So what does a clutch do, anyway?

This thread reminded me that I have no idea what that crazy third pedal does in manual transmission cars. From reading the Wikipedia article, I get that it has to do with transmitting torque from the engine to the wheels (or something) yet automatics manage to do this automatically.

It basically connects the engine to the wheels. If you press in the clutch pedal, the two are physically separated. The engine will continue to turn but the wheels will receive none of that effort.

it disengages a motor from the mechanical contraption it’s sending power to. Found in all sorts of gizmos from cars and trucks to meat grinders. In automobiles it allows you to shift gears, rearranging the internal physical configuration of a transmission’s parts which would be very difficult to do and damaging (in most cases) if it were actively receiving mechanical power from the engine. It also makes it possible to start the car from a standstill, by letting you gradually apply torgue from the engine - a part of the car that’s moving - to the transmission and wheels - which are not moving.

To add to what IntelSoldier and ntcrawler have said, a difference between manual and automatic transmissions is that in some automatics, there’s the torque converter, which uses a type of thick oil to engage between drive and driven, with inherent losses of efficiency, because it’s not a solid physical connection, like you get with a manual’s clutch.

I think…


How Clutches Work.

Includes a whole mess of animated diagrams.

In automobiles -

It’s best not to think of the clutch/clutch pedal as an “on/off” switch. Yes, at the extremes (fully depressed or fully released) it will act like an “on/off” - push the clutch pedal fully in and no power will be delivered from the the engine to wheels, let the clutch fully out and full power (minus mechanical/friction losses) will be delivered to the wheels.

But a big feature of what the clutch does in an automobile is to modulate how much power from the engine goes to the wheels. If you hold (for just a few seconds) the clutch at maybe 50%, the clutch will “slip” - some of the engine’s power is getting transmitted to the wheels but not all. The remaining energy goes to heat (friction) as the clutch slips.

This sounds bad, and indeed if you let the clutch slip constantly and too much it will wear it down. But it’s necessary for smooth shifting when driving a manual transmission. If, for example, you’re at a standstill with the clutch fully depressed:

  1. You shift into first gear
  2. You release the clutch quickly and completely

then the engine (in most cars) will stall. There will also be a nasty jerk as the engine makes it best effort then stalls. There’s too much mechanical resistance for the engine to be able to get the car moving to a speed where the engine can be running at an acceptable rate.

So you need to let the clutch slip a bit as you launch from a standstill into first gear.

Same goes for first to second gear in most cars, although it’s not as dramatic. It’s difficult to stall a car going from first to second gear, but without some slip there will be a neck-wrenching jerk if you just let the clutch pop.

Past second gear the gear ratios make the shifts much smoother and you can let the clutch pop out as you please.

If you take a light bulb for an analogy - I think of the clutch not in terms of a light switch (on/off) but as a light dimmer switch (0%-100% with everything in between).

The clutch is like a parent that occasionally steps in to seperate two combative children.

Of course, for evolution (as well as your car) to accomplish anything, one has to occasionally let the parts fight it out.

A clutch can take many forms but they all work basically the same. They are a mechanism that allows a rotating object to drive another rotating object by means of intermediary friction. In the case of a car you have what looks like 2 plates facing each other. The clutch peddle separates them and when you let the clutch out you are bringing them together slowly so that there is some slippage between them.

In “olden days” (after the “days of yore” but before your mother got pregnant) it was necessary to double clutch. You had to do this because manual transmissions didn’t have synchronizers in them (little elves that go screeeeeee if you miss a gear).

In a modern transmission with synchronizers you don’t need the clutch once the car is moving. You can shift up and down by matching engine speed to the respective road speed of the gear your in. You’ll understand what respective is because it involves the aforementioned elves.

The way my dad described it to me when teaching me to drive is that you envision your transmission as a long pole that’s rotating. Now you cut the pole in half and put two flat plates with sand paper on them facing each other with the pole halves going back out the other side. When the clutch peddle is out (i.e. not being depressed), the two plates are pushed tightly into one another, when you push it in, the two plates are slowly backed away from each other. These two sandpaper plates and the system for connecting and disconnecting them are “the clutch.”

Now one half-pole is connected to the wheels and the other is connected to the output of the engine. If you change gear, the output of the engine will cause it’s half-pole to start rotating at a significantly different speed than the half connected to the wheels–being turned by the inertia of the car and nothing else. So you can’t just change gears while they’re connected or either you’ll suddenly shoot forward, suddenly brake, or most likely cause your engine to seize and stop. So what you do is disconnect the two halves, swap the gear, adjust your foot on the gas to either less or more to bring the engine output rotation speed into near-synch with the wheel input rotation speed, and then slowly let the two plates reconnect. The slower you do this, the more gently the friction between the two will bring them into perfect synch–though of course also the more wear there will be on your clutch and the sooner you’ll have to replace it. After driving with a clutch for a week or two, you get good enough at synching up the two speeds by feel that it really only takes a half second to do the whole process and it’s still perfectly smooth, with almost no wear on the clutch.

How do you do all that and still manage to concentrate on the road? It sounds like you need to be a fighter pilot to keep track of everything. But more importantly, how to automatic transmissions manage to solve this problem without involving the driver in any way?

I learned to drive on manual transmissions. You just practice, practice, practice, until it becomes second nature. One of my driving teachers (a family friend, who in a past career had taught soldiers to drive tanks) got me to drive down a city street changing gears down at every intersection, then changing up again. You soon get used to the feel of the clutch pedal, and you learn to sense the interaction between engine, transmission and wheels.

Can’t answer the automatic transmission problem, but the clutching / shifting stuff does become second nature after a fairly short time - a few hours of driving, maybe.

I learned how to drive on automatic transmission, then in college my parents bought me a used car - with manual transmission. I’d driving manual precisely once before that. It lead to some interesting times, but within a week I was doing it without any distraction from the road.

As many folks who’ve switched between the styles find, if I’m driving manual transmission for a few hours, then go back to an automatic car, I’ll find myself trying to press the nonexistent clutch and reach for the nonexistent shift lever :slight_smile:

I gather the mileage differences between manual and automatic transmission cars are not as wide as they once were, due to improved technology. Used to be that was a pretty strong reason to stick (heh) with manual transmission if you knew how to drive it. Of course, this made driving in stop-and-go traffic especially sucky - lots of fatigue on the left foot and right arm.

It’s just another pedal. Most automatic transmission drivers manage to use the brake, gas, steering wheel, wipers, turn signals, etc and still keep up a conversation or a cell phone call, and often without any memory - when you snap out of a train of thought - of how you drove the last five miles. A manual gearbox is just one other thing on top of the half dozen or so you’re already doing.

How do you get used to it without losing concentration off the road? It becomes second nature the same way that other tasks are. The same way you don’t have to manually count “one thousand one, one thousand two”, etc to determine your following distance, the same way you don’t have to concentrate to keep your car directly in the center of the lane, the same way you don’t have to look down when you want to lower or increase the radio’s volume (you get used to where the knob is, etc), the same way you can ride a bicycle without falling off. Once your body and mind get used to the task and develop a feel for the operation of the clutch, it becomes second nature. With enough practice, you no longer have to think about it, and with enough experience you can switch between different cars (and unless the clutch is waaaay different) and still drive them smoothly.

A number of the descriptions in this thread are inaccurate and do not properly describe what is actually occurring in a manual transmission car. Sublight’s link to HowStuffWorks does provide a proper explanation. Going through it, and the accompanying “How manual transmissions work” article, will reveal that

  • The clutch does NOT “connect the engine to the wheels”.

  • “Shifting gears”, like “Stepping on the gas”, is a misnomer. The actual gears inside a manual transmission that make up the varying gear ratios do not move, ALL the gears are powered and spinning ALL the time. “Shifting” involves disconnecting the output shaft from one set of engaged gears and hooking it up into another set of engaged gears.

  • The actual process of properly making this whole mess work properly is even more complicated than most of the posters here make it sound. The driver is not simply matching the rotational speeds of 2 items (wheel and engine), but 3 - the engine, the wheel, and the transmission which has its own rotational speed/energy independent of the engine and wheel.

  • Vin Diesel, in addition to being one of the greatest actors of our time, is also a bearing of useful advice on driving. If you go through everything, you can see that “double clutching”, or more specifically using the engine’s(1) power to match the speed of the transmission(2) to the wheels(3) before connecting the wheels(3) to the transmission(2), is a good idea, even in a car with a synchronized gearbox. Why wear out valuable and difficult to replace synchronizer rings when you can do their work yourself, and turn the daily commute into an even MORE masochistic experience? For me, it’s a no-brainer.

:rolleyes: No, no, no.

You’re supposed to tell them that we’re a breed apart, wrestling the power betwixt engine and wheels in our bare hands, fingers directing brake horse power in a second. Then walk away chest out and head held high, knowing that they’re looking on in astonishment and envy.


My wonderful SO, once picked up my drunken ass and, for some reason, I didn’t want to leave my car at the bar. So, instead, she left hers and drove my stick shift home, having never driven a stick shift before. Granted, it was only about 4 miles home at 3 a.m., but she made it the whole way, stalling only once. I did offer some basic advice, but stick really isn’t the mystery people make it out to be.

Now, if it were an old, old stick with no synchro-mesh, that’s a wee bit harder. But on a modern stick, there’s a whole lot of leeway you get with gear shifting.

Where they having a sale on nits by you?

Personally, from the first time I learned to drive a stick shift, I’ve always preferred it. The driver has more control with a stick. Of course, you pay for it by having to do more work, but you can get more acceleration, faster, with better control, in a stick. I did have to give it up from when I got my (left) hip replacement until after I got the knee replaced and it healed, because there is significant pressure on those joints when clutching. But I’m back to a stick now.

I understand that they’ve (mostly) closed the gap between fuel efficiency now, so that an automatic doesn’t burn that much more gas than a stick. But there’s still the control issue. Kinda like the difference that used to exist between a Mac and a PC - Mac for power; PC for control - except that with the stick you get more of both.

As for adapting any time I drive an automatic, I’ve always used my left foot for braking then. I suspect I have lots of company. It gives me something to do with my left foot. :wink:

Pffft, try writing poetry with it.