[jungo75]

Is it the DCCD that makes the car pull on uneven terrain, like a messed up street or something like that or is it a problem with the car? I was reading that the car puts the power where it needs it, so I was thinking its the car and not a problem. Does anybody have a different take on this one?

[donmei]

If the center diff is locked, and the car starts to push, your natural instinct to lift will save the day. It is NOT the fastest way around a snow cross course, but in the real world where you are NOT concentrating 100% on driving, understeer is king.

Also, at high speeds understeer is king. Thats why at short tracks like NHIS the nascar guys like the car "loose" i.e. oversteering. But at long, fast tracks they want a little "push" i.e. understeering, because it is more predictable.

There is little more terrifying than running out of steering lock while powersliding your way around a corner and going into the weeds sideways.

ZONE - no flame intended, but I suggest you go to a high performance driving school. your comments above show some gaps in your knowledge of the theory of how a car goes around corners.

In either situation, whether the diff is locked or not lifting will cause the car to tighten its line. Its just that in one case that happens by the front end tucking back in (nice and predictable) and in the other case it is caused by the back end stepping out. (not so predictable, but fast and fun).

Botom line is that if the sh1ts on the ground and you need to get home , lock the diff.

Don

[donmei]

I just dug this up from another post I did in 1995. Its pretty explicit on how the dccd settings affect things:

************************************************** *******

Hoo boy. I can shed some light, but can't figure how to do it without taking up 5 pages.

First, I think the original poster just wanted to understand the system, he didn't want to make his car into a rwd car.

Jumping in. All differentials, even open differentials can be configured to give more torque out one axel than the other. (think of our center differential like a rear differential turned 90 deg.

Our differential is mechanically set to give 65% power to the rear wheels. Just like the rear axel on an old Chevy is designed to give 50 % power to each wheel. Ahh, but what does this mean . . 50%.

Ok lets say that one wheel is on ice and the other is on pavement. The differential will only be able to send as much torque to the wheel on pavement as it can to the one on ice.

So in this case, the ice limits the amount of torque that can be applied.
Since power is torque x rpm and the wheel on pavements rpm is 0, then power to that wheel is 0. The wheel on ice has the same low torque value, but is turning, so all the power is going through that wheel. This stuck car scenario is easier to visualize than an awd system to me. So lets continue.

Now lets replace our stuck car's rear differential with one that can be locked. Now the axels are locked together. As you step on the gas, both wheels turn at the same speed. But the torque applied to the icy wheel is limited to traction. This wheel slips, but does not spin. (you can't see it) The tire on pavement receives the majority of the torque. And since its turning at the same rpm as the wheel on ice, you have the wheel on the pavement now receiving the bulk of the power.

There's no "intelligence" to a locked axel. But a locked axel does in fact, automatically give more power to the wheel with more traction. (since rpm of each wheel is the same, which ever one has the traction to take more torque, is taking more power)

So, back to our car. Now imagine that differential, rather than distributing left to right, distributes front to back. 65% of TORQUE goes to rear wheels. When the system is open, that is always how it is. Thats why guys can do donuts with the rear wheels spinning and the fronts just appearing to be along for the ride. The fronts are getting 35% of the torque, not enough to break them loose, the rears with 65% are spinning up. Now we roll the dccd control forward, all the way forward. Now front and rear MUST move at the same speed. Torque is now biased based on traction. In theory its 50/50, but in certain cases it could be nearly 100% front or nearly 100% rear. For example, where the front of the car is on dry pavement and the rear is on wet ice. In this case, with the diff locked, almost all the torque (and since f and r are turning at same rpm) almost all the power will go to the front. Now lets go back to open differential. Same situation. The rear wheels spin. Lets say that while spinning they can only produce 65 units of torque (whatever that unit is) Now, the front only gets 35 units of torque. So, the slipery conditions at the rear of the car limit how much torque the front gets. And if this isn't enough to get you going, you could be stuck with one end on perfectly dry pavement.

What our computer does is continuously vary the lockup of the diff in order to adjust power for the best handling and/or traction. In manual, using the dccd, you can go from open 65/35 to locked. When locked, it could be as much as 100% front, or 100% rear.

Hope this is of value.

Don

[doublejz]

put it in lock and do donuts in a nice drive asphalted parking lot

[wrbsti666]

I like locked or auto in the snow also

[karr1008]

Nice write up Don.

[jungo75]

Very helpful for newbies like me don, appreciate it.

[boondocksaint]

Don is t3h king. Lock in snow, +1. Also, be aware that 06+ MY have a slightly different torque split (although I cannot remember it for the life of me). 35/65 was 04 (and 05 maybe) only. :dontknow:

[RichXKU]

04-05 was 65/35
06+ is 59/41, also the 06's have a gunked up center diff (viscous) no longer able to be truly full-open. Bad for donuts and slight increase in understeer, but good if...
you're trying to make a u-turn on very uneven pavement and the rear axle gets one wheel off the ground, so you hit the gas and the rear spins wildly for a split second then you feel the DCCD go BUPUP-B-B-B-B-BBBB as it tries to match a stopped front axle with a free spinning rear axle.

So aside from making it easier for the DCCD to match axle speed when there's a large difference, I can't think of much benefit to the adding of goo, and changing of tq split in the 06+ center diff. Someone prove me wrong.

[Shomen]

Nice write up Don. I actually like "auto" in snow as well. I was driving up to Tahoe in about 1+ ft of snow and remember the posts here and decided to try "lock" while waiting to go through chain control.(FYI I didn't have chains on) Long story short the faster I went (35-45) it felt like the back wanted to step out aka fishtail. I put the car in auto and the car calmed down and felt alot better (so much so I was cruising at 50 passing people till I looked at the speedo and slowed down) Of course I'm going to experiment again this winter but that's my experience.

[RichXKU]

Originally Posted by Shomen

decided to try "lock" while waiting to go through chain control.(FYI I didn't have chains on) Long story short the faster I went (35-45) it felt like the back wanted to step out aka fishtail.

That is pretty much the opposite of what you should have experienced... Usually locking the front and rear together causes heaps of understeer!

[Moderato]

Originally Posted by donmei

I just dug this up from another post I did in 1995. Its pretty explicit on how the dccd settings affect things:

************************************************** *******

Hoo boy. I can shed some light, but can't figure how to do it without taking up 5 pages.
..............................Hope this is of value.

Don

Excellent write up, my thoughts exactly.

[donmei]

oops above should read "another post I did in 2005, not 1995.