On cars with manual transmissions, when in low gear (typically 1 or 2), why does accelerating and then taking your foot off the gas make the car lurch forward with that uneven, jerking motion?

Question

Why wouldn’t the car just decelerate smoothly when you take your foot off the gas? And why does it often continue even if you step on the gas again?

In: Engineering

Answer 1

Taking your foot off the gas starves the engine, so the combustion firing is no longer fast enough to keep up with the wheels’ rate of turn from the car’s inertia pushing it forward. Since, with the clutch engaged, the wheels are still directly connected to the engine, it’s being pushed faster than it naturally “wants” to go for this reduced amount of fuel, so you get “engine braking”. In a higher gear and/or higher speeds, the engine braking feels smoother, but at lower speeds it is jerky because you “feel” the cylinders individually resisting the turning and there may be some misfiring cylinders. If you disengage the clutch (push the clutch pedal) the engine and wheels can both turn smoothly again at different rates.

If you have a good “feel” for this, you absolutely *can* let off the gas to slow down, or first downshift and let off the gas to really slow down without braking. But you have to recognize that below a certain RPM (speed of rotation) the engine will be very unhappy with this.

Why does it still jerk if you push on the gas again? Because, after slowing down without shifting gears, the engine now has to overcome a much higher resistance, without mechanical advantage of a lower gear ratio, to accelerate the wheels again. So the engine strains and skips, or stalls completely. That’s why you would want to downshift before trying to accelerate.

I’ll often downshift to engine brake into a corner, use the real brakes to adjust the speed for best cornering path, and, since I’m already in a lower gear, accelerate out of the corner rapidly. But this is at a relatively high speed.