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Fwd problems

I jacked up the front of my car and noticed that both front wheels turn when in park. They do full rotations. But that’s not it. When you turn one wheel forward, the other wheel turns backwards and vice versa. It’s fwd. Im guessing these things arent supposed to happen. So I’m wondering if it’s axle problems or something else.

That’s how an open differential works.

When you raise both tires off the ground with an open differential, and spin one tire, the tire on the opposite side will spin in the opposite direction.

If you do this with a lock-up differential, both tires will spin in the same direction.



I’ve notice that happening w/my manual transmission Corolla, but would that behavior be expected with an automatic transmission in P also? With my RWD truck I don’t think I can manually turn the wheels at all when the trans in P. It’s has a limited slip diff though.

What will a FWD with posi do when you try and rotate the wheels when the car is in park but both tires are off the ground?

Sounds like there’s no problem here

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Depends on the type of posi. A viscous posi will rotate opposite like an open diff but with some drag. A gear type posi will do the same with virtually no drag. RWD clutch type posi is not used on FWD cars as it make steering rather stupid.

Yes it would.

Your guess would be wrong. It’s normal.

Why is kind of hard to explain, but it’s because of the way the differential gears are set up.
I posted a copy of the differential for a RWD vehicle because I thought the internal movement would be easier to visualize, but a FWD works the same.
Envision if you can turning the axle shaft with the big ring gear. Now follow the turning movement through the spider gears from the axle on the side you’re turning through the spider gears to the gear on the opposite axle. You’ll find that the center spider gears will cause the opposite axle to turn in the opposite direction.

From TSM’s diagram above it looks like while the pinion and ring gears won’t turn in P, the spider and side gears will allow the wheels to turn opposite directions, provided both wheels are off the ground and it isn’t a locking-style differential.

This isn’t the image I’d hoped to find, but I was unable to find a better one. In a FWD system it isn’t as easy to understand (I think) because of the eccentric nature of the designs, so I chose this image for illustration purposes.

To understand the image, one needs to imagine the spider gear on the right hand side as well, and to understand that the spider gears in the upper and lower positions in the image will be held in their planetary position by the gear casing. When the car is moving in a straight line, they’d be operating in an orbital path around the center of the unit but not turning on their own axes. When the car is turning, they’ll be orbiting but also turning, each at a different speed.

Differentials are difficult to explain verbally. But I think they’re one of the most ingenious and straightforward approaches to one of the most complex problems in a car’s design; how to apply continued torque to both wheels while allowing the wheels to turn at different speeds. The simplicity with which they do this is IMHO absolutely ingenious.