Question On My Awd Drivetrain

Baja 252 · January 1, 2011

My drive way has an incline to it. In the winter if it has ice or packed snow on it my old truck in 2hi would slide down the drive. If I put it in 4hi it would lock all 4 wheels in park and stay put.

When my new AWD is in park does the drivetrain lock all 4 wheels or just the back ones? I assume the parking brake is still only holding the rear wheels.

Stevens03 · January 1, 2011

AWD is engaged all the time. There is a power differential from the front to the rear, but the front is always engaged. And yes, the parking brake only sets the rear brakes.

MikeNH · January 1, 2011

Isn't the center diff on the AWD systems totally open? If so then I don't think it will act the same way as a truck locked in 4Hi with a locked center diff, it would do the same as a truck in 2WD.

Matt_ · January 1, 2011

Isn't the center diff on the AWD systems totally open? If so then I don't think it will act the same way as a truck locked in 4Hi with a locked center diff, it would do the same as a truck in 2WD.

No, it would not act like a 2wd.

With all 4 tires on the ground it will act just like a 4wd. The front tires wont roll unless the back tires are rolling (unless the transfer is in Neutral).

MikeNH · January 1, 2011

Isn't the center diff on the AWD systems totally open? If so then I don't think it will act the same way as a truck locked in 4Hi with a locked center diff, it would do the same as a truck in 2WD.

No, it would not act like a 2wd.

With all 4 tires on the ground it will act just like a 4wd. The front tires wont roll unless the back tires are rolling (unless the transfer is in Neutral).

I'm not so sure- on a traditional 4x4 the t-case/center diff is locked so no matter what, power is split 50/50. This is why you get the binding in turns. On the AWD system, the t-case has an open center diff so I'd imagine the front tires can still spin freely whereas the traditional 4x4 the fronts will be locked because there is no place to "give" between the rear and front axles.

cgalford · January 1, 2011

Even if the center diff is completely open if the rear wheels aren't turning and the transmission is locked then the fronts cannot move. Fully constrained system.

Matt_ · January 1, 2011

Isn't the center diff on the AWD systems totally open? If so then I don't think it will act the same way as a truck locked in 4Hi with a locked center diff, it would do the same as a truck in 2WD.

No, it would not act like a 2wd.

With all 4 tires on the ground it will act just like a 4wd. The front tires wont roll unless the back tires are rolling (unless the transfer is in Neutral).

I'm not so sure- on a traditional 4x4 the t-case/center diff is locked so no matter what, power is split 50/50. This is why you get the binding in turns. On the AWD system, the t-case has an open center diff so I'd imagine the front tires can still spin freely whereas the traditional 4x4 the fronts will be locked because there is no place to "give" between the rear and front axles.

I'm 100% positive they wont spin. I know how the system works.

Just like Chris posted above.

tonygxp · January 1, 2011

AWD rules

MikeNH · January 2, 2011

Isn't the center diff on the AWD systems totally open? If so then I don't think it will act the same way as a truck locked in 4Hi with a locked center diff, it would do the same as a truck in 2WD.

No, it would not act like a 2wd.

With all 4 tires on the ground it will act just like a 4wd. The front tires wont roll unless the back tires are rolling (unless the transfer is in Neutral).

I'm not so sure- on a traditional 4x4 the t-case/center diff is locked so no matter what, power is split 50/50. This is why you get the binding in turns. On the AWD system, the t-case has an open center diff so I'd imagine the front tires can still spin freely whereas the traditional 4x4 the fronts will be locked because there is no place to "give" between the rear and front axles.

I'm 100% positive they wont spin. I know how the system works.

Just like Chris posted above.

Unless something has changed than it is impossible for the AWD trucks to "act" like a traditional 4x4- the T-case to the best of my knowledge is strictly an open diff so in theory on ice all power could go to one axle.

Matt_ · January 2, 2011

Isn't the center diff on the AWD systems totally open? If so then I don't think it will act the same way as a truck locked in 4Hi with a locked center diff, it would do the same as a truck in 2WD.

No, it would not act like a 2wd.

With all 4 tires on the ground it will act just like a 4wd. The front tires wont roll unless the back tires are rolling (unless the transfer is in Neutral).

I'm not so sure- on a traditional 4x4 the t-case/center diff is locked so no matter what, power is split 50/50. This is why you get the binding in turns. On the AWD system, the t-case has an open center diff so I'd imagine the front tires can still spin freely whereas the traditional 4x4 the fronts will be locked because there is no place to "give" between the rear and front axles.

I'm 100% positive they wont spin. I know how the system works.

Just like Chris posted above.

Unless something has changed than it is impossible for the AWD trucks to "act" like a traditional 4x4- the T-case to the best of my knowledge is strictly an open diff so in theory on ice all power could go to one axle.

That's not the question. If the transmission is in park and the back wheels are locked, the front wheels wont roll, period. I don't understand why this is so hard to comprehend.

What you're saying it would do is like spinning the rear driveshaft without either of the rear tires spinning... not going to happen. One tire would need to be up in the air for that to happen.

MikeNH · January 2, 2011

That's not the question. If the transmission is in park and the back wheels are locked, the front wheels wont roll, period. I don't understand why this is so hard to comprehend.

What you're saying it would do is like spinning the rear driveshaft without either of the rear tires spinning... not going to happen. One tire would need to be up in the air for that to happen.

This is how I see it-

The truck is in park with the e-brake set, the park pawl in the transmission is locked tight as are the back wheels but since the front wheels have an open diff between them and the locked up trans or the rear axle, couldn't the front wheels spin/rotate still?

Matt_ · January 2, 2011

That's not the question. If the transmission is in park and the back wheels are locked, the front wheels wont roll, period. I don't understand why this is so hard to comprehend.

What you're saying it would do is like spinning the rear driveshaft without either of the rear tires spinning... not going to happen. One tire would need to be up in the air for that to happen.

This is how I see it-

The truck is in park with the e-brake set, the park pawl in the transmission is locked tight as are the back wheels but since the front wheels have an open diff between them and the locked up trans or the rear axle, couldn't the front wheels spin/rotate still?

With the tcase being an "open diff", either the rear driveshaft or the transfer would still have to spin for the front driveshaft to rotate.

If it was 4wd (or a locked diff for the tcase) then the front driveshaft would only rotate if the rear was also rotating, even if the trans was in neutral. That's the difference.

So per the question posed by the OP, the front tires will not roll with the trans in park and the ebrake on.

I also have a hunch that you don't need the ebrake on.... not 100% sure on this but I don't believe the tcase will allow the rear driveshaft to spin with the trans in park.

BrockwayMT · January 2, 2011

Center diffs always screwed with my head. I had to stop and think about this one for a minute.

In theory if there is no limited slip diff then you only need one wheel to slip in order to roll backwards. BW4485 has 40/60 F/R split.

A)If one front wheel were on rollers or ice: For the truck to roll backwards at a wheel rpm of lets say -10 rpm, then a front wheel would be turning forward at 40 rpm (-4 times the travel rpm of the other three wheels).

Math:

Both rear wheels at -10 rpm so rear carrier turns at the average of the two wheels which is -10. Rear propshaft turns at ring gear * axle ratio(ar) which is -10ar. Park pawl in trans holds center diff carrier so rear prop at -10ar makes front prop turn at -10ar*(-60/40)=15ar. Front carrier spins at 15ar/ar = 15 rpm. Since one front tire is turning -10rpm and the front carrier is turning 15rpm then the slipping front tire is turning 40 rpm because 15 is the average of -10 and 40.

B)If one rear wheel were on rollers for the truck to roll backwards at -10 then one rear wheel would be spinning forward at 23 1/3 rpm or negative two and two thirds the travel rpm of the other 3 wheels.

Same math but rear prop turns at -10ar*(-40/60) which is 6 2/3 rpm. Thus with one rear tire rolling backwards at -10rpm then the other rear tire is going forward at 23 1/3 rpm.

By this logic if any one tire can spin the truck will roll backwards. If the park brake is on holding both rear tires then the vehicle will only roll backward if both rear tires and one front tire slips.

MikeNH · January 2, 2011

Center diffs always screwed with my head. I had to stop and think about this one for a minute.

In theory if there is no limited slip diff then you only need one wheel to slip in order to roll backwards. BW4485 has 40/60 F/R split.

A)If one front wheel were on rollers or ice: For the truck to roll backwards at a wheel rpm of lets say -10 rpm, then a front wheel would be turning forward at 40 rpm (-4 times the travel rpm of the other three wheels).

Math:

Both rear wheels at -10 rpm so rear carrier turns at the average of the two wheels which is -10. Rear propshaft turns at ring gear * axle ratio(ar) which is -10ar. Park pawl in trans holds center diff carrier so rear prop at -10ar makes front prop turn at -10ar*(-60/40)=15ar. Front carrier spins at 15ar/ar = 15 rpm. Since one front tire is turning -10rpm and the front carrier is turning 15rpm then the slipping front tire is turning 40 rpm because 15 is the average of -10 and 40.

B)If one rear wheel were on rollers for the truck to roll backwards at -10 then one rear wheel would be spinning forward at 23 1/3 rpm or negative two and two thirds the travel rpm of the other 3 wheels.

Same math but rear prop turns at -10ar*(-40/60) which is 6 2/3 rpm. Thus with one rear tire rolling backwards at -10rpm then the other rear tire is going forward at 23 1/3 rpm.

By this logic if any one tire can spin the truck will roll backwards. If the park brake is on holding both rear tires then the vehicle will only roll backward if both rear tires and one front tire slips.

And this is why we have engineers.

Matt_ · January 2, 2011

In theory if there is no limited slip diff then you only need one wheel to slip in order to roll backwards. BW4485 has 40/60 F/R split.

This is not true.

Let's make this simple.

2WD truck with an OPEN rear differential. If you put the truck in park and jack one tire off the ground, it will NOT spin. Therefore, if you were parked on an incline with the other tire on a perfect frictionless piece of ice, the truck will not roll backwards.

Just because a differential is "open" doesn't mean you can spin any of the 3 rotaries (two wheels and a driveshaft) at any given time. TWO always need to be free to allow rotation.

In that 2WD truck i mentioned, if you jacked one wheel up and put the trans in neutral, then the tire would spin and so would the driveshaft. Edit, And if you were in Park and jacked both rear tires off the ground, they would spin in different directions.

Also, you did a whole lot of math for nothing. 60/40 torque split is not the same as 60/40 RPM split. The front and rear driveshaft spin at the same RPM, not 60/40 RPM (hence, the front and rear differentials have the same gear ratio).