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I think most of us refer to systems that actually lock the center diff'l as PART-TIME, because they cannot, should NEVER be driven on high traction surfaces with the diff'l LOCKED. Therefore PART-TIME, for use ONLY on adverse roadbed conditions rain, ice, packed snow, or off-road in poor traction conditions.
Full-Time or Permanent 4WD systems simply run with the center diff'l open.
AWD systems generally have some way of automatically locking, or "slightly" locking (viscous fluids) the center diff'l upon wheelspin.
Many (most??) modern day AWD systems run with the center diff'l continuously open and use braking to prevent wheelspin and thereby maintain a high level of torque overall.
1) Most owners of part-time 4WD don't understand when NOT to be using the system. I have seen many crashes on freeway exit ramps, especially curved declines.
2) Many AWD systems that utilize viscous fluids still bind while turning which is detrimental while on extremely slippery surfaces (ice). My '98 JGC w/QuadraTracII would still bind considerably while turning although not as bad as some others that have more forward bias. Unintentional resistance to rotation while on slippery surfaces can have extremly bad results!
3) Open differentials w/stability and traction control is best in my opinion for the typical driver on paved roads (yes, I do understand that there may be better options for off-roading but that is not what I am addressing). The advantage of 25% torque to each wheel at time=0 is a huge benefit. I am always amazed at the difference and the fact that in many cases, I have to intentionally try to break a wheel free to even activate the traction control system.
Why have a multi-wheel drive system when it isn't usable 99% of the time? The extremely limited benefits of part-time 4WD systems are being sold to the general public to their detriment. Varying quality of road surfaces makes these systems unsafe in most cases.
- Eric
If you GMC or what ever bravada was gripping in a turn somthing was wrong with it or they had the work peramiters, a Viscous coupler will not creat that, it works by spinning exesivly for front or back, and then gets tougher to create the "soft lock"
2. Engine torque is adjusted proportionally, increased, to the rear with stearing inputs, allowing the front tires' contact patch to be allocated to directional control.
3. During startup or acceleration, as G-forces climb, proportionally more engine torque is routed rearward.
4. During coastdown or braking, engine drag torque is routed only to the rear.
Miss anything?
I think your talking about low, but part time systems can engage 4wd High too, or atleast in my opinion the good ones should.
Thats fine but when you said that you had seen many bad wrecks on highway onramps it means that the person driving that car was not using their brains in getting on the inersate with their center diff locked, A four wheel drive system with an unlocked center differential but power to all corners will not make going down a curved on ramp a strugle for life.
CV joints are wonderful, but even my 85 Jeep equipped with them could be a serious threat to my knuckles at times.
And I think, the idea of only locking the diff'l is low range is a fairly recent developement. Likely a result of too many inexperienced owners driving inappropriately in 4hi locked mode. I could run both of my Jeeps, an 85 and a 92, with the diff'l locked, hi or low.
A Full Time system will have a center diff and a transfer case, but I've never heard of a Part Time system with a center diff. That's why it's a Part Time system is it not?
Of course I certainly DO NOT claim to be a expert of any kind.
If the vehicle can be run without the potential for mechanical damage to the driveline on high traction surfaces with both front and rear driven, it must have something between the two with the functionality of an open diff'l.
So many definitions out there, but I was using and agreeing with wwest on his. Part-time to me is locking front and rear for limited situations. AWD uses some sort of clutch (viscous is popular) to link together when required. My 98 Jeep Grand Cherokee (and many others) use viscous clutch that will begin to bind simply by turning the wheels and moving in a parking lot. It is a noticible effect and not the ideal situation when on slippery surfaces; that was really my point. Many AWD vehicles (Jeep Liberty, Dodge Durangos, etc.) will bind to some degree when turning tight radiuses. My point is that zero binding is ideal.
As far as vehicles on the freeway crashing, yes, that was probably due to a locked differential (Jeep CJ-7s are common).
wwest, I agree with almost everything on your Ideal AWD post, other than on slippery surfaces while slowing or stopping, I would rather not have torque applied to any wheel and allow my smart braking system to slow me down, 100% without mechanical linkage to the drive system. Torque bias to any wheels will begin to turn the vehicle in an uncontrollable fashion and then the braking system is hampered in its task. Trick years ago on rear wheel drive cars was to put into neutral while stopping on slippery surfaces and starting to spin. Car would immediately straighten out.
Can we have an anti-lock system for/when engine braking torque is responsible for "locking" the drive wheels on an icy, very slippery surface? That would likely prevent a lot of accidents, especially for FWD vehicles.
Restated: I don't want to HAVE to understand when to engage or disengage the 4wd system, I just want to drive in the most efficient and safe manner. True full-time 4wd vehicles allow this without any thought. Example: My wife is truly ignorant that the vehicle has 4wd and I feel 100% comfortable that she won't do anything stupid (just don't push THAT button and lock the center differential honey, and you will be OK - alright, maybe slight concern, but hopefully you get my point).
Most of my concern regards part-time 4wd (or whatever you call the technology where you don't want to drive on dry pavement due to binding).
I purchased an S-10 Blazer in 1988 that had a lever that you pulled to put into 4wd. Definately was part-time and didn't want to drive curved, dry pavement with it! There are still many vehicles on the road that have this "almost useless for driving on the pavement" technology. Only off-roaders can truly appreciate this type of system. I wouldn't even want to use it on wet roads and would question its use on varying road conditions. SO, what good is it for the typical consumer?
I'm getting deeper into this than I intended; sorry.
I can remember many times over the years when my rear-drive manual transmission car would start to turn on me when I was in too low of a gear and the engine was braking the wheel. I suppose that the phenomenon would be different in the Cruiser with better distributed potential traction, but still think that the option of NO braking in icy conditions would be beneficial.
We had some pretty nasty ice/snow mix in OH last week and I was "playing" with my Avalon and was travelling on a freeway at slow speeds and simply switched the transmission from OD to 3rd gear. I could immediately feel the front begin to slow and the rear end start to come around on the icy roadway! That is what I was thinking of when I responded to your post.
With experience it can be done safely with RWD vehicles since loss of directional control will not be simultaneous with loss of traction at the driven wheels.
The latter is also why RWD and/or rear torque biased AWD is inherently safer, MUCH safer, on adverse roadbed conditions than FWD or front torque biased AWD.
Tlcman is using "part-time" as if there is a center differential that can be either locked or unlocked. That is not my definition of a part-time system. Part-time 4wd is a front and rear driveline powered by a transfer case. There is no center differential. This is like the systems on many pickups and Jeep Wrangler.
The full-time 4wd and AWD systems have some type of system to account for differing driveline speeds, be it a viscous coupling, electromagnetic clutch, center differential, or electronic diff (like Acura).
So all of the above comments seem correct from a certain point of view. I agree with Qualitynut's original assertion--the average Joe is unfamiliar when he/she may properly and safely use a part-time 4wd system because of a lack of understanding of the dynamics involved. Like Tlcman has said, though, put a center diff in there and it takes away the issues.
Regards,
Tom
Assume the (inexperienced) driver has sensed a bit of understear, followed their natural instincts and lifted the throttle. The roadbed was/is very slippery and the front tires had obviously lost enough traction to significantly reduce lateral control.
I've driven many rear-wheel-drive cars. Countersteering is second nature to me. I can correct for that.
The only thing you can do for understeering is wait it out and hope you get some grip before you hit something...
Did I get that right?
So I guess that means that since I do know how to recover from overstearing I'm one of the lucky ones who can continue to purchase RWD only and possibly thereby extend my life duration.
"around the corner same situation but your front eng pushes, most average drivers and me (dono about you) would pull a little back on the accelerator and lighten up on the steerin angle, every time ive understeered in my honda that is what I did and i regained full control in less time and in a safer manner than oversteer step out."
The average driver is not thinking of all of the posibilities while on the highway on ramp so when the rear starts to slide or "step out" It takes them longer to realize what is going on, thus longer to recover and less of chance of a safe recovery. Oversteer causes the backend to swing out which is very dangerous on cars and SUV's which can roll pretty easily. Few cars are equiped with rolling airbags more but still not alot have side airbags, and almost every new car has front airbags now, so an understeer where it can be corrected by lessening the steering angle and not jumping out of, but rather giving less gas to the engine. Doing this I have regained control out of every understeer that I have ever got in with a front wheel drive car. and lest say that you dont recover from the understeer or the oversteer example. Not recovering from an over steer, means that your car/ SUV is sliding sideways, and if its an SUV can roll pretty easily, causing a more severe accident, if you cant regain control on a understeer, then you will hit somting from the front and most cars are designed to take frontal impact, and many have multi stage airbags, its safer than rolling your car, plus if you cant regane control and happen to go off the road, and there are no obsticals then you can still control your car as you play around in the mud trying to come to a stop. Do you understand me now?
it seems the natural response when something wrong happens, is to let up on the throttle,
in an oversteer situation, this helps the car regain composure, as the rears stop spinning (from applied power) and regain traction and fall back in line.
in an understeer situation,when accellerating around a corner, in some front wheel drive cars, letting off on the throttle, very often transfers weight back to the front wheels, giving them add'l traction, and at the same time unweighting the rear, which causes an oversteer situation.
regards
Crawl ratio = rear end gear ratio X lowspeed transfer case gear ratio X 1st gear tranny gear ratio.
P.S. I've always favored Toy's & Chevy's for this reason...never got them stuck like I got my Fords stuck & stuck & stuck...you get the idea.
http://www.toyota.com
-Mike
And lifting the throttle for recovering from understear or plowing in a FWD vehicle does not always, or maybe even rarely, result in recovery.
Cadillac actually installed an over-running clutch in their FWD vehicles with the high HP/Torque Northstar engine to help prevent loss of control resulting from your recommended recovery procedure.
In many FWD vehicles lifting the throttle will oftentimes result in enough engine drag torque to the front wheels that when "added" to the lateral forces from stearing will still exceed the front tires' traction coefficient.
I am extremely partial to vehicles that allocate the front tire contact patches to directional control and the rear to drive and drag.
and secondly
when you want to accelerate and turn at the same time you put two forces on the tyre; directional changes, and acceleration. The combination of these and the ammount is what starts understeer, the tyres can not keep traction with the steering angle and the amount of torque that is being put to them. These two reasons are why the tyres break loose, so if you correct them, then you will correct the probblem, thats too simplistic but i could write a page on it if you would like.
"over-running clutch"
Im assuming this means that the changing of gears is mutch more smooth, this means that like the word implies, the clutch mechanism gives mroe slip so that there is less force applied to the front tyres right away which could cause them to break loose, and on a car not equiped witha over-running clutch you would correct that by lifting the throttle like I said.
Get off the gas fully, front tire traction fully dedicated to maintaining directional control.
NOT!
Just as the AAA is now suggesting for recovery from understearing FWD vehicles, slipping the transmission into neutral is somewhat cumbersome but will always work.
You guys need to get your terms straight or you're going to confuse people that are reading this forum for the first time and have little knowledge of these systems.
Steve, Host
I believe that a CENTER DIFFERENTIAL is an accessory to a 4WD system while a TRANSFER CASE is fundamental to a 4WD system.
AWD never (that I know of) has a TRANSFER CASE.
AWD systems are presumed to be FULLTIME, but many 4WD systems were, and are, fulltime. I think the "marketeers" wanted to differentiate AWD for 4WD for good reason. AWD is generally taken to mean fulltime 4WD with no driver intervention.
Most traditional 4WD systems, even those with fulltime mode, have other drive modes, thereby require driver intervention.
The new 4runner, for instance, has a RWD mode, an AWD/4WD mode, and a part-time 4WD mode wherein the center differtial is locked.
Disregarding its off-road capability entirely, the 4runner's fulltime system is likely one of the best in the market today.
Oh, sorry, almost forgot. Most transfer cases also have a differential in the form of a planetary gear set. I think, not sure, the term "transfer case" has come to mean a 2 speed gearbox, by default.
VEVSC w/DAC
Includes Vehicle Stability Control (VSC) with Traction Control (TRAC), automatic limited slip differential, hill-start assist control (HAC) and downhill assist control (DAC). REQUIRES OG or PT or SJ or SN.
But this is not listed as an option on the buyatoyota site. Does anyone have this option? Is this NOT an option with the vehicle I have described? I'll be doing in-town, occasional snow/ice, and some beach driving - is this package worth $800?
No ice or snow required.
B