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 here. Our C5 project has been down for a while waiting for the LS3 engine transplant. Most of the parts were here, but a few key elements were held up by virus-related supply chain slowdowns, giving us ample opportunities to find other upgradeable items “while we were in there.”
One of those was the clutch. We had already replaced our stock clutch with a high-performance, single-disc unit paired with an aluminum flywheel, but we felt that a further upgrade was warranted since we were again increasing performance and moving the car more toward dedicated competition use.
Our opportunity was realized when Mantic Clutch USA released a new dual-disc clutch that was a bolt-in option for our Corvette. With a simple part swap, we could reduce overall weight and engine-accelerated rotating mass while increasing clamping force. Lots of wins in that column.
So let’s look at some of the advantages of a multi-disc clutch and how it’s like getting a free lunch performance-wise.
First, the ability of a clutch to produce enough friction to adequately transfer engine power to the drive wheels is dependent on several factors, but total surface area of friction surfaces is critical. Load capacity scales very favorably with surface area, so increasing surface area means more ability to get all that power where it needs to go.
With a single-disc clutch, increasing surface area means increasing diameter, and there’s only so far you can go before you hit bellhousing on the outside or output shaft on the inside. In other words, a single-disc clutch is physically limited in total surface area by the available real estate.
However, adding additional discs allows you to dramatically increase total surface area while maintaining or even reducing overall diameter. After all, if you double the number of mating surfaces, a 20% reduction in diameter is a small tradeoff.
But that reduction in diameter has a bonus as well: It reduces the diameter of the rotating mass of the clutch, meaning it has less leverage on the output shaft and is easier to spin. Less resistance to spinning the engine means faster engine acceleration, which means faster car acceleration, which means more trophies.
Finally, favorable materials can be used to further increase strength and clamping force while reducing that all-important rotating mass. In the case of our unit from Mantic Clutch USA, super-lightweight titanium drive blocks are used, meaning the outermost components of the clutch—which would have the largest impact on rotational moment of inertia—have been optimized to be as light as possible while still being incredibly strong.
But how does a multi-plate clutch actually work? Well, anyone who has ever replaced a motorcycle clutch is probably familiar with its basket-style clutch fingers that hold the multiple plates. An automotive multi-plate clutch operates with similar hardware, albeit in a highly beefed-up fashion.
To do the heavy lifting on the the working principles, we’ll turn you over briefly to a much younger, more fluffy-haired version of Jason Fenske from Engineering Explained. Time-traveler-from-the-past Jason does a great job of explaining the operating principles of a multi-plate clutch in this video:
To paraphrase, the clutch plate drives the friction discs through a series of fingers, or “drive blocks,” which allow the clutch plates to float independently of each other when tension is taken off them by the clutch actuation cylinder.
When pressure is reapplied, the discs are pressed together, and the discs driven by the drive blocks on their outside diameter attached to the clutch housing spin the discs attached to the driveshaft, and everyone is happy.
In the case of the $2750 Mantic Clutch USA twin-disc clutch, the effort was put into reducing overall mass and keeping the remaining mass as close to the center of rotation as possible. Hence the titanium drive blocks. Function dictates they be placed on the outer edge of the clutch housing, so the priority was to make them as light as possible.
Even the aluminum flywheel has a subtle conical shape—thicker toward the middle and thinner toward the outside edge—which places more of the mass near the rotating center, further reducing polar moment.
Overall, the clutch assembly and flywheel weighs in at 32 pounds. Our previous single-disc, high-performance clutch and aluminum flywheel weighed 38 pounds. The stock setup weighed more than 55 pounds.
Now all we have to do is wait for our shop assistant to clear quarantine so we can get it bolted in. Thanks, 2020!
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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.
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