When it comes to clutches, as with buffets, more plates are more better. “But,” we can hear you screaming over your keyboards, “isn’t lighter better when it comes to clutches? And won’t more plates be heavier? Why are you doing this to us?”
Follow along folks, and we’ll get to all that in a minute.
First, here’s how we got …
I understand the idea of reducing the polar moment of inertia as far as engine response and parasitic losses go but I wonder about the difference in 'total clutch plate weight' as far as how it relates to high rpm shifting and synchro function? If the overall weight hasn't increased by more than the polar moment decreased, it should be an advantage in that area too.
The weight feeds in to the calculation of Polar Moment of Inertia (PMI for brevity), but as far as impact on rev change performance the PMI is all that matters. Most demonstrations of PMI are 2-dimensional, as it's way easier to draw and to understand, but you have to think in 3D to really understand the effects of a change. You also have to keep in mind that PMI is basically a structure's resistance to twisting in the same way that leverage is a structure's resistance to bending. Applying torque to one end of the flywheel/clutch/driveshaft system causes those parts to twist, and once that twist overcomes the friction in the system that system will start to turn. The polar moment of inertia is the resistance to a change in that twist, and lowering that value at any point in the system makes that resistance lower. Changing RPM changes how much twist is being fed into the system and a lower PMI will resist that change less. Less resistance means less work for the synchros, and easier shifting regardless of RPM.
Higher overall system mass may have some minor harmonic damping effects, but their impact would be minimal compared with PMI changes.
BikingEngineer said:You also have to keep in mind that PMI is basically a structure's resistance to twisting in the same way that leverage is a structure's resistance to bending.
Best description of this I've ever heard and now that it's appeared on our website I'm totally within my rights to steal it someday :)
Why does going to more friction area increase friction force? If the force of friction is F=u*N why do we care about surface area?
In reply to buzzboy :
Because multi plate clutches let you "use" the same force multiple times. Two pieces of paper on top of each other are easy to slide apart, but interleave a phone book, and it's impossible to pull apart. The same force clamping the first clutch plate is also clamping the second, so it's actually F=2 * u * N for a twin disk clutch.
The weight feeds in to the calculation of Polar Moment of Inertia (PMI for brevity), but as far as impact on rev change performance the PMI is all that matters.
Thanks for the clarification that weight is already part of PMI, but i still don't think it's guaranteed that PMI goes down by decreasing diameter but then doubling the number of clutch discs. I would guess that whatever clutch plates are in twin disc and triple disc clutches don't have sprung hubs or 'facing springs', so two of them might still be no heavier than a single conventional disc of a larger diameter. This is all probably common knowledge among people who are more familiar with multi-disc clutches for manuals. I have never looked too far into it because I've never built a car to be seriously competitive in a timed even where the weight of the assembly or the shift time was a significant factor, plus AFAIK everything i own can get enough clamp load with a 'regular' clutch assembly to run 10s in the 1/4 mile if that was the goal. I feel like you have to be competing for fractions of seconds or running a single-digit 1/4 mile to have a legitimate use for one.
Vigo (Forum Supporter) said:The weight feeds in to the calculation of Polar Moment of Inertia (PMI for brevity), but as far as impact on rev change performance the PMI is all that matters.
Thanks for the clarification that weight is already part of PMI, but i still don't think it's guaranteed that PMI goes down by decreasing diameter but then doubling the number of clutch discs.
That's because it isn't guaranteed, but it's related to the 4th power of diameter, so it doesn't take a lot of diameter decrease to make it happen and the mass will come down in proportion with the diameter squared, assuming a solid disk of uniform material.
JG Pasterjak said:BikingEngineer said:You also have to keep in mind that PMI is basically a structure's resistance to twisting in the same way that leverage is a structure's resistance to bending.
Best description of this I've ever heard and now that it's appeared on our website I'm totally within my rights to steal it someday :)
I summarized it from the Wikipedia page for Polar Moment of Inertia, so feel free.
RX8driver said:Vigo (Forum Supporter) said:The weight feeds in to the calculation of Polar Moment of Inertia (PMI for brevity), but as far as impact on rev change performance the PMI is all that matters.
Thanks for the clarification that weight is already part of PMI, but i still don't think it's guaranteed that PMI goes down by decreasing diameter but then doubling the number of clutch discs.
That's because it isn't guaranteed, but it's related to the 4th power of diameter, so it doesn't take a lot of diameter decrease to make it happen and the mass will come down in proportion with the diameter squared, assuming a solid disk of uniform material.
To add to this, while it isn't guaranteed that PMI will go down with diameter it's very likely that it will go down due to the exponential effect that diameter has. I would imagine that any Design Engineer making a multi-plate clutch application would do the very basic mathematical calculations for PMI before they did almost anything else to get an idea of the scale of the improvement.
buzzboy said:Why does going to more friction area increase friction force? If the force of friction is F=u*N why do we care about surface area?
The Coefficient of Friction is actually a fairly complicated thing to pin down due to all of the factors that feed into it. For a clutch system the biggest factors are microscopic scale (or smaller) inconsistencies on the friction surfaces (asperities, in technical jargon) sliding against each other and providing a resistive force. Imagine two toothed surfaces sliding against each other and you'll be in the ballpark. As you press these surfaces together these teeth start to mesh together and will resist motion, that resistance is what's causing your Coefficient of Friction to exist. There are other effects related to temperature, velocity, atmosphere, and a huge variety of other things, but that's a discussion for Tribologists, not gearheads. When you add plates to a clutch pack you're adding a bunch of these surfaces, while keeping the Normal Force (N) relatively constant, and your CoF goes up because of it. Usually your clamping force will actually go down with the addition of plates in the pack, so you have a more useable clutch pedal.
In reply to BikingEngineer :
You should post more frequently. I think we'd all get smarter as a result.
AngryCorvair (Forum Supporter) said:In reply to BikingEngineer :
You should post more frequently. I think we'd all get smarter as a result.
Thank you. There's definitely a balance though There's only so much engineer brain that any particular discussion can take before it gets a little dense, it's just that an article about clutch plates lends itself to more than usual.
Thank you. There's definitely a balance though There's only so much engineer brain that any particular discussion can take before it gets a little dense, it's just that an article about clutch plates lends itself to more than usual.
The fact that you recognize and admit that makes me and probably a lot of others here more willing to listen! I'm an instructor/trainer (I think AngryCorvair is too) and tailoring your level of specificity to your audience is one of most important traits a teacher of any kind can have.
I'm leaving the discussion with the impression that in general, these multi-plate clutches will have less PMI for the engine AND the synchros. Cool. 
I think in a clutch or brake application, the main reason for increasing surface area is their capacity to absorb and dissipate heat and has little to do with generating adequate friction. That, plus longer service lives, which is also partly related to heat.
The multi-disk clutch pack can also be easier on the synchros in the transmission. With a single-disk clutch the synchros have to accelerate or decelerate the clutch disk to the proper speed to allow the sleeve to engage the next gear. In the multi-disk pack described in the article, the plain steel plates are the ones coupled via splines to the transmission's input shaft. Those plates are relatively thin and relatively small diameter, so it appears that shifts could be substantially faster.
However those shift speeds won't come close to what's possible with a dual-clutch transmission, in which the next gear is pre-selected and one clutch can engage while the other is disengaging.
Something that wasn't mentioned in the article is that multi-disk (dry) clutches have a reputation for abrupt engagement due to high clamping force and small diameter. Any comments on that?
Martt
I used a multi-disk clutch in my Cobra kitcar. Are they all inherently noisy (rattles) when disengaged, or was that just a "feature" of my particular supplier?
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