Tandem axle traction question

[john in la]

Help me settle a disagreement of terms we had at work today.
Class 8 truck with tandem axle.

How many wheels pull under normal driving conditions????

When you lock in the axle interlock what is this doing????

My answer is 1 tire under normal conditions and the interlock locks the two rear ends together so you now have 1 tire on each axle pulling.

Another guy at work swears that 1 tire from each axle pulls all the time under normal conditions and the interlock makes the rear end positive traction locking the wheels on either side of the truck together.

I may have to eat crow tomarrow at work but I can take it. Who is correct????

 

[D&Dservice]

I get this question alot at my shop, When unlocked the power takes the path of least resistance, usually to the rear axle due to less rotating mass in the rear axle. think of it as a full time 4x4 pickup, in unlock the axle with the least amout of resistance gets 90 percent of the power due to less rotating mass IE: shorter driveline to front axle and smaller axle components. Just like the rear on a truck, All that is happening when you lock it up is you are locking both drivelines together, sending equal power to both axles, there is no locking of the wheels together, the next time you get one stuck you will see one tire on each axle spinning when locked, unlocked you will only see one tire on the whole truck spin, the one that has the least amount of traction, It is called an interaxle diffrential, it is exactly that, a differential, just like the one in the rear axle of your pickup, its just laid in there lengthwise instead of cross ways. So your class 8 has three diffs, in the rear axles , hopefully my explantion is slightly clearer than mud, and BTW you are right he is wrong, I have been a diesel mech for 20 years and own my own shop, I work on many of these! so Im not pullin your leg.

 

[john in la]

Thanks
Someone will be eating crow at work tomarrow.
At least it will not be me.

 

[Anonymous-0]

The ones I remember work like he stated below, when locked, seems opposite wheels on each axle, diagonal would spin if slippery. Remember the label on the switch, said for slippery conditions or something? I know of one guy who had a trucking operation who used to disconnect them so the drivers would not run with them on.

I've always wondered what a power divider is or does, is it the same thing?

I understand how those rears work like in your question, but I remember the time I blew a power divider to pieces in a triaxle freightliner, 8 spd low hole - deep reduction, and I was using lo range, owner had sent me to work on site, excavator operator totally overloaded me and i did not know it, should have stood on the cab protector while he loaded, this was a road truck, would handle site work, but you can't heap the 28 yard box with heavy gravel and run in soft conditions, ground was stable but spongy enough to strain the driveline, just too much weight, it let loose like a grenade, pieces of gears punched right through the cast housing, I have one piece of each in a jar, that would have really hurt someone standing near it, always wondered what that power divider actually did. I had to help repair the darned thing, but we just put a new one in, always wondered what would have caused a failure like that, better to use a heavy ole AutoCar, or Mack on site, they always took the abuse.

 

[Leland]

It depends on how it's set up some have full lockers which all 4 on the tandem pull ,and some are set up like a dog leg rear where just as said above only one set of wheels per axle pull .Even single axles have an air locker so you have a posi rear end .

 

[Anonymous-0]

As far as the slippery condition switch,that was the tractor protection valve for locking out the air to your steering axle brakes on slippery roads to prevent jackknifes.These were outlawed by the feds many years ago.Another misconception is you can't engage the power divider on the go.It can be engaged while moving so long as both axles are turning the same speed.Never engage while one tamden is spinning.Damp gave a good explanation of how a standard tamden power divider works,but they now have available severe service diffs where you can lock in axles side to side also.

 

[RodinNS]

Think of it this way. The power divider IS an Interaxle Differential. So, a differential between the axles works on the same principle as any other non locking differential. The wheel with the least grip of the 4 spins. If you lock the power divider, it's the same as locking any other differential.... so it divides power equally, fore and aft, and thus will spin one wheel on each axle. If you have 4 way lockups, then you lock the power divider and you lock the differential in each axle so all 4 corners are locked solid....

Rod

 

[john in la]

So I guess we were both right depending on what type of set up you have seen in the past.

I did not even know they made a setup that locked all 4 wheels together.

 

[Anonymous-0]

I can remember older trucks having that switch, but never knew what it was for, that and the one that locks the rears, which obviously is not the same and I can remember using it once in awhile.

Funny you mention what that other switch actually does, on that freightliner, it had the big floatation tires on the front, and I had to keep those slack adjusters backed off enough to not skid, especially when slippery, that was a real hazard, and at that time it was illegal too. Then you had to be D.O.T certified to adjust your brakes, so if they got you at the checkpoint, you had to call someone in, could not do it yourself, that was a joke, if you think about it. Every morning, before heading out, I would raise up the dump body and adjust all the rear brakes, tighten em down and turn em back 1/2-3/4 of a turn, that truck always stopped well with a full load, pusher or 3rd axle had brakes too if I recall, been awhile, never knew that trucks had a switch for those fronts, if the feds outlawed it, what was the solution to the problem? Those big flotation fronts skidding was no joke, I'd rather take the ticket, boss agreed, I can remember getting material tickets at the asphalt plant for 29 ton, lot of weight, the truck had good brakes and you did not miss those fronts being backed enough to keep em skidding.

 

[Anonymous-0]

The solution now is anti-lock brakes,like on cars.I have a 03 Trail King 40 ton lowboy and it has them on it.Didn't think I would like them at first,but it keeps from flat spotting and ruining a set of tires in an empty panic stop.

 

[RodinNS]

I spose 4-way's may not be overly common in some areas. They're mostly only spec'd here on logger trucks where they're offroad and down in every hole within 300 miles of the mill... Otherwise they're seldom used. When you see a 'Star for sale here with a 14,6 front and 46 rears and a two line wet line, odds are, it hauled wood... and 99% of them have 4 way locks.

Rod

 

[Leland]

execpt for the mack tandems no power divider they are just hooked together like the old duce and a halfs .

 

[Anonymous-0]

I've been out of the trucking loop for awhile, in '05 I ran a new freightliner shipping horses every few weeks, lot different from the last one I ran, '68 and 72 AutoCar's, nice creature comforts in that new one. Yeah anti lock brakes would do it, that truck was an '87, things have changed.

I had a front brake cam over on a R model mack once, the flatspot ruined a brand new tire, I had to run like that for a day too.

 

[trucker40]

Damp has a good explanation.I think when its not in lock,the power goes to the right rear wheel.Right rear duals wear out first.When you lock it,power goes to the left front duals and right rear duals.
If you have antilock brakes it somehow distributes it to all of the drivers,if its set up for it.My 96 KW has it and you can feel it working on a slick road.It has an interaxle differential along with the other differential,but most of the time the power is going to the right rear drive tires.When you flip the switch it locks the drive shaft to both drive axles instead of just the back drive axle.The front drive axle doesnt do much unless that that switch is on.

 

[guido]

Hello john in la.
The third differential allows both tandem to run at different speeds.That is why you can spin one set of tires. When you lock the differential both front and rear units are functioning as one differentials, you are locking the third differential. Both drive shafts are turning at the same speed. When in the unlocked position the differentials function as a normal differential but if one wheel spins the axles run at different speeds, through the third differantial. Hope it makes sense to you.
Guido.

 

[guido]

Hello john in LA.
There is still a little confusion on how the tandem axles work. Under good road conditions inter-axle differential unlocked each wheel receives 25% of the power say 100 units of power input.
With poor road conditions 100 units of input inter-axle differential unlocked, each wheel receives 10 units of torque for a total of 40.
Poor road conditions inter-axle differential locked front differential 20 units 10 each wheel, rear axle 50 units 25 each wheel for a total of 70. The inter-axle is used in rain as well as snow conditions and can be shift it at any time is needed.
As you can see in the locked position the rear unit has the bulk of the load. On a wet road the system make it for a safer driving condition.
These are the rockwell specs.
Guido.