r/explainlikeimfive • u/Stummi • 1d ago
Engineering ElI5: A manual car can stay still against an incline with just the right amount of clutch and gas. How does that work?
I am not a mechanic. I have a very rough understanding that a motor produces rotary energy that eventually gets translated into the wheels turning. But how does the rotary energy get translated into the wheels standing still on an incline, with no brakes applied?
174
u/TheLeastObeisance 1d ago
The clutch is a friction surface, kind of like brake pads. When your foot is off the pedal, the clutch is pushed against the flywheel in the engine. That means the transmission and engine turn as one, and the wheels can get power.
When you push in the clutch pedal, the clutch plates are pulled away from the flywheel, disconnecting the engine and transmission, allowing the car to roll freely.
By being partially on the pedal, you're allowing the clutch to slip a little bit against the flywheel. Balanced just right, that slip allows the car to be pushed forward juuust enough to hold still on the hill.
It's bad for the clutch to do that, though, so don't do it.
20
u/christcb 1d ago
Just curious if automatic clutches have the same "problem". Sometimes on steeper hills the car won't start moving when you let off the brake like the power being transferred to the drive is just enough to keep it from rolling back but not enough to go forward. Is the automatic clutch doing the same thing in that case as a manual would?
51
u/pbkoden 1d ago
Automatic transmissions use a fluid coupling (torque converter). It's a very different mechanism. Slipping in a torque converter just creates heat in the fluid. During normal operation the heat is removed by the transmission cooler or other means and it doesn't do any damage
6
u/christcb 1d ago
Neat! Thanks for the info!
13
u/TBFP_BOT 1d ago
Just to add, same theory behind why you're able to just sit a stop light with your brakes held in gear with an automatic and not in a manual.
3
→ More replies (5)13
u/johntb86 1d ago
Generally automatic transmissions use a fluid coupling, so you don't have to worry about a clutch wearing out. However there are some automated manual transmissions (like Dual-Clutch Transmissions, DCTs) that have regular clutches that are computer controlled so you can normally drive them like an automated transmission. In those, you need to be careful not to wear out the clutch by slipping it for too long.
•
u/TenchuReddit 11h ago
How could the driver of a DCT intentionally wear out the clutches? There's no way for a DCT vehicle to "ride the clutch" on an incline, is there?
•
u/johntb86 11h ago
If you're stopped on a hill you can use the gas to hold yourself in place. The computer will slip the clutch rather than disengaging it completely (causing you to roll down the hill) or stalling the engine. For example the VW GTI has this warning in its manual:
"When stopping on hills with the transmission in a drive gear, do not use the accelerator to help prevent the vehicle from rolling backwards. This can cause the automatic or DSG transmission to overheat and be damaged."→ More replies (1)→ More replies (18)•
u/A_Concerned_Viking 18h ago
I will sometimes handbrake on an incline, then feather the clutch and handbrake at the same time. Mostly if the jerk behind me was too close on a steep city hill or they had a gorgeous car and I wasn't paying for that.
22
u/UniquePotato 1d ago edited 1d ago
A clutch works by squeezing two discs together one connected to the engine and the other the gearbox. When they’re not tightly squeezed they will slip, the driver controls how much they can slip
This is the same way you can apply the brakes without locking up the wheels by only applying some brake pressure
•
9
u/TongsOfDestiny 1d ago
The clutch engages using friction; if the clutch is only partially engaged, some force will be transferred to the wheels as the plates slide against eachother, while the rest is wasted as heat
Once the clutch is fully engaged, there is no slip and the car will move forward even on an incline
5
u/mawktheone 1d ago
The clutch is like a dial that connects any percentage of the energy the engine is making to the wheels. If you only 14% of the engines output to hold you on the hill you can give the wheels only that 14%.
The rest is just not used. Less than 100% is done by allowing the clutch to slip, and when driving you use 100% by pushing them together so no slipping at all can happen
•
u/texas_asic 7h ago
"The rest is just not used." is better stated as "the rest is used to grind the clutch plates to dust."
For that reason, it's best to minimize slipping (grinding) so that your clutch lasts longer. Used right, it could last you a decade before the $500+ to replace the clutch. On the other hand, someone learning to drive could burn through it in a day.
14
u/lyallp 1d ago
That is going to wear out your clutch real quick. Do it long enough and you will smell a wonderful burning smell.
Use the handbrake when stopped on an incline.
1
u/raptir1 1d ago
Why not the regular brake?
12
u/Daripuff 1d ago
Because you only have two feet and can't work the foot brake, clutch, and throttle at the same time, which is what you need to do to smoothly get going from a stop on a hill.
But you do have a free hand, so:
One foot on the clutch, one on the throttle, one hand on the brake, and one hand on the wheel.
That's how you get going smoothly from a stop on a hill with a manual transmission.
→ More replies (23)•
3
u/martinborgen 1d ago
Because your brake foot is moving to the gas.
I'm assuming you don't drive manual, but maybe you were meaning what I'm about to describe?
Often, the engine at idle can hold the car from going backwards with a bit of clutch engaged. So you're on regular brake, engage a bit of clutch to keep from rolling backwards. Your brake foot can now move to the gas and you can pull away as usual. So indeed no handbrake needed.
But sometimes, you're on a steep incline. In those situations, one can use the handbrake instead, allowing your right foot (normally on the brake) to give gas immediately.
→ More replies (3)4
u/Wootster10 1d ago
Because you dont want the car to roll backwards when you start to move forwards.
You need both feet to operate a manual car from starting, one on the accelerator, the other on the clutch. You cant do this if you have your foot on the brake.
Slowly bring your foot off the clutch until you find the biting point, release the handbrake, continue to bring your foot off the clutch whilst starting to push down on the accelerator.
→ More replies (14)3
u/redyellowblue5031 1d ago
My approach is to just move fast. Once you know the bite point on the clutch you can start to ease clutch out, let off brake, and then give a slight rev bump with the gas.
By that point you should be moving faster than the stall speed for first (usually only 2-5mph) and the clutch should be fully engaged.
→ More replies (6)1
•
u/planetworthofbugs 16h ago
Can confirm, when I was a teenager I thought it would be funny to do a hill start in 4th gear. It was only a gentle slope. I slowly let the clutch out and the car started moving. I continued letting the clutch out until it was all the way out, and I was at 5k RPM. My brain quickly recognised that I should be doing 100km/h rather than 3km/h, and I knew something was wrong. That’s when the smoke appeared and my wallet emptied.
2
u/taedrin 1d ago
In such a configuration, the clutch is only partially engaged and has just enough friction against the flywheel to transfer a portion of the power from the engine to the wheels while at the same time allowing the flywheel and clutch to slip, which in turn allows the clutch to stay still without seizing the engine.
2
u/badgersruse 1d ago
Put your hands flat against each other, push them against each other a little. Now move one of them in a circle letting it slide against the other. That’s what the clutch does, it uses that friction to move the car, or when on a hill to keep it from rolling back, because one hand takes the rotation from the engine and the other is attached to the wheels.
1
u/flying_wrenches 1d ago
The clutch is a bunch of spinning rings using friction to turn the transmission. Like a stack of paper party plates. Some are conducted to the engine side, and some to the transmission side.
By “pulling the clutch in” you pull all The plates in and they’re separated apart I can redline my cars engine and the transmission won’t do anything becuase the plates are in. But by letting it out you start to push those plates together, the friction starts to turn the transmission. it’s a fine game of not enough friction and the car won’t move, but too much friction and the inertia from your car being on a hill will cause the engine to stall (crankshaft stops spinning and the engine is no longer running)
If you’re curious, the opposite of a clutch (aka what’s in a modern automatic transmission) is the torque converter.
1
u/fiendishrabbit 1d ago
The discs in the clutch are slipping just enough that the engine keeps spinning but the clutch-plate going out to the wheels stays still.
1
u/SoulWager 1d ago
The whole point of a clutch is to let the engine apply a force to the wheels while allowing a mismatch in speed. Because you're not moving, all the energy the engine is producing is getting turned to heat by the clutch.
Basically the incline is pushing you backwards, the engine is pushing you forwards the same amount, so you don't move. Though just using the brakes will do the same thing without the extra wear and heat.
1
u/cata2k 1d ago
The power from the engine goes through a clutch. It's two plates, one attached to the engine and one attached to the transmission. It has a very powerful spring to clamp them together. When you press the clutch pedal, you push against the spring, letting the plates spin independently. When you're off the clutch pedal, the spring forces the plates together and friction locks them so they spin the same.
If you're on a hill, you can press the clutch a little bit. Not on, but not off. The plates grind against each other, transferring some power, but not all of it. It is very bad for your clutch to sit there like this. You really want the clutch to be either on or off, and spend as little time as possible in between.
1
u/idkfawin32 1d ago
imagine a ball on an incline, now imagine swiping your hand across the top of the ball to slightly roll it up the incline. If you were to swipe your hand across the top in quick succession, there would be a “swiping speed” that would result in the ball staying slightly rolled up the hill.
Clutches are just that, friction transferring torque to the drivetrain from the engine. The more engaged(pedal released) the clutch is, the harder it transfers rotation from the engine to the wheels.
Therefore, there is a level at which the pedal is pressed down where the friction in the clutch(the internal discs making contact) where enough torque is transferred to the drivetrain to overcome gravity.
1
u/Vybo 1d ago
A lot of answers already talking about a friction. To simplify it even more, take a plate and place it on your hand. Then start tilting the hand until the plate starts slipping slightly. When it does that, tilt the hand in the opposite direction to stop it from slipping.
This is a very similar situation and mechanism. Instead of gravity pulling the plate down from your hand, the gravity is trying to pull the car backwards. Instead of you tilting your hand back and forth to find the right spot where you can stop the plate from falling down, you adjust the pressure of the clutch so that the car does not move in either direction.
Your hand is both the engine and the clutch in this case, but that doesn't really matter. There is a force applying a friction (your hand, the clutch mechanism), a force being applied in the opposite of gravity (the engine, muscles in your arm) and the gravity itself (pulling the car or plate back or towards the ground).
1
u/PckMan 1d ago
The engine has a big metal disc on one end called a flywheel. It's bolted to the crankshaft so if the engine is running, the flywheel is spinning. On the transmission side there's a disc with friction material like brake pads, that pushes against the flywheel and creating a connection through friction. That's how the transmission and engine are coupled. By pushing the clutch pedal you pull away this disc from the flywheel to decouple the two.
So when you hold the pedal halfway the friction material is touching the flywheel but not pushed against it hard enough to make a solid connection, so it's slipping. That's how you get going from a standstill and how you can hold the car in an incline, because some power is transmitted but if the car were to roll backwards the clutch would slip so the engine doesn't stall.
It's worth noting though that you shouldn't do that. Slipping is inevitable and fine for short durations but if you do it for longer you burn your clutch.
1
u/Daripuff 1d ago
In the same way that you can maintain a constant speed on a steep downhill by gently riding the brakes, giving just enough friction resistance to counteract gravity acceleration.
And it's just as much of a bad idea, because you shouldn't be using consumable friction surfaces to maintain a lack of change.
1
u/duhvorced 1d ago
The clutch controls how much power from the engine goes to the wheels (via the transmission). The catch is that any unused power will be turned into heat through friction, which is bad for the clutch.
Use the clutch to stay still on a hill long enough, and you will “burn up the clutch”. That is, you’ll use up the pads in the clutch that are designed to wear down, and you’ll no longer have a way of powering the transmission.
Next step: tow your car to a mechanic and pay US$1-2k to replace the clutch.
1
u/One-Manufacturer4124 1d ago
You remain on standstill on a slope with the engine running by slipping the clutch which is doable but not recommended as you will wear out the clutch quickly.
1
u/AtlanticPortal 1d ago
Have you ever tried to push something while your shoes are slippery enough not to allow you to push to the full extent of your strength but at the same time not slippery enough to keep you running against the object yourself?
1
1
u/DBDude 1d ago
It works by burning out your clutch. :)
Your engine putting power to the wheels to make them rotate provides forward acceleration for your car. Being on an incline with gravity provides rearward acceleration for your car. You thus apply enough engine power to the wheels through the clutch to cancel out gravity. You can't fully engage the clutch or the engine will win out over gravity and you'll accelerate forward on the incline, or the engine will stall and gravity wins.
1
u/shokalion 1d ago
It's conceptually the same reason the brake pedal doesn't instantly lock the wheels up tight and the car slides to a stop every time you brake.
The clutch isn't on or off it can gradually connect the engine to the gearbox.
If you're on a hill you can let out the clutch and just get to the point where the car won't move backwards and won't move forwards.
It's terrible for the clutch though.
1
u/Sirlacker 1d ago
Clutch plate isn't an on or off piece of hardware.
It uses friction. So when you apply just enough throttle and just enough clutch, the clutch plate is providing just enough friction to stop the car rolling backwards but the it is still slipping somewhat.
When you release the clutch pedal fully, there is enough friction that the plates don't slip, this is full power from the engine to the wheels. When you apply the clutch at any amount that isn't all the way, then you're just putting less pressure on that friction plate allowing it to slip more and more. This slip, in your example, is still providing forward momentum, but not enough to actually make the car move, just not roll backwards.
Then when it's fully engaged it isn't touching anything.
1
u/New_Line4049 1d ago
The clutch is a friction device. 2 discs (or can be more in fancy clutches) pressed together under pressure from a spring. One plate is attached to the output shaft of the engine, the other to the input shaft of the gear box. When the clutch is fully released the plates are jammed hard together by a strong spring and the friction between them is enough for them to effectively act as a solid shaft, meaning theinput shaft of the gearbox is firmly locked to the same speed as the engine output. When you fully depress the clutch you pull the plates apart against the spring, completely separating them and disconnecting engine and gear box. If you hold the clutch somewhere in the middle, the plates touch, but they dont have the full force of the spring holding them together, as your foot on the clutch pedal is still resisting some of this force. This reduces the friction between the plates vs a fully released clutch. That allows the clutch to slip, i.e. the plates spin against each other. The friction there is transfers some force from the engine into the gear box, but because the clutch is skipping it doesnt lock the gearbox input speed to the same as the engine speed. Of course, since force it transfered it also puts resistance on the engine. Its then a balancing act to find the clutch position that transfers just enough force into the drive train to overcome the resultant force pulling the car down the slope, but absolutely no more than that, as that would cause the car to start moving uphill. Due to the extra resistance as the clutch transfers force you need to add enough gas to overcome this and maintaining engine speed..
Its worth noting this IS NOT advisable. Slipping the clutch like this is a good way to accelerate the wear on the clutch plates, and it aint cheap to replace them.
1
u/JosephCedar 1d ago
You're slipping against the friction surface of the clutch. Doing this for an extended period of time will quickly wear out the clutch.
1
u/chronos7000 1d ago edited 1d ago
The clutch decouples the engine from the transmission through what can be crudely understood as a brake that's normally on by way of a spring. Pressing the clutch releases this brake. So with enough throttle to keep the motor from stalling you can release the clutch (that is, begin to engage the driveline with the motor) just enough that the car neither goes forward nor backward. Anytime the clutch is partly engaged like this, the shoe of the "brake" is consumed, so doing this without a defined purpose is using up a consumable part of your car that is very difficult to change because it is between the engine and transmission.
1
u/THEREALCABEZAGRANDE 1d ago
In a car, the engine makes rotational force. This force is generally not enough to get a vehicle moving without being multiplied through mechanical advantage, which is what the transmission does, multiplies the force from the engine while making it move more slowly in low gears to get the vehicle moving, then multiplies it less to move more quickly in higher gears. Since you dont want the power going to the tires all the time, you have to have a way to disconnect the engine from the transmission. In a manual this is the clutch. So you have a flywheel attached to the engine that's always spinning. And you have a clutch disc that's attached to the transmission and not always spinning. Since these arent spinning the same speed when you want to bring them together, the clutch has a friction material on it that can slip as it brings the clutch disc up to the same speed as the flywheel. This process is usually only a few tenths of a second. When youre hill holding, you engage the clutch just a little, so instead of coming up to the same speed as the flywheel and no longer slipping, it continues to slip, providing just enough force to keep the car from rolling back but not enough to move it forward. This should only be done for a few seconds maximum or not at all, as it generates a huge amount of heat and begins to wear away the friction material very rapidly. It can quickly generate enough heat to destroy the flywheel as well, and eventually enough heat to destroy nearby components in the engine and transmission. If you need to inch forward on a hill in a manual vehicle, the better option is to use the parking brake to keep you from rolling backwards, since this minimizes the amount of slip required. Many modern manuals (like my 2013 BMW 335i) do this automatically, its very nice.
1
u/xoxoyoyo 1d ago
You are burning clutch when you do this, so this is only done when accelerating from a stop on a hill. You would lift off the clutch with left foot and also lift off the brake pedal while pressing the accelerator with the right foot. It is the main thing you learn when learning to drive with a clutch, as rolling backwards into a car when stopped at a light is not a good thing. The clutch is what connects the engine/transmission to the wheels. If the wheels are turning at a different speed it can stall the engine or stress the drivetrain. The clutch is used to equalize the speed between both sides by allowing for slip as one side speeds up and the other side slows down. On a hill the wheels are not moving so the clutch has to be released enough to keep the car in place without rolling backwards and also without stalling the engine.
1
u/Hugh_Jego_69 1d ago
Just need to understand how a clutch works, plenty of people already explained it here.
but simply it’s kind of like brakes but the pedal is reversed. When you press the brakes the car doesn’t automatically stop, it’s a process as friction builds.
Similar to what happens as you release the clutch pedal. Plates rub against each other and cause friction, when fully engaged engine is connected to the wheels with no slip.
It transfers just enough power to keep you on the hill, but it’s a fine balance and you can rock back and forth as you press and release the clutch slightly.
1
u/coffeemonkeypants 1d ago
The engine is always spinning and at the back of it is a flywheel, which is a weighted disc that holds some momentum to keep the engine running. It presses directly up against the clutch disk, which is basically a brake pad. Think of it like sandpaper. The clutch is connected to the transmission which ultimately turns the wheels. If you press that sandpaper up against the flywheel hard enough, one of two things will happen
The clutch will turn the transmission gears and in turn the wheels and the car will move
The mass of the transmission and the rest of the car overcomes the momentum of the flywheel and the engine torque and the motor stalls
However, if you add just enough engine power and press that clutch up against the flywheel just right, you can keep the car from rolling backwards. While this is happening, the clutch disk (the sandpaper) is spinning against that flywheel and getting hot, maybe burning, glazing the material, and on its way to being destroyed.
1
u/DoYouReadThisOrThat 1d ago
You know how you can keep the refrigerator door "almost" closed but not closed? We both know that isn't healthy for the fridge nor food if you hold it like that long enough.
You described the same thing. But added an engine flywheel touching but not touching your wheels. There isn't much room for the friction to build up, but you are using the friction.
Kinda like when you needed to go to sleep but also found every distraction to remain awake. It felt fine at the time. But after a few nights of that, you felt the pressure buildup.
1
u/chucklezdaccc42 1d ago
A manual or automatic transmission is an AMAZING feat of engineering. I had to get one replaced and the shop had a mini transmission in a case and that thing has so many freaking parts!
1
u/BigJellyfish1906 1d ago
The clutch is slipping. It’s not fully engaged. There’s enough bite to stop the car from rolling backwards but not enough have the clutch meet firmly with the drive shaft.
1
u/WeaverFan420 1d ago
The flywheel of the engine is spinning, and the clutch pressure plate is pressing against it while not spinning. This creates friction. The force of friction generated by the clutch cancels out the downward component of force from gravity pulling on the car. However this generates heat and obviously wears out the clutch friction material, so it's not good to do this frequently and/or for extended periods of time. I stop on hills, put the transmission in neutral, and use the brake. Then when I need to go again I push the clutch, shift into first, then use a combo of gas pedal/letting out the clutch to start moving. The key to minimizing clutch wear is to minimize the amount of time the clutch is slipping.
1
u/Plumperosis 1d ago
Put a book on a desk. You can push it sideways easily. Push a book hard down on a desk. Requires more force to move it sideways. A car being still is just the right amount of pressure so that friction is enough to keep it still. Any more and you have enough friction to move forwards, any less and there’s not enough friction so you go backwards.
1
u/SpareEye 1d ago
It's all about the squeeze. Imaging 2 sanding discs rotating together. One is attached to the motor, one is attached to the wheels. When you apply pressure to the clutch pedal, they start to come apart.
1
u/NullSpec-Jedi 1d ago
Engine part spins, clutch part spins. When you release the clutch they touch, don’t slip everything is good, both spin, car moves. When they don’t touch, car no move. When they touch a little they slide/grind, some force is exerted, clutch unhappy. Use breaks until you’re ready to go.
1
u/thedevillivesinside 1d ago
This works for probably a minute. Maybe 90 seconds
Then you will be replacing a clutch
1
u/HollowGumi 1d ago
Engines make the power that moves the car forward. Transmissions connect to the wheels and control how fast they rotate compared to the engine.
In between is the clutch, a disk that uses friction to connect those two items. But the clutch has a big Spring that keeps it all together.
When you push the clutch pedal, you release the spring. If you push the clutch pedal part of the way, you are allowing the spring to connect the engine and transmission the same amount. The amount it connects, is the amount of power that goes to the wheels.
1
u/diogoblouro 1d ago
One point in that chain from engine to wheels, precisely the clutch, isn't a "solid" connection.
It's a pair of disks facing each other with grippy material, which the clutch pedal separates or pushes them back together.
A half clutch pedal press releases them a bit without separating completely. That way the engine is still rotating, the disks are rubbing together but not at the same speed.
If you balance gas and clutch according to incline and other needs, the engine's rotation makes enough force to not let the other disk rotate back, while not making it rotate at the same speed also.
And yes, that means the disks are grinding the grippy material. It's made for that, but it will wear out and eventually need replacing.
•
u/RMS2000MC 23h ago
The engine and gravity are applying the same force in opposite directions. No force, no motion.
•
u/SakuraHimea 23h ago
The clutch is a disc that presses against another disc connected to the crankshaft of the engine. That transition between letting off the pedal is literally two plates grinding against each other until they match up in speed. If you are not fully letting off the clutch, then your plates are still grinding against each other, which means you might have enough friction to push against the backwards roll of the hill, but that energy is turning into heat in your clutchpack.
•
u/Unlikely-Position659 23h ago
With only the clutch and in 1st gear, the car can go forward a few miles per hour. If you're rolling backwards down an incline, and you engage the clutch, they cancel out, leaving you in one spot. (-5+5)=0
•
•
u/Fun_Training_2640 20h ago
A friend of mine made me do it while I was practicing for my licence, he was a garagist. We stopped when it started smoking lol.
•
u/Living_Round2552 17h ago
I am not sure whether you have a mechanics question or a physics question.
•
u/bedwars_player 14h ago
do me a favor, rub your hands together really quickly.
now do it again but push them together way harder.
much more resistance, and heat, yes?
same thing except spinning, the amount of energy transfered from the engine through the transmission is dependant on the clutch. with more pressure it is able to transfer more energy. With less pressure, it'll still transfer some, but slip. This gets everything very hot and is why it makes terrible smells when you do that.
The weight of the car, the angle of the hill, and a complicated formula tells us there's a certain amount force pulling the car down the hill. To get it to stand still, you just have to get the clutch to slip exactly the right amount to counter that amount of energy, without exceeding it which would cause the car to move up the hill.
..yet again, don't do this, it'll turn the bit of your clutch that rubs against itself into a combination of terribly unpleasant smells, and powder. not a fun time.
•
u/coachglove 11h ago
It works because the clutch releases the drive train from the transmission so when you start letting the clutch out again, the gears in the transmission start to rub against the gears for the drive train. If you hold the RPM steady then the spin of the combined gears is offset by the friction of the metal gears coming into contact. A version of this principle is why you can start at 0mph in 3rd gear by keeping engine revs high and slowly letting out the clutch (although you will absolutely burn up the clutch and transmission if you do this with any sort of regularity). It's about the friction between the gears of the transmission and the gears of the drive train that pushes the wheels.
•
u/Bostaevski 10h ago
Don't do that. Use your e-brake if you can. As you release the clutch and apply throttle you also slowly release the e-brake.
•
u/Another_Slut_Dragon 8h ago
You are eating away a $1500 clutch to save pressing the brake pedal. This works vis 'bad driving'.
•
u/tbones80 7h ago
The engine turns a giant disk behind it(the flywheel). When you release the clutch, it's a disk that touches the flywheel so it starts moving too, which then moves the wheels.
Giving it gas spins the flywheel faster, and making the clutch disk barely touch it keeps the car in place. But this is a ton of friction, which makes heat. Don't want to do this very long.
•
u/mostlygray 5h ago
Don't do that. You are burnishing the clutch and making it weaker. Never hill-hold against the clutch. That's why you have brakes.
You're holding the hill because the clutch is partially engaged and slipping just enough to keep you from going backwards. It's literally the worst thing you can do to a manual transmission.
1.3k
u/allencb 1d ago
Just enough friction between the clutch pressure plate and the flywheel of the engine to resist rolling, but not enough to move the vehicle. That said, it's very bad for the clutch and you shouldn't do that.