CDL Locked Snow on Road "No ABS"? (1 Viewer)

[SUMOTOY]

Kurt, you should just read Sumotoy's epic tome on the subject if you have 8 or 9 hours to kill...

LOL! It shouldn't take that long, but I have more information if needed. That said, looking at what you wrote below, I'm concerned my drivel didn't help much....

Seriously though, at least for me, having the center VSC active (unlocked) and the ABS active is the best for on-road snow and ice conditions. You can lock the CDL and still have your ABS if you do the "trick" where you drive straight at ~30 mph for a 1/4 mile or so. I guess I like the ABS on the street cause I like to stop in a straight line regardless if it takes a greater distance. Regardless, a good set of snow & ice tires make a huge difference.

PROBLEM to your "trick". CDL engaged and ABS engaged is the most dangerous state of driving in any awd vehicle, and creates the biggest potential loss of control. With the CDL locked, the condition and state of the LSPV is irrelevent, since brake force will travel through the locked driveshafts. With CDL on/ABS on: The driveshafts are locked, and if ABS engages any wheel, those abs pulses will travel through the driveshaft to the apposing axle wheels, which will then cause the ABS to re-engage. With all this happening, driver control under braking is gone, and your best hope is dumb luck. There isn't a single AWD vehicle that has ABS active in a center differential=locked state.

To the OP, until you 'fix' the LSPV, CDL locked will give you the best braking for what you have. Without CDL locked, the LSPV on a lifted truck is the same as running totally unloaded, so you have the least amount of rear brake proportioning. With the fronts doing all the work, expect to have them lock up first. Add the adapter plate to the LSPV mounting at the axle, you can do this with home depot parts for less than 5 bucks, btdt on my lifted 4R years ago.

In the meantime, if ABS is running you afoul, until you fix it, pull the relay and/or the fuse. An intermittent 'ABS active' state is not safe to you or others. I won't comment on why you don't fix what you have, only explain what can happen if you don't

A lot of misconceptions on how CDL affects braking, but in a straight line with front/rear driveshafts locked, you have ideal brake force distribution impending lockup (because brake force exactly follows weight transfer with a locked center diff). Read: The shortest stopping distance in a straight line will come from CDL locked.

ABS only gives driver control under brake-avoidance maneuvers as only one here pointed out. FYI in the late 80's and early 90's, Audi center locker awd included a ABS disable switch on the dash in open center diff mode... As gravel, pebbles, slush , snow and other loose materials will cause longer stopping distances with abs than a wheel under lockup (the buildup of material actually stops the vehicle shorter).

Aside... We just had our first major snow in Chicago yesterday. Last night I went out to my favorite abandoned industrial park and worked with rally-tossing the 80 (Blizzack DMV1) with both the CDL on(no abs) and CDL off (with abs). I gave up after about 1/2 hour, as even with the supercharger, it took a lot to throw the truck out of sorts on purpose =- got bored. And, the CDL-on(abs off) gave better dynamic handling in all scenarios last night, by a large margin.

HTH, IMO, IME, and my .02

Scott J
94 FZJ80 Supercharged
Eventmaster and Senior Instructor: Steamboat Winter AWD High Performance Driving Experience 1999-2012

 

[clownmidget]

Scott, it did certainly take you long enough!

But your reply seems to misinterpret my post by referring to choosing to engage the CDL and keep the ABS as mine. I was simply informing the OP that one could decide to do so if they wished. Nothing more. I already posted that I do better with the CDL open and ABS active on public roads when ice & snow are the issue.

 

[SUMOTOY]

Scott, it did certainly take you long enough!

But your reply seems to misinterpret my post by referring to choosing to engage the CDL and keep the ABS as mine. I was simply informing the OP that one could decide to do so if they wished. Nothing more. I already posted that I do better with the CDL open and ABS active on public roads when ice & snow are the issue.

Bottom Line: ABS Active with CDL Engaged should never be driven on the street, not even as a test. If you engage the CDL and keep the ABS you have tricked the ABS into thinking it can operate as a 3channel/4 channel abs. By definition, ABS Active with CDL Engaged creates a "single" channel abs system... until the effect of the first pulse through the attached locked driveshaft causes a reaction from the other 2/3 channels.

My best hope was I explained why CDL w/ABS creates a major safety problem, not a preference

Cheers

Scott J
94 FZJ80 Supercharged

 

[Grench]

Bottom Line: ABS Active with CDL Engaged should never be driven on the street, not even as a test. If you engage the CDL and keep the ABS you have tricked the ABS into thinking it can operate as a 3channel/4 channel abs. By definition, ABS Active with CDL Engaged creates a "single" channel abs system... until the effect of the first pulse through the attached locked driveshaft causes a reaction from the other 2/3 channels.

My best hope was I explained why CDL w/ABS creates a major safety problem, not a preference

Cheers

Scott J
94 FZJ80 Supercharged

Anyone who has read the other threads knows that Scott and I don't agree on much. In this case he's right though. You should never try to drive with the center locked and the ABS active/ready. It is dangerous.

 

[clownmidget]

I actually think I got this wrong - the "trick" is to have the CDL off (VSC active) while having the ABS off.

From my memory of the old 80scool stuff - having the center diff "open" you can enjoy the AWD nature of the truck, and with the ABS off you can then power slide it into corners.

Sorry for the confusion. Carry on.

 

[SUMOTOY]

For more reading on CDL and ABS, this is a good summary
http://4x4abc.com/4WD101/ABS_parttime.html

In my 20+ years of teaching awd performance driving and awd chassis dynamics study, the biggest obstacle to education and technique, usually can be traced to misunderstanding how awd systems work. With so many technical terms now interchanged, and more 'active' differential technology introduced, it's easy to get confused. But certainly ABS active with a center differential locked defines dangerous = not an opinion.

And I will put forth that IMO/E tire technology has advanced enough to be the biggest variable in driver dynamic control of the FZJ80 in winter conditions.

Scott J
94 FZJ80 Supercharged

 

[landtoy80]

Has anyone ever tested how much wheel spin either from the front or rear, with the CDL unlocked, there is before the VC locks up? Say hook up a strap to something stationary and try to drive on ice or snow.

I was on a iced over bridge and going about 5 mph or less. The bridge was banked. I was trying to get in the other lane as a sports car hit the bridge. I was at about a 45 deg angle trying to get in the other lane but I just keep driving forward at about a 45 deg angle (drifting at 5 mph). I ended up locking the CDL and got into the other lane. I have not thought about it till now that maybe at such a slow speed the rear wheels may of been spinning and not the front wheels do to the VC not locking up.

 

[Ebag333]

Low speed the VC won't be inclined to lock up. If it was, then we'd have problems with it engaging on the trail when running open diffs (for those who have done the 7 pin mod). Since it's not an issue with the VC engaging at low speeds, it's unlikely that it would have engaged in your scenario either.

You can goose the throttle to get the VC to engage (force wheel spin), but that is likely to get more force/momentum than you would like in that scenario as it takes quite a bit of throttle. Better in that scenario to lock the CDL as you did, that's where having a button comes in handy.

 

[SUMOTOY]

I actually think I got this wrong - the "trick" is to have the CDL off (VSC active) while having the ABS off.

Turn the CDL on above 30mph, ABS goes off.... Turn the CDL back off above ~30mph, the ABS will stay off until the next restart cycle of the truck.

From my memory of the old 80scool stuff - having the center diff "open" you can enjoy the AWD nature of the truck, and with the ABS off you can then power slide it into corners.
Sorry for the confusion. Carry on.

More confused. With the ABS *on* or *off* you can power slide the truck into corners. In fact, with CDL *on* or *off* you can power slide the truck into corners. Read: A power slide is not an ABS or CDL variable.

Has anyone ever tested how much wheel spin either from the front or rear, with the CDL unlocked, there is before the VC locks up? Say hook up a strap to something stationary and try to drive on ice or snow.

I was on a iced over bridge and going about 5 mph or less. The bridge was banked. I was trying to get in the other lane as a sports car hit the bridge. I was at about a 45 deg angle trying to get in the other lane but I just keep driving forward at about a 45 deg angle (drifting at 5 mph). I ended up locking the CDL and got into the other lane. I have not thought about it till now that maybe at such a slow speed the rear wheels may of been spinning and not the front wheels do to the VC not locking up.

4 conditions must be present for VC lockup *progression*. Speed differential of 2 attached axles, heat (viscosity), applied torque (engine), and allowed slip (VC). At 5mph, speed differential is minimal, heat then is minimal, applied torque is minimal, so allowed slip (between two axles) is maximum.

The VC is always preloaded, usually at 5%, so technically it was engaged, and likely over 5% in your scenario.. The VC will start to lockup at any speed differential, but at 5mph, much less than just locking the center. That said, as you describe your scenario, remember a VC will uncouple if all wheels are spinning at the same rate. VC can't identify/isolate a traction problem, it reacts only to attached axle speed differential.

I don't agree with any statement that a VC is not engaged while wheeling, as it's always engaged. Even when the CDL is locked, the VC is engaged. The CDL only assures that the VC always 'sees' equal attached axle speeds. But if the VC is preloaded 5%, it's engaged, slipping, heating, and preloading torque even with CDL *on*.

HTH

Scott J
94 FZJ80 Supercharged

 

[clownmidget]

Jezzus Scott you come off so pompous in nearly every one of your posts it's really hard not to take the thread to the Chit-Chat section.

Yeah, I knew how to disable the ABS before you ever owned a truck, so thanks for taking the time to teach me something...

Braking hard before entering a turn, locking the tires up in the process, throwing the truck into a sideways slide, then once in the turn, hitting the throttle hard to power out of the turn. That's pretty much what I've heard folks refer to as a "power slide". I'm not a race car driver or pretend to be one. Having the ABS on makes that a whole lot more difficult. That seems pretty obvious to me.

I gotta say on a personal note, I don't really find much of what you post with regards to this thread to be worth much. Your only saving grace is that you always tend to end it with "in my opinion" or "experience". Regardless it's always still very preachy in tone. So you've driven around on flat tracks and parking lots. IME, once you enter very steep ascents/descents and throw in some off-camber situations to boot, with some kind of serious consequence if you leave the confines of the road you can appreciate ABS and what it does better than old-school experience with playing with the brake pedal.

Kurt, good luck with driving this winter in Colorado.

 

[landtoy80]

Driving the 80 in CO winter is nothing like trying to stop a semi with an empty trailer on slick snow/ice. What makes thing worse is ABS in tracker but no abs on trailer. The trailer brakes always lock up.

What I do notice here in CO is that with the use of magcloride (sp) the roads are more slick than when they used ground up granite. Yes no more hood dings and chipped windshields but driving on hard packed snow was not an issue back before they started treating roads with the mag.

How does tire pressure affect snow/icy roads? Is more or less better?
My tires have 65psi max. I usually air the tire so that the tread of the tire is flat on the road.usually about 55 psi. I have my new tires at 56psi and notice that I need to air down a little as the edge is not all the way on the road. Its only about a less than the thickness of penny and maybe a 1/4 in in from the edge of tire. 285/75/16 open lug mud tire. So I need to drop a few psi.
My thinking is with more psi, less chance of hydroplane as there is less tire surface on the snow but with less psi you get more tire surface on the road so there is more traction. I guess it would depend on if it were wet or dry snow.

 

[SUMOTOY]

Jezzus Scott you come off so pompous in nearly every one of your posts it's really hard not to take the thread to the Chit-Chat section.

Yeah, I knew how to disable the ABS before you ever owned a truck, so thanks for taking the time to teach me something...

With all due respect, I'm sure you mean to say things that aren't confusing in the thread above. I read them, and thought that many sections of the posts you put up were either plain wrong, or conveyed a basic misunderstanding of mechanical and dynamic handling of awd/abs systems (IMO).... There is no 'trick' to disabling ABS is my point. My other more important point, is any trick to have ABS activated with CDL locked is dangerous. Both points not well presented by you -- first?

Braking hard before entering a turn, locking the tires up in the process, throwing the truck into a sideways slide, then once in the turn, hitting the throttle hard to power out of the turn. That's pretty much what I've heard folks refer to as a "power slide". I'm not a race car driver or pretend to be one. Having the ABS on makes that a whole lot more difficult. That seems pretty obvious to me.

Sounds like you are referring to a Scandanavian Flick-type power slide. Braking hard can be a component of it, but locking the tires isn't a necessary or common component of a power slide. I only 'think' I'm pretty well versed on that, as I was taught that technique in ProRally School by WRC drivers almost 30 years ago, with a lot of practice since.

...So you've driven around on flat tracks and parking lots. IME, once you enter very steep ascents/descents and throw in some off-camber situations to boot, with some kind of serious consequence if you leave the confines of the road you can appreciate ABS and what it does better than old-school experience with playing with the brake pedal.

Kurt, good luck with driving this winter in Colorado.

Thanks, and FYI, I spent 4 years as a competitor in ProRally, so I guess I've had a few 'situations' of which you speak. In two weeks time, I will also spend 4 days (my 18th year doing so btw) at Steamboat Center for Driving Sciences Ice track teaching awd high performance driving. If you've ever been to that track, you would understand there isn't a flat section on the 4 miles of ice. I appreciate ABS for exactly what it is capable of doing, and I also spend a lot of time educating folks on what it can't do... Since I also took my SC 80 to that venue for 4 days in 2006, I'm pretty sure I can speak knowledgeably the 80 chassis dynamics

I'm not 'old school' at all, I'm very well versed on almost every current AWD/ABS/EDL/TCS system that comes out. Because I need to be when teaching chassis dynamics. The root of practical driving technique, is understanding how systems work, and how they react to driver input and real world driving. If you can't communicate that knowledge well, it's tough to speak to "serious consequences"...

And my passion is AWD chassis dynamics. I study, practice, educate and compete using my knowledge and understanding of it. Which you may take offense to, but this is not my intention.

IMO/IME and my .02 "playing with the brake pedal"

Scott J
94 FZJ80 Supercharged

 

[SUMOTOY]

80 tire pressures on snow/ice

...
How does tire pressure affect snow/icy roads? Is more or less better?
My tires have 65psi max. I usually air the tire so that the tread of the tire is flat on the road.usually about 55 psi. I have my new tires at 56psi and notice that I need to air down a little as the edge is not all the way on the road. Its only about a less than the thickness of penny and maybe a 1/4 in in from the edge of tire. 285/75/16 open lug mud tire. So I need to drop a few psi.
My thinking is with more psi, less chance of hydroplane as there is less tire surface on the snow but with less psi you get more tire surface on the road so there is more traction. I guess it would depend on if it were wet or dry snow.

Here's my .02 testing the 80 tire and air bag pressure at Steamboat Ice Track FWIW (pretty sure you can get my full report with a search of the archives 2006).. IMO/E the 80 has the best dynamic control with the tires planted on the snow/ice. The best handling/traction I got was with tire pressures at the lowest range (25-26) and air bags lowest as well (6psi). Which tends to support IMO/E , it's not the tread contact patch variable as much as it is the 'effective spring rate' of the tire itself that affects 80 dynamic handling in low cf conditions.

A high tire pressure will tend to build spring energy in the tire, causing it to lose contact patch, as the respective corner oscillates up and down. One of the main reasons ice tires tend to 'squirm' on dry pavement is from a combination of the sipes and the very low energy sidewall.

You can test this yourself buy lowering the tire pressure down to the minimum spec for the weight of the truck, and trying different snow/ice conditions with it that way. This is pretty common to do with AWD cars as well, and it follows what the Bridgestone test drivers recommend at the Steamboat corporate track.

HTH

Scott J
94 FZJ80 Supercharged

 

[landtoy80]

Tire pressure, Another question I have.

On a tire that has 35psi max and a tire that has 65 psi max.

If you air a tire that has 35 psi max to 25 psi its not under inflated.
If you air a tire down to 25 psi that has a 65 psi max, is it under inflated?
Will a tire with 65 psi max have the same sidewall flex and amount of tread on the road as a tire that has a max of 35 psi?

Every time I contact tire manufacture they give the Lawyer answer, set tires to manufacture spec. In the Camry if I set the tires to factory spec the out side tread wear out long before the center tread.

I have come to the conclusion that the tire needs to be aired so that the tread is flat on the road but not sure if thats best for traction in all on road conditions.

 

[Everett80]

Jezzus Scott you come off so pompous in nearly every one of your posts it's really hard not to take the thread to the Chit-Chat section.

Yeah, I knew how to disable the ABS before you ever owned a truck, so thanks for taking the time to teach me something...

Braking hard before entering a turn, locking the tires up in the process, throwing the truck into a sideways slide, then once in the turn, hitting the throttle hard to power out of the turn. That's pretty much what I've heard folks refer to as a "power slide". I'm not a race car driver or pretend to be one. Having the ABS on makes that a whole lot more difficult. That seems pretty obvious to me.

I gotta say on a personal note, I don't really find much of what you post with regards to this thread to be worth much. Your only saving grace is that you always tend to end it with "in my opinion" or "experience". Regardless it's always still very preachy in tone. So you've driven around on flat tracks and parking lots. IME, once you enter very steep ascents/descents and throw in some off-camber situations to boot, with some kind of serious consequence if you leave the confines of the road you can appreciate ABS and what it does better than old-school experience with playing with the brake pedal.

Kurt, good luck with driving this winter in Colorado.

hehe... I'm sure I'm just stirring the pot here, but wow... sounds like someone is jealous deep down inside...

 

[SUMOTOY]

Tire pressure, Another question I have.

On a tire that has 35psi max and a tire that has 65 psi max.

If you air a tire that has 35 psi max to 25 psi its not under inflated.
If you air a tire down to 25 psi that has a 65 psi max, is it under inflated?
Will a tire with 65 psi max have the same sidewall flex and amount of tread on the road as a tire that has a max of 35 psi?

Every time I contact tire manufacture they give the Lawyer answer, set tires to manufacture spec. In the Camry if I set the tires to factory spec the out side tread wear out long before the center tread.

I have come to the conclusion that the tire needs to be aired so that the tread is flat on the road but not sure if thats best for traction in all on road conditions.

Wow that's a whole 'nother thread IMO. The easy answer is 'no', or at best ='it depends' to all of the above. The same size tire 'generally' has the same load ratings up to the lower maximum pressure. IOW, two 285/75 16 (35psi max on the Pmetric, and 65psi on the Reinforced) tires on stock wheels will generally have the same respective load ratings at 25-35psi. The 65psi tire will continue to increase it's load rating at 36-65psi, the standard tire will have the same 35psi load rating at any pressure above 35.

There are charts available from most major tire manufacturers that give the load ratings of a each size/model tire at various pressures. If you know the front and rear axle weights, you can calculate more closely than the door sticker, optimal tire pressures of any vehicle quite easily. FYI, when using Pmetric tires on an SUV, Federal Motor Vehicle Standards uses a correction factor of .9 x load rating.

Optimal tire pressure based on 'looks' is tough to do these days, because there are so many differences in sidewall/tread compounds, including some that use a higher wear center tread compound. The good news is these newer compounds allow a larger variation in 'incorrect' tire pressure for a given load without catastrophic failure.

HTH

Scott J
94 FZJ80 Supercharged Blizzack DMV1@32psi

 

[Walking Eagle]

What is wrong with locking the CDL on snow roads? Its no different then driving in 4hi on the FJ60, FJ62 or any 4x4.

1. You loose ABS - but you've already lost that at all times.

2. The front and rear will fight each other during corners. When the center diff is locked, and you go around a corner, a wheel has to slip to allow the turn. A slipping wheel, by definition, has no traction. So, when you go around corners the truck slips. The truck also pushes cause it doesn't want there to be a difference between wheel speed front and back. That's where you see most 4x4's go in the ditch on snow.


It's not horrible, but it's not as good as leaving the CDL open. This is of course talking about snow covered roads, throw 2ft of snow and 3+ ft drifts in there and the equation changes from control to just getting any forward movement.

 

[SUMOTOY]

...

2. The front and rear will fight each other during corners. When the center diff is locked, and you go around a corner, a wheel has to slip to allow the turn. A slipping wheel, by definition, has no traction. So, when you go around corners the truck slips. The truck also pushes cause it doesn't want there to be a difference between wheel speed front and back. That's where you see most 4x4's go in the ditch on snow.

That's got a lot of variables attached to making such a generic statement. All tires slip when the truck moves, longitudinally and laterally. In chassis dynamics study, it's the 'significance' of the amount of slip that dictates oversteer, understeer, or total lack of control (100% slip). The variables studied and isolated to car 'control' are normally divided into 4 main groups to identify significance
Vehicle Dimensions and weight distribution (wheelbase, cog, axle load)
Vehicle elasto-kinematic suspension & steering characteristics (alignment, steering angle, suspension frequency, shock dampening, sprung:unsprung weight ratio, springs)
Tire design characteristics
Torque distribution to the wheels

A lot of studies around on AWD center differentials isolating the above variables. In mixed cf conditions (ice on one side and pavement on the other), the car control with a center differential in a locked state doesn't vary much from an open center differential at 60mph (measured steering angles follow each other almost exactly)

It's not horrible, but it's not as good as leaving the CDL open. This is of course talking about snow covered roads, throw 2ft of snow and 3+ ft drifts in there and the equation changes from control to just getting any forward movement.

I encourage folks to lock that center differential on snow and or ice and compare car control. There is an intrinsic belief that ABS is a unassailable car control device. It *can* be, but it's control advantage over a locked center diff is only if the CDL braking caused a lockup of a wheel. Until that point, there are a lot of proven advantages to a locked center diff in car control on low or mixed cf conditions.

I don't believe for a second that center differentials of any type are the causes of awd vehicles laying in the winter-ditch. I spend a lot of time teaching that every AWD system gives a false sense of 'traction', and IMO/E it's over-driving the tires that causes most awd vehicles to leave the road in the winter. AWD cars/trucks with all season tires are the worst (Audis, bmw x drive, SUV) IMO. At the Steamboat Ice Track Events, we don't even allow all season tires, and actively encourage dedicated ice tires. It saves on the participants wallet in the long run...

What is wrong with locking the CDL on snow roads? Its no different then driving in 4hi on the FJ60, FJ62 or any 4x4.

Nothing, it's absolutely the same as driving in 4hi in any 4x4, as the CDL locked defines part-time 4x4 'hi' mode.

IMO/IME and my .02 arbitraged thru the peso

Scott J
94 FZJ80 Supercharged

 

[Devil's Slide Doug]

A lot of studies around on AWD center differentials isolating the above variables. In mixed cf conditions (ice on one side and pavement on the other), the car control with a center differential in a locked state doesn't vary much from an open center differential at 60mph (measured steering angles follow each other almost exactly)

Lots of really neato ice track/rally/vehicle dynamic stuff. Very important info for all the folks that load up their families in their 80's and drive down that snowy icy road at 60 mph.

I encourage folks to lock that center differential on snow and or ice and compare car control.
Scott J
94 FZJ80 Supercharged

This, I actually do agree with.

 

[Walking Eagle]

That's got a lot of variables attached to making such a generic statement.

No, it's well documented fact. Lock the CDL and turn at full lock in a gravel lot, you'll hear it slip. Measure the turning radius. Then do it with the CDL unlocked. Open center gives less slip and tighter turning radius (it's why people do the 7 pin mod). It lets the front and rear spin at different speeds just like an open diff on an axle does. Just like a locked diff on an axle makes one tire skip, so does a locked center diff. It's also why you're not supposed to lock diffs (center or axle) on dry pavement.

It's not nearly as confusing as you're trying to make it.

Anybody who you've 'taught' that a locked center diff does not make a car push in corners should demand a refund. Right after they get pulled out of the ditch of the corner they just went straight through.