4wd hi turning radius ???

[paul2005TB]

I have just tried my 4wd hi/low mode. In a parking lot in 4wd the TB can not turn at any reasonable radius certainly nothing near what it can in the 2wd mode. It is hard to turn the wheels and it "feels like" the brakes are being applied in proportion to the steering angle.

Question: Is this normal?... it sure doesnt feel right. the vehicle is brand new and has 250 miles on it.

[mjr15197]

Its normal.

[jwstewar]

I sure wouldn't be doing anymore tight turns on pavement while in 4wd. That is a good way to break something. Even though the T-Voys aren't coupled as tightly as some other 4Xs, it still isn't a good idea.

[inphoenix]

Dad always told me to take my truck on bare dirt once a month and run it in 4wd. A good five minutes or more, to keep the tran 4wd gears in good shape. Put 65k on my 03 TB and never had a tran problem. Those runs were the only time I took it "off road". What your feeling is totally normal, don't let it worry you.

[paul2005TB]

Greatly appreciate your replies Mjr, Jwstewar, Inphoenix.

It simply struck me as odd since front wheel drive vehicles do not behave like this.

Inphoenix, are you saying that on sand it would not feel like the brakes are on and the turning radius increased as it is?

Certainly is true that if you need 4wd you are certainly NOT going to be on a dry paved parking lot.

OK Ill put it to rest but would welcome some comment on (technically/mechanically) why 4wd does this.

[Frostee]

you might be feeling the 'wheel hop'. it happens when the surface youre on doesnt have any give (cement, asphalt, etc.) because all 4 wheels are attempting to move the vehicle. the rear ones forward, the front ones in whichever direction they are pointed. so if youre turning, the vehicle is being pushed forward by the rears, and 'pulled' in another direction by the fronts. the rear wheels pushing cause the fronts to 'hop' when the surface isnt prone to letting go.

on loose or sliperry surfaces (snow, ice, dirt, gravel) you wont feel this as much, since the road surfae has 'give' and the wheel can still rotate freely while engaged in 4wd mode.

ive noticed it on my TB, but only on the 'no give' surfaces

my :m2:

[mjr15197]

Check out the following link. They can explain it better than I can. The section on the transfer case is relevant to what you are experiencing.

http://auto.howstuffworks.com/four-wheel-drive.htm

[the roadie]

I have just tried my 4wd hi/low mode. In a parking lot in 4wd the TB can not turn at any reasonable radius certainly nothing near what it can in the 2wd mode. It is hard to turn the wheels and it "feels like" the brakes are being applied in proportion to the steering angle.

Question: Is this normal?... it sure doesnt feel right. the vehicle is brand new and has 250 miles on it.

I see you've been exploring the edges of the envelope where I've been poking around as well. I noticed this early on in my off-road adventures, and did some tight circles on hard-packed dirt so I could look at the tire scrub marks. Wasn't able to conclude anything from that, but it will be raining today, and I bet I can get somebody to watch the wheels in the parking lot at work and see which one scrubs.

So I read the shop manual and theory of operation for the transfer case, front differential, and differential disconnect unit. Everything I've read says that the front differential is totally open and unlocked, which is a bit disappointing with the rear locker option being sooooooooo useful on steep and slippery trails. An open diff would not exhibit this effect of scrubbing during turns, unless it was a front-to-back locking effect caused by the transfer case. But the explanation of the clutch operation in the transfer case leads me to believe that it can act as a slip joint in 4HI and 4LO mode

Now, on a few steep and slippery trails, with ruts that cause one or another wheel to be in the air (articulation is pretty lame until I modify some Moog sway bar links to give me a quick disconnect feature), I've noticed that the rear automatic locker (G80), a fantastic option BTW, often activates to keep me going when a rear wheel loses traction.

Oddly, I never seem to have a spinning tire problem with the front end! There isn't an automatic locker, and the manual says the front diff is open, so a tire in the air should spin like mad, taking half of the torque from the transfer case, and letting the rear wheels do all the moving.

So either I'm not noticing wheel spin in the front when it happens (in the excitement of being on such a trail in the first place.), or perhaps the front differential disconnect unit also serves to lock the front diff? Or it's another unknown behavior of the transfer case clutches?

It bothers me to not know all about how it works, and it's definitely related to your turning radius question, and I was thinking to run some driveway experiments soon. Put it all on jack stands with the tires about an inch off the ground, and chock 1, 2, or 3 wheels at a time and *carefully* see what happens to the no-traction wheel. I'm also interested if both back tires are spinning (as has happened to me in sand) but the front tires are digging on rocks, will I get decent (or any) traction from the front tires to pull me through. A locking transfer case (which we don't have but TJWilly likely does on his Jeep) would guarantee front traction.

Our superior turning radius is so useful otherwise, and really helps me pick my way through rocks and trees, compared to a Yukon/Tahoe, for instance - but it is a big surprise to see it scrub in parking lots. I also have to try it in A4WD mode, since that mode engages the front diff disconnect spline shaft, but leaves the transfer case in "ready but not transferring torque until it detects slippage" mode. If it doesn't scrub in that mode, then the root cause is going to be related to the transfer case, not the front diff.

Any 4WD specialists care to comment?

[paul2005TB]

Thanks Roadie, Im starting to understand what is implied by 4wd.. not as simple as 2wd from the front when the the rear wheels are pushing too.. I will have to re-read your post a few times to digest it all. Great content. Im so thankful and grateful for this forum.

I certainly hope I didnt compromise anything by pushing it in an overly tight radius.. I did it for at least 1/2 a turn until I heard that rubbing friction sound and braking feeling.

[the roadie]

I certainly hope I didnt compromise anything by pushing it in an overly tight radius.. I did it for at least 1/2 a turn until I heard that rubbing friction sound and braking feeling.

Good thought. I haven't heard of it on this forum yet, but weak CV joints can be broken if the tire friction is high enough on dry pavement. GM might have designed them to be stronger than stock tires, but they can't know about upgraded tires.

A paranoid person might think the too-short distance from the tire to the upper control arm was engineered in ON PURPOSE to prevent us from putting on bigger tires and that allowed them to use wimpier CV joints. I haven't broken anything expensive on the trail yet, but I predict my CV joints will be the first item. As long as I am carrying the tools to remove the halfshaft, it won't be a disabling failure.

[MTPockets]

Yep, it's the front & back axles trying to rotate @ the same speed that causes the binding on dry pavement in 4x4. The back is trying to push the front, the front is trying to pull the back.:duh: That in combination with the TB's incredibly tight turning radius. These babies will turn on a dime.:thumbsup: Front CV joints & half shafts are usually the weak link in a IFS 4x4. Roadie, one way to keep the free wheel from spinning on an open diff is mash the brake pedal. Use the parking brake for the rear axle. A locked front diff is a bear turn with + our front axle disconnect wouldn't work with a front locker. Best way to keep your 4x4 components exersized is to pop on the 4x4 for a short burst as you pull out of your driveway every other week. Straight aways are best.

G/luck
Joel

[the roadie]

Yep, it's the front & back axles trying to rotate @ the same speed that causes the binding on dry pavement in 4x4. The back is trying to push the front, the front is trying to pull the back.:duh: That in combination with the TB's incredibly tight turning radius. These babies will turn on a dime.:thumbsup:

Got it. I tried the parking lot in A4WD, and it didn't scrub at all. In that mode, the clutch in the transfer case is commanded by the encoder motor to put the clutch plates close, but not engaged until the transfer case detects significant RPM difference between the front and rear shafts.

So using a hint on a web site, and a few Envoy specs , then I put together this to-scale drawing. The thing to think about is that the turning point is in line with the rear axle, 185 inches from the rear differential. The front tires are both farther from the turning point than their rear counterparts, and the distance from the front differential to the turning point is 218.5 inches.

If we had a viscous clutch in the transfer case like some SUVs and Jeep Grand Cherokees, this difference in shaft speed would not cause bind-up. But the TB/Envoy transfer case clutch pack, in 4HI and 4LO modes, attempts to lock the front and rear shafts together. I thought from the factory manual that this clutch pack would slip, but it seems from parking lot experiments that it's a pretty tight coupling. I like it!

SO in a curb-to-curb left turn, with 36 foot turning diameter, the right front tire is going to describe the longest line, of approximately 36 feet /2 X 12 in/ft. times pi or 678 inches. A stock 29" dia tire has 91" circumference, so it's going to rotate about 7.5 times in the turn. The left front (with its turning radius shorter than the right front) is going to rotate about 6 times.

The right rear rotates about 6.6 times, and the left rear rotates 4.7 times.

The front differential, since it's unlocked, will average out the two front tires, and (ignoring the gear ratio inside the differential) turn its shaft 6.75 times (the average of 7.5 and 6). The rear will try to turn its shaft 5.65 times, the average of 6.6 and 4.7.

This is about a 20% difference between the front and rear driveshafts, and the transfer case clutch pack must be doing its best to keep them locked together, so naturally the drivetrain binds up.

Isn't math wonderful?:cool:

The Visio drawing also tells me that the right front tire is at an angle of 26 degrees in all this, and the left front at 32 degrees. Assume our steering geometry is similar to this ideal case.

http://www.roadie.org/EnvoyTurning.JPG

[iDontKnow]

The TB/Envoy is a part-time 4WD system. You'd need a transfer case with a center differential or viscous coupler.

Research the NP242 transfer case. That was a viscous coupler transfer case used in the Jeep Grand Cherokee in the early 90's.

Below is a very helpful link.

http://www.4x4abc.com/4WD101/index.html

Hope that helps!

[the roadie]

The TB/Envoy is a part-time 4WD system. You'd need a transfer case with a center differential or viscous coupler.

Research the NP242 transfer case. That was a viscous coupler transfer case used in the Jeep Grand Cherokee in the early 90's.

Below is a very helpful link.

http://www.4x4abc.com/4WD101/index.html

Hope that helps!

Actually, I'm very happy *not* having a viscous coupling. If I'm on dry pavement, A4WD is fine, and doesn't scrub. On the trail, I want the transfer case to lock up as much as possible.

That web site is by a guy relatively local to me, who also offers expert off-road training *and* a special Rubicon run for lightly modified SUV's! I've been looking longingly at it for months!

http://www.rubicon-trail.com/Rubicon/ot.html

"Let's face it - the Rubicon Trail is a mean one.
Most everyone agrees you need at least big tires and low gears to make it through - to make it look easy.

But what about if you don't care about making it look easy? What if you don't want to jack up your truck and put monster mudders on it just to drive some frickin' ol stage coach road? Drive it anyway!

I'll help you save your face and your truck. Group size limited to 4 vehicles.
We'll do a Rubicon Trail trip with stock or close to stock 4x4. Not to prove it can be done - but to have a ball!

When?
August 2, 3, 4 and 5, 2006

The Rubicon can get crowded on weekends. Starting on Thursday gives us a good headstart and a better choice of camp sites. Leaving the entire Sunday for your drive home gives us a safety cushion in case something goes wrong and we need extra time on our way out. Also, the trail is officially closed on Friday for the annual Jeep Jamboree and a start on Friday would not be possible.

How?
You sign up, pay your fee, we'll meet and I'll use every trick in the book to get you through. Unscathed. It'll be hard. Lots of work. Lots of sweat. To make sure we get it right, we need some extra time. 3 full days."

See you there!

[nheidmann]

im in, but where is that at?? Im in atlanta, so I am gonna need directions, a price, and a permission slip (for my parents):rotfl: they will let me go!!:D

[the roadie]

im in, but where is that at?? Im in atlanta, so I am gonna need directions, a price, and a permission slip (for my parents):rotfl: they will let me go!!:D

If you have to ask where the Rubicon is, you're not worthy to ride it. :rolleyes:

:D

I'll start a new thread soon for Rubicon planning, assuming Mrs. Roadie gives me the gift of the August guided run for our anniversary or something. I already got the PullPal winch anchor for my birthday.
:offtopic:

[iDontKnow]

Actually, I'm very happy *not* having a viscous coupling. If I'm on dry pavement, A4WD is fine, and doesn't scrub. On the trail, I want the transfer case to lock up as much as possible.
Yeah, a viscous coupler transfer case is definitely not for the off-roader. I had looked into a NP242 because I'm not a big fan of A4WD and don't really plan on doing the off-roading like you do... but would like the 4WD for snow and rain.

I did find a couple sites that made NP242 adapters for earlier 4L60E trannys, but none for the newer ones...

It'd probably be more work than worthwhile... but I like dreaming up projects. :)

[Dacomputernerd]

:undecided This thread is very interesting reading!

[nheidmann]

hey now, Im only 17, so I dont have a clue other than Im sure its out west.. maybe colorado, around lake tahoe?? would be cool, but Im sure the tb would be beat pretty good, and need quite a few mods..

[the roadie]

It'd probably be more work than worthwhile... but I like dreaming up projects. :)

As do I.....as do I......

Actually, it's a pretty easy mod (I imagine) to fool the transfer case controller into overdriving the clutch through the encoder motor, and increase the front-to-back locking. Hmmmmm. :undecided

[the roadie]

hey now, Im only 17, so I dont have a clue other than Im sure its out west.. maybe colorado, around lake tahoe?? would be cool, but Im sure the tb would be beat pretty good, and need quite a few mods..

Even a 17YO can acquire clue. :cool: Don't put yourself down, but use the net.

It's definitely in the Tahoe area, which is in CA, east of Sacramento which is about in the center of the state. I'm currently running through my local friends to see who wants to ride shotgun and be cameraman. My budget for the trip is about $4K, half of that would go for fresh mods. Oh, and I'm 54 years old on Sunday, and my only daughter's married off at this point, so this *is* my hobby. :yes: