We were driving along today and started hearing a wheel speed "click click click" from the driver's side rear, so pulled into the first mostly empty parking lot to examine that tire. I got out and my wife drove slowly away, a few feet at a time. Found a big rock jammed between the treads, popped it out, and problem solved. Much better than the bolt or nail all the way through it could have been. So here's the odd part. Every time she moved the car a few feet forward the back bobbed up and down. I had never seen it do that before. When we got home after completing our errands I had her watch the back while I drove it forward as she had, and it wasn't bobbing. Then I remembered that she had said that she needed to give it a little gas to make it move, so figured she must have had the parking brake on. Applied that brake, and sure enough the Prius went bob bob bobbing along. I understand why having the parking brake on is enough to keep the car from creeping, but how does it make the car bob when gas is applied?
The torque of the front axle transfers a downward force on the braked rear axle. Pretty obvious, I would have thought.
Sure, but the the motor always turns the axle and the car doesn't bob normally when it starts up. Having the back wheel locked is needed. I think it goes something like this: The motor applies torque to the front axle. Under normal operating conditions, ie, no parking brake, this will cause the front tires to rotate around that axle a lot and the body a negligible amount. When the parking brake is on the front tires cannot rotate (initially) because the back tires are essentially locked to the ground. The torque has nowhere else to go and the rotations reverse: the wheels don't rotate around the axle but the body does until the forces reach equilibrium - it displaces vertically by a few inches. Eventually enough torque is applied that the force at the contact patch on the front wheels, which is similar to the force at the rear wheels, is so much the parking brake cannot keep the back wheels locked and they and the front wheels turn. At that point torque is applied more and more to the front wheels and less and less to the body. The car body displaces back towards its initial vertical position.
I expect some of this is simply a resulting force at the rear axle. The car moves forward and the rear tires rotate around their center. The partially applied parking brake (that's mounted to the rear suspension trailing arm) resists that rotation. The resulting force tries to rotate the trailing arm at its pivot towards the body. At some point the coil springs can't compress more and rebound. If either brake drum is slightly out of round, then brake effect will vary some as the wheel rotates - which magnifies the phenomenon. Posted via the PriusChat mobile app.
