work for people who think they should or can. And, be very careful about what angle you are sitting at, etc. They have a different geometry than a dump truck and will tip right over. Hope this helps.
  9. Albemarle Lawn

    Albemarle Lawn LawnSite Bronze Member
    Messages: 1,544

    I am considering Kenworth and Mack, also. I have easiest access to a Volvo dealership, since buying new and Volvo is WAY less expensive than Kenworth, and the cheapest to insure (airbag for driver).

    The truck would have the 8LL trans and Meritor axles, so the only Volvo stuff would be the motor and chassis. I'm really not too worried about the engine, Volvo has a huge diesel engine business with a variant of the D12 engine going into yachts, generator, etc. It is a seasoned engine, really I'm more weary of Caterpillar (likes to use o-rings to seal everything) and Cummins (have heard of ISX motors that are finicky and even pistons thru blocks).

    Not too crazy about the outdated Kenworth cab, doesn't it still use rivet construction?

    Mack....definitely a contender. Just wish a dealer was near me.

    I do some hobby off-roading and definitely understand that if you are spinning you will break something lockers or not. That is something I will very much instill in my drivers.

    Finally....any thought on bodies? American Rolloff, Galbreath, etc.....who makes the best 60K or 75K units. Do you really need a 75K or are you just driving around in a heavier rig you will seldom use?
  10. nac

    nac LawnSite Member
    Messages: 126

    Well i just bought a Volvo VHD axle foward went with 465HP, 8LL, 20F-20L-46R and a 75,000 American extandable tail (custom hoist lots of extras heavy spec steel more rollers mand then some.

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