Biting Point Question

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amp88

amp88

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Last week I had a bit of a thought and today I was discussing it with a friend in his car. My thought goes like this...

You're sitting in your car, on a relatively steep incline. It's a manual car and you're holding the car still on the hill in first gear on the biting point. The engine's currently revving at 2500rpm. What happens if you increase the engine revs from 2500rpm to 5000rpm, whilst keeping the clutch pedal still?

Thoughts...

:)
 
The noise from the engine increases as the rev's rise.

Obviously.
 
@live4speed: :lol: I was thinking more along the lines of does the car move or not ;)

@neanderthal: No, the handbrake is off
 
Yeah I know, but that still happens :lol:. I think if your right on the biting point then the car will move a you increase the revs but not much, it could depend on the car though I guess, what clutch is used and the power, weight of the car ect would all play a part.
 
Hmm, good question. First answer is you start eating through your clutch plate like none other. Then I'd say you move up the hill a bit.

Then, thinking about it....

All you do is burn clutch. The friction isn't increasing since you aren't moving the pedal. So there's no extra force being exerted. There is more energy going up in more burnt clutch, but it's force that we need here. Basically, the force of friction within the clutch and the force of gravity hauling you downhill.
 
In my experience the car starts to move backwards. I guess because the faster they rub, the less friction between them.
 
In this order:

1. Not a lot.
2. Your clutch explodes.
3. You roll backwards at an alarming rate, in a cloud of acrid smoke.
 
DQuaN
In my experience the car starts to move backwards. I guess because the faster they rub, the less friction between them.
Click to expand...

If the car rolls backwards my guess would be it's because the clutch heats up, and the coefficient of friction drops.

It seems possible that it could happen the other way too, the clutch could get some more grip as it heats... I guess I'd need to see a plot of clutch coefficient of friction vs. temp.

Or you cuold just go try it, I won't as my clutch has a lot of miles on it, and I don't want to have to replace it until it gets warmer out :)
 
For that to happen to me, I would actually have to hate my clutch enough to use it as a friction brake... :lol:

Bu seriously, you'll start to move forward. If the clutch is biting enough to hold the car on the incline, then increasing the revs will start you moving forward. If you increase the revs, the car will start moving forward around 3500, then the clutch starts slipping at 5000rpm unless you let the pedal all the way out. If you've got maybe a hundred thou kays on your clutch, it may blow.
 
retsmah
It seems possible that it could happen the other way too, the clutch could get some more grip as it heats... I guess I'd need to see a plot of clutch coefficient of friction vs. temp.
Click to expand...
Coef of static friction > Coef of kinetic (moving) friction for all materials

So you will always roll backwards...
 
But he's already got the clutch at the biting point. The car will move forward. Whether it'll keep on moving forward at 5000 rpms depends completely on the condition of the clutch.
 
skip0110
Coef of static friction > Coef of kinetic (moving) friction for all materials
Click to expand...

The clutch is already slipping at 2500rpm, it's just slipping more at 5000.

Anytime you are holding the car on a hill using the clutch and the gas, the clutch is slipping.
 
I would not expect the car to move forwards. Basically the clutch is slipping, so the coefficient of friction you get is the kinetic coefficient of friction. Your frictional force is coefficient of friction x normal force. So the only way you can get more force out is if you increase the normal force, meaning you let the clutch out more. Giving it more gas isn't going to change the normal force, and it shouldn't change the coefficient of friction.

That said, friction is a weird thing, you can't be certain without looking at some data from a clutch manufacturer or actually trying it out. I'm sure the coefficient of friction will change with temperature, how exactly again is something you would have to try out.


Interestingly, on a friction related note, for a while people thought that a car would not be able to accelerate at more than 1g (without downforce) because it was assumed a coefficient of friction could not be greater than 1.
 
The point is, even if the clutch is already slipping, more power will push the balance the other way. If you keep the clutch at that point and increase the revs, the clutch will EVENTUALLY fry, but the car will still move forward.

This is one of those questions where the answer ought to be: IT DEPENDS.
 
My route to work involves an incline that I quite often ride the clutch instead of sitting on the brake or handbrake. From my experience if you sit at 2500rpm on an incline and then rev up to 5000rpm you dont move. The incline is what is holding you there, there isn't enough drive going through the clutch to move the car.

However if you do it on a level surface I'm pretty sure your car will creep more at 5000rpm then at 2500rpm.
 
when im teaching people how to drive stick, thats what i tell them to do when the car first moves; hold the clutch and add gas. normally this means smooth movement from a standstill, not the jerking kangaroo hops novices usually evince. of course, thats on flat ground.

in my 2002, when i hold on an incline (like at a light) and add revs, it creeps forward. im not going to go into the physics of if, coz i cant, but i figure that if the power being transmitted through the clutch at the biting point at idle is just enough to hold the car in place, then adding power will result in forward momentum. even if there is a loss of friction, because we are talking about a 400% increase in rpm (5-600 at idle vs 2-2500 to takeoff. i highly doubt by reviving slightly that you are decreasing the friction by 400%)
 
retsmah
The clutch is already slipping at 2500rpm, it's just slipping more at 5000.

Anytime you are holding the car on a hill using the clutch and the gas, the clutch is slipping.
Click to expand...
Oh, right :banghead:

So you would go forwards, maybe to a point until the clutch gets greasy or something...
 
In a Fiat 126 2500 - 5000rpm means you go backwards , the biting point would be how much torque gets transmitted to a clutch w/ a reasonable friction coefficient .
 
Enough with the theoretical stuff. I just backed my car down to the sloped portion of my driveway and used the clutch to hold the car on the hill. Once it was stationary I gave it more gas. Revs increased, car stayed planted. So there is your answer.

This is what is happening. Your engine output is spinning and applying a torque to the flywheel. The clutch can only transmit a torque equal to the Normal Force on the clutch multiplied by the coeficient of friction of the clutch. We've already said that we aren't going to touch the clutch pedal, so the normal force cannot change. So, if you want to transmit more torque to the wheels, then you have to increase the coefficient of friction.

Increasing engine speed will not increase the coefficient of friction. I felt pretty confident in this already, even more so after trying my little driveway experiment, but just to be sure i consulted my Mechanical Engineering Design text, and in the section on clutches and brakes there is no mention of any kind of material that would increase the coefficient of friction due to an increasing engine speed.

When you are on a hill slipping your clutch to hold the car, all of the engine's power is being used to create heat in the clutch. You rev the car more, you make more power, and that power goes towards making more heat.
 
DQuaN
In my experience the car starts to move backwards. I guess because the faster they rub, the less friction between them.
Click to expand...

Roughly what i was thinking too. And you would also seriously wear the clutch.
 
retsmah
Enough with the theoretical stuff. I just backed my car down to the sloped portion of my driveway and used the clutch to hold the car on the hill. Once it was stationary I gave it more gas. Revs increased, car stayed planted. So there is your answer.

This is what is happening. Your engine output is spinning and applying a torque to the flywheel. The clutch can only transmit a torque equal to the Normal Force on the clutch multiplied by the coeficient of friction of the clutch. We've already said that we aren't going to touch the clutch pedal, so the normal force cannot change. So, if you want to transmit more torque to the wheels, then you have to increase the coefficient of friction.

Increasing engine speed will not increase the coefficient of friction. I felt pretty confident in this already, even more so after trying my little driveway experiment, but just to be sure i consulted my Mechanical Engineering Design text, and in the section on clutches and brakes there is no mention of any kind of material that would increase the coefficient of friction due to an increasing engine speed.

When you are on a hill slipping your clutch to hold the car, all of the engine's power is being used to create heat in the clutch. You rev the car more, you make more power, and that power goes towards making more heat.
Click to expand...

Or:

Famine
In this order:

1. Not a lot.
2. Your clutch explodes.
3. You roll backwards at an alarming rate, in a cloud of acrid smoke.
Click to expand...
 
Just to throw a fart into the hurricane...

I've had a sort of opposite experience. On a 1993 Suzuki GSXR 750 I was practicing racing starts, just so I knew I could fully annoy Nova driving retards in the UK. This horrendous clutch abuse generated heat in the plates and on one really vicious takeoff the clutch locked up. Gearchanges were possible once moving, either letting off on the upchange or a stamp and blip to go down the box. But stopping was impossible making for a fun ride home.. Lesson learned.

Stripping it down showed the steel plates had warped due to heat and practically locked the clutch up when the lever was in.

So, I know cars usually have single plate dry clutches compared to a bike's multiplate wet clutch, but could thermal expansion possibly increase the Normal force on the pressure plate?

Personally, I doubt it would be enough to move the car, but what do you think?

Kurtis.
 
I dunno about that, it seems in theory it could. I would think the coefficient of friction would drop a lot faster with the added heat, might not though.

I might try the same thing in a bike next time I have mine out, I think a bike may go up the hill by increasing revs. It's got some viscous friction going on I would think, which is speed dependent. I'll have to try it :)
 

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