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Can 4WD (part time) actually cause skidding?
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=3&t=3827		 Page 1 of 2
Author:  bennybmn [ Wed Nov 30, 2005 7:34 pm ]
Post subject:  Can 4WD (part time) actually cause skidding?
So thanksgiving turned out quite snowy in good Ol' VT. I offered to run to the store for Mom since "I have 4WD!!". Now I've seen PLENTY of SUV's off the road up there over the years. All the flat-landers who think they need an SUV for the skiing trips to VT, and they get going in a straight line, then the first turn they come to with snow or ice and they are in a ditch.

But thanksgiving was different. I wasn't going fast (maybe 20 mph), and I was going around a left hand bend in the road, and I felt the back end wiggle around a little, then all of a sudden it was coming around! I did a 180 (counter clockwise) and ended up in the yard on the LEFT side of the road! 10 feet further, and I would have either hit a culvert or maybe even rolled it because of the slope.

Anyway, I've had a theory for a while that I'd like to run by you all. Call me crazy, but with no center diff, the front and rear wheels want to spin at the same rate. But when you turn, they need to spin at different rates because the rear wheels take a different path than the fronts, similar to how the left and right spin at a different rate in a turn. So in theory, at least one of the tires needs to give, it needs to "skid" or something to releave the tension, which people call "drive line binding" in tight turns. Is it possible having it in 4 actually caused the skid?

Side note: Last winter I had my Revos on and never had a problem, this year I was on 24 hour old stock goodyears, so I know what a idfference tires make! But I still want to explore my theory. Let me know what y'all think! Thanks
Benny
Author:  Ed [ Wed Nov 30, 2005 9:23 pm ]
Post subject: 
Short answer? Yes.

For the reasons you mention. When you're trying to turn in part-time 4wd (i.e. centre diff locked) in the snow your front wheels are gonna want to turn at the same rate as the rear ones, which is probably going to make them slip, which means they've lost traction, hey-presto; no steering and you go straight on!

Having said that, from your description it sounds like oversteer, rather than the understeer that'd be caused by being in part-time 4wd. It could be that the road was just too slippery, even for 4wd at 20mph. No traction's no traction, doesn't matter how many wheels you've got on the road!

Personally though, I always use full-time 4wd in the snow.
Author:  fiend [ Wed Nov 30, 2005 11:56 pm ]
Post subject: 
on my command trac t-case i only have 4hi and 4lo, my question is, is the 4hi on mine the same the select-trac full time?
Author:  Guest [ Thu Dec 01, 2005 12:51 am ]
Post subject: 
4wd on our KJs is like part time. the fulltime is when the center diff isn't locked.
Author:  ManicMechanicJoe [ Thu Dec 01, 2005 2:48 am ]
Post subject: 
I live in minnesota and we began to get snow within the past couple weeks. Since it is my first snow with the KJ, I have been experimenting with both full and part time four wheel drive. To me, it seems much easier to "fishtail" the KJ in full time. I think this is because the rear wheels are getting more power due to them slipping. Full time works great on the high way and leaving a stop but I prefer the old stand-by part time four wheel drive for anything else. Isn't it great to live in a place where precipitation turns any road into a 4x4 trail?
Author:  SD-KJ [ Thu Dec 01, 2005 3:13 am ]
Post subject: 
i too have been experimenting here. I have noticed the same effects of Full time wanting to come loose in the rear. We had about 5-6 inches of fluffy snow today and it was hard for ANY 4WD Vehicle to stay straight on the road. It was quite funny actually to see everyone drive in it. The KJ took it with stride but i did have to be careful going around corners because it was really slippery.

I have found that for the most part 4 part time is the way to go in deep snow when you need max traction. However if there is any dry pavement at all i put it in 4 full. Seems to work for me, but i am open to be proved wrong.

Snow = FUN! :twisted:

-nick

oh and btw, your theory sounds good to me, in fact i have proved it several times now.
Author:  bennybmn [ Thu Dec 01, 2005 11:10 am ]
Post subject: 
Well guys, if I HAD full time I would have used it for sure!! :D

Your right, I did oversteer, but I think that's because I was correcting for some initial understeer, now that I think about it... Either way, I think it would depend on which wheel decided to break loose first, ya know? Bottom line, tires make all the difference!! And yes I think part time works great in deep snow. I was pushing snow with my bumper is was so deep last yeear, no problems! I think this snow was just real wet and slick.
Author:  Jeepjeepster [ Thu Dec 01, 2005 12:08 pm ]
Post subject: 
short answer#2, put revos back on!!
Author:  sstenner [ Thu Dec 01, 2005 3:12 pm ]
Post subject: 
ran into a little snow in new york last week... discovered that too much air in the revos made a big difference too - was running about 35psi, aired down to 32 and they stuck like glue. wasn't even using 4wd at the time.
Author:  bennybmn [ Thu Dec 01, 2005 6:02 pm ]
Post subject: 
Jeepjeepster wrote:
short answer#2, put revos back on!!


I have to agree with that one.... They'll go on before I go home for christmas!! I'm still waiting to hear on a job change, so the KJ may be going away soon... but I'm hanging onto the Revos in case I can find a stick KJ, or maybe a Wrangler. So if it's possible I'll be switching before the holiday, I'm not gonna bather swapping the tires just yet.
Author:  roadpup [ Fri Dec 02, 2005 1:41 pm ]
Post subject: 
One thing to keep in mind with winter driving... the tires that lose traction will always try to overtake those that still have traction. That why the rear end of your KJ swapped ends on you.

I got a better understanding of the dianamics of skids on a skid pad up in Green Bay a few years ago. It's amazing how easily the tail end of a semi-trailer can come around. Because the rigs we drove were equiped with restraining cables, the tractor/trailer combination were unable to go into a full jack-knife. I did manage to do a 360 :lol:

We were instructed that when a vehicle goes into a skid, take your feet off all controls, quickly pick out a reference point you want to go and lock your eyes on that point. 10 times out of 9 you unconsciencously steer towards that reference point. So if you lock onto the center of the road, that's the direction you tend to go. I didn't believe it until I tried it. It worked. Allowing the tires to roll on their own restores traction and then control.

The only thing different with a four wheeler (cars) is that they tend to spin whereas 75' of tractor and trailer tend to slide/plow.
Author:  Tightwadsjeeper [ Fri Dec 02, 2005 5:44 pm ]
Post subject: 
fiend wrote:
on my command trac t-case i only have 4hi and 4lo, my question is, is the 4hi on mine the same the select-trac full time?


NO. Your command trac is PART-TIME 4WD, and locks the front and rear driveshafts together allowing for no slippage. That is why (as mentioned above) it's actually possible to have LESS traction (especially as the turns tighten or speed escalates) than you might have with 2WD since you're causing the wheels to slip in order to maintain the same rotational speed.

4WD part-time is really only meant to be used in conditions where you have minimal traction and don't need to go fast.

Luis
Author:  SD-KJ [ Fri Dec 02, 2005 6:22 pm ]
Post subject: 
Since we are on the subject of 4wd i want to ask a question. Is it hard on the transfer case to switch back and forth from 4full to 4part time? The other day i found that driving around town the main roads were pretty much dry but the side streets hadent been plowed yet. And i didnt want to damage anything by running 4part on the main roads so i would put it in 4 part when i went on the side streets then back to 4full on the main roads. -Will this hurt anything? I was shifting properly (slow down...shift to 4part...then move on)

Thanks

-nick
Author:  Jeepjeepster [ Fri Dec 02, 2005 7:01 pm ]
Post subject: 
If it was me, i would just leave it in full time. It should be good enough for snow on the road. But it shouldnt hurt anything to switch back and forth. Dont do it to much though. The only thing it can do is wear out the shifting mechanism.
Author:  bennybmn [ Fri Dec 02, 2005 7:28 pm ]
Post subject: 
I also remember hearing/reading somewhere that shifting into neutral lets all the wheels do their own thing too, much better for re-gaining traction. Now I don't see how shifting the transmission into neitral would make much of a difference since the center diff is still locked, and I don't think I would have the time/presence of mind to grab the T-case lever and toss it into neutral, but it's worth a try if you're experimenting in a parking lot or something.
Author:  SKIDOG [ Fri Dec 02, 2005 8:40 pm ]
Post subject: 
I think that is actually true to a point.I drove a 18 wheeler for Schnieder at one time and the film they showed us during training showed a truck jack-knifing on ice and as soon as the driver pushed in the clutch the bed came around and the truck was back to going in a straight line.You would have to see it to believe it.From what I remember putting it in neutral takes the drive off the wheels and there is no force pushing them one way or another.But like you said who's going to grab for the lever while your fishtailing.The only thing would be to try it somewhere were no damage would occur.
Author:  bennybmn [ Sat Dec 03, 2005 12:51 am ]
Post subject: 
As soon as we get some more snow I plan on experimenting a lot!! There's a road right near me that's real wide and basically vacant on weekends/nights. Whenever I go there in the snow there's a lot of donut marks and what not! Good place to play. nice and flat, nothing big and hard to run into :D
Author:  ManicMechanicJoe [ Sat Dec 03, 2005 3:24 am ]
Post subject: 
here's a good one if you have SelecTrac: place transfer case in full-time, put the transmission in reverse, crank the steering wheel to "lock" on one side, then give it gas (or diesel for you CRD folk). The vehicle will rotate on a center axis, something only Audis and Subarus used to do. Its a blast to watch you buddy in his (insert brand name here) fullsize pickup attempt this as it only works with an open center differential. It looks incredible and does not require a lot of space.
Author:  SD-KJ [ Sat Dec 03, 2005 3:58 am ]
Post subject: 
ManicMechanicJoe wrote:
here's a good one if you have SelecTrac: place transfer case in full-time, put the transmission in reverse, crank the steering wheel to "lock" on one side, then give it gas (or diesel for you CRD folk). The vehicle will rotate on a center axis, something only Audis and Subarus used to do. Its a blast to watch you buddy in his (insert brand name here) fullsize pickup attempt this as it only works with an open center differential. It looks incredible and does not require a lot of space.


Oh yes! I do this a lot, You really dont have to give it much gas at all and when you get good at it you can do it rolling! Lots o fun! :lol:

The first time i ever did it i was sort of freaked at how easily it did it but now that i have mastered it its sooo much fun, more fun than in a pickup because in my old 2wd pickup you can get stuck but not in the Jeep!!!!

-nick
Author:  waddy whiteguts [ Sat Dec 03, 2005 1:58 pm ]
Post subject: 
Oh I have to try that, come on snow! With my old Isuzu Pickup 5 speed in 4wd in snow I could accelerate great but if I backed off the gas quickly without pushing in the clutch it would go into a full skid like I had slammed on the breaks. I had an Axiom also (big mistake) that had an auto tranny that had some kind of problem where it wouldn't freewheel properly when decelerating and that got a little hairy in the snow. My Liberty is the best vehicle I've driven in snow so far! I use full time and part time depending on how deep the snow is here.
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