What causes kangarooing in a car

[Slick Rick]

If im driving really slow, put the car into first, lift off the clutch and then press the accelerator slightly the car starts jerking a lot. Why is this?

 

[Sage]

You’ve just given it a crapload of torque – first gear is really designed to just get your car moving from a complete standstill. If you’re already crawling, try putting it into second – it should be a lot smoother.

 

[Duke]

First gear is very short, meaning that a lot of torque is transmitted to the drive wheels, as Sage says. So if you give it a little gas, the car jerks forward, and all the slack in the driveline - motormount bushings, gear play, CV joint play, twist in the driveshafts, etc. is tightened up. Also, you yourself are moved backwards in the seat, and your foot lifts slightly off the accelerator, making the car slow down. Then, again since first gear is very short, the reverse process happens: all of that driveline windup relaxes and carries on to the deacceleration side... causing you to move forward in you seat slightly, thus pressing on the gas a little more, and repeating the cycle.

 

[daan]

Australian petrol.

 

[Slick Rick]

Thanks people, explains it perfectly.

 

[daan]

Thanks people, explains it perfectly.

No problem. Glad to help, cobber.

 

[Stevisiov]

Australian petrol.

 

[wfooshee]

There's also the factor that at such low speeds, the driveline winding back and forth on its mounts is actually a significant fraction of the wheel speed, so the engine rocking back and forth actually accelerates and decelerates the wheels, even without changing throttle and engine speed. It feeds back on itself until either the engine stalls, you depress the clutch to release the oscillation, or you accelerate out of it (which is the worst choice - if you're in the bump bump bump driveline lash, press the clutch and try again to re-engage it smoothly.)