All Wheel Drive Class 101

[LCPilot]

I saw this thread and decided to do a little "seat of the pants" testing in the ice and snow today. When I went into a deliberate slide by accelerating in a corner the CDL engaged made the truck push to the outside a lot more than when it was disengaged. I tried the same corner multiple times to verify.

When starting from a dead stop with CDL on the truck had less slippage and more control going straight.

When driving on icy snow packed roads at 50 mph the CDL switch position seemed to make no difference.

So, to put the results of my testing into practice, I plan to keep the CDL off when I am driving around and only engage it when I am starting off from a stoplight on an icy hill.

(Thanks for the excuse to get out and play).

 

[Nay]

A basic misunderstanding of locked center differentials and awd chassis dynamics. It's accepted theory and physics that a locked center differential gives ideal brake force distribution in a straight line up to lockup (read: front and rear brake force is balanced exactly to weight distribution thru the locked driveshafts). Which means exacty that it will brake and steer better than a car with or without ABS. Remember, ABS needs to be ACTIVATED before an ABS equipped vehicle has close to ideal brake force distribution. Prior to activation, an ABS vehicle will have mechanical proportioning only, which is far from ideal brake force distribution.

What ideal brake force distribution does BEFORE you steer is reduce the front end loading of braking force, which allows more ability for the front wheels to steer. For maximum traction on dry pavement, there are several awd race cars that have welded center diffs, and have been known to win championships. This means that awd and locking differentials is not a lo mu surface advantage only.



I say the arguments presented are marketing, not physics or acceopted practice. Change your brake fluid as PM, make sure your rotors and pads are in good shape, Front and rear. Ck your tire pressures regularly, and general tire condition. Pack your vehicle thinking about heavy things at the bottom and forward in the rear space. Train yourself and other drivers in your house with educational drivers schools that include lane change maneuvers and limit of adhesion exercises. Place your hands at 9 and 3 o'oclock position on the wheel as habit, any other position won't allow you an avoidance maneuver. These are the things that make more of a difference in an avoidance maneuver than if your center diff is locked or not.

Saying that a locked center diff is anymore unsafe on dry pavement than unlocked with ABS is disengaging the driver from proper skillset. In AWD, the misconception is that the vehicle dynamics under braking are the same as in RWD or FWD. That can't be further from the truth. Chassis dynamics is a complex issue, but hardly rocket science or above basic skillset learning.

Grench et. al. happy to point you to some SAE articles on the subject of traction, braking and awd dynamics.

ST

My friend, I like a lot of what you have to say, but in this case, any theoritical proposition of driving with a locked center diff in a 4x4 is just flat out contradictory to the experience of doing so. I have been in the situation where the t-case would not unlock after a day of wheeling, and it is not pretty. You cannot drive on dry pavement in this situation. Racing experience has nothing to do with driving to the store. You can argue equally that a locked rear diff is a major advantage to an open diff in racing, especially in low traction situations, but that has nothing to do with locking your rear diff going to the mall. I know people who live in the formula 1 world and race lifted and locked trucks on ice for fun, and to a degree of advantage. They would never claim that spools are good for daily driving. Calling that apples and oranges is unfair to fruit.

When you lock the center diff around town, the bind in even moderate turning is so bad that the vehicle will not turn properly. This is not understeer. This is a vehicle that will effectively only go in a straight line unless you have tires that have so little traction they can spin on dry pavement without any noticeable input to the driver.

This has nothing to do with ABS, non-ABS, or anything else. You cannot drive with a locked transfer case on drive pavement unless the road you are on is virtually straight. I have a hard time believing that this is a debatable point, and racing is not a comparable perspective. Anybody who disagrees should engage the CDL (just drive in low range if you don't have a CDL) in your garage and go to buy some bread. Leave it locked for the entire trip. Feel free to post the results of your experience.

Nay

 

[SUMOTOY]

... When you lock the center diff around town, the bind in even moderate turning is so bad that the vehicle will not turn properly. This is not understeer. This is a vehicle that will effectively only go in a straight line unless you have tires that have so little traction they can spin on dry pavement without any noticeable input to the driver.

This has nothing to do with ABS, non-ABS, or anything else. You cannot drive with a locked transfer case on drive pavement unless the road you are on is virtually straight. I have a hard time believing that this is a debatable point, and racing is not a comparable perspective. Anybody who disagrees should engage the CDL (just drive in low range if you don't have a CDL) in your garage and go to buy some bread. Leave it locked for the entire trip. Feel free to post the results of your experience.

Nay

Horsepucky. And let me put for the record (again) I teach car control for street drivers, I haven't 'raced' competitively since 1997. For S&G I ran locked center just yesterday (AWD hi, CDL on), for the whole day, in chicago. I was testing the handling of my 80 in turns from a stop light and overall feel of the braking and handling (dry pavement, super sticky Blizzack stock size ice tires). Actually it is understeer Nay. At full lock it can be difficult to force tire slip. But I had no full lock in normal city driving yesterday, and mild understeer.

I won't parrallel park, or do some other tight maneuver with my foot to the floor, but certainly for on road driving, I personally like the phenomenal difference in brake feel and proportion, and the more even handling thru turns.

I run locked on pavement all the time, and have done so prior to my 80 with my quattros. Last year, almost my whole trip to Steamboat was with CDL on. And in my spirited climb thru the passes outside of Fort Collins and Rabbit Ears with a gaggle of quattros on mostly dry pavement, I ran the CDL on the whole time. Any other mode would have put my supercharger advantage to shame.

I only advocate the *understanding* of awd chassis dynamics, and have a issue with the unsupported 'unsafe' statements. Lift kits, roof rack over/loads, improper training, bad PM, bad tires, bad brakes, bad driving position, bad driving habits, all have bigger effects on 80 safety in terms of chassis dynamics than the CDL on or off. You can choose to lock it or unlock it based on your driving involvement, your driving comfort zone, or your concerns for accelerated driveline wear. Choosing to advocate no CDL on pavement because it's *unsafe* is buying the marketing.

I go thru this mindset every year at Steamboat Center for Driving Sciences, and at many pavement Driving School venues. It really doesn't bother me much, it's easy to demonstrate. I get involved in these discussions because my passion is awd chassis dynamics, and has been for 15 years. I teach car control at cliinics and track venues. I don't buy hype, I don't buy ABS as safer than understanding and training.

And, I have spots open at Steamboat for anyone that wants to have one of the best times of their lives learning car control in an environment that allows you to regularly exceed the laws of physics without any damage to your vehicle, only ego.

ST
Eventmaster
Gruppe-q Steamboat 2007
www.gruppe-q.com

 

[SUMOTOY]

I saw this thread and decided to do a little "seat of the pants" testing in the ice and snow today. When I went into a deliberate slide by accelerating in a corner the CDL engaged made the truck push to the outside a lot more than when it was disengaged. I tried the same corner multiple times to verify.

When starting from a dead stop with CDL on the truck had less slippage and more control going straight.

When driving on icy snow packed roads at 50 mph the CDL switch position seemed to make no difference.

So, to put the results of my testing into practice, I plan to keep the CDL off when I am driving around and only engage it when I am starting off from a stoplight on an icy hill.

(Thanks for the excuse to get out and play).

Everyone should get out and play! A couple more comparos might be in order. First, a deliberate slide is just that. I would expect understeer from the front alxes inability to travel faster than the rear. However, is it more or less predictable than open diff? I found in a deliberate slide, it was almost impossible to control the front end with the cdl unlocked. Specifically, the predictability of the chassis handling increased with CDL locked.

The other test is turning under acceleration. We know that the CDL gives ideal torque distribution under acceleration and braking in a straight line. (Jumping ahead in my AWD dynamics series) What happens while turning. In turns, the rear axle will try to turn more slowly causing the engine torque distribution to go rearward. Up to the point where the front wheels are forced to slip and lose traction due to the fact the F and R driveshafts force this slip. The result is the understeer you feel.

In working through chassis dynamics, we need to speak to a skid/slide control vs non skid control. It's really tough to speak to both, because IMO/E slide control is when you have already exceeded the laws of physics. Look at torque proportioning during a non slide is the first step as very few really explore 80 traction at the limit and beyond.

As a rule, I find the 80 chassis to be extremely stable and predictable with the center diff locked. As with the early audi quattros, I find this to be true with chassis that use basically the same suspension front to rear. Combined with the fact that this is a dual rigid axle system, weight shift and suspension movements are basic (simple works!).

I encourage everyone to play some more. Really work through how the truck handles locked vs unlocked. My experienced with the locked center in the 80 chassis is one of a noticeable rise in dynamic chassis control, not reduction. I will present some basics to understanding chassis dynamics to help reduce many misunderstandings of awd vehicle control.

HTH

ST

 

[firetruck41]

Having driven on gravel roads, and short stretches of pavement with the CDL engaged or in PT4wd vehicles, I can not imagine driving on (dry) pavement with CDL, and just going to the grocery store. Talk about increased steering effort and tire scrub, I can't think how that contributes to increased safety when goin other than straight at speed. I almost have a hard time believing ST drives with CDL locked, on dry pavement, if he has never experienced that before. Though I acknowledge he has been teaching driving on snow/ice for 15 years, so he doesn't have to reiterate that again.

 

[SUMOTOY]

Having driven on gravel roads, and short stretches of pavement with the CDL engaged or in PT4wd vehicles, I can not imagine driving on (dry) pavement with CDL, and just going to the grocery store. Talk about increased steering effort and tire scrub, I can't think how that contributes to increased safety when goin other than straight at speed. I almost have a hard time believing ST drives with CDL locked, on dry pavement, if he has never experienced that before. Though I acknowledge he has been teaching driving on snow/ice for 15 years, so he doesn't have to reiterate that again.

I drive that way all the time in chicago. In normal driving, especially rush hour, I find the front rear bobbing to be much less with the CDL engaged, and the braking to be stronger and much more linear. I also find understeer to be a educated condition of chassis dynamics, not a 'dangerous' part of chassis dynamics. In terms of predictability (which is my pursuit in the most basic of driving habits/patterns) there is no question that locked center understeer is the most predictable part of the 80 chassis, up to and beyond the limits of adhesion. BTDT.

That it somehow forces you to steer into an accident, or *not* allow you to have steering control, is a basic misunderstanding of understeer. If I know I'll be in a full steeering lock situation, it's easy to reach over and shut off the CDL. I've run CDL locked for thousands of miles, in the city and on the open road, without full lock bind induced axle jumping antics. My RF birf was clicking when I bought the truck 50k ago from the PO, and it still does the same amount...

I find no abnormal tire wear on my gummy blizzacks from regularly running CDL locked on the street. I find and propose *safety* to be far down the list in terms of this CDL on/off debate in relation to chassis dynamics/vehicle control. It's hard enough to find agreement here on how CDL and braking systems work. Let's not jump too far ahead of the game. I advocate that proper education and understanding of CDL and chassis dynamics is better than the blatent "don't use that button on dry pavement" postulate.

Get out and play, have fun, on pavement and off. Get a *feel* for what feels good to you and your truck. The very first and main reason I use CDL-on a lot, is that I found the brake prop valve on my 80 to suck, bad or worse than the one on my 4R (which I replaced with a manually adjustable eventually). I can't think of the last time I had an ABS event, so I'm quite comfortable with using the CDL locked more often.

ST

 

[-Spike-]

Those of us who have gotten a 'feel' for driving with the CDL locked on pavement take issue with your constant insistence that we have a 'basic misunderstanding' of what's going on. I think you've deluded yourself into thinking that there's a great conspiracy among the ABS/auto manufacturers to kill us all, and that has blinded you to the 'feel' of your entire drivetrain binding with the slightest turn on pavement. Or, your CDL is broken.

-Spike

 

[TCH2FLY]

^ X2 .....exactly my thoughts.
With my center diff locked my 80 performs exactly like each of my previous part time 4x4s when they are running in 4-HI. I just can't imagine running my FJ-40 around in 4-HI all year long.

If, as you suggest, running with a locked center diff is better and has no ill effects why are manufacturers wasting millions to add such complexity to these vehicles when they could just lock it at the factory and forget it.

 

[turbocruiser]

Guys, you know the old saying on wrestling with pigs - everyone gets extremely dirty but that's the way piggy wants it!!!

Yer not gonna win with the foolosopher we have here; you cannot possibly teach him anything at all cause "he's already been there, already done that, got the tshirt, etc , etc , etc" the only one who can teach to him is him. That's cool as long as no one takes all this advice too much to heart. So many statements here have been flat foolishly wrong. That's cool too cause we all have mistaken moments, as long as no one screws something up by driving around adding wear to the truck or even worse adding windup to the transition between some friction and no friction in a corner!

I'm not trying to take an attacking tone, and I apologize for the fact that it is, but it is hard to counter everyone's collective experience with one person's old "BTDT" each and every time. Look, driving with the CDL locked all the time is simply stupid in so many ways, yes I've done the testing myself, so, I counter the original BTDT with my own BTDT! There, the argument is forever suitably solved, wow that works!!! So, my advice, get up, hose off the mud (IH8MUD) and stop wrestling with the piggy.

 

[Brian Macgyver]

OK gents; Everybody is going every which way on this. To be simple, you want to be able to slip a bit in every way when the road is icy or when traction is minimal. That is what AWD was made for. When the going is slippery, do not disable your center diff or lock your axles. But you need to have the choice when the road is rough or when the front or rear diff needs work. I have run my truck without the rear axle, or the front. I once removed my rear diff and axles for repair and still drove the truck all over Arizona in front wheel drive. Try that in a Ford!! I have locked everything to pull stumps or to pull over rough spots with my wimpy tires. It is the selectable lockers that set the LC80 above all of the other factory 4X4's out there. There are times when you don't want to lock it up. Snow and ice are one of those times.
I live in the lower elevations of the Prescott Metro area where we seldom see any icy driving. But I usually have to go up to the main town where this is common to work. I buy material at Walmart and Home Depot where the parking lots are sheer ice all over. Those retail outlets are at the highest elevations in town. I drive on sheets of ice up to a mile long every day. I seldom notice the ice. With full-time and the lockers off, it drives like it is on dry pavement. I have really wimpy road tires, (well, really light factory off-road 175-75-16 tread) not Nittos or other real tires. If I was to lock up any part of the drive train it would slide around like a sled. It would slip like rum on an ice floe! But it handles perfectly well unlocked. My Celica with limited slip would be useless up there! But my LC doesn't really care! It drives great on that slick stuff. I often tow a heavy trailer, and it slides a bit, but the truck rocks steady. I don't even notice the ice. It is not an issue.
The full-time, all-locker system is priceless. Find ways to override it selectively, but when dealing with regular slippery roads, let it do its thing.

 

[SUMOTOY]

I entered this discussion because I find the 80 chassis with the center diff locked to have much better handling than with it unlocked. I drove for the last 3 days in Chicago traffic with the center diff locked. I do this regularly, and have for many years with the quattros I drive as well. Does this mean everyone should do it? Not sure I care. What I do believe to know pretty well, is how AWD systems work, their chassis dynamics behavior, and regularly practice driving at the limits of adhesion (and beyond).

There is a lot of misunderstandings of *how* AWD systems work, from torque distribution to braking. It's in the archives here. I put forth a lot of information on some of this, entertained discusssions and arguments to the contrary. I put my 80 thru at the limit (same as emergency avoidance maneuvers) last year at Steamboat. With CDL on and CDL off (ABS on). I can compare thethe chassis dynamics of running each mode (in theory and application) easily up to and at the limit of adhesion.

I learned long ago that "safety" and education have a direct and high correlation. Anyone is welcome to buy into the marketing of awd systems, I'm just a chassis dynamics guy with a lot of questions.

ST

 

[Hayes]

Differentials are over-rated.
I'm welding all three this weekend.

 

[Nay]

Horsepucky. And let me put for the record (again) I teach car control for street drivers, I haven't 'raced' competitively since 1997. For S&G I ran locked center just yesterday (AWD hi, CDL on), for the whole day, in chicago. I was testing the handling of my 80 in turns from a stop light and overall feel of the braking and handling (dry pavement, super sticky Blizzack stock size ice tires). Actually it is understeer Nay. At full lock it can be difficult to force tire slip. But I had no full lock in normal city driving yesterday, and mild understeer.


ST
Eventmaster
Gruppe-q Steamboat 2007
www.gruppe-q.com

Let me put for the record that I can't get out of my driveway with CDL on while driving at idle speeds. I didn't realize that was actually just understeer

I believe the appropriate cliche is "experience is the best teacher".

 

[SUMOTOY]

Let me put for the record that I can't get out of my driveway with CDL on while driving at idle speeds. I didn't realize that was actually just understeer

I believe the appropriate cliche is "experience is the best teacher".

Agree. I just have a problem with the CDL locked on road as a no-no. It's not only an offroad device. Audi built hundreds of thousands of road cars with center lockers. Already awd with ABS, one has to wonder why reading this thread.

Nay, you really want to speak to idle speed chassis dynamics? I didn't figure you for a newbie.

Cheers

ST