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Old 02-20-2014, 10:59 AM   #1 (permalink)
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Manual transmission Clutch Question...

So, I am in another forum and made a slightly off topic post in my thread. This is how it went:

I made this statement.
Quote:
Technically, there is always a slight amount of slip, at all times, since there is only a friction connection between the flywheel and clutch disk. But, we won't get into that since that amount of slip is probably less than 1%.
Edit: I should have said, "Technically, there is always a slight amount of slip, under certain conditions, since there is only a friction connection between the flywheel and clutch disk."

This is the reply.
Quote:
BS. It's zero.
A properly working clutch allows zero slippage while engaged. None.
Locked in a 1:1 ratio.
Real world: 1% slippage would be a huge problem for a clutch.

Sorry, you have misinformation and I felt an overwhelming need to educate.
This is my response.
Quote:
Let's discuss, shall we?

A clutch relies on FRICTION! It doesn't "lock" in a 1:1 ratio, unless of course it is some kind of clutch that can physically lock. As far as I know, the 7th Gen Civic clutch is not a lock up design. As you know, the clutch disk is sandwiched between the pressure plate and the flywheel. There are no tabs, locks, mechanisms that "lock" the flywheel to the clutch disk. It is FRICTION that holds the clutch disk to transmit the engine's power to the transmission.

Stick-slip phenomenon - Wikipedia, the free encyclopedia Here is a great Wiki article to read up on. It explains stick-slip. "Stick" is what is described when the clutch is engaged. Stick can be overcome by the engine's torque, but there are factors that can affect the holding properties of the clutch. The coefficient of friction can change due to temperature, friction material properties, oil contamination, water, rate of torque application, velocity, the list goes on. You can read some more here: http://www.pemltd.com/pdf/determining-the-static.pdf

If we use the example of driving on a straight and level road, with no wind, hills or other factors, the "stick" of the clutch should not be overcome if the auto manufacturer properly chose a clutch. But, add in those real world factors and the "stick" of the clutch can be overcome. Think of stomping on the gas trying to make it up a hill. The rate of torque changes and it is very possible that it can overcome the coefficient of friction. From the above Wiki article, “If an applied force is large enough to overcome the static friction, then the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movement.” Or put in a different way, if the engine puts out enough torque, the clutch will not be able to hold it and will slip.

The coefficient of friction is what differentiates the holding power of an organic material versus a sintered iron material. Above I mentioned some other factors that can change the coefficient of friction. When the coefficient changes, so does the stick of the clutch and therefore the amount of torque the clutch can transmit without slipping.

Here is some more info on friction and clutches.

Coefficients Of Friction

Friction Materials

This paper is pretty math intense, but it mathematically shows how engine torque can overcome the clutch when sticking. "The sticking of the clutch sustains as long as the torque transmitted through clutch (Tc) remains below the maximally transmittable torque Tmax c..."
http://www.mate.tue.nl/mate/pdfs/4159.pdf

You said that slipping at 1% would be a huge problem for a clutch. To correct you, I said, "Less than 1%". Less than 1% is a range of numbers from .999(repeating)% down to an infinitely small percent. I didn't specify because I don't have the data to estimate this, so I generalized. This percent could be .0000001% over a 30 minute drive at 60 mph (which would be something like .007296th of a revolution) and the percent slip does not mean the clutch is constantly slipping. Simply put, it slips here and there. I should have worded my previous post to be, "The average slip is probably less than 1%". However, even though the percent is very, very small, it still means the clutch is slipping.

Not to start another debate, but the same slip can be found with auto belt systems, tires rolling over the ground, etc. Friction can allow slip.
So, I am looking for some feedback on my post since I feel that forum doesn't have an abundance of technically savvy people. The person I am debating with is a very knowledgeable mechanic. I am not looking to be right or wrong as much as I am looking for some evidence that a clutch does or does not slip when fully engaged (I don't mean always slipping, but will slip under certain circumstances). I am looking for a friendly discussion. I have my flame proof suit on, just in case...


Last edited by sarguy01; 02-20-2014 at 12:34 PM..
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Old 02-20-2014, 11:15 AM   #2 (permalink)
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I would not say that a clutch "always" had slip. Normally, slippage only occurs as the clutch is being engaged or disengaged. If the clutch were slipping as it were fully engaged, then I'd say that you'd have problems.
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Old 02-20-2014, 11:19 AM   #3 (permalink)
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Quote:
Originally Posted by t vago View Post
I would not say that a clutch "always" had slip. Normally, slippage only occurs as the clutch is being engaged or disengaged. If the clutch were slipping as it were fully engaged, then I'd say that you'd have problems.
You would then disagree that a clutch can slip when engaged, even when outside conditions and changing parameters are constantly changing?
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Old 02-20-2014, 11:23 AM   #4 (permalink)
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Quote:
Originally Posted by sarguy01 View Post
You would then disagree that a clutch can slip when engaged, even when outside conditions and changing parameters are constantly changing?
A clutch can slip when engaged. There is no argument, there.

What I am saying is, if a clutch does slip when it's fully engaged, then there's something wrong. The idea behind fully engaging the clutch is to not have any slippage, because at that point where the cluch is fully engaged, there is no more need to synchronize the driven member to the driving member.
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Old 02-20-2014, 11:38 AM   #5 (permalink)
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Quote:
Originally Posted by t vago View Post
A clutch can slip when engaged. There is no argument, there.

What I am saying is, if a clutch does slip when it's fully engaged, then there's something wrong. The idea behind fully engaging the clutch is to not have any slippage, because at that point where the cluch is fully engaged, there is no more need to synchronize the driven member to the driving member.
Okay, then it seems as if I need change the wording in my original post. I wasn't suggesting a constant slip when engaged.

I don't think a negligible amount of slip, under certain conditions, when the clutch is fully engaged always means there is something wrong. OEM clutches are made to transfer torque, but also to last, be quiet, not vibrate, etc so there are going to be trade off's in holding capacity.
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Old 02-20-2014, 12:06 PM   #6 (permalink)
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Quote:
Originally Posted by sarguy01 View Post
Okay, then it seems as if I need change the wording in my original post. I wasn't suggesting a constant slip when engaged.

I don't think a negligible amount of slip, under certain conditions, when the clutch is fully engaged always means there is something wrong. OEM clutches are made to transfer torque, but also to last, be quiet, not vibrate, etc so there are going to be trade off's in holding capacity.
Your statement below definitely suggests that the clutch is always slipping. As to the above, a negligible amount of slip means the clutch is going out and needs to be replaced. While the clutch is engaged to the flywheel, there should be absolutely zero slip. Even a small amount while constantly engaged will overheat the disk causing it to fail.

Quote:
Technically, there is always a slight amount of slip, at all times, since there is only a friction connection between the flywheel and clutch disk. But, we won't get into that since that amount of slip is probably less than 1%.
The statement below is spot on. Slippage will cause the clutch, pressure plate and flywheel to overheat. As with many things in life, overheating will ensure a much shortened life.

Quote:
BS. It's zero.
A properly working clutch allows zero slippage while engaged. None.
Locked in a 1:1 ratio.
Real world: 1% slippage would be a huge problem for a clutch.

Sorry, you have misinformation and I felt an overwhelming need to educate.
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Old 02-20-2014, 12:08 PM   #7 (permalink)
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Quote:
Originally Posted by poomanchu View Post
Your statement below definitely suggests that the clutch is always slipping. As to the above, a negligible amount of slip means the clutch is going out and needs to be replaced. While the clutch is engaged to the flywheel, there should be absolutely zero slip. Even a small amount while constantly engaged will overheat the disk causing it to fail.

The statement below is spot on. Slippage will cause the clutch, pressure plate and flywheel to overheat. As with many things in life, overheating will ensure a much shortened life.
Please explain to me how a clutch will lock in a 1:1 ratio.
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Old 02-20-2014, 12:14 PM   #8 (permalink)
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Quote:
Originally Posted by sarguy01 View Post
Please explain to me how a clutch will lock in a 1:1 ratio.
The pressure plate 'clamps' it into place when the clutch pedal is released, often with a ton or more pounds of force. Press in the pedal, release the clamp.

Think of it this way. If the clutch were slipping and you are not in a 1:1, you basically have a manual variable ratio (the non-1:1 ratio), where your revs in a certain gear has nothing to do with speed. This is not true, unless your clutch is needing replacement.

Seeing as how we can accurately judge speed in a manual based on gear and RPM, we would have to assume a 1:1 ratio. In this case, the assumption is correct lol.
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Old 02-20-2014, 12:32 PM   #9 (permalink)
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Quote:
Originally Posted by poomanchu View Post
The pressure plate 'clamps' it into place when the clutch pedal is released, often with a ton or more pounds of force. Press in the pedal, release the clamp.

Think of it this way. If the clutch were slipping and you are not in a 1:1, you basically have a manual variable ratio (the non-1:1 ratio), where your revs in a certain gear has nothing to do with speed. This is not true, unless your clutch is needing replacement.

Seeing as how we can accurately judge speed in a manual based on gear and RPM, we would have to assume a 1:1 ratio. In this case, the assumption is correct lol.
First off, the clutch cannot LOCK in a 1:1. So his statement was not “spot on”. In the world of friction, there can always be a chance of slip. Two metal plates with a disk sandwiched in between is hardly an absolute locking device. If there were some teeth, gears, etc that locked it, fine. But there aren't any mechanical engagements on the Civic clutch. It works on the principles of friction. A clamp relies on friction as well. Have you ever had a part, that was clamped tight, move? I have and it's because the force acting on that part overcame the friction holding it in place.

Again, I am not suggesting the clutch is slipping at all times. I am suggesting an engaged clutch can slip under certain circumstances. I am not saying it WILL, I am saying it CAN. I do not think that while I am driving my clutch is constantly slipping.

There is a lot of clamping pressure. But, that is not directly correlated to the amount of torque a clutch can hold under given circumstances. One ton of clamping pressure doesn't mean the clutch can hold one ton of the engine’s torque. There are a lot more factors, like the constantly changing coefficient of friction, as sited in one of the articles in the original post. Also cited in those articles, were the explanations about the differences between dynamic and static friction, outside factors, etc. It was actually some interesting reading.

In our world, we can assume all we want about the clutch not slipping. But, under the physics world, a clutch can slip. Do you disagree??
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Old 02-20-2014, 12:53 PM   #10 (permalink)
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Quote:
Originally Posted by sarguy01 View Post
Again, I am not suggesting the clutch is slipping at all times. I am suggesting an engaged clutch can slip under certain circumstances. I am not saying it WILL, I am saying it CAN. I do not think that while I am driving my clutch is constantly slipping.
But that's how I read your post - that a clutch is never fully engaged and that some slippage was always occurring.
Quote:
In our world, we can assume all we want about the clutch not slipping. But, under the physics world, a clutch can slip. Do you disagree??
CAN slip, sure. DOES 'slip', a tiny bit, i.e. the flywheel and pressure plate do not maintain perfect 1:1 rotation, under certain circumstances, when the operator would otherwise assume that it was not slipping, sure. But *always* slipping? I don't buy it.

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