80 in ice & snow + question re CDL

[IdahoDoug]

I like Walking Eagle's first paragraph. Dead on and I think sums up the tradeoff. When turning (even on large radius highway curves), each tire follows a different line and they need to turn slightly differently. A locked CDL will force slippage in this instance, no matter how slight. On low friction surfaces like snow/ice this slight slippage matters a lot as you already have compromised traction. Just another way of saying what WE said and his paragraph is pretty much my policy in the absence of any compelling factor to change it.

DougM

 

[SUMOTOY]

Assume the driver panics and slams the brake to the floor. All models except ABS fail instantly.

Every single example you have given ignores wheel lockup and it's inherent loss of control. With the CDL on, wheel lockup occurs later than it does in a non-ABS no-CDL vehicle. However, when it does occur in the CDL example it is more sudden as all 4 wheels slide at once. Without the CDL the typical vehicle will lock the rear wheels first, giving the driver time to correct the pressure before the front end slides.

A locked wheel has no directional stability. A vehicle with all 4 wheels locked (CDL or non-ABS) will move/slide/crash in the direction of it's greatest momentum.

Two things to consider... First, every vehicle manufacturer wants fronts to lock first, that's why you have a proportioning valve without ABS and select low with ABS. Understeer.

Second, statistics.... prior to ABS, single vehicle avoidance accidents were 50% *lower*, and multivehicle crashes were only 14% higher.

A locked wheel has not directional stability. Statistics seem to indicate this isn't a significant factor to Vehicle Accidents or Accident Avoidance Maneuvers.

ST

ST

 

[SUMOTOY]

I like Walking Eagle's first paragraph. Dead on and I think sums up the tradeoff. When turning (even on large radius highway curves), each tire follows a different line and they need to turn slightly differently. A locked CDL will force slippage in this instance, no matter how slight. On low friction surfaces like snow/ice this slight slippage matters a lot as you already have compromised traction. Just another way of saying what WE said and his paragraph is pretty much my policy in the absence of any compelling factor to change it.

DougM

Forced tire slip isn't automatically to the front axle. In fact, physics would dictate that forced slip during turns goes to rear wheel first. In a turn, the rear driveshaft is attempting to turn slower. This causes engine/brake torque allocation to go rearward, up to tire spin. This allows better steering control up to wheel spin.

I might point out that all tires "slip", always. Wheel spin is 100% slip, and that's when equal driveshaft *speed* causes the front tires to have more understeer properties in AWD chassis dynamics.

A lot of successful racing efforts in AWD vehicles with the center diff locked (as in welded) solid, on dry pavement. That's driving at the limit of adhesion, or close to it, always. All other chassis dynamics are optimized. We need to think of that as we speak to CDL on in dry pavement. Brake forces are closer to ideal up to lockup, and handling dynamics are better up to the limit of adhesion.

Much of these discussions want to focus 'after' the limit of adhesion is exceeded. That defines loss of control IMO. After the limit of adhesion ABS "might" help bring a driver back. Not much supporting evidence in the real world to support that ideal.

YMMV

ST

 

[Walking Eagle]

I don't agree with this assessment.

That's a shocker.

Back up to the axles. If you turn, the torque allocation goes to the rear, up to wheel spin, so forcing tire slip isn't necessarily going to cause loss of steering. In fact, it allows more steering up to wheel spin.

You do realize that with the center differential locked, that there isn't more torque allocation going to the rear don't you? I didn't say which tire would slip did I? A tire has to slip. It's usually the front that pushes in a corner with CDL locked (classic 4wd). But, I'll give you that theoretically, since the tires have to slip with CDL in a corner, it could be a rear tire that loses traction. Lost traction is lost control. The more traction you lose, the more control you loose. Any loss due to the fact that the front and rear travel different paths means there is that much less for steering, accelerating and braking, or if the rears are the ones slipping, lateral stability, accelerating and braking.

The dynamics of differentials and vehicle dynamics change when you compare front, rear and center..

It's the same type of deal, the slipping and not slipping and locked and not locked, and different paths of front tires, and left and right tires. Not identical, not interchangable, but simular, and therefor same type of deal.

Not true, see Ideal Brake Force Distribution test.

I don't feel like looking up your ideal brake force dist. test. Don't need to, it's simple. You have 4 tires. 4 Contact patches. 4 places for traction. The quickest you can stop is to have all 4 at the very edge of locking up. If you put it in CDL or 'normal' 4wd, you have not changed those contact patches, the number of tires, or the amount of traction those 4 contact patches give. So you haven't increased the ability to stop.

Not true, see Split mu chart, CDL steers better than unlocked..

Again, not going to look up your split mu chart. 'normal' 4wd CDL does not steer better. Go to a gravel lot. Drive in a circle with the steering wheel locked. Then do the same thing with CDL on. The circle will be larger cause the front is 'pushing'. This is pretty common knowledge to anyone with any 4wd experience.

That *can* happen, it's not inherent to locking a center differential.

ST

That DOES happen. See it all the time. And no, the car doesn't magically control the driver, but you see it all the time. I'm guessing you're one of 'those' people.

 

[Walking Eagle]

I like Walking Eagle's first paragraph. Dead on and I think sums up the tradeoff. DougM

Thank you.

 

[Walking Eagle]

Two things to consider... First, every vehicle manufacturer wants fronts to lock first, that's why you have a proportioning valve without ABS and select low with ABS. Understeer.

hummm..... could the proportioning valve be there to provide more braking force to the front cause that's where all the weight is going? Could that be why disks were added to front first?

Second, statistics.... prior to ABS, single vehicle avoidance accidents were 50% *lower*, and multivehicle crashes were only 14% higher

A locked wheel has not directional stability. Statistics seem to indicate this isn't a significant factor to Vehicle Accidents or Accident Avoidance Maneuvers.

ST

ST

"He uses statistics as a drunken man uses lamp-posts... For support rather
than illumination."
-- Andrew Lang (1844-1912)

Don't suppose the added speed limits might have effected it? Higher HP FWD cars? People driving faster cause 'ABS will save me'. I remember when ABS first came out, they required lables on the back of cars with it in Germany cause rear endings went up as the guy in front with ABS out braked the guy behind him.

 

[SUMOTOY]

hummm..... could the proportioning valve be there to provide more braking force to the front cause that's where all the weight is going? Could that be why disks were added to front first?

The proportioning valve is to reduce braking to the rear. That doesn't mean it 'adds' brake force to the front.

Don't suppose the added speed limits might have effected it? Higher HP FWD cars? People driving faster cause 'ABS will save me'. I remember when ABS first came out, they required lables on the back of cars with it in Germany cause rear endings went up as the guy in front with ABS out braked the guy behind him.

The problem with rollovers, side impact, and fatalities is, they don't lie, they are history. I put it all more simply, ABS <not equal to> Vehicle Control. You can try to assign other reasons, the SAE studies I referenced didn't attribute ABS = Vehicle Control, quite the contrary.

ST

 

[SUMOTOY]

You do realize that with the center differential locked, that there isn't more torque allocation going to the rear don't you? I didn't say which tire would slip did I? A tire has to slip. It's usually the front that pushes in a corner with CDL locked (classic 4wd).

With the center locked, all braking force follows weight transfer. Period. It's not usually the front that slips, it's the rear that slips due to turning radius causing torque allocation rear.

It's the same type of deal, the slipping and not slipping and locked and not locked, and different paths of front tires, and left and right tires. Not identical, not interchangable, but simular, and therefor same type of deal.

We can agree to disagree. When you put a locker in the center, they aren't the same type of deal. When you put a locker in the front, they aren't the same type of deal. Vehicle dynamics is much more complex than that.

I don't feel like looking up your ideal brake force dist. test. Don't need to, it's simple. You have 4 tires. 4 Contact patches. 4 places for traction. The quickest you can stop is to have all 4 at the very edge of locking up. If you put it in CDL or 'normal' 4wd, you have not changed those contact patches, the number of tires, or the amount of traction those 4 contact patches give. So you haven't increased the ability to stop.

That's just plain not true, read the post. In CDL on mode, all brake force follows exactly weight distribution. Period. In CDL off mode, all brake force follows the amount of brake force at the front and the amount of brake force to the rear allocated by the prop valve. You exactly *have* increased the ability to stop.

Again, not going to look up your split mu chart. 'normal' 4wd CDL does not steer better. Go to a gravel lot. Drive in a circle with the steering wheel locked. Then do the same thing with CDL on. The circle will be larger cause the front is 'pushing'. This is pretty common knowledge to anyone with any 4wd experience.

The axles must travel at the same speed, that creates understeer. A big jump to say understeer hinders the ability to steer better.


ST

 

[Walking Eagle]

The proportioning valve is to reduce braking to the rear. That doesn't mean it 'adds' brake force to the front.

ST

Where did I say that it 'adds' brake force to the front. It provides more. aka it gives a greater PROPORTION to the front. Cause that's where all the weight is transfered to durring braking. Duh!

 

[Jeremiaheaster]

ST, thanks for the facts. I for one appreciate them. (of course it helps that I agree with you )

 

[firetruck41]

Second, statistics.... prior to ABS, single vehicle avoidance accidents were 50% *lower*, and multivehicle crashes were only 14% higher.

A locked wheel has not directional stability. Statistics seem to indicate this isn't a significant factor to Vehicle Accidents or Accident Avoidance Maneuvers.

ST

ST

As I pointed out in the other ABS post, this is not correct and not supported by the NHTSA which has been tasked to study ABS. Below is a quote from the abstract of an NHTSA ABS study from 2000, this was a revision to the 1998 study, that ST may be referring to.

ABS still seems to have a beneficial effect in preventing each crash type except for side impacts, where it is appears to be associated with a higher response rate especially for passenger cars. However, it appears to be beneficial in preventing pedestrian crashes, rollovers, run-off-road crashes and frontal crashes with another moving vehicle.

The previous study indicated several disbenefits in fatal crashes. The only statistically significant one remaining is rollovers of LTVs.

and another quote from the abstract of another NHTSA study:


The braking performance of nine high production passenger vehicles was evaluated in eighteen stopping situations. These situations were comprised of various road surfaces, driver steering actions, and vehicle speeds. Testing was performed with lightly and heavily laden vehicles, with ABS active and disabled, and used two brake pedal application techniques. The selected vehicles included at least one ABS from each of the eight current , major, ABS manufacturers.

This study found that for most stopping maneuvers on most surfaces, ABS-assisted full pedal brake application stops were shorter than those made with the ABS disabled. The one systematic exception was on loose gravel where stopping distances increased by an average of 27.2 percent overall. Additionally, the vehicular stability during testing was almost always superior with the assistance of ABS. For the cases in which instability was observed, ABS was not deemed responsible for its occurrence.

Based on results to date, the authors of this study believe ABS braking performance deficiencies are not responsible for the apparent increase in ABS-equipped, single-vehicle, run-off-the-road crashes.

 

[SUMOTOY]

Nothing has changed in the studies in reality, just claims that ABS is not 'responsible' for the crashes noted.

http://www-nrd.nhtsa.dot.gov/vrtc/ca/capubs/lvabstask1_crashdatareport.pdf

See tabse 4 and see table 5. Note they are logging *changes* (that's *changes*) from the 98 study. There are no significant ones, other than the INCREASE in LTV rollovers.

Note, these studies only compare ABS to non ABS. There is no comparo of ABS to non ABS with CDL.

ST

 

[firetruck41]

Nothing has changed in the studies in reality, just claims that ABS is not 'responsible' for the crashes noted.

http://www-nrd.nhtsa.dot.gov/vrtc/ca/capubs/lvabstask1_crashdatareport.pdf

See tabse 4 and see table 5. Note they are logging *changes* (that's *changes*) from the 98 study. There are no significant ones, other than the INCREASE in LTV rollovers.

You've had no problem using conclusions from the 1998 study, which has been revised and no longer supports your view. I don't have any problem drawing some conclusions, as all the other studies they have done seem to be showing a benefit to ABS, as well.

To me it is surprising they came to the conclusion they did in 98, not the 2000 conclusion.

Note, these studies only compare ABS to non ABS. There is no comparo of ABS to non ABS with CDL.

ST

Big deal, I wouldn't expect NHTSA to test a condition that less than 1% of vehicles will, or are even able to) employ.

 

[TCH2FLY]

It's the same type of deal as running a locker in the rear of a RWD car. If one wheel is on dry and one on wet, it's going to want to spin the car. Only now you're doing that front to back. Open diff you may not move at all, as the tire on ice spins wildly and the one on dry pavement just sits, but you'll stay straight.

Not accurate, I drove a RWD car with a spool in the back on the street as my DD for a year and a half and the tire with the GREATEST resistance (traction) prevents the tire with least resistance from rotating any faster and so there is nothing to cause the car to want to "spinout". Both tires need to lose adhesion before the backend will come out (excess throttle on dry pavement or both tires on slick surface) Does the rear locker in a LC cause the truck to "spin" when applied??

Where did I say that it 'adds' brake force to the front. It provides more. aka it gives a greater PROPORTION to the front. Cause that's where all the weight is transfered to durring braking. Duh!

You mistaken....the valve REDUCES the proportion of braking to the rear to avoid lock-up on the unloaded axle as the weight shifts forward not to increase or give "more" front braking and to re-quote you.... DUH.
(I realize you may see this as a glass 1/2 full or 1/2 empty answer but it isn't)

I don't defend ST's posts because he can tend to come off as arrogant but he has a lot of knowledge in this area. You need to get some facts straight before you attack him.

 

[Walking Eagle]

The axles must travel at the same speed, that creates understeer. A big jump to say understeer hinders the ability to steer better.


ST

Well, after you've seen a few 4wd vehicles go straight, when they mean to be turning (including my 40 once or twice when I was young), that jump isn't so big.

Sumo - as always, you'll be right in one and only one place, your mind.

 

[Walking Eagle]

As far as disabling ABS goes, and using CDL in snow and ice - If you think you can out think and out respond the computer - by all means, run with your CDL on at hwy speeds. You're allowed your opinion.

 

[Walking Eagle]

Not accurate, I drove a RWD car with a spool in the back on the street as my DD for a year and a half and the tire with the GREATEST resistance (traction) prevents the tire with least resistance from rotating any faster and so there is nothing to cause the car to want to "spinout". Both tires need to lose adhesion before the backend will come out (excess throttle on dry pavement or both tires on slick surface) Does the rear locker in a LC cause the truck to "spin" when applied?? .

Well, if one wheel is getting traction, and the other isn't, that one wheel is going to push that side of the truck/car, and the other side isn't doing anything (assuming glare ice). If you have a force on the left and no force on the right, it's going to cause a moment (torque) about the center of the axle, and want to twist the car. How much depends on a bunch of factors, like is the other tire really not providing any force, are the front wheels on dry or ice, if dry they may have enough traction to counteract this force (sum of moments=0), does the axle twist enough to make it effectively countersteer the vehicle and on to infinity. Get your wheels spinning good on the ice, and let one hit pavement with the other still on ice, you're going to get a jerk to the ice side (aka, right tire hits pavement, car is going to jerk left). If you have the rear locker in a LC on in this situation, maybe, depends on what the front end is doing.

You mistaken....the valve REDUCES the proportion of braking to the rear to avoid lock-up on the unloaded axle as the weight shifts forward not to increase or give "more" front braking and to re-quote you.... DUH.
(I realize you may see this as a glass 1/2 full or 1/2 empty answer but it isn't).

How about we phrase it this way? 'The Proportion valve controls the proportion going to the front and back brakes.' Cause it also increases the pressure to the rear when the vehicle is heavily loaded. Either way, hitting the CDL switch doesn't change the four little contact patches that are going to stop you!

I don't defend ST's posts because he can tend to come off as arrogant but he has a lot of knowledge in this area. You need to get some facts straight before you attack him.

You are correct that he comes off as arrogant. And he will not admit he's wrong, even if he is. That has been proven more than once on this forum. And in my whinnyist 4 year old voice 'He attacked me first'. And I'd suggest he get his facts straight. Firetruck has presented quotes (aka facts) from NHTSA to debunk everything he's said. As far as these 'facts' he's presented

Originally Posted by SUMOTOY
Second, statistics.... prior to ABS, single vehicle avoidance accidents were 50% *lower*, and multivehicle crashes were only 14% higher

They're not facts at all, they're statistics, and statistics are dangerous. What makes them dangerous is how the person is using them. You can use the same data to show many things if you jack with them. What he has here is a classic 'Ice-cream causes shark attacks' analysis. In June, July and August there is an increase in Ice-cream sales at the beach. Also in June July and August there is an increase in Shark attacks at the beach. Obviously, the increased ice-cream sales is causing the sharks to attack!!! Stop selling ice-cream this instant!!! It's correlation without causation. The real cause of both more ice-cream sales and shark attacks is that there are more people at the beach and in the water in June, July and August. Prior to ABS single vehicle avoidance accidents were 50% lower than after ABS. What else could have changed in that time frame? Seems to me, when ABS was first coming out in the 90's, also happens to correspond to the time that SUV and truck sales went up - minivans were also at their peak. Think heavy poorly handling SUV's in the hands of soccer moms with a false sense of security driving too fast might cause more single vehicle avoidance accidents to go up? As much as we love our SUV's, they don't handle as good as your average Honda Accord. They don't stop as quick either. So, if I'm trying to avoid a crash, I have a better chance in the better handling better stopping 4-dr sedan. The number of SUV's on the road goes up, so does the number of times a person can't stop or turn quick enough.

This is why I say he used statistics like a drunk uses a lamp-post.

Just to make this really painfull, lets go back to my first post and see what facts are there:

'You're not going to get a difinitive answer - only opinions.'

This one certainly is being proven in this thread!

'I run CDL off on the hwy in ice and snow.'

Yep, I can confirm that as fact, been doing it alot in Tulsa this past week.

'Steers much better.'

If we assume that by better, I ment that it is better for the front end not to push as much, that we've agreed on that.

'CDL locked makes it want to go straight, and only straight.'

Yep, pretty factual on that one.

'The whole reason you're not supposed to run CDL on dry pavement is it won't allow the wheels to slip when turning right?'

Wording might be a little weak, but true. Turn on dry pavement with CDL locked, or in your RWD with a spool, you'll hear the tires 'bark' as they are forced to loose traction and skid / slip.

'Well, if you run it in the snow / ice and turn, the CDL is going to make the tires slip.'

yep, no different than the barking on dry pavement, or the skidding you'll hear in gravel.

'Your tire only has so much traction that can be used for stopping accelerating and turning. '

Yep, it's a fact that any tire can only provice so much traction. That's why there has been a return to RWD in many cars, you can't get the front tires on a Dodge Magnum to support the vehicle, steer the vehicle, and take the torque of a Hemi. Rear tires support weight and push, and front support weight and steer, better balance of duties.


'If you're forcing them to slip some just to keep the drivetrain from binding, you're loosing some of that traction that could be used for turning, stopping and accelerating. '

See above - only so much traction is there, and you're loosing some forcing the tires to spin - sure looks like one and one makes two.


'Unless you're spinning tires easily from accelerating, I don't see any need to hit the CDL button.'

Yep, it's a fact, don't see a need.

'And if you are in slippery enough conditions to use the CDL, you shouldn't be running at highway speeds.'

Perhaps not perfectly clear, but implied that this is also 'what I see' or what my oppinion is, and it's a fact that I don't think you should be running hwy speeds if it's so slippery you need the CDL to get going.

Seems like my facts are pretty solid. Better than statistics that are recited without context anyway.

 

[TCH2FLY]

How much depends on a bunch of factors, like is the other tire really not providing any force, are the front wheels on dry or ice, if dry they may have enough traction to counteract this force (sum of moments=0), does the axle twist enough to make it effectively countersteer the vehicle and on to infinity.

Agreed, I only disgree with the simple statement in your earlier post.

Either way, hitting the CDL switch doesn't change the four little contact patches that are going to stop you!

I have never disagreed with you on that subject , the first paragraph of your initial post was right on track. I know what feels best to me....I couldn't care less about the physics involved.

 

[Jeremiaheaster]

Think heavy poorly handling SUV's in the hands of soccer moms with a false sense of security driving too fast

Maybe your facts are solid but you sound sexist. Aren't men statistically responsible for more accidents, lol.

I wish I knew as much as you guys about the chasis dynamics so I could actually participate meaningfully in this conversation. But at least humor me with answering this question:

When I am starting to spin out (wheels spinning/car turning away from direction of travel) in in my LC while in standard drive I find it difficult to right that motion.
When I am in 4lo (CDL engaged) righting the spinout is as simple as pointing the front wheels in the direction of intended travel and punching the gas. The LC straightens right out.
If CDL reduces traction and therefore control, then why do I find it easier to control the vehicle with CDL engaged in a loss of control situation?

 

[KliersLC]

Maybe your facts are solid but you sound sexist. Aren't men statistically responsible for more accidents, lol.

I wish I knew as much as you guys about the chasis dynamics so I could actually participate meaningfully in this conversation. But at least humor me with answering this question:

When I am starting to spin out (wheels spinning/car turning away from direction of travel) in in my LC while in standard drive I find it difficult to right that motion.
When I am in 4lo (CDL engaged) righting the spinout is as simple as pointing the front wheels in the direction of intended travel and punching the gas. The LC straightens right out.
If CDL reduces traction and therefore control, then why do I find it easier to control the vehicle with CDL engaged in a loss of control situation?

They are talking about braking...