# The moment a clutch begins to engage...

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**46**Tech Regular

In the light of my bad sarcastic jokes ...a little nice movie i made, that`s me..

https://www.youtube.com/watch?v=zHpAifN_2Sw

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**47**In the light of my bad sarcastic jokes ...a little nice movie i made, that`s me..

https://www.youtube.com/watch?v=zHpAifN_2Sw

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**48**Tech Regular

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**49**The most difficult part is the fact that most engineers dont want to share the knowledge, they see it as their secret power to make money. Most of them find it hard to share and have patience to explain over and over. In my opinion we should share more. A lot of people are easily offended if their theory is taken in doubt. The mechanical world is really black or white, it`s confronting.

point 1 and 3 are understanded ok, point 2 is the problem i think:

The centrifugal force is always in radial direction of the cranck axis, the value is determined by the mass of fly weight, radius of the center of gravity of the fly weight and the tangential speed of the CoG of the fly weight.

point 1 and 3 are understanded ok, point 2 is the problem i think:

The centrifugal force is always in radial direction of the cranck axis, the value is determined by the mass of fly weight, radius of the center of gravity of the fly weight and the tangential speed of the CoG of the fly weight.

Obviously some people don't like being taught and we end up regurgitating the basics over and over again. Chemistry taught me the scientific method and the concept of re-entrant use of data and findings into new hypothesis gives me a buzz! Learning isn't linear

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**50**Tech Regular

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**51**Tech Regular

Back to the task at hand...

I think I've come to the conclusion that the weight of the throw and the hardness of the spring or tension applied with preload has more to do with the acceleration of the clutch shoe than the amount of gap. The amount of gap as RBakker said has to do with the forces being applied by the spring to cause slippage. How to measure the differences in forces applied and tension in the spring is my next quest and I'll likely need more help from those of you who like the physics as much as the performance.

This is because the radius change in the throw is negligable and even a 1mm gap will only change the distance on a 30mm clutch (hence velocity) from 94.25 mm per revolution, to 97.38mm per revolution an that's only a change of around 3%. So while the forces do change slightly, they arent significant.

So here goes, I need you keeping check on this RBakker!!

1g throw, 30mm clutch, 10k rpm engage (for one throw only)

At 10k/rpm the velocity of the throws are prior to engagement:

94.25mm * 10,000 / 60 = 15707.96 mm/s or 15.7 m/s

97.38mm * 10,000 / 60 = 16231.56 mm/s or 16.2 m/s

Centrifugal force calcs

15.7^2/.030 * 1g = 8216.3

16.2^2/.031 * 1g = 8465.8

Again a 3% increase in force.

Increasing centrifugal force.

A Grub screw, a large one may weigh .2 grams. this is a significant factor of up to 20% more force!

who cares about the gap...mass and the spring..!!

But what about the accelerating and decelerating of the clutch shoe, when coming into contact with the bell?

Doesn`t the gap determines the speed the clutch shoe can reach..? what about impact and impuls..?

https://www.youtube.com/watch?v=_3KkYcKm4uY

*Last edited by rbakker; 03-11-2015 at 07:28 AM.*

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**52**
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**53**Check (diameter or radius in the formula)

who cares about the gap...mass and the spring..!!

But what about the accelerating and decelerating of the clutch shoe, when coming into contact with the bell?

Doesn`t the gap determines the speed the clutch shoe can reach..? what about impact and impuls..?

https://www.youtube.com/watch?v=_3KkYcKm4uY

**TKS DONE**,who cares about the gap...mass and the spring..!!

**HMMM!!**But what about the accelerating and decelerating of the clutch shoe, when coming into contact with the bell?

**YOU ARE SO MEAN! So this I ask you to elaborate on, seeing you posed the question!! lol**Doesn`t the gap determines the speed the clutch shoe can reach..? what about impact and impuls..?

**That's the next question, it would be a difference of the forces the throws apply versus the opposing forces of the spring and I havent considered that yet! Because.. While the gap will slightly have an effect of the centrifugal forces, it can also change the spring forces at play through the range of spring compression and I will begin to research how springs behave!**https://www.youtube.com/watch?v=_3KkYcKm4uY

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**54**Tech Regular

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**55**Tech Regular

a force working against this axial force based on the spring constant and clutch shoe position (so clutch weight / gap)

An acceleration of the clutch shoe based on the resultant force of the centripetal minus the spring.

Pit that in a excel sheet and you can look at the differences between setups and get a better understanding whats happening in the clutch, only to compare setups. The theory you understand now.

This is also the point where it all stops...

The point of contact between shoe and bell including the decelerating, friction and force spikes is a bridge to far.

For calculation of that, you need to model that in FEM software like Ansys or Abaqus. That`s a mechanical study and 5 years professional experience away.

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**56**Yes, so we have a force from the centripetal acceleration, thats converted by the angle of the fly weights to the axial force of the clutch shoe.

a force working against this axial force based on the spring constant and clutch shoe position (so clutch weight / gap)

An acceleration of the clutch shoe based on the resultant force of the centripetal minus the spring.

Pit that in a excel sheet and you can look at the differences between setups and get a better understanding whats happening in the clutch, only to compare setups. The theory you understand now.

This is also the point where it all stops...

The point of contact between shoe and bell including the decelerating, friction and force spikes is a bridge to far.

For calculation of that, you need to model that in FEM software like Ansys or Abaqus. That`s a mechanical study and 5 years professional experience away.

a force working against this axial force based on the spring constant and clutch shoe position (so clutch weight / gap)

An acceleration of the clutch shoe based on the resultant force of the centripetal minus the spring.

Pit that in a excel sheet and you can look at the differences between setups and get a better understanding whats happening in the clutch, only to compare setups. The theory you understand now.

This is also the point where it all stops...

The point of contact between shoe and bell including the decelerating, friction and force spikes is a bridge to far.

For calculation of that, you need to model that in FEM software like Ansys or Abaqus. That`s a mechanical study and 5 years professional experience away.

one aspect that hasn't been made clear is why a larger gap means a harder clutch engagement. I believe this is due primarily to the centrifugal force growth with rpm. your expression v^2/r is correct but is unclear.

In our case v=r*w (where w is angular velocity in radians/sec or 2*pi*rpm/3600) so the flyweight forces grow with r*rpm^2.

the means a larger gap has 2 effects, when the flyweights close the larger gap it is with a larger radius and at an increased rpm. the rpm^2 means the change in clutch shoe force is greater for the same change in rpm. (engagement is occurring on a steeper slope of the centrifugal force curve, F vs rpm). the increased radius at engagement further increases the clutch shoe pressure.

to recap, the increased radius does increase the force (assuming a constant rpm).

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**57**I don't think you need to stop when the flyweights close the gap. I suspect friction and vibration are minor effects.

one aspect that hasn't been made clear is why a larger gap means a harder clutch engagement. I believe this is due primarily to the centrifugal force growth with rpm. your expression v^2/r is correct but is unclear.

In our case v=r*w (where w is angular velocity in radians/sec or 2*pi*rpm/3600) so the flyweight forces grow with r*rpm^2.

the means a larger gap has 2 effects, when the flyweights close the larger gap it is with a larger radius and at an increased rpm. the rpm^2 means the change in clutch shoe force is greater for the same change in rpm. (engagement is occurring on a steeper slope of the centrifugal force curve, F vs rpm). the increased radius at engagement further increases the clutch shoe pressure.

to recap, the increased radius does increase the force (assuming a constant rpm).

one aspect that hasn't been made clear is why a larger gap means a harder clutch engagement. I believe this is due primarily to the centrifugal force growth with rpm. your expression v^2/r is correct but is unclear.

In our case v=r*w (where w is angular velocity in radians/sec or 2*pi*rpm/3600) so the flyweight forces grow with r*rpm^2.

the means a larger gap has 2 effects, when the flyweights close the larger gap it is with a larger radius and at an increased rpm. the rpm^2 means the change in clutch shoe force is greater for the same change in rpm. (engagement is occurring on a steeper slope of the centrifugal force curve, F vs rpm). the increased radius at engagement further increases the clutch shoe pressure.

to recap, the increased radius does increase the force (assuming a constant rpm).

__if__the rpm remains constant, the engine is supplying more power to maintain that revolution and continues to accelerate as the spring resists it.

I was also working acceleration in metres per second, so I divided by 60 because of RPM is in minutes.

The current findings were more a revelation that adding or reducing throw weights and spring stiffness would be far more effective than expanding the gap. While the spring compression is linear, if the force of the throws is greater then the gap will effect the acceleration of the shoe as it apporaches the bell.

If the spring force and throws are equal then the gap will have less effect. eg, soft spring, large gap, heavy throws versus stiff spring, smaller gap etc... Again, this is all subject to the engine that's driving it too! Let me know what you think! And to re-iterate... Im not a qualified mechanical engineer, I do enjoy science though.

Thanks for the input, all ideas are thought provoking and as RBakker said, the hinge factors and impact are beyond the scope of this thread unless someone feels they want to contribute.

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**58**Tech Regular

one aspect that hasn't been made clear is why a larger gap means a harder clutch engagement. I believe this is due primarily to the centrifugal force growth with rpm. your expression v^2/r is correct but is unclear.

In our case v=r*w (where w is angular velocity in radians/sec or 2*pi*rpm/3600) so the flyweight forces grow with r*rpm^2.

the means a larger gap has 2 effects, when the flyweights close the larger gap it is with a larger radius and at an increased rpm. the rpm^2 means the change in clutch shoe force is greater for the same change in rpm. (engagement is occurring on a steeper slope of the centrifugal force curve, F vs rpm). the increased radius at engagement further increases the clutch shoe pressure.

to recap, the increased radius does increase the force (assuming a constant rpm).

We have:

time

rpm

mass flyweight

distance CoG of flyweights

amount flyweigths

angle flywheel or pressure plate (reverse clutch)

spring constant

friction coefficient

gap distance

How do we proceed with the resistance of the mass of the car in Nm at the flywheel in time..?

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**59**We could assume a simple friction coefficient and forget about static/dynamic friction and heat buildup. Let`s dare Bliss to make a model in excel with all the formulas including friction and slip/engaging time, that would be fun and good to use..!!

We have:

time

rpm

mass flyweight

distance CoG of flyweights

amount flyweigths

angle flywheel or pressure plate (reverse clutch)

spring constant

friction coefficient

gap distance

How do we proceed with the resistance of the mass of the car in Nm at the flywheel in time..?

We have:

time

rpm

mass flyweight

distance CoG of flyweights

amount flyweigths

angle flywheel or pressure plate (reverse clutch)

spring constant

friction coefficient

gap distance

How do we proceed with the resistance of the mass of the car in Nm at the flywheel in time..?

Hang on a moment.. I'm Engaged!! HAH!!

For now I'll stick with the throw weights and the spring forces.. You are welcome to get the accountants chessboard out and formulate referenced cells, would be a good submission. I want to focus on the Nm for the throws and springs.

So clutch that spreadsheet like a handbag Rbakker and engage yourself for a moment!

Twill be for the good of many, for I am only one.

A speck of dust on the shoe, alone it is no fun.

I've slipped and dusted trying, 50 years of inertia in my way,

The spreadsheet will be for all of us, to get faster every day.

The engineer that dares me, he's smirking from many kilometres away,

I'll joke and make it fun to do and dream of racing in Holland one day.

Met Willy while I was at the worlds, your nation's pride now at stake,

That spreadsheet wont even make you sweat, to you it's a peace of cake!

So don't be teasing this old man, with genius up your sleeve,

I will bow in lowly ignorance before I let your talents leave.

Think of how many will gain from the gift you give when they go to the track,

So with this poem I raise my glass to salute and with it dare you back!

h

*Last edited by blis; 03-12-2015 at 03:57 PM.*

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**60**
Blis, this is great reading

You have many talents

keep it going.

You have many talents

keep it going.