Question: Tandem drive “Over the Road” semi tractors as it relates to 4 wheel drive?

[fkovalski]

We all know that driving a regular 4WD truck like our Dodge’s is not recommended for dry pavement because the drivetrain can “bind up” on itself if there is no slippage of the tires causing damage. Hence all wheel drive cars (AWD) have a center differential of sorts to prevent this.



My question is this: On semi tractors that have 2 sets of rear drive axles… are the diffs positively locked (i. e. no slippage) … or is there some sort of differential between the two? If it is the former situation, how is it that no damage occurs. I would assume that there are situations where the wheel speed of front to rear axle…and…right to left would be different enough to cause a ‘winding- up” condition similar to a 4WD on dry pavement.



Just curious,

-frank.

 

[BJMarshall]

Trucks are equipped with a power divider which is essentially a differential. With the power divider unlocked, a truck can actually spin one set of tires on one axle on a slippery surface. Most are equipped with an air activated lock on the power divider which will send power to both axles when engaged.

Some are equipped with a differential lock on one axle and some have it on both axles.

 

[fkovalski]

So... when the 'divider' is in the position that allows both drive axles to be engage... is it similar to 'full time 4WD?" That is... a driver should not use this in all around normal driving scenerios? It should be used in situations only when needed?



I would assume that it helps in winter driving conditions. Often I see chain laws in my area calling for chains on single drive trucks but not for all semis. I take it is this way under certain weather conditions because the dual drive trucks do in fact provide signicantly more traction in slippery conditions.

 

[BJMarshall]

You are correct. The power divider remains unlocked most of the time.

 

[SRadke]

They are a one wheel wonder until you lock it in too. Few things more helpless that a bobtail semi without an interlock.

 

[JFaulkner]

The power divider only locks the front and rear axle together, right and left are still open. (right side in mud and left on concrete your still not moving) You can buy a "Full locking differential" which locks all together, mine was an $1100. 00 option. Then it'll go forward but it won't turn. guess you can't have it all.



"Few things more helpless that a bobtail semi without an interlock"



No kidding, my last truck got stuck on an asphalt parking lot covered with snow on level ground WITH a trailer WITH a diff lock, I was really impressed. :-(

 

[fkovalski]

... . a bobtail semi without an interlock.

What is meant by "bobtail semi?"

 

[SRadke]

Tractor without a trailer. IE, minimal weight on the drivers.

 

[surfbeetle]

What is meant by "bobtail semi?"

that's just the tow portion (cab) of the rig, no trailer.

 

[fkovalski]

... Then it'll go forward but it won't turn. guess you can't have it all.

Will it not turn... even on snow or mud? If not- is this assuming a non- drive front axle? If it were '6WD', would the chances of turning be better?



Just trying to visuallize the scenerio. Thanks for all the information!

 

[NIsaacs]

They are a one wheel wonder until you lock it in too. Few things more helpless that a bobtail semi without an interlock.

Unless it is a single drive bobtail with a tag axle that does not lift!



The main difference between the traction of a single drive semi and a tandem drive, is the weight placement. Most single drive trucks pull three axles, as in doubles, so only about one fourth of the gross load is on the drive axle, not counting the steer axle. On the tandem drive truck, they only pull two axles or one half the weight of the gross load.





"NICK"

 

[JoshPeters]

Will it not turn... even on snow or mud? If not- is this assuming a non- drive front axle? If it were '6WD', would the chances of turning be better?



Just trying to visuallize the scenerio. Thanks for all the information!

Those trucks with the weight and torque they have... . if they turn they can/will twist or break an axle.



So lets say your stuck, you hit the lock between the axles, then lock the axles themselves. After that you hit the Cell phone for a tow. All of the "locks" are air operated and Very strong units. The weak link becomes the axle. (this has been told to me by my Tech school teacher, I may be entirely wrong, as that was 7 years ago)



Josh

 

[JFaulkner]

Will it not turn... even on snow or mud? If not- is this assuming a non- drive front axle? If it were '6WD', would the chances of turning be better?



Just trying to visuallize the scenerio. Thanks for all the information!

In mud or snow it'll just push the steering tires. I would assume that front drive axle would stop the pushing.



If you want a wakeup call price the front drive option on a class 8. I know for a Kenworth T300 (class 7 medium duty) single axle, Just to add the front drive was $13000. 00 extra on a $53000. 00 truck. That would pay alot of tow bills.

 

[fkovalski]

I would assume that it helps in winter driving conditions. Often I see chain laws in my area calling for chains on single drive trucks but not for all semis. I take it is this way under certain weather conditions because the dual drive trucks do in fact provide signicantly more traction in slippery conditions.

... The main difference between the traction of a single drive semi and a tandem drive, is the weight placement. Most single drive trucks pull three axles, as in doubles, so only about one fourth of the gross load is on the drive axle, not counting the steer axle. On the tandem drive truck, they only pull two axles or one half the weight of the gross load.

Aha!... The latter explains the former! That makes sense! It is more a factor of weight than it is drive wheels (or both?)

 

[GaryCarter]

Might as well spec out the CTIS, too, while ordering the front drive. Heck, it's only money.



Central Tire Inflation System before someone asks. Air down for low traction- air up for high speed. Sweet.





Gary

 

[SGibson]

If I go into mud, snow, deep sand etc with my big Dodge I'll engage the interaxle lockout, usally I'm in double under when I crawl out and never go past 2nd or 3rd under locked up but I can still steer.

 

[rbattelle]

Fascinating... thanks for the education!



While we're on the subject, what about trucks with triple axles? Are all 3 axles driven, or is one pair "dummy" just to support additional load. What's it like to turn something with that many axles (and only 1 steering axle)?



-Ryan

 

[JFaulkner]

You can get all three to drive but most trucks with 3 rear axles only have the rear two powered and the front is usually a liftable axle and sometimes also "steerable", only needed for those "Heavy " loads

#ad

 

[EMD Diesel Power]

Its pretty funny watching drivers burn up or blow up power dividers.



It doesnt take much... . that little weenie thing is taking the same amount of power and splitting it as the big rears do. One of the reasons that its not a full-time device and there are warning labels all over by the switch for it. :-laf :-laf



Some of the restrictions have been... do not engage during wheelspin, and dont use over 15 mph. That may have changed... its been a few years since I've done trucks.

 

[JFaulkner]

"build somthing idiot proof and someone builds a better idiot"