i am assuming this is from the front end having limited slip?

would this be an accurate assumption?


stock tire size.

 

It comes from you back tires pulling and nothing slipping, I have bent a driveshaft by turning too tight in 4x4 before. I would be darn careful on hard surfaces!

 

U joints...I think...

 

yeah, u-joints. not good on them turning sharp on a hard surface.

 

Here is what is happening:

Your front tires travel in a farther path (from the center of the turning radius circle) than the rears do. If you look at the paths in which the tires travel, you'll notice that the inner rear tire makes a smaller circle than the outer front. The front tires have to spin at a faster speed to travel a longer distance in the same amount of time than the rears do. This results in a bind issue since the two driveshafts HAVE to spin at the same RPM because they are mechanically linked in the transfer case. However, the front one needs to spin faster to cover a greater distance but can't. It's essentially the same scenario as an open differential vs. one that is "locked". There HAS to be a speed differential device or binding will occur. If no differential is allowed, something has to give....hopefully traction gives instead of a mechanical part.

Also, the u-joints in the front axle shafts (in the knuckles) ONLY provide driving force to the wheels at a steady speed when the wheels are pointed straight ahead (no u-joint angle). Once the wheels begin to steer in a direction other than straight, the driving force on the tire side of the u-joint begins to speed up and slow down twice per revolution. The higher the angle of operation, the larger the discrepancy in the output speed.

 

If you ever take the axles shafts out you will notice the u joints do not bend (for lack of a better term) in a way to allow us to turn sharply enough on flat surfaces. I have to do a 3 point turn everytime I pull into my driveway with 4x4 on!

Or what Cody said above!

 

Great explanation Cody- you nailed it.

 

It's not the u-joint's fault that you can't turn sharp. The angles they operate at are the same whether or not they are being "driven".

When you're trying to turn on a slick surface while in 4X4, the truck will "push" through the corner (continue to go straight). This is because there is no differential speed in the driveshafts (front and rear). The tires are forced to turn at the same speed and since the front tires are refused the opportunity to spin faster, they begin to "drag" and overcome friction on the road surface so they slide sideways.

 

I just had another thought.....

Research a little on Full-time 4x4 trucks. What is different on those that allows turning on hard surfaces with substantially less "bucking"? Answer....there is a differential in the transfer case.

 

Now I understand it better, great explanation Cody.

 

I guess I should have clarified. I was out in the shop working when I had an epiphany and I should clarify before someone beat me to it.

The above quoted post relates to older full-time cases. The new trucks (specifically half-ton based trucks) don't even have a traditional front drive mechanism. There is no "positive" engagement method on newer AWD or "auto 4x4" transfer cases. Instead of using a positive dog tooth lock to drive the chain that turns the front output yoke (and ultimately, the driveshaft), a clutch pack is used and varying amounts of pressure are applied to it during various operating conditions by the shift motor by way of cam action. This clutch pack is between the main shaft 4x4 shift collar and front drive chain. No mechanical "solid" link exists. The Transfer Case Control Module uses a couple inputs to decide when and how much to apply the clutches. If "auto" is selected, the clutches remain very lightly compressed until rear wheel slip is detected and then pressure is applied to "even out" the driveshaft speeds front to rear. The actual "4x4" position will command maximum clutch pressure at all times....but it's still not mechanically locked.

Depending on make, a front axle disconnect system will be used. "Auto" will lock this disconnect together and therefore the front drive axle will be ready for any input from the transfer case and front driveshaft.

Ok...now you're confused.

 

Great explanation Cody!!!

 

Thanks for that. I know we all benefited from it.

And just to clarify, it is the tire "slipping" or "giving" way as if it is catching up with something else...


Thanks,
Andy