Myth Busters

[rickeieio]

In sitting around the campfire or chatting online with folks, I've heard lots of myths and flat out wrong ideas about large trucks, meaning our HDT's.  So, in the interest of putting some of these incorrect ideas to rest, I invite everyone to listen, and comment, on things we've been taught, that just don't seem right.  

I'll start off with the myth that a "live tandem"  (two drive axles in the rear) has one axle being the "drive" and the other just being along for the ride.  UNTRUE.  There is a gear case built into the front axle (called a Power Divider) which houses a differential, providing both axles with the same amount of input torque, so that both axles drive together.  If one axle loses traction the power goes there, allowing that axle to spin.  Unless, you engage the lock in the power divider, then once again, both axles receive equal input.

Now the unfortunate part.  For as long as I've been on this forum (14 years?), self proclaimed experts have been stating this myth, and worse, singling trucks by removing one axle.  Did they not think that the reason they had to have special drive shafts made was because the remaining axle was never intended to have that much input torque?  The proper way to single is to remove the tandem and replace it with a heavier axle built to handle the torque.  Look at any factory single.  I could name at least three participants of this forum who over the years have made this same mistake, and I can also point out customers who've experience axle failures because of it.  Will they all fail?  Of course not, but why do it wrong. even after it's been brought to your attention that you're providing a dis-service to the customer?

So, I'll relinquish my soapbox so that someone else can expose other myths about these trucks.

[HERO Maker]

Good question!  But how much torque is really needed with our light loads, and would it really put stress on a single axle?

[GeorgiaHybrid]

Rocky, all it takes is to get mired down and stuck and a tweek of the throttle. A driveshaft can be twisted in half or a U-joint can break. A start on a steep hill will also load up the torque required to get moving. That is why most trucks that are factory single axles come with 23K rears or better.

We are lighter than most at 50K+/- with some on this forum pushing 70K. Those are not "light rigs" and will often push a truck to the breaking point.

[rickeieio]

Great catch Rocky.  You bring a question that I doubt can be answered, but I'll counter with the question....why take the risk?  It doesn't cost much to go to a junk yard and get a proper axle, or even better, start with a factory single or 6x2, as mentioned by Georgia in another thread.

[Darryl&Rita]

If you want to get down in the mud with the barnyard animals, fuel efficiency quests have led to 6x2 trucks. These have a dead axle in the rear position, that can't easily be raised, like a true lift axle.

[GeorgiaHybrid]

Rick. How about fuel milage? Had one guy at a rally insist that he got 20mpg plus bobtailing and 14 to 15mpg towing a 26K fiver. And that was with a DD15.....

If you are honest about it, most of us here will say 5.5 to 9.0mpg when towing our fivers. That is day in, day out, uphill, downhill, winter, summer and caught sitting in traffic.

[HERO Maker]

I agree with all you guys and that is why I never singled.  Especially with the weight I had with that Teton.  7K on pin.

Good discussion around the campfire though, as long as the libations didn't flow too much!

[rickeieio]

Agreed on the fuel consumption.  We get anywhere from 6.5 to nearly 10 mpg, for short runs.  But at the end of the day, an average of about 7.5-7.8 is normal, with a total gcvw of just under 50k#.  And that's by hand calculation, not believing the lie-o-meter on the dash.

But back to my original post, I just hit the tip of the iceberg with that one myth.  There are lots more, but that one is fresh in my mind since it's been on the forum recently.  It just bugs me that people who should know better, don't engage their brains and see how silly these ideas really are, and continue to toss them about.

[spindrift]

2 hours ago, rickeieio said:

I'll start off with the myth that a "live tandem"  (two drive axles in the rear) has one axle being the "drive" and the other just being along for the ride.  UNTRUE.  There is a gear case built into the front axle (called a Power Divider) which houses a differential, providing both axles with the same amount of input torque, so that both axles drive together.  If one axle loses traction the power goes there, allowing that axle to spin.  Unless, you engage the lock in the power divider, then once again, both axles receive equal input.

Rick, maybe I'm not understanding what you've said here.  You can spec a tandem axle tractor with a power divider that gives you the ability to have two drive axles.  You can also spec that same truck without a power divider where you're drive axle is only the rear axle. 

In the case of the truck with the power divider, you would NEVER engage the power divider under normal driving conditions so why wouldn't that front axle be "along for the ride"?  Again, assuming you haven't driven over a banana and need to engage the power divider. 

[lappir]

My first truck (which I still Own). Was singled long by it's previous owner that converted it to an "RV Hauler".  It's getting close to 1 million miles on it. I am much more happy with my "New to me" truck that still is Tandem and will be as long as i own it. Sure it was more expensive to put new rubber on it and maybe I will get worse fuel mileage, but looking down at the dash and not seeing the "Brake" message because the person who singled it didn't fix the computer or what ever it would have taken to prevent that message is worth the cost. There are probably a few other things, but that one comes to mind. 

 

Rod

[ddm502001]

39 minutes ago, spindrift said:

Rick, maybe I'm not understanding what you've said here.  You can spec a tandem axle tractor with a power divider that gives you the ability to have two drive axles.  You can also spec that same truck without a power divider where you're drive axle is only the rear axle. 

In the case of the truck with the power divider, you would NEVER engage the power divider under normal driving conditions so why wouldn't that front axle be "along for the ride"?  Again, assuming you haven't driven over a banana and need to engage the power divider. 

That is incorrect, can order a Tag or Pusher tandem where there is but a single driving axle and a Sleeper trailer style tube axle.  With two differentials there is a Necessity for a Power Divider differential.  As to Axle weight and durability, that after fifty years of changes on trucks is also a misnomer.  Tandems anymore only come as 38, 42 or 46 thousand capacity, much of which is Axle Housings weight, axle shafts sizing and suspension.   KW eight bag is the heaviest tandem air ride, Pete and Freightliner spring leaf or IH Spring leaf are the Lightest where a few are complexity nightmares of bushings, torque rods and arms or just far too many small parts as the Pete setup.  NEWAY Aftermarket the least complex and around same weight as well to consistency of function for SA use.  Third members or Differential "Pumpkins" are relatively interchangeable 38&42k weights Banjo Housings only Slight increase in heaviness, 46k will have again heavier banjo housings, casings and bearings as well axle to axle differential gear train.  Singling a Tandem ANY truck except a KW requires reuse of the Forward Suspension onto the Rear housing, including Spring Seats to banjo housing, the Ujoint 'Flange' will need to be converted from the smaller Ujoint series rear shaft to a Larger which a reasonable gear train shop can ID.  A reuse of the Front Housing with the Rear Differential is possible, just have to Blank Off the Power Divider Shaft point.  The primary issue of the Tandem Rear is they are generally constructed HYPOID Gearing and I am no engineer so have NO Clue why they did that other than to alter angle of attack for the R&P to "Non Parallel" to reduce affects of severity of Ujoint motions.  Rear Differentials set in a Reverse Angle to Forward, point UP drastically as opposed to up relative to Trans flange angle on forward.  Gets really complicated where used to repair these and ran into all manner of foul up repairs had to remediate.

On RV use ANY rearmost Differential can be utilized for a Single Out but CAUTION need be followed to not overload the mechanism by adding seriously heavy Frame Extension, boxed all steel bodies with extensive tooling boxes, support structures behind cab and above all else Ramps/platform to add a Auto or Motorcycle On the Platform Box behind cab.  Truck Capacity Weights are Cumulative as to Anything at springs Up including Drive Axle itself and the Tongue of the Toad.  Single Bag air rides will force frame Torque UP as accelerate, monitor your where with all as to Overhangs and overall trailer heights when stationary.

 

Some light Reading:

https://www.globaltransmissionsupply.com/eaton-fuller-transmission-troubleshooting-guide/driveline-angularity

[spindrift]

4 minutes ago, ddm502001 said:

That is incorrect, can order a Tag or Pusher tandem where there is but a single driving axle and a Sleeper trailer style tube axle.  With two differentials there is a Necessity for a Power Divider differential.  As to Axle weight and durability, that after fifty years of changes on trucks is also a misnomer.  Tandems anymore only come as 38, 42 or 46 thousand capacity, much of which is Axle Housings weight, axle shafts sizing and suspension.   KW eight bag is the heaviest tandem air ride, Pete and Freightliner spring leaf or IH Spring leaf are the Lightest where a few are complexity nightmares of bushings, torque rods and arms or just far too many small parts as the Pete setup.  NEWAY Aftermarket the least complex and around same weight as well to consistency of function for SA use.  Third members or Differential "Pumpkins" are relatively interchangeable 38&42k weights Banjo Housings only Slight increase in heaviness, 46k will have again heavier banjo housings, casings and bearings as well axle to axle differential gear train.  Singling a Tandem ANY truck except a KW requires reuse of the Forward Suspension onto the Rear housing, including Spring Seats to banjo housing, the Ujoint 'Flange' will need to be converted from the smaller Ujoint series rear shaft to a Larger which a reasonable gear train shop can ID.  A reuse of the Front Housing with the Rear Differential is possible, just have to Blank Off the Power Divider Shaft point.  The primary issue of the Tandem Rear is they are generally constructed HYPOID Gearing and I am no engineer so have NO Clue why they did that other than to alter angle of attack for the R&P to "Non Parallel" to reduce affects of severity of Ujoint motions.  Rear Differentials set in a Reverse Angle to Forward, point UP drastically as opposed to up relative to Trans flange angle on forward.  Gets really complicated where used to repair these and ran into all manner of foul up repairs had to remediate.

On RV use ANY rearmost Differential can be utilized for a Single Out but CAUTION need be followed to not overload the mechanism by adding seriously heavy Frame Extension, boxed all steel bodies with extensive tooling boxes, support structures behind cab and above all else Ramps/platform to add a Auto or Motorcycle On the Platform Box behind cab.  Truck Capacity Weights are Cumulative as to Anything at springs Up including Drive Axle itself and the Tongue of the Toad.  Single Bag air rides will force frame Torque UP as accelerate, monitor your where with all as to Overhangs and overall trailer heights when stationary.

 

Some light Reading:

https://www.globaltransmissionsupply.com/eaton-fuller-transmission-troubleshooting-guide/driveline-angularity

What part of what I said is I correct?

Secondly, in a tandem axle/rear drive axle configuration,  why is the rear axle used as the drive axle and not the front?

[rickeieio]

Spindrift, I think what he meant is that I am incorrect.  But notice, I clearly wrote "live tandem", which in olde pharte lingo is a tandem which has two driving axles with power divider.  I also referenced the possibility of using a factory single or a 6x2.

So, I stand by my statement.  

Edited December 4, 2023 by rickeieio

[Darryl&Rita]

Back to your myth, try watching the Coquihalla circus spectacular, and not cringe every time someone says, "His power is going to the front axle, so that's what's chained up" while the rear axle spins, and the truck sits. Helps perpetuate the dumb.

[spindrift]

11 minutes ago, rickeieio said:

Spindrift, I think what he meant is that I am incorrect.  But notice, I clearly wrote "live tandem", which in olde pharte lingo is a tandem which has two driving axles with power divider.  I also referenced the possibility of using a factory single or a 6x2.

So, I stand by my statement.  

But in your live tandem example, that front axle is still "dead" 99.99% of the time...give or take.

[rickeieio]

It's no more dead than the rear.  If the rear was doing all the pulling, those tires would wear out more quickly, which they don't.  I've heard others claim it's the rear axle that's dead.

C'mon Tom, you've worked on plenty of trucks.  Why is the front yoke so much beefier on the front axle than the rear?  Cuz the power gets divided roughly equally.

Do as I did, and google "live tandem truck axles".

Myths die hard.

Edited December 4, 2023 by rickeieio

[spindrift]

14 minutes ago, rickeieio said:

It's no more dead than the rear.  If the rear was doing all the pulling, those tires would wear out more quickly, which they don't.  I've heard others claim it's the rear axle that's dead.

C'mon Tom, you've worked on plenty of trucks.  Why is the front yoke so much beefier on the front axle than the rear?  Cuz the power gets divided roughly equally.

Do as I did, and google "live tandem truck axles".

Myths die hard.

So what you're saying is that you drive your truck with the power divider locked?

Don't forget, you can have both an interacted lock and a differential lock.  Interaxle is like limited slip on both axles. Diff lock is like having positraction on both axles. I always start with interaxle then lock the diffs if that is not enough.

Maybe we need a dictionary before we try to kill old myths.

Edited December 4, 2023 by spindrift
Further clarification

[GeorgiaHybrid]

Hopefully, this link will clear it up, BOTH axles are always driving the wheels and can run independently as far as speed goes. When you lock the divider, it is no longer dividing the power but locks them together.

https://desitrucking.com/understand-your-power-divider/

[Deezl Smoke]

 Boys!....play nice!! Don't make me stop this rig.🤭

[Deezl Smoke]

 Another "myth" is that auto-shift, or automated shift is automatic.

[GeorgiaHybrid]

Yep, a true automatic in our rigs would be an Allison. The rest are more properly "automated manuals" like most of our smart cars.

[rickeieio]

11 minutes ago, GeorgiaHybrid said:

Hopefully, this link will clear it up,

Beautifully done.  Thanks G. H.

Two myth's down, many to go.........

[GeorgiaHybrid]

2 minutes ago, rickeieio said:

Beautifully done.  Thanks G. H.

Two myth's down, many to go.........

LOL, do we need to get into axle lockers and the power divider locker and when they can and should be used?

[rickeieio]

I think your link did a fine job. If anyone doesn’t understand it after reading the full link, they have no business operating a truck. Much less singling and charging customers. 

[SuiteSuccess]

1 hour ago, GeorgiaHybrid said:

Hopefully, this link will clear it up, BOTH axles are always driving the wheels and can run independently as far as speed goes. When you lock the divider, it is no longer dividing the power but locks them together.

https://desitrucking.com/understand-your-power-divider/

Good explanation.