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4WD and AWD systems explained

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Comments

  • bwhbwh Posts: 76
    It is most likely caused by the fact that your truck has a heavy live axle in back. When one side is "bumped" it transfers directly to the other wheel.
  • ateixeiraateixeira Posts: 72,587
    I.E. the non-independent rear suspension. I'd agree.

    -juice
  • gluizgluiz Posts: 17
    The key point is that locking a diff does not "increase traction". The goal is to make better use of the available traction. In the example, if one side slips (assume the flat side) with no effort at all, implying that all traction is on the bump side, then you do use all the traction.

    However, let's say both sides have traction but the bump (left) side offers 2x of the flat (right) side for a total of 3 units of traction. If you were locked, you would have to loose the 1x on the right (b/c wheel must slip). The bump side must also provide some torque to cause the slip, at most 1 unit (some comes from engine). Hence, instead of 3 units of traction, you can only use between 1 and 2.

    Now, I am not arguing for not locking up, that it's a bad thing. I am just pointing out that they are not ideal. Their binary operation is what makes the inconvenient but they are the cat's meow when you need them.
  • bwhbwh Posts: 76
    I think you are still missing my point a bit so how about this appraoch. Say the bump is really big on the left. It is big enough to lift the right tire nearly off the ground. What type of traction device would you like in this scenario? An open diff defeats you immediately, the right tire spins the vehicle stops. A limited slip in most cases is a marginal tool. I assure you the LS will slip badly on the right whil trying to clamber for traction on the left.

    I maintain that the traction you sacrifice with the locker is traction you didn't need anyway. So if it drags the opposite tire along and the left one doesn't lose any what was your NET loss....0.

    I have seen guys in Jeeps with LS diffs in back. On Christmas day this year, on a trail in California I had to pull just such a vehicle free. He had solid footing on one side and the LS would not transfer enough power to move the Jeep. I went through first locked up with no problem, even if I drug a tire now and then. As long as you have control over your lockers there is no better traction device.

    OOps I had to come back and address one more issue. You say that a locker can't increase traction, it can utilize what is there. Tell me, what increases traction, tires or road surface are the only possible answers.
  • What is the difference between Auto 4WD in an lets say Explorer vs. AWD ?
  • paisanpaisan Posts: 21,181
    In general Auto will no apply any power to the front axle until there is significant/noticeable slippage of the rear wheels, and when it applies it I don't think it's variable, just a fixed split. On AWD you get some power to the front axle at all times and it's less of a reactionary, more of a pro-active system. In general terms AWD usually is not disableable.

    -mike
  • hengheng Posts: 411
    When you are in the rough slippery stuff, so what if a wheel drags, the other one is pulling you out.

    If these AWD systems wait until wheel slip is detected you've lost it already. On pavement its probably OK but out in the rough you'd be stuck.

    Question: aside from equiping your vehicle with ARB air lockers, what vehicle comes factory with locking diffs?
  • gluizgluiz Posts: 17
    I think we are making the same point. Your example of the tire totally of the ground is the perfect time for the locker. I do not disagree. The original point was that being locked does not maximize traction under all situations and I was giving an example. You also point out a limitation regrading the LS.

    The final point about increasing traction ... I was just responding to your original post saying "... found locking diffs to do anything but increase traction ...".
  • gluizgluiz Posts: 17
    Readily available in the US - Land Cruiser (front and rear). Not so available - G-wagon (one importer in US). If you go US "exotic" - Unimog, M35's (and other heavy trucks). You don't find auto-lockers much because of the liability. Some year Hummers had a Detroit as an option in the rear, that's the only produciton vehicle I know of with a factory auto-locker. Also, traction control systems that can control each wheel under severe conditions will become a more prevasive option in the future, IMHO.
  • bwhbwh Posts: 76
    The front locker is gone from the new V8 Land Cruisers. When they dropped the solid front axle the locker went with it. The Gwagen will be at selected Mercedes dealerships this fall. It will remain rare and expensive though.

    Traction control systems are excellent for road situations. I doubt they will ever take hold in the off-road community. They simply are not the best solution. I have read that they are very prone to overheating the brakes under heavy off-road use. But for most they are better than a locker.

    gluiz, you caught me, what I should have said was that lockers are the most effective way to maximize the available traction. This is not an on-road solution though. Using lockers on a snowy highway for exaple could prove disasterous. They would definately cause instability at speed. I have always found the fully open diffs to very effective in snow. Generally speaking one wheel "could" spin but the remaining three do not maintaining stability.
  • paisanpaisan Posts: 21,181
    Anyone care to take a stab at why Front LSDs aren't more popular?

    -mike
  • ateixeiraateixeira Posts: 72,587
    Packaging & expense would be my guess.

    -juice
  • bwhbwh Posts: 76
    Well expense is a good reason. The extra $3-400 to add one to the rear of most trucks keeps them scarce. Packaging is not an issue, they really don't consume any more space than an open diff.

    The real reason is that it would screw up the steering something fierce. Now we enter the world of liability. The lsd is not like a locker that you can select to operate, the lsd is always on call. For anything short of a hard core off roader this would be a bad idea. Even in a dedicated off-roader the locker is far better. From experience I can tell you any lock up in front is dangerous except at crawling speed.
  • paisanpaisan Posts: 21,181
    Just curious because now LSD is going to be on the new Sentra SER and IIRC the Integra has it as well. How would a front LSD differ from a rear one (as far as safety wise)?

    -mike
  • drew_drew_ Posts: 3,382
    image
  • bwhbwh Posts: 76
    ..... those systems are electronically controlled to minimize the effect on steering. It also depends on the % of slip control built into the unit. You could build a 20% LSD into a front driver and not cause a safety issue. At the same time you haven't added a lot of traction. My Porsche has 40% LSD in back, you can hear it fighting itself on tight sandy corners. 20% would probably be enough to limit inside wheel spin on a front driver without causing a problem. This is a very different application. LSD on a rear driver can use much more lock up before you "feel" it in the wheel. If I lock the rear on my truck and try turning on dry pavement it is resistant. If I lock the front and try the same maneuver it is nearly impossible to turn. So getting back to the original query, LSD in front adds cost to what are normally cost conscious cars being front drive and all. Unless you are trying to harness quite a bit of power while cornering ie SER or ITR, it is a waste of money because you will never apreciate the benefit. Originally I thought your question pertained to a truck type application. I would think this would require a higher lock up percentage and hence adversely affect steering. Again you could put a looser LSD in front of a truck as well but the benefit would be negligible.
  • paisanpaisan Posts: 21,181
    It was in reference to trucks. I thought it might be a good alternative to the more expensive lockers. I do notice that in order to get my rear LSD to engage I need to yank on the parking brake slightly. Is that normal?

    -mike
  • gluizgluiz Posts: 17
    LSDs have two components to resistance - preload and dynamic. Preload is the amount of resistance (or locking) w/o any power being applied. If you were to raise one wheel up and try to spin it by hand, this is what you feel. Dynamic is how much locking occurs based on input torque. In a mechanical LSD, preload is the default pressure on the clutch pack, dynamic is the ramp angle.

    When you get an LSD like bwh refers to on his Porsche, it has high preload. I have a sports car with a viscous LSD and you don't feel much at all. However, it I spin off the line, it will "lock" up right away.

    For a front LSD, you need low preload or you get funny steering effects as bwh pointed out. Furthermore, you have to worry about braking while turning in which case you cannot have any dragging/bias to one side or the other. Even if you have no preload, most LSDs cannot tell the difference between engine braking and acceleration. Torsens are becomming more common that LSD b/c they can tell the difference.

    And finally about the parking brake trick - this works b/c you are putting more torque into the diff. This means the dynamic lockup comes into play which means you have a higher lockup. Is it a domestic vehicle?
  • drew_drew_ Posts: 3,382
    Sort of...paisan has an Isuzu Trooper ;-)


    Drew
    Host
    Vans, SUVs, and Aftermarket & Accessories message boards
  • bwhbwh Posts: 76
    What are you trying to use the truck for? For all but the most severe off-road situations a front traction device is unnecessary. If you can lock up the center and rear and leave the front open this will take you just about anywhere. The front locker is nice when rock crawling though. If you need traction in front I think a locker is the only sensible choice. To put an aftermarket LSD in front probably locks you into a clutch type LS. You will wear it out off road quickly, the same goes for a clutch type in the rear, this is better suited to road use. If you have a LSD in back dump it and put in a locker. You will be amazed how much better it is. Then pass on a front device unless you plan to get really crazy off-road.
  • drew_drew_ Posts: 3,382
    But of course, your G-wagen has a stock front locking diffy :-) BTW, hope that you can join us in the MB chat tonight at 6-7pm Pacific. Pop in to any one of the MB topics in SUVs or sedans, and you'll see the link to the chat near the top of the screen, just underneath the discussion topic title.
  • bwhbwh Posts: 76
    ...I get pretty crazy with it off-road too! We were in Big Bear, CA at x-mas time wheelin. That is where I had to pull the Jeep over a rock. He was bashing u-bolts and grinding the transfer case skid plate trying to horse that baby up over the boulders. The Jeep had 33" tires and a rear LSD. I managed to walk the G, all locked up, through the same section without any drama (on smaller tires). I had my wife, her mother, and my 4 yr old twins in the truck. I have some great rock pics held hostage in my mother-in-laws camera. As soon as she gets them developed I'll put them on the web. The video we took was awesome, you could barely walk the trail, but the mighty G transported the family with ease.
  • paisanpaisan Posts: 21,181
    Well over the weekend I was up at my house in the adirondacks and had a nice snow pack/melting parking lot that I was able to test out various things with. I pulled into a mound and was able to spin 1 front (alternating) and in the rear I could only spin both rears if I engaged the parking brake and even at that it didn't lock up as well as I would have thought an LSD would. I'm bring the truck into the dealer for some other stuff and wanted to see if I should have em check out the rear LSD since it didn't seem to be working very well. I don't do much off-roading, and the few times I've gone, never got stuck, so I'm not too worried about that.

    -mike
  • bwhbwh Posts: 76
    The situation you describe does not sound unusual to me. I have a '99 F350, when you put one tire on the dirt and one on the tar, it spins the tire on dirt like crazy. This HD truck has a LSD, the amount of lock up is pretty low, but it still helps. The reason behind this is that too much lock up will make the truck unstable at highway speeds on snow etc. Fully locked will cause bad fishtailing. Too much in a LSD will do the same thing. Letting one wheel spin in a situation like that keeps the other moving at road speed and avoids the fishtailing. So they are loose for a reason. Appling a little brake is the perfect way to handle it, you just wouldn't want to do it all of the time.
  • paisanpaisan Posts: 21,181
    Thanks for the info, I didn't want to go crying wolf to the dealer if in fact that was a normal situation on LSDs. I've only gotten her stuck 2 times and both times were my own doing (raming into a 3.5' snow bank with 2' of snow on the backside) We were so stuck my buddy had to climb out the back door in order to get out!

    -mike
  • bwhbwh Posts: 76
    Snow is a tough one. I innocently went to turn around on a narrow road this winter. I got a little too far off the shoulder and the lean of the ditch sucked me in. I very slowly tried to back out in low range with the diffs locked. The snow was about three feet deep at the bottom. No matter what I did I slid further down. I was downright embarrassing to have one of the guys from work pull me out with his Cummins Dodge. The moral is , snow can be tough for any 4wd.
  • hengheng Posts: 411
    Yes, I even got my 4WD to go sideways some on level ground. Deep granular, backing up from stuck position.

    Comments Please: If only big buck SUVs (Land cruiser, G wagon, Hummer, etc) have locking diffs and LSD is only effective to various degrees, it sounds almost economical ($400 to $600?) and more effective to install a driver selectable axle lockup setup (like a ARB air locker) in the rear of a plain jane 4WD with open diffs (<$30,000) versus the $35,000 to $50,000 SUVs with LSD, traction control or other systems.

    Keep in mind the diff would be locked only for deep snow, sand and other 'tow me out' situations.
  • bwhbwh Posts: 76
    What you get on a Land Cruiser and a G-wagen is durability in the extreme. Something an Explorer or Pathfinder type vehicle cannot offer. They are not necessarily bad just not super tough. Remember the G is a millitary vehicle, and a much tougher one than the Hummer. That and the real world utility and MB driving characteristics are what the premium price reflects.
  • tincup47tincup47 Posts: 1,508
    taught by Camel Trophy veterans. One of the things that was impressed upon the class is that all off road vehicles can and will get stuck eventually. Having a winch and recovery tools and learning how to use them was recommended higher than spending money on locking diffs. (The question was asked in class.)
  • bwhbwh Posts: 76
    Hmmmmmm, Camel Trophy, Land Rover. Adding lockers to a LR is an iffy proposition in my opinion. While it has been done many times, lockers in combination with large tires on Land Rovers leads to inevitable driveline failure. The axles and diffs are not designed for the additional stresses imposed by lockers. Land Rover uses a different appraoch, they have a very flexible suspension designed to keep the wheels in contact with the ground even in extreme situations. I think they may also have been considering a different set of circumstances than I personally do. Camel Trophy events take place in remote areas and face quite a bit of mud, deep sand etc. Lockers may get you further but you very well may get stuck anyway. I use mine in the mountains not the jungles and rarely encounter the need to traverse mud holes and the like. For rock crawling the lockers are a better investment. In addition to having lockers I carry a high lift jack and straps for recovery. Adding a winch is the next step. This is a spendy addition on my truck so the two to three trips a year have yet to justify it. I suppose after my first good "stuck" I will run out and buy one though.

    So I guess I agree and disagree with that appraoch. In a Camel Trophy type situation in a Land Rover I would get the recovery tools first. If I had say a Toyota pick-up and used it in the Rockies I would travel in a group, have a strap and a good jack, and buy the locker.
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