Crawl control discussion

[M1911]

If the slipping wheel keeps constant speed turning in a locked axle, then what is making it turn? The same torque that the other wheel is getting.

No, it is not getting the same torque. Torque is a function of force at a distance from the point of rotation. In this case, since the friction on the slipping wheel is much lower, the torque applied is also much lower.

I agree with Wes, don't think about it as a % of torque. Instead, think about locking a diff as simply forcing both output axles to turn at the same speed.

 

[Madtiger]

No, it is not getting the same torque. Torque is a function of force at a distance from the point of rotation. In this case, since the friction on the slipping wheel is much lower, the torque applied is also much lower.

I agree with Wes, don't think about it as a % of torque. Instead, think about locking a diff as simply forcing both output axles to turn at the same speed.

Ok fair enough. I stand corrected.

Either way, the traction of a fully locked vehicle is better than CC.

 

[Chewbacca]

This discussion is quickly turning into whether the plane on the treadmill will take off or not...

 

[slipkid13]

I was under the impression that ATRAC in the 200 Series is active at all times. Not sure where I got that information, but I remember it coming from someone who I regarded as a knowledgable. Does anyone have documentation either way?

Edit to clarify my understanding:

ATRAC = On at all times
MTS = Engaged in 4L
CC = Engaged Via Toggle Button only available in 4L
TA = Engaged Via Toggle Button only available in 4L

This is correct, a quick look at the owner's manual confirms this.

 

[linuxgod]

Either way, the traction of a fully locked vehicle is better than CC.

On that we absolutely agree!

 

[ElJayBird]

I’ve used crawl control a handful of times and it’s basically dark magic. It failed me once but I was buried to the frame in hard packed snow.

I've used it a handful of times and this is the only time its failed me as well. Seems the only cure for being stuck in packed, heavy snow is a shovel and lots of throttle.

 

[afgman786]

Back to the OPs question. CC is great unless you need momentum is what I've learned. CC has gotten me thru situations where being open could not, but there was traction and it just wasn't being sent to the proper wheels consistent enough. I have also been in muddy climbs/slick climbs where the only thing that was going to get me thru was momentum and the speed of CC was too slow to get me thru that. Also the fact that CC grabs at wheels individually to keep traction can hinder you when all you need is fully locked and you can keep constant power going to every wheel without technology grabbing at the wheels.

Seeing you're in FL, I'd assume your trails can get like snot like other parts of the east coast I wheel where nothing but lockers would get you thru when its wet.

 

[highfructose]

Maybe this will simplify things:
Imagine a 2wd truck with its rear end in the air.
the engine produces 400 lbs of torque

I grab the right rear wheel while it’s under power. I stop the right rear wheel by applying 200 lbs of resistance. The left wheel continues to turn though because it’s an open differential. The left wheel now is receiving 400 lbs of torque since differentials are designed to send the power to the wheel with least resistance and I’m disallowing torque to work it’s way through my wheel on the right.

now we try the experiment again. I weld the spider gears of the diff together and Lock the left and right wheel together. They no longer can spin at a differential rate. I grab the right wheel and apply 200 lbs of resistance as it is under power. The right wheel AND left wheel stop at this point. Since there’s no differential, the excess energy cannot transfer across the axle.

thus a locked differential allows half of the power to go to each wheel at all times....while a differential allows a “differential” in the power application...sometimes 100% to one wheel...sometimes 100 to the other. Sometimes 50,50...sometimes 70,30, etc.

can an engineer or physicist point out which parts are wrong?

 

[M1911]

Yeah, not so much.

If you lock a differential and put one wheel in the air, even though that wheel is turning it is getting very little torque because there is very little resistance being provided by the air. Even though both axles are turning at the same speed, the wheel on the ground is getting the majority of the torque.

Think about it this way, the wheel in the air has very little resistance to turning, If you could measure the rotational strain on that half shaft, it would be negligible. The other wheel is on the ground and that ground is providing resistance to the axle. So if you could measure the rotational strain on that half shaft, you would find it significant. In other words, there would be little twisting (i.e., torque) on the half shaft with the tire in the air, but there would be considerable twisting on the half shaft with the tire on the ground.

Don’t think about locking differentials as apportioning torque. Think about them as simply causing both output shafts to turn at the same speed.

To think about it yet another way, you can’t have torque on an axle without resistance. If you don’t have resistance, then any torque applied would result in angular acceleration (tire would spin faster and faster)

 

[highfructose]

Yeah, not so much.

If you lock a differential and put one wheel in the air, even though that wheel is turning it is getting very little torque because there is very little resistance being provided by the air. Even though both axles are turning at the same speed, the wheel on the ground is getting the majority of the torque.

Think about it this way, the wheel in the air has very little resistance to turning, If you could measure the rotational strain on that half shaft, it would be negligible. The other wheel is on the ground and that ground is providing resistance to the axle. So if you could measure the rotational strain on that half shaft, you would find it significant. In other words, there would be little twisting (i.e., torque) on the half shaft with the tire in the air, but there would be considerable twisting on the half shaft with the tire on the ground.

Don’t think about locking differentials as apportioning torque. Think about them as simply causing both output shafts to turn at the same speed.

I understand, but if a free spinning wheel in the air is going very fast...and the wheel across the axle is not spinning at all...and a brake is applied to the free spinning wheel..SOMETHING is transferred to the non spinning wheel which then causes it to bite and move. This is how the traction control system works.

if not torque, what is the term for what has been transferred from the braked free spinning wheel to the grounded non spinning wheel just before that wheel bites and moves the vehicle forward?

and thanks...I feel like im getting close to figuring the out.

 

[grinchy]

I am a physicist, but only undergrad. Nothing interesting happens until post-doc.

I did hear a tree fall in the forest last time I was there.

 

[grinchy]

I understand, but if a free spinning wheel in the air is going very fast...and the wheel across the axle is not spinning at all...and a brake is applied to the free spinning wheel..SOMETHING is transferred to the non spinning wheel which then causes it to bite and move. This is how the traction control system works.

if not torque, what is the term for what has been transferred from the braked free spinning wheel to the grounded non spinning wheel just before that wheel bites and moves the vehicle forward?

and thanks...I feel like im getting close to figuring the out.

Isn't what is happening is that the braked wheel is now not free to turn, so the wheel on the ground must turn? That's what an open diff would do, right? It spins the wheel that has the least amount of traction.

 

[highfructose]

Isn't what is happening is that the braked wheel is now not free to turn, so the wheel on the ground must turn? That's what an open diff would do, right? It spins the wheel that has the least amount of traction.

Yes...I guess it’s a multi part question...

the first part is: is it technically torque being transferred? Is there another word for what is transferred across axle when a traction system brakes a free spinning wheel in order to give traction to a non spinning wheel?

and that question is only so we don’t get lost in terminology issues...

The second part is: whatever is being transferred...is it 100% of the available amount or 50% when you completely arrest the movement of the spinning wheel?

And the third part is, if you locked the axle thereby not requiring the transfer of whatever that was that was transferred initially...is each wheel then receiving 50% each of that stuff?

the relevance being, if you completely arrest the spinning of one tire, and thereby send 100% of the available stuff (torque or whatever) to the wheel across the axle, are you actually doubling the stuff (torque) on the other wheel?

I think first I need the terminology though...I call it torque but it appears people are saying it’s something else. Once I know what it is, the questions are less unwieldy.

 

[Lester did that]

To the OP. Depends what you are using it for? The CC is a gem in sand, thick mud and very loose stuff going down hill. It can also make you look pro'ish in some other situations. This is only my personal opinion. Air lockers lock and unlock with out fail every time unless they where installed wrong. If you break the airline it is 20 min repair. IF the stock Elockers go wrong it is not cool. You might get lucky smacking things with a hammer. But if you get stuck in 4L with the diffs locked and have a long drive home that is a bad day. The air lockers are dependable the Elocker not so much. The air lockers also engage in an instant not so much the Elockers.

So it really depends what you are doing. Air lockers are overkill for most folks. You can do more with that truck all stock than every SUV on the market today and still make it home. If you decide to keep it stock and use the CC put it in 4L and lock the diffs as often as you can. Obviously not on tarmac. The electronic system needs to be used to keep it "fresh". There is electronic grease in there that needs to get moved around from time to time and parts that need to touch. If it sits not moving for years its gets like an old man sitting in a chair too long.

 

[afgman786]

To the OP. Depends what you are using it for? The CC is a gem in sand, thick mud and very loose stuff going down hill. It can also make you look pro'ish in some other situations. This is only my personal opinion. Air lockers lock and unlock with out fail every time unless they where installed wrong. If you break the airline it is 20 min repair. IF the stock Elockers go wrong it is not cool. You might get lucky smacking things with a hammer. But if you get stuck in 4L with the diffs locked and have a long drive home that is a bad day. The air lockers are dependable the Elocker not so much. The air lockers also engage in an instant not so much the Elockers.

So it really depends what you are doing. Air lockers are overkill for most folks. You can do more with that truck all stock than every SUV on the market today and still make it home. If you decide to keep it stock and use the CC put it in 4L and lock the diffs as often as you can. Obviously not on tarmac. The electronic system needs to be used to keep it "fresh". There is electronic grease in there that needs to get moved around from time to time and parts that need to touch. If it sits not moving for years its gets like an old man sitting in a chair too long.

E lockers have a come a long way and are a lot more reliable than what the horror stories of the past offer. Also E-lockers engage almost instantly, I believe the newest of eaton/harrop are less than 1/4 turn to engage forward to reverse. I'd consider that instant enough on a 200.

 

[linuxgod]

Yes...I guess it’s a multi part question...

the first part is: is it technically torque being transferred? Is there another word for what is transferred across axle when a traction system brakes a free spinning wheel in order to give traction to a non spinning wheel?

and that question is only so we don’t get lost in terminology issues...

The second part is: whatever is being transferred...is it 100% of the available amount or 50% when you completely arrest the movement of the spinning wheel?

And the third part is, if you locked the axle thereby not requiring the transfer of whatever that was that was transferred initially...is each wheel then receiving 50% each of that stuff?

the relevance being, if you completely arrest the spinning of one tire, and thereby send 100% of the available stuff (torque or whatever) to the wheel across the axle, are you actually doubling the stuff (torque) on the other wheel?

I think first I need the terminology though...I call it torque but it appears people are saying it’s something else. Once I know what it is, the questions are less unwieldy.

with an open diff, you're sending 100% of torque to the diff. the diff is then sending 100% of power to the "least grippy" wheel. If both wheels have grip, then they get ~50/50 distribution, but for instance in a turn the outer wheel taking the longer path gets a higher percentage than the inner wheel.

1. braking the spinning wheel is just friction (or "negative acceleration" if you prefer - as I said, IANAP, I just play one). in an open diff
2. In an open diff, you send 100% of torque to the non-braked wheel once it can move freely. Note the last part as while the brake is applied to one wheel and the other is in contact with the ground and has traction, *temporarily* you've got the equivalent of both wheels having "grip" (resistance, really) so the torque being sent down the driveshaft and into the diff is getting spread across both shafts. It's only once the tire starts to move (either because it has traction or because it loses traction and then spins freely) that >50% torque is being directed to that side.
3. Note this is in contrast to a locked diff where torque is applied to any wheel that has contact. If both wheels are on the ground then each one gets 50% of the torque. If one is in the air and one is on the ground, then the wheel in the air spins, but it doesn't take much force to move it so the physics effect is that all of the torque from the driveline is being directed to the wheel on the ground, thus giving the wheel with traction 100% of available torque.

(Side note, which I think applies to your "relevance" question: if ATRAC/MTS is braking your wheel that's high up in the air then that side is in fact receiving some of the force from your driveline so 100% of torque is not being applied just to the wheel on the ground (though it may still be >50% - so if you use lockers you want to turn off ATRAC/MTS if possible)

Remember torque is the twisting force. It has nothing to do with the speed of the twist (that's RPMs), or how quickly you get there (that's determined by HP). Think about using a screwdriver. On a loose screw it requires very little effort to twist the screwdriver and push the screw in. You might even be able to do it with your fingers. That's low torque. However if the screw is nearly stuck (like you used a 3" long screw in a piece of solid oak and didn't drill a pilot hole) then it takes a LOT of (slow) force for you to twist the screw in without stripping it.

 

[linuxgod]

The air lockers are dependable the Elocker not so much. The air lockers also engage in an instant not so much the Elockers.

Air lockers do engage instantly. They also don't disengage (and then need to re-engage) when you switch to going in reverse.

I think e-lockers are more dependable than air lockers though. I've seen many people complain about leaking air hoses, blown seals, etc and thus the air locker not functioning when it's needed. There are certainly stories about early e-lockers breaking, but I believe based on my completely anecdotal experiences and forum reading that those days are mostly gone.

FWIW your CDL is an e-locker and it's very durable. Reliable too, except for the damn flashing CDL light we seem to get...

 

[highfructose]

with an open diff, you're sending 100% of torque to the diff. the diff is then sending 100% of power to the "least grippy" wheel. If both wheels have grip, then they get ~50/50 distribution, but for instance in a turn the outer wheel taking the longer path gets a higher percentage than the inner wheel.

1. braking the spinning wheel is just friction (or "negative acceleration" if you prefer - as I said, IANAP, I just play one). in an open diff
2. In an open diff, you send 100% of torque to the non-braked wheel once it can move freely. Note the last part as while the brake is applied to one wheel and the other is in contact with the ground and has traction, *temporarily* you've got the equivalent of both wheels having "grip" (resistance, really) so the torque being sent down the driveshaft and into the diff is getting spread across both shafts. It's only once the tire starts to move (either because it has traction or because it loses traction and then spins freely) that >50% torque is being directed to that side.
3. Note this is in contrast to a locked diff where torque is applied to any wheel that has contact. If both wheels are on the ground then each one gets 50% of the torque. If one is in the air and one is on the ground, then the wheel in the air spins, but it doesn't take much force to move it so the physics effect is that all of the torque from the driveline is being directed to the wheel on the ground, thus giving the wheel with traction 100% of available torque.

(Side note, which I think applies to your "relevance" question: if ATRAC/MTS is braking your wheel that's high up in the air then that side is in fact receiving some of the force from your driveline so 100% of torque is not being applied just to the wheel on the ground (though it may still be >50% - so if you use lockers you want to turn off ATRAC/MTS if possible)

Remember torque is the twisting force. It has nothing to do with the speed of the twist (that's RPMs), or how quickly you get there (that's determined by HP). Think about using a screwdriver. On a loose screw it requires very little effort to twist the screwdriver and push the screw in. You might even be able to do it with your fingers. That's low torque. However if the screw is nearly stuck (like you used a 3" long screw in a piece of solid oak and didn't drill a pilot hole) then it takes a LOT of (slow) force for you to twist the screw in without stripping it.

So it sounds like lockers always put 50% to each wheel.

open diff with braking traction control sometimes puts MORE than 50% to a particular wheel, correct?

 

[chilibit]

I would think about torque if I was in boulders or rock ledges. I am more interested in managing traction in slippery conditions and the CC is masterful at that. I do recall from freshman physics that friction from a full stop and friction from motion are mathematically different. The CC is faster and its algorithms are better than I have ever been and I started this in 1975. My FZJ-80 was triple locked. It convinced me that it was no panacea. But my challenges are the slippery surfaces.

 

[GordJ]

Let me try this, to respond to the pencil analog, if we cut each axle and put a “meter” between the diff and the wheels and this meter measures the torque being applied to the end of the axle (like an inline ammeter) the one in the air will read differently than the one on the ground. Same thing if we put a shear pin type of connector, one shears and the other spins.