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I don't know much about the inner workings of a car. For driving one, I find that having an rough idea about what the gearbox, engine and clutch do and where they are is often enough.

However for the life of me I can't figure out one thing. You know when you want to get the car moving again from a complete stop, you raise the clutch to the biting point, release the brake and start to accelerate? As the clutch gets to the biting point yet before the car is able to move, for the wheels are kept still by the brake, the front of the car lifts up. What is lifting the front of the car up? Surely if the wheels are not actually rotating, inertia isn't the cause. What's at work here?

Cheers.

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  • Is the clutch engaging at all at the point you're talking about?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Feb 10, 2016 at 23:43
  • @ᴘᴀᴜʟsᴛᴇʀ2 When it's half way up, like what you do when you start the car.
    – Einheri
    Commented Feb 10, 2016 at 23:54
  • You star the car with the clutch half engaged? But I'd imagine it would be the engine applying what torque it can to the chassis.
    – Ben
    Commented Feb 10, 2016 at 23:55
  • @ben Isn't that normal? Turn the key, slowly raise the clutch, release the handbrake, release the clutch and accelerate?
    – Einheri
    Commented Feb 10, 2016 at 23:58
  • Starting the car is starting the engine, the process you described is starting the engine (if that's what the key turn is) and pulling off at an uphill.
    – I have no idea what I'm doing
    Commented Feb 11, 2016 at 8:07

2 Answers 2

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The reason why the car lifts even if its stationery is because of the torque produced by the engine and the suspension.

The engine is not connected to the wheels with one single shaft, there is the transmission in between which will have a very minute slack in the gearings.

So when you let go of the clutch the torque from the engine tries to push the car forward now to things happen here. Assuming the wheels are braked in.

  • In the minute slack in the various parts of the transmission the engine gets a very small space to move.
  • The engine tries to move the tyres but it cant since they are braked in.
  • SO obviously the only other thing which can move are the suspension which have a little bit of flex.

All this combined lifts the car up basically due to the torque of the engine and the flexibility of the suspension.

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12

Newtons Third Law

For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object.

The lateral center of gravity (CG) is above the axle as well.

So you have two different forces acting upon the chassis.

Since the lateral CG is above the axle line for most vehicles inertia takes effect. The chassis is trying to stay in the same place. The rear wheels are driving the car forward. There is more weight above the axle than below it. Car wants to lift.

With the torque on the rear wheels, Newtons Third Law law takes effect. When you dump the clutch and accelerate the opposite must occur in an equal way. Front end lifts and NTL proves itself, again.

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  • I think this answers the question why the front of the car wants to life up when the car is accelerating. However, what I'm baffled by is that the car's nose lifts up when the clutch is raised to the biting point. Is it because the engine, which sits in the body of the car, is pushing itself up trying to move the tyres, which are kept still by the brake?
    – Einheri
    Commented Feb 11, 2016 at 11:47
  • If you read the other response. They both say the same thing. I'm simple explaining it using physics. Take the torque portion and think equal and opposite reaction. Take the front end lift and suspension depression and think inertia. It's the same
    – DucatiKiller
    Commented Feb 12, 2016 at 2:11
  • Inertia is the tendency to resist acceleration. The car is not accelerating, which is why I get mighty confused with the discussion about inertia. I know, the centre of gravity is above the axle, so there is a torque rotating the chassis backwards when the car accelerates. But in this case, the brake is keeping the car from accelerating. I really don't think your one and the other answer are explaining the same phenomenon.
    – Einheri
    Commented Feb 12, 2016 at 2:13
  • The car is motionless. It's state is static. At the moment you begin to engage the clutch that mass of the vehicle is being acted upon by a force. At that moment inertia is defined. The chassis moves up because it wants to remain in its current state. That's basically it.
    – DucatiKiller
    Commented Feb 12, 2016 at 2:33
  • As the clutch starts to bite, there is torque applied through the driveshaft (assuming rear wheel drive) through the differential and out to the tires. The tires are trying to get thee ground to move, but it doesn't want to move, so the axle is twisted in the opposite direction which, along with the front springs of the car, lifts the front of the vehicle.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Feb 12, 2016 at 3:27

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