Crawl control discussion

[Madtiger]

If the axle is locked and each tire spins at precisely the same speed, do they not each receive the same amount of engine output? Wouldn’t that mean a triple locked vehicle only sends 25% of power to each wheel?

and, vice versa, if you limit traction on ALL wheels except 2, on a center locked vehicle, then 50% of of that energy goes to each of the tree spinning tires? This is the situation most people that get stuck in standard 4wd’s have. They are sending 50% of the energy to one of their front tires that has no traction...and 50% of the energy from one of the rear tires that has no traction.

so if you can use your crawl control system to, essentially, arrest all spin on various tires on the front and back as needed, you increase the energy going to a particular tire at a rate of 50%, similar to how a stuck vehicle spinning a tire on the front and back would be...except with crawl control, due to braking, you are just swapping that 50% away from the tire with no traction and to the tire with traction.

im not an engineer, btw, hence the request for discussion/dialogue.

No. In theory, a triple locked vehicle has 100% of power going to each wheel at same time. Not 25%.


If you turn a pencil on one end, then does the other side get turn too? the other side gets 100% of torque you applied, right? Same principle. Because it is locked at 3 points, all engine torque flow continuously To each corner…and all corners get 100% (in theory) torque.

 

[WesSiler]

If the axle is locked and each tire spins at precisely the same speed, do they not each receive the same amount of engine output? Wouldn’t that mean a triple locked vehicle only sends 25% of power to each wheel?

and, vice versa, if you limit traction on ALL wheels except 2, on a center locked vehicle, then 50% of of that energy goes to each of the tree spinning tires? This is the situation most people that get stuck in standard 4wd’s have. They are sending 50% of the energy to one of their front tires that has no traction...and 50% of the energy from one of the rear tires that has no traction.

so if you can use your crawl control system to, essentially, arrest all spin on various tires on the front and back as needed, you increase the energy going to a particular tire at a rate of 50%, similar to how a stuck vehicle spinning a tire on the front and back would be...except with crawl control, due to braking, you are just swapping that 50% away from the tire with no traction and to the tire with traction.

im not an engineer, btw, hence the request for discussion/dialogue.

Depends on how much traction each wheel has. Thinking in terms of power % is just a confusing way to look at it. Torque follows the path of least resistance. Locking diffs apportions it evenly to the axles or wheels that diff controls.

 

[TeCKis300]

This other thread/video is pretty relevant to this thread showing how different traction control systems work. They put the the 200-series traction/CRAWL systems 4th place compared to other manufacturers systems.

4wd test- Large (stock) SUV comparison video w/ great commentary

forum.ih8mud.com

 

[linuxgod]

Locking diffs send torque to the wheels with traction. The wheel with no traction gets no (or extremely little) force applied, even though it's spinning freely

What is a locking differential?

A locking differential can lock the axles together to provide 100% of available torque to the wheel with traction.

www.eaton.com

What is a locking differential?​

Locking differentials (generically referred to as “lockers”) can lock the axles together to provide 100% of available torque to the wheel with traction. During turns, a locking differential operates like an open differential - the wheels can rotate at different speeds. However, when traction is needed, the axles can be mechanically locked together forcing the wheels to rotate at the same speed. This is especially helpful in off-roading situations when one wheel is off the ground or on an otherwise very low traction surface. When locked, the wheel in the air doesn’t receive any torque because there is no traction and the wheel on the ground receives all the torque, allowing the vehicle to move.

 

[WesSiler]

Locking diffs send torque to the wheels with traction. The wheel with no traction gets no (or extremely little) force applied, even though it's spinning freely

What is a locking differential?

A locking differential can lock the axles together to provide 100% of available torque to the wheel with traction.

www.eaton.com

What is a locking differential?​

Locking differentials (generically referred to as “lockers”) can lock the axles together to provide 100% of available torque to the wheel with traction. During turns, a locking differential operates like an open differential - the wheels can rotate at different speeds. However, when traction is needed, the axles can be mechanically locked together forcing the wheels to rotate at the same speed. This is especially helpful in off-roading situations when one wheel is off the ground or on an otherwise very low traction surface. When locked, the wheel in the air doesn’t receive any torque because there is no traction and the wheel on the ground receives all the torque, allowing the vehicle to move.

Both wheels would rotate at the same speeds. Stop thinking of this as % of power/torque and as wheel speed and it'll be a lot easier.

 

[linuxgod]

No. In theory, a triple locked vehicle has 100% of power going to each wheel at same time. Not 25%.


If you turn a pencil on one end, then does the other side get turn too? the other side gets 100% of torque you applied, right? Same principle. Because it is locked at 3 points, all engine torque flow continuously To each corner…and all corners get 100% (in theory) torque.

In an open diff (similar to the pencil analogy) torque goes to the wheel with slippage - in this case the outer end of the pencil swinging wide in the arc, not the inside.

In a locked diff torque goes to the wheels with traction, and the ones that slip get (virtually) none. The *speed* at which they rotate is constant, but the torque is not.

With CC/MTS, you have an open diff, but the system applies the brakes to the free-spinning wheel in order to generate torque on the grippier one. I *believe* in that case you do send all your torque to the grippier wheel as the brakes + open diff is acting as if the braked wheel is on the ground and the spinning wheel is the one slipping, but I may be mistaken

 

[linuxgod]

Both wheels would rotate at the same speeds. Stop thinking of this as % of power/torque and as wheel speed and it'll be a lot easier.

You are correct, they will rotate at the same speed, but the amount of torque being applied from the drivetrain is being sent exclusively (or almost exclusively) to the wheel with traction.

 

[Madtiger]

You are correct, they will rotate at the same speed, but the amount of torque being applied from the drivetrain is being sent exclusively (or almost exclusively) to the wheel with traction.

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.

 

[TheGrrrrr]

4-wheel traction control is on all the time, including daily driving when needed. It does slow down/pulsate brake of slipping wheel(s) but is not aggressive and is more for all-weather traction.

A-TRAC was an old system that was present on FJ Cruiser and was the precursor to CRAWL. And that was ON only in 4 low.

But people use ATRAC and 4-wheel TRAC interchangeably all the time since the demise of FJC.

VSC is what I think of as controlling the vehicle in commute settings. I have definitely had ATRAC limit wheel spin going up an obstacle in AWD with the Center Diff locked, not 4L, so its definitely functioning beyond what you would see in a commute. MTS seems to be a an adjustment tool for how the ATRAC functions. CC adds the element of acceleration control.

ATRAC limits wheel spin. MTS allows for adjustment of how ATRAC limits spin based on the preset selectable profiles while in 4L. CC takes all of that and adds controlled acceleration to the mix.

 

[GordJ]

Just for fun, if you measure the “twist” on the axle with traction (which I believe is torque) the axle with traction and load will show some ft/lbs while the axle in the air has virtually zero. There is the same torque available to the axle with a floating wheel but there will be none measured while the wheel is in the air.

 

[Madtiger]

VSC is what I think of as controlling the vehicle in commute settings. I have definitely had ATRAC limit wheel spin going up an obstacle in AWD with the Center Diff locked, not 4L, so its definitely functioning beyond what you would see in a commute. MTS seems to be a an adjustment tool for how the ATRAC functions. CC adds the element of acceleration control.

ATRAC limits wheel spin. MTS allows for adjustment of how ATRAC limits spin based on the preset selectable profiles while in 4L. CC takes all of that and adds controlled acceleration to the mix.

VSC is active when your vehicle is going in different direction than what your steering wheel says…it is more complicated but that’s the gist. In other words, VSC is about keeping you stable in a cornering or emergency maneuver...by braking the appropriate wheel(s) to keep you pointing in direction of steering wheel.

4-wheel traction (braking slipping wheel) is about keeping you going forward. Two differnet job…one helps you when you slip while cornering; the other helps you move forward at a stop light in wet conditions.

So yes, TRAC or, if you prefer, ATRAC is active while you drive everyday and while off-roading...MTS and CRAWL are just more advance modes of TRAC.

 

[Madtiger]

Just for fun, if you measure the “twist” on the axle with traction (which I believe is torque) the axle with traction and load will show some ft/lbs while the axle in the air has virtually zero. There is the same torque available to the axle with a floating wheel but there will be none measured while the wheel is in the air.

If everything is locked, The twist force is the same on both. Of course, only one is actually being applied to the ground.

 

[GordJ]

If everything is locked, The twist force is the same on both. Of course, only one is actually being applied to the ground.

So if we measure the torque on the driveshaft, which is the torque available, it would measure 100% while the loaded axle would be virtually the same while the unloaded axle would be virtually zero. If both wheels had traction each axle would be half of the driveshaft torque, wouldn’t it? Of course rear end gearing would change the axle measurements so the percentage is used and not ft/lbs. I’m not trying to discount your post but merely discussing the situation.

 

[Madtiger]

With CC/MTS, you have an open diff, but the system applies the brakes to the free-spinning wheel in order to generate torque on the grippier one. I *believe* in that case you do send all your torque to the grippier wheel as the brakes + open diff is acting as if the braked wheel is on the ground and the spinning wheel is the one slipping, but I may be mistaken

This is wrong. Sorry, my keyboard broke. Can’t explain In detail. CC and TRAC help decrease loss of torque leaking to the slipping wheel (on same axle). Thus, the wheel with traction still gets some torque (more with CC mode). CC clamps down on slipping wheel very aggressively…to help maintain torque to the wheel with traction…thus cannot be used at higher speeds.

The reason why CC or TRAC cannot transfer more power is because some of it is lost to heat from braking a spinning wheel. Thus CC does have a limit before it has to shut down due to overheating the braking system…I think 15 min of continuous use is the limit??

 

[Madtiger]

So if we measure the torque on the driveshaft, which is the torque available, it would measure 100% while the loaded axle would be virtually the same while the unloaded axle would be virtually zero. If both wheels had traction each axle would be half of the driveshaft torque, wouldn’t it? Of course rear end gearing would change the axle measurements so the percentage is used and not ft/lbs. I’m not trying to discount your post but merely discussing the situation.

Use the pencil analogy. If you tie two pencils together with superglue (or Flex Tape ), then you turn one pencil by rotating it. What happens to the other pencil its glued to? Does it twist with the same exact torque you applied? Yes. Does it matter if it is on the ground or not? No.

 

[linuxgod]

Just for fun, if you measure the “twist” on the axle with traction (which I believe is torque) the axle with traction and load will show some ft/lbs while the axle in the air has virtually zero. There is the same torque available to the axle with a floating wheel but there will be none measured while the wheel is in the air.

Yep, that is what Eaton is saying in the link I provided. I found the same (or similar) explanation on several other reputable sites.

IANAP (physicist), but torque is the twisting force applied. If a tire is in the air, it's spinning "freely" (even if it's in conjunction with the differential) and there's no torque being applied. The twisting force is being applied to the wheel on the ground. The fact that the other wheel in the air is moving is irrelevant to the amount of torque being applied to the wheel. Or perhaps another way for folks to visualize it is if you have a very long screwdriver and you grab it in the middle to tighten a screw, all of the force (torque) is being applied to the end with the screw, not the end which is floating in the air even though both sides are turning at the same speed...

 

[WesSiler]

Yep, that is what Eaton is saying in the link I provided. I found the same (or similar) explanation on several other reputable sites.

IANAP (physicist), but torque is the twisting force applied. If a tire is in the air, it's spinning "freely" (even if it's in conjunction with the differential) and there's no torque being applied. The twisting force is being applied to the wheel on the ground. The fact that the other wheel in the air is moving is irrelevant to the amount of torque being applied to the wheel. Or perhaps another way for folks to visualize it is if you have a very long screwdriver and you grab it in the middle to tighten a screw, all of the force (torque) is being applied to the end with the screw, not the end which is floating in the air even though both sides are turning at the same speed...

But, the second you applied resistance to the wheel in the air to measure torque, you'd be transferring torque to it.

The idea of torque apportionment becomes much more relevant when we get into mutli-motor EV trucks. No one is talking minute details yet, but by actively driving wheels with traction, they should be able to find more traction than ICE 4x4s are able to.

 

[TheGrrrrr]

"I am not a physicist, but..." is always my favorite way to start a sentence. Glad to see one of my Mud favorites use it as well! Sad that it was totally used in context, as I really only use it out of context. Carry on....

 

[M1911]

If the axle is locked and each tire spins at precisely the same speed, do they not each receive the same amount of engine output? Wouldn’t that mean a triple locked vehicle only sends 25% of power to each wheel?

No.

Power is a function of resistance.

For example, think about turning a merry go round at a constant speed. Merry go round A is on fresh bearings and turns easily. So it requires little effort for you to turn it at the target speed -- you are doing little work. But merry go round B is on very worn bearings that haven't been lubricated in decades. It is very hard to turn. Turning merry go round B at the same speed requires much more power than turning merry go round A at the same speed.

You are probably thinking "wtf does this have to do with tires and traction"? Think of those merry go rounds as the tires, and replace the resistance of the central bearing with friction at the surface of the tire.

 

[linuxgod]

Use the pencil analogy. If you tie two pencils together with superglue (or Flex Tape ), then you turn one pencil by rotating it. What happens to the other pencil its glued to? Does it twist with the same exact torque you applied? Yes. Does it matter if it is on the ground or not? No.

I don't think this analogy is correct. Yes the other pencil moves, but we're torque is the force applied at the wheels. See my screwdriver analogy I just posted. The twisting force applied, which is the torque, is being applied to the side with the screw, not to the end of handle. Yes the handle spins, but unless there's resistance the force is directed to only one end despite them both rotating in concert.

If you're saying the torque is split evenly between the two tires when they are both on the ground that is true. But if you are saying it is split evenly when one wheel is on the ground making good contact and the other is spinning freely (up in the air, or on ice) you are arguing that Eaton's explanation I linked to earlier is wrong. Wikipedia has another similar explanation:

Locking differential - Wikipedia

en.wikipedia.org

a locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal. (Unequal torque, equal rotational speeds).

But, the second you applied resistance to the wheel in the air to measure torque, you'd be transferring torque to it.

Correct. And it's really a lie to say 100% of torque goes to the wheel on the ground anyway since resistance in the driveline on the lifted wheel would be at least a small parasitic loss anyway.