In a manual transmission car when clutching halfway why throttling does not move the car?

Question

I have a manual transmission car.

Preconcept: I thought the clutch worked with two disks, when there's more friction between them, the transmission disk makes the other disk move. The friction is higher when there's more pressure between the disks, but also if the transmission disk moves faster, it makes the other disk move faster.

Situation: I'm at a slope mantaining the car stopped with the gear 1, throttling a little bit, with the clutch pressed just at the right spot to mantain the car stopped without going backwards.

Question: If I release the clutch, the car goes forward up the slope. But if I press the throttle maintaining the clutch in the sweet spot (not moving it at all), the car does not move forward. Why is that?

PS: Sorry I couldn't even articulate a succint question.

Answer 1

Physics dictates that dynamic friction is a force applied based on two parameters:

• The coefficient of friction between two given materials (determined experimentally)
• The amount of force between the two objects

The relative velocity of the two objects is not present in this equation.

Edit: that equation is very simply

F_d = u_d * F_n Where

• F_d is the dynamic friction force
• u_d is the coefficient of dynamic friction between the two surfaces
• F_n is the normal force. The amount of force applied perpendicular to the interface between the two surfaces

In practice, it's quite possible you're generating extra heat and causing friction fade (the coefficient of friction is often temperature dependent), actually reducing the amount of force/torque applied to the input shaft of the transmission.

The reason you add throttle is:

• Add energy to the flywheel
• Get the engine into a range where it can generate more torque without stalling

Thus allowing you to be more aggressive on the clutch.

Obligatory: Don't hold a car on a hill for long periods using the clutch

Additionally: u_d is a complicated thing, which is why it's easiest to determine it experimentally. Things that can affect it:

• temperature
• glazing (overheating causes a layer of much lower friction material to develop. This can sometimes cost the clutch)
• Water/oil presence (though this is no longer a dry u_d so one could argue this)

Answer 2

A manual transmission with a "conventional" clutch, not a multi-plate version, will have a smooth metal surface on the flywheel and a smooth metal surface on the pressure plate.

Friction material similar to that of brake pads is bonded to the clutch disk, which engages the transmission.

When the clutch pedal is depressed fully, the flywheel/pressure plate is free to spin with the engine speed without transferring movement to the transmission. As the pedal is slowly released, the pressure plate closes on the clutch disk, providing force to be applied to all three components.

It's important to note that using the clutch to hold position is akin to pressing on your brake pedal while you drive, creating excessive wear and extreme heat. You will eventually burn out the clutch, preventing the automobile from moving, and destroying all three components.

The web site How a Car Works has a few useful images and a full explanation, photos below from linked site:

This photo shows the clutch with the pedal depressed fully, while the one below shows the pedal fully released:

Your question about why one specific action creates movement on the roadway while the other does not is likely that the clutch position has changed from one to the other without being detected, as each action should create additional movement. You may not be able to "maintain the sweet spot" when you increase the throttle. Also, the additional heat from running a slipping clutch may also be destroying the clutch lining preventing movement.

If you wish to avoid expensive repairs and have a working vehicle for an extended period, cease to slip the clutch in the manner you describe. Brakes are to prevent slipping back, not clutches.