What are safe speeds to engage the front axel?

[Hooty481]

Hey just what the questions says. Is it even safe to engage the front end while the truck is in motion? I have never tried but i have wanted to before. what do you all know about it?

 

[Wildchild467]

with low gear ratio that the deuce has, the drive shafts are spinning very quickly... i always come to a complete stop. Who knows if i have tire slippage at the time when i realize i need to lock in the front axle, so better safe than to risk damage to the T-case. i dont feel like replacing a T-case for something i could have prevented, so i try to take the best care of everything i can.

 

[Recovry4x4]

Unless you have lockout hubs, you can ejgage it at any speed. The ifront and rear diffs are turning at the same speed.

 

[deathrowdave]

I have to agree any reasonable speed the the T/C should engage without any problems everything rotating at same speed for this reason.

 

[stumps]

Unless you have lockout hubs, you can ejgage it at any speed. The ifront and rear diffs are turning at the same speed.

Yes, and no.

If all wheels have traction with the ground, then it is safe to shift into AWD at any speed.

If the rear wheels are spinning due to lack of traction, and the front wheels are rolling along the ground, you will grind the gears in the transfer case if you shift it into AWD.

If the front wheels are skidding due to lack of traction, and the rear wheels are driving along the ground, you will grind the gears in the transfer case if you shift into AWD.

And, if you have lockout hubs, and they are locked out it is never safe to shift the transfer case while in motion.

The only time I will shift while in motion is when I KNOW that I have traction on all wheels, and I am expecting to soon need AWD. Times like when I am driving from a gravel road into a muddy rutted up section of the road, or from a gravel road into a stream (ford)... All other times I stop first.

-Chuck

 

[91W350]

I would think disengaging the clutch would allow the case to safely shift. All tires and shafts should be close with no engine torque involved. That is how I shift mine anyway, prefer rolling with load load to stationary, thanks for the interesting thread.

 

[stumps]

I would think disengaging the clutch would allow the case to safely shift. All tires and shafts should be close with no engine torque involved. That is how I shift mine anyway, prefer rolling with load load to stationary, thanks for the interesting thread.

Most of the time that is probably true, but consider this: The rear of the truck has 4 wheels on the ground, and as such has pretty good traction... especially if there is a load. If the front tires are skidding in some slime, and the rear wheels are rolling when you shift, you will grink the gears.

I had just this situation happen to me once when I was driving in the snow. The truck was very cold, and the oil in the differentials was like molasses. I seemed to have good enough traction in RWD that I was just driving that way. I hit a small hill, and the front wheels stopped rolling, and were skidding. I put in the clutch, and took my foot off of the gas, and tried to shift into AWD, but the transfer case started grinding. When I did this, I was driving a PU truck with a manual transfer case shift, so I didn't force things... but if it had happened with the deuce, the air assist unit would have forced the transfer case into gear anyway.... well it would have tried.

It is always perfectly ok to shift when the truck is stopped. It is always ok to shift when the truck has good traction on all wheels. All other conditions can potentially cause damage to the transfer case.

-Chuck

 

[clinto]

I think somebody should post an image from the the TM that shows how the front axle engagement in these transfers operates.

That would be quite illuminating.

 

[stumps]

I think somebody should post an image from the the TM that shows how the front axle engagement in these transfers operates.

That would be quite illuminating.

OK.

Everything, gear wise is constantly engaged, but when you shift to FWD, a dog clutch (#24) engages to couple power to the front drive shaft. If the speeds between the front drive shaft, and the rear drive shafts are equal, everything works nicely. If the drive shaft speeds are different, the dog clutch grinds its little teeth off.

-Chuck

 

[Recovry4x4]

Yes, and no.

If all wheels have traction with the ground, then it is safe to shift into AWD at any speed.

If the rear wheels are spinning due to lack of traction, and the front wheels are rolling along the ground, you will grind the gears in the transfer case if you shift it into AWD.

If the front wheels are skidding due to lack of traction, and the rear wheels are driving along the ground, you will grind the gears in the transfer case if you shift into AWD.

And, if you have lockout hubs, and they are locked out it is never safe to shift the transfer case while in motion.

The only time I will shift while in motion is when I KNOW that I have traction on all wheels, and I am expecting to soon need AWD. Times like when I am driving from a gravel road into a muddy rutted up section of the road, or from a gravel road into a stream (ford)... All other times I stop first.

-Chuck

I forgot to consider the lack of common sense required to determine that all wheel ends are turning at the same speed before engagement.

 

[91W350]

My front axle scales in at 6400 pounds with my old white dog and myself in the cab or 3200 pounds per tire. To equal that on the rear tires I would have to be 8000 pounds overweight. I would think the front tires would be rolling the same as the rear. That being said I have pushed the front end turning in snow or even wet pavement, when taking off from a stop under power and trying to turn tight at the same time. I doubt the fronts are going to be sliding much without drive to the rear axles. If they are sliding I would like to think I would recognize that right away. Not saying it cannot happen, but I would like to think that would be pretty obvious. Probably accompanied with an Oh Sxxx! Thought at the same time, white knuckle death grip on the wheel..... Glen

 

[73m819]

I forgot to consider the lack of common sense required to determine that all wheel ends are turning at the same speed before engagement.

YOU ought to see what happens in the above case to a power divider. just listen for the BANG when the rear rear is spinning at a high rate of speed ( stuck)and the dumb a$$ truck driver locks in the power divider, or the same DATD spinning them self out of the mud with the power divider locked in with rpm close to redline and have the #2 axle grab the hardtop, BANG

 

[stumps]

I forgot to consider the lack of common sense required to determine that all wheel ends are turning at the same speed before engagement.

I'm not quite sure what you are trying to say with that sentence, but common sense doesn't really apply here.

Over the years, there have been a large variety of different schemes used to handle the transfer case function. Some have simple dog clutches and should be shifted only when all wheels are all turning at the same speed, and others have friction or fluid differentials built in, and can be shifted anytime at all.

The OP used extremely good common sense in asking the question. I figure that we owe it to him to give him a complete and correct answer.... Chest thumping is, as always, optional.

-Chuck

 

[stumps]

My front axle scales in at 6400 pounds with my old white dog and myself in the cab or 3200 pounds per tire. To equal that on the rear tires I would have to be 8000 pounds overweight. I would think the front tires would be rolling the same as the rear. That being said I have pushed the front end turning in snow or even wet pavement, when taking off from a stop under power and trying to turn tight at the same time. I doubt the fronts are going to be sliding much without drive to the rear axles. If they are sliding I would like to think I would recognize that right away. Not saying it cannot happen, but I would like to think that would be pretty obvious. Probably accompanied with axn Oh Sxxx! Thought at the same time, white knuckle death grip on the wheel..... Glen

The only vehicle where I have gotten caught, wishing I was in AWD, with the front wheels stopped is a pickup truck... an empty pickup truck... which balances out to about the same relative axle weights. The rear wheels were indeed being driven, but the front wheels skidded to a stop in the snow from the drag of the cold differential oil... and very quickly I might add.

Kind of rare, perhaps, but when it does happen, you end up needing to compress several minutes of thinking into a couple of seconds.

-Chuck

 

[steelypip]

Put simply: if you're really worried about it, don't shift into AWD unless you're sure all the wheels are turning the same speed - zero is always safe. I'm guessing that the dog clutch on the M35 TC is pretty forgiving about engaging if the dogs are pointing at each other - not all are. We had a mid '70s Land Cruiser that was a bear to get into 4wd some times- would do things like put two wheels on the dirt shoulder to generate some shaft motion and allow the splines to line up.

I'm guessing you guys all have air shift, so if the dogs aren't actually engaged when you select AWD, it will pop in as soon as there's a little relative motion between front and rear shafts.

Probably worth mentioning that getting out of AWD can sometimes be challenging as well - there can be preload (shaft bind) between front and rear shafts that prevents the clutch releasing. Again, stopping and backing up with wheels on dirt is a big help in this case.

The only examples I can name that I would actually be comfortable with engaging AWD with any significant speed up would be the ones with cone clutches in the TC in place of dogs or splines. You can still break parts with these if the shaft speed delta is too high, but it's not as likely.

 

[dittle]

I'm not quite sure what you are trying to say with that sentence, but common sense doesn't really apply here.

Over the years, there have been a large variety of different schemes used to handle the transfer case function. Some have simple dog clutches and should be shifted only when all wheels are all turning at the same speed, and others have friction or fluid differentials built in, and can be shifted anytime at all.

The OP used extremely good common sense in asking the question. I figure that we owe it to him to give him a complete and correct answer.... Chest thumping is, as always, optional.

-Chuck

I think what he was saying is that he assumed the operator would know not to try to engage the transfer if one set of wheels was spinning while the other was not. If the operator does not know that and still tries to do engage the front axle they get what they get. In other words......you can't fix stupid.

 

[Capt.Marion]

I'm leaving for lunch now, but somebody please post a screenshot of the -10 Operator's Manual where it deals with engaging the front axle, so we can get the official word instead of a bunch of folks's interpretations of the TM. (Even though most of them are mostly correct.)

 

[gringeltaube]

............... I'm guessing that the dog clutch on the M35 TC is pretty forgiving about engaging if the dogs are pointing at each other ...................................
I'm guessing you guys all have air shift, so if the dogs aren't actually engaged when you select AWD, it will pop in as soon as there's a little relative motion between front and rear shafts...........

That's correct.
From having seen many high-mileage "junk" T-cases inside, abused and worn out: these dog clutch halves are well designed and virtually indestructible. The picture below shows parts that came out of a TC that was going to be scrapped!
So even if the switch was accidentally flipped ON while the truck was stuck already and with rears spinning it will hold up fine. And just the resulting noise will normally deter anyone from doing this to often.... yes, this has to do with common sense.....

Probably worth mentioning that getting out of AWD can sometimes be challenging as well - there can be preload (shaft bind) between front and rear shafts that prevents the clutch releasing. Again, stopping and backing up with wheels on dirt is a big help in this case.
........................

Very true!



G.

 

[AGE|kshaufl]

I will note that while in the Philippine Islands I was driving an A2. We were doing jungle training and was slipping in the muck and had engage the TC on the fly. The rear wheels were spinning in the mud. The noise was horrible and I will tell you that the front wheels did not engage. According to the mechanics that repaired the TC the teeth were sheared off.

TM's are usually gospel but!!! If I know that I am going into a sticky situation I would engage them before I start slipping. I believe that the damage had occurred due to the rear wheels spinning and the front were not.

 

[wreckerman893]

The idea behind the air shift switch is that you can engage the front axle before you get into trouble as opposed to the sprag that kicks the axle in when the rears are already spinning.

I have shifted the front axle in while moving with no issues...just like dropping into low range (split shifting) on the fly......it is an aquired knack and you have to know what you are doing and use some common sense.

Shifting with the rears spinning and the RPMs are maxed out is reacting rather than using your head and anticipating a problem.

Driving these vehicles is not rocket science.....don't read too much into it and don't try to make it more complicated than it really is.