Crawl control discussion

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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.
When you apply the brake to the spinning wheel, that is just like putting that tire on the ground — there is now resistance and and then it gets torque. But without the brake applied there was very little torque. Since the wheel was free in the air, it was very easy to spin, thus requiring little torque.

As I said, don’t think about locking a diff as dividing torque equally. What it does is cause the two output shafts to turn at the same speed — that is all it does.
 
So it sounds like lockers always put 50% to each wheel.
No, No, no, no, no, no.

When a differential is locked, the two output shafts turn at the same speed. The amount of torque each output shaft sees is a function of the resistance to turning on that shaft — that is, the amount of traction for that tire.

A locked diff doesn’t parcel out torque to each shaft, trying to keep it equal. All it does is cause the two output shafts to turn at the same speed. That is all.
 
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.

Exactly!
 
I have a question to those who retrofitted lockers on the 200 series, how does the A-Trac or CC manage with locked diffs?
 
Crawl/atrac/mts: allows much of the benefit of lockers without the steering penalty, and unlike lockers doesn’t CAUSE slip on ice due to binding. Also can basically dig itself out.. pretty awesome.

Locker: gives better/more positive control of keeping the axles turning at the same rate. Atrac/etc is a very coarse method to try and slow the slipping wheel. Watch it in action and it is nowhere near as smooth as a locked diff. Meanwhile, for the short periods that the slipping wheel is totally stopped by the brake, the diff is now spinning the other tire with traction at twice the speed, and therefore it is getting only half of the torque for that short period of time.

Neither is perfect. Combined, they are awesome.

In reality crawl will do anything I should be putting my truck on, so that’s how it’ll stay.

I had lockers on my 80 and they arguably allowed me to get into the situation in which I rolled it. Without the lockers I probably never would have been there. Ultimately that’s my fault and not the lockers, but it’s not a completely uncommon phenomenon.

$85k USDM 200s don’t have lockers because the vast majority of them would rust away before they ever got engaged.
 
Crawl/atrac/mts: allows much of the benefit of lockers without the steering penalty, and unlike lockers doesn’t CAUSE slip on ice due to binding. Also can basically dig itself out.. pretty awesome.

Locker: gives better/more positive control of keeping the axles turning at the same rate. Atrac/etc is a very coarse method to try and slow the slipping wheel. Watch it in action and it is nowhere near as smooth as a locked diff. Meanwhile, for the short periods that the slipping wheel is totally stopped by the brake, the diff is now spinning the other tire with traction at twice the speed, and therefore it is getting only half of the torque for that short period of time.

Neither is perfect. Combined, they are awesome.

In reality crawl will do anything I should be putting my truck on, so that’s how it’ll stay.

I had lockers on my 80 and they arguably allowed me to get into the situation in which I rolled it. Without the lockers I probably never would have been there. Ultimately that’s my fault and not the lockers, but it’s not a completely uncommon phenomenon.

$85k USDM 200s don’t have lockers because the vast majority of them would rust away before they ever got engaged.
Stupid question: so if MTS/CC is braking the wheels without traction, in order to drive the wheels that have traction, then what benefit is locking the center diff? If only one tire had traction, wouldn’t CC brake the others and direct torque to the one wheel not spinning?
 
Stupid question: so if MTS/CC is braking the wheels without traction, in order to drive the wheels that have traction, then what benefit is locking the center diff? If only one tire had traction, wouldn’t CC brake the others and direct torque to the one wheel not spinning?
Like a locking axle diff it can be a more direct mechanical linkage and not need to get to the point of slip to do it’s job.

Little bit of a tangent, but I personally feel many of us are too quick to engage the CDL given the reality that it causes binding between the front and rear axles when turning tightly, and how capable the truck is with it off. If we run 4Lo and keep CDL off until we need it we can turn tighter and put less strain on the driveline. Then when we approach a difficult obstacle it is just a button press away. Defaulting to off means we don’t need to wait for it to disengage before approaching a tight turn, but conversely there is usually no issue getting it to engage quickly.

This doesn’t apply to deep sand and mud, for the record. I’m thinking of the stuff I usually spend time on in Colorado.
 
"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....
I usually follow IANAP with HMB (hold my beer)

I actually was a physics student long ago but I had to drop out because I couldn't do the math nor afford grad school, so I became an engineer instead. Its easy to get wrapped around the axle when talking about differentials... :rofl: rather than add my own $0.02, I found this: Open vs Locked Differential - Torque Transfer - Explained I think the kid in the video has got it essentially right.
 
I have a question to those who retrofitted lockers on the 200 series, how does the A-Trac or CC manage with locked diffs?

As all tyres does run exactly at the same speed, these elecronic systems dont have to do anything. You can even use crawl, but you dont have the advantage out of it.


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

What means TA? Maybe my english skill isnt good enough?

Thanks for clarifycation!
 
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?
No, No, no, no, no, no.

When a differential is locked, the two output shafts turn at the same speed. The amount of torque each output shaft sees is a function of the resistance to turning on that shaft — that is, the amount of traction for that tire.

A locked diff doesn’t parcel out torque to each shaft, trying to keep it equal. All it does is cause the two output shafts to turn at the same speed. That is all.
ok so torque isn’t transferred across the axle when brakes are applied to the free spinning wheel.

does the grounded wheel temporarily spin at twice the speed when the brake is applied to the lifted free spinning wheel? Is it extra wheel speed that’s transferred to the wheel with traction? Or is it just the normal amount of wheel spin that’s transferred?
 
I usually follow IANAP with HMB (hold my beer)

I actually was a physics student long ago but I had to drop out because I couldn't do the math nor afford grad school, so I became an engineer instead. Its easy to get wrapped around the axle when talking about differentials... :rofl: rather than add my own $0.02, I found this: Open vs Locked Differential - Torque Transfer - Explained I think the kid in the video has got it essentially right.
This is exactly the video I needed to see. This answers everything. Wheel spin is transferred when brake is applied to free spinning wheel...that wheel spin is then applied to the ground in the amount it can (determined by a relationship between downforce of the vehicles weight and friction coefficient of the ground against the tire) thereby directing extra potential torque to the non spinning wheel. In lay person talk: torque is transferred across the axle when the brake is applied.

where I was wrong: this same thing happens with a locker. The locker doesn’t insist that it is 50% to each tire at all times because torque to the ground isn’t just a function of the driveshaft driving the differential...torque to the ground is a function of twist of the shaft PLUS downward force of the vehicle AND, most importantly, nonslipperiness of the ground.

Those two other variables...ground slipperiness (friction coefficient) and downward force/weight of the vehicle.. were the things I was missing from my mental model

All that said:

my lay persons understanding of what’s going on with crawl control, in all likelihood, is that the vehicle is braking all wheels and only letting small amounts of torque to each tire up to but not exceeding the amount of torque each wheel CAN transmit to the ground, given the dowsed force/weight on a tire and friction coefficient/slipperiness of the ground that the computer thinks the terrain will allow.

I.e. it gives each tire only enough juice that it can that will go forward while also keeping it from slipping. The moment it feels slip it’s says “ya nah,” and tries to give some juice to one of it’s buddies...likely also giving just enough juice to the slippery one to keep it at the same speed as the alternate tire it’s searching for to give torque to...that way you’re not dragging said slippery tire and wasting forward momentum or causing the vehicle to steer via stopped wheels as the computer blindly searches each wheel looking for the one that spins and also allows forward momentum.

what’s interesting is that whatever gyroscope that the vehicle has that is telling the vehicle that it is actually maintaining forward momentum...and not just spinning the wheels stuck in one place...must be quite advanced. It’s not like the car has eyes that it’s using to determine it’s physical location relative to the scenery.

also, this answers my question as to why crawl control seemed superior to triple and double locked vehicles in my videos I posted:

driver error. The locked drivers were exceeding the amount of torque that the slipperiness of the terrain and downward force/weight over the tires would allow...thereby causing their stickiest wheel to slip/spin without transmitting torque to the ground.

Crawl control, alternatively, was using the brakes to make sure that the stickiest tire DIDNT exceed its allowable torque, thereby allowing the vehicle to apply its potential torque to the ground without spinning a tire waywardly.

Had the triple locked Chevy driver applied less throttle, theoretically, he could have discovered the correct amount of torque that the terrain would allow and crawled up...

i suppose the point of interest there would be: what if crawl can vary the amount of torque at a quicker and more appropriate level than a human driver?

in situations such as that, crawl...or a vehicle with an automatically and partially engaging lockers combined with traction control...would be superior to the human driver with selectable lockers.

this would explain why Land Rover and others have integrated partially and automatically engaging differential locks rather than selectable lockers.
 
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This is exactly the video I needed to see. This answers everything. Wheel spin is transferred when brake is applied to free spinning wheel...that wheel spin is then applied to the ground in the amount it can (determined by a relationship between downforce of the vehicles weight and friction coefficient of the ground against the tire) thereby directing extra potential torque to the non spinning wheel. In lay person talk: torque is transferred across the axle when the brake is applied.

where I was wrong: this same thing happens with a locker. The locker doesn’t insist that it is 50% to each tire at all times because torque to the ground isn’t just a function of the driveshaft driving the differential...torque to the ground is a function of twist of the shaft PLUS downward force of the vehicle AND, most importantly, nonslipperiness of the ground.

Those two other variables...ground slipperiness (friction coefficient) and downward force/weight of the vehicle.. were the things I was missing from my mental model

All that said:

my lay persons understanding of what’s going on with crawl control, in all likelihood, is that the vehicle is braking all wheels and only letting small amounts of torque to each tire up to but not exceeding the amount of torque each wheel CAN transmit to the ground, given the dowsed force/weight on a tire and friction coefficient/slipperiness of the ground that the computer thinks the terrain will allow.

I.e. it gives each tire only enough juice that it can that will go forward while also keeping it from slipping. The moment it feels slip it’s says “ya nah,” and tries to give some juice to one of it’s buddies...likely also giving just enough juice to the slippery one to keep it at the same speed as the alternate tire it’s searching for to give torque to...that way you’re not dragging said slippery tire and wasting forward momentum or causing the vehicle to steer via stopped wheels as the computer blindly searches each wheel looking for the one that spins and also allows forward momentum.

what’s interesting is that whatever gyroscope that the vehicle has that is telling the vehicle that it is actually maintaining forward momentum...and not just spinning the wheels stuck in one place...must be quite advanced. It’s not like the car has eyes that it’s using to determine it’s physical location relative to the scenery.

also, this answers my question as to why crawl control seemed superior to triple and double locked vehicles in my videos I posted:

driver error. The locked drivers were exceeding the amount of torque that the slipperiness of the terrain and downward force/weight over the tires would allow...thereby causing their stickiest wheel to slip/spin without transmitting torque to the ground.

Crawl control, alternatively, was using the brakes to make sure that the stickiest tire DIDNT exceed its allowable torque, thereby allowing the vehicle to apply its potential torque to the ground without spinning a tire waywardly.

Had the triple locked Chevy driver applied less throttle, theoretically, he could have discovered the correct amount of torque that the terrain would allow and crawled up...

i suppose the point of interest there would be: what if crawl can vary the amount of torque at a quicker and more appropriate level than a human driver?

in situations such as that, crawl...or a vehicle with an automatically and partially engaging lockers combined with traction control...would be superior to the human driver with selectable lockers.

this would explain why Land Rover and others have integrated partially and automatically engaging differential locks rather than selectable lockers.
Based on your description of CC, it still seems to me that engaging the CDL would be limiting Crawl’s ability to do it’s best job? I guess I’m still confused as to when a locker (especially the CDL that is stock) offers a benefit versus just using MTS or Crawl with center diff unlocked, and why a locked CDL would be better?
 
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?

ok so torque isn’t transferred across the axle when brakes are applied to the free spinning wheel.

does the grounded wheel temporarily spin at twice the speed when the brake is applied to the lifted free spinning wheel? Is it extra wheel speed that’s transferred to the wheel with traction? Or is it just the normal amount of wheel spin that’s transferred?
I think I can answer all of these:

Lockers put 50% to each wheel when on the ground. If only one wheel is on the ground, they put 100% to that wheel

Open diff is similar but reversed. It puts 50% to each wheel when on the ground. If only one wheel is on the ground, it puts 100% to the wheel in the air.

With an open diff, when the brakes are applied to a spinning wheel, torque is directed to the non-spinning wheel. That one becomes the "spinning" wheel and the braked wheel becomes the non-spinning one. (Note: your brakes are far stronger than your engine, which is why people can do "brake stands" where you stand on the brakes and gas the engine and the vehicle doesn't move. It's also why the unbraked wheel will end up being the one that is free-spinning and gets all the power).

Torque has nothing to do with the speed of the wheel. That's why in a locked diff both wheels spin at the same speed but all the torque only goes to the one which has traction. Remember, torque is the twisting force. It's not how fast you spin a screwdriver, that's horsepower. Torque is how you turn a *really* tough screw.

this answers my question as to why crawl control seemed superior to triple and double locked vehicles in my videos I posted:

driver error. The locked drivers were exceeding the amount of torque that the slipperiness of the terrain and downward force/weight over the tires would allow...thereby causing their stickiest wheel to slip/spin without transmitting torque to the ground.

Crawl control, alternatively, was using the brakes to make sure that the stickiest tire DIDNT exceed its allowable torque, thereby allowing the vehicle to apply its potential torque to the ground without spinning a tire waywardly.

Precisely. This is why CC will often get people out of tough spots that lockers don't. The ECU can brake/unbrake the tires and modulate the throttle right up to the point of traction loss better than humans can. In many situations even a very skilled driver would have trouble accomplishing what CC can do.

IMO lockers are best when you're going into an obstacle (mud, rocky hill, etc) and you want to start off with the max traction. CC and MTS will approach those obstacles assuming you have traction and will adjust wheel speed to compensate when you don't (or when CC senses you're not moving along as expected). If you use lockers correctly you may not need CC/MTS assistance at all.

Why did Toyota opt for CC/MTS but not lockers in our trucks? Quite frankly it's cost and capability. Lockers cost $, whereas using the brakes to modulate wheel spin to gain traction is basically free (aside from the cost of the ECU). Plus CC/MTS can achieve 99% to 101% of what skilled humans can. And most drivers are just not that skilled (or would even know when they should/shouldn't use lockers), so in those cases CC/MTS is way better than they are.

Side note: CC is cool, but lockers are more fun. I've used CC on some tough-looking rocky obstacles, and it just ate up the terrain. Just pointing the wheel forward and letting it do it's thing was quite frankly, BORING. Modulating the throttle and brake myself is a rewarding challenge.
 
Based on your description of CC, it still seems to me that engaging the CDL would be limiting Crawl’s ability to do it’s best job? I guess I’m still confused as to when a locker (especially the CDL that is stock) offers a benefit versus just using MTS or Crawl with center diff unlocked, and why a locked CDL would be better?
Theoretically I would tend to agree.

Also at issue is that, technically, the center diff is a torsen...meaning even if you don’t engage it via the “locking feature,” there is only a certain amount of slip that it will allow before it “automatically” locks itself at least somewhat...unless I misunderstand the nature of our torsen center differential.

I’m not sure how much slip our torsen center diff allows before engaging, but, hypothetically, your vehicle is, unlike part time 4wd systems, ALWAYS capable of 4wd...and always capable of locking its own diff-even if you lose the electronic ability to engage those features manually. This is a plus for the land cruiser.

I wish it had a manual lever for engaging low range, rather than electronic. That way you’d ALWAYS have 4wd, low, and a locking center diff, even if your electronics failed you.

anyone aware of a “hack” to go under the vehicle and manually engage or disengage low range?
 
I think I can answer all of these:

Lockers put 50% to each wheel when on the ground. If only one wheel is on the ground, they put 100% to that wheel

Open diff is similar but reversed. It puts 50% to each wheel when on the ground. If only one wheel is on the ground, it puts 100% to the wheel in the air.

With an open diff, when the brakes are applied to a spinning wheel, torque is directed to the non-spinning wheel. That one becomes the "spinning" wheel and the braked wheel becomes the non-spinning one. (Note: your brakes are far stronger than your engine, which is why people can do "brake stands" where you stand on the brakes and gas the engine and the vehicle doesn't move. It's also why the unbraked wheel will end up being the one that is free-spinning and gets all the power).

Torque has nothing to do with the speed of the wheel. That's why in a locked diff both wheels spin at the same speed but all the torque only goes to the one which has traction. Remember, torque is the twisting force. It's not how fast you spin a screwdriver, that's horsepower. Torque is how you turn a *really* tough screw.



Precisely. This is why CC will often get people out of tough spots that lockers don't. The ECU can brake/unbrake the tires and modulate the throttle right up to the point of traction loss better than humans can. In many situations even a very skilled driver would have trouble accomplishing what CC can do.

IMO lockers are best when you're going into an obstacle (mud, rocky hill, etc) and you want to start off with the max traction. CC and MTS will approach those obstacles assuming you have traction and will adjust wheel speed to compensate when you don't (or when CC senses you're not moving along as expected). If you use lockers correctly you may not need CC/MTS assistance at all.

Why did Toyota opt for CC/MTS but not lockers in our trucks? Quite frankly it's cost and capability. Lockers cost $, whereas using the brakes to modulate wheel spin to gain traction is basically free (aside from the cost of the ECU). Plus CC/MTS can achieve 99% to 101% of what skilled humans can. And most drivers are just not that skilled (or would even know when they should/shouldn't use lockers), so in those cases CC/MTS is way better than they are.

Side note: CC is cool, but lockers are more fun. I've used CC on some tough-looking rocky obstacles, and it just ate up the terrain. Just pointing the wheel forward and letting it do it's thing was quite frankly, BORING. Modulating the throttle and brake myself is a rewarding challenge.
Solid breakdown.
 
Not sure that’s right. My understanding was 60/40 rear bias in 4hi, 50/50 in 4lo OR when CDL is locked. Can someone verify?
4Hi or 4Lo, it's definitely only 50/50 when the center diff is locked.

I thought it was 70/30 but it could be 60/40. I saw a Toyota brochure saying the Prado is 70/30 rear bias, but I don't see anything detailing the 200-series in a quick search.
 
Like a locking axle diff it can be a more direct mechanical linkage and not need to get to the point of slip to do it’s job.

Little bit of a tangent, but I personally feel many of us are too quick to engage the CDL given the reality that it causes binding between the front and rear axles when turning tightly, and how capable the truck is with it off. If we run 4Lo and keep CDL off until we need it we can turn tighter and put less strain on the driveline. Then when we approach a difficult obstacle it is just a button press away. Defaulting to off means we don’t need to wait for it to disengage before approaching a tight turn, but conversely there is usually no issue getting it to engage quickly.

This doesn’t apply to deep sand and mud, for the record. I’m thinking of the stuff I usually spend time on in Colorado.
To add to this technically running without CDL we aren't in 4wd but awd hi or lo. Is the bind really that drastic to warrant not having it engaged though? Especially since the higher mileage 200s tend to have a mind of their own when trying to engage CDL and don't always engage when asked. So for certain terrain it might be smart to engage ahead of time if you know you'll need it (IMO).
I do agree though that our rigs are more than capable with CDL not engaged and just letting ATRAC do its thing. I've been on trail rides where CDL wouldn't engage and I didn't have too many issues because the computer was smart enough to keep traction where I needed it.
 
It's 70/30 rear/front bias
I’m curious what differential in percentage outside the 70/30 (or 60/40) specification the torsen will allow before it begins to bind and lock.

anyone know?
 

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