I have towed and launched many a boat in my day. I used to be a semi-pro slalom skier. I have NEVER had a truck 'roll' backwards, or forwards, while in Park. I have NEVER had a single vehicle 'roll' while in Park. 90% of all people never use the e brake. I hardly ever use the e brake.

 

Once again, I have NEVER had a vehicle 'roll' when it was in Park. I hardly ever use the e break. The point of the e break is exactly that, for emergencies. For that one odd time your brakes fail.

 

Others have already explained it, but you refuse to understand or even acknowledge how a rear differential operates.

In the video the driveshaft is NOT moving. Period. And I can prove it without the driveshaft being visible in the video.

Watch the video again carefully starting at 1:35. At 1:37 you'll notice the driver's side rear wheel is trying to spin FORWARD. You should ask why that is.

Learn how a rear differential operates, and you can correctly answer why that wheel was spinning forward as the truck was rolling backwards.

 

I do understand how a differential works. I was a line mechanic at a Ford dealership back in the 70's. If the transmission is in park, the driveshaft can't rotate. If the drive shaft can't rotate, neither will the rear wheels.

 

With that answer there, it clearly means you do NOT understand how a differential works. Furthermore you refuse to even try to learn.

And you avoided answering the question as to why that wheel was rotating forward as the truck was rolling backward.

Let's be honest, you've successfully lost all credibility in your argument at this point. The answer to the question was already posted in this thread by a Ford engineer, all you had to do was copy and paste and you would have been correct. Yet you were unable to identify the terms "pinion", "spider gears", or even the word "stationary".

You're only arguing for the sake of arguing at this point. There's nothing further to discuss.

 

Pocket, I am not trying to argue. Please explain to me why my truck doesn't roll down the hill when I have it in park WITHOUT the e brake in use? When a vehicle is in park, it shouldn't roll. That's all I'm saying.

 

Just like doing a burnout with an open differential, you will loose traction with one wheel and not both if that one wheel has less grip.

when you are parked and the transmission is in P, if one wheel is held still, the other can still rotate, thus the truck can roll backwards.

 

So what you are saying is that this can only happen if one wheel has the ability to slide?

 

that's what I think. One wheel will stay still (while sliding), and the other will rotate.
Although I'm not nearly as much of an expert as some of the guys above.

 

Correct.

If both tires have equal grip, it's equivalent to driving in a straight line. The differential keeps both wheels spinning at the same speed. In park, it keeps both wheels locked equally from moving.

If one tire has grip but the other doesn't, the differential is designed to allow the two wheels to move at two different speeds. It's how vehicles turn smoothly without wheel hop in the rear.

Ice is a prime example of how easy it is to make this happen when the transmission is in park, as one wheel can very easily lose grip on an incline - as shown in the video.

This also happens on boat ramps if they are steep enough, and if there's sufficient water, slick algae growth, or enough gravel to allow a wheel to lose grip.

If say you're stuck in mud and doing the one-wheel spin, you actually have two things spinning - the driveshaft and the loose wheel. If you're in park and rolling, the driveshaft isn't spinning, so both wheels spin - in opposite directions as the spider gears walk. At least 2 things have to spin, either a driveshaft and wheel or both wheels.

This isn't easily accomplished with a truck on a lift and trying this by hand. With enough force you can duplicate it, but most people never try so it's not exactly something observed on a daily basis. But in the video, you can clearly see the driver's side wheel moving forward, so that means the other wheel is spinning in the opposite direction, and the driveshaft is not moving because it's locked in place with the transmission in Park.

 

Thank you. I buy that. The ice/slippery surface is what threw me.

 

Ahhhh, I found a perfect demonstration. Watch the entire video, the 2nd half explains what we see in the ice slide video:

 

If the truck is on ice, then there is no park. You're saying, repeatedly, that you don't know how a differential works. I can extrapolate from these posts that you don't know what park does, either.

Park holds the output shaft of the transmission from turning. The driveshaft is attached to the output shaft on one end, and the differential pinion on the other end. Since the output shaft and driveshaft can't turn, either can the pinion. If both rear tires have traction the truck cannot move.

Since this truck was on ice, one of the wheels lost traction. From the video it was the right tire, because the left was rolling in the direction that the truck moved. You couldn't see it, but the other wheel was rolling forward, the opposite of the way the truck was rolling.

That's how park holds a vehicle from rolling. The vehicle can only move if one of the wheels rolls in the opposite direction of the vehicle movement. This is not unique to Ford, it is true for ANY vehicle that has an open differential. It's why owner's manuals tell you to set the parking brake in addition to placing the trans in park.

 

All they have to do to that truck, once it's out of the pool, is drill a 3/16 weep hole in the intercooler.

 

I just spit out my coffee reading that.