This is from our Youtube video: how do dual disc clutches work. The link is at the bottom of this page if you’d prefer to watch vs. read 🙂 But we figured you may want to study up and read along at your own pace, so enjoy!
Dual Disc Clutches can be a bit of a conundrum. How do they even work? In this video, we’ll explain what they are, how they’re designed, what advantages they have, and nail this question down!
Remember back in high school physics class when you learned F=un? Yes, you were probably daydreaming about that awesome super car you’d own some day…
Or getting in trouble throwing paper airplanes around
So maybe you just weren’t even paying attention; but, regardless, we’re going to take a look at why dual disc clutches seem to defy this basic physics principle.
You might be getting a drift of what I’m referring to here and what the problem is, but if you’re not yet, we’re going to dive deeper into this.
What is a dual disc clutch?
Dual disc clutches are a high-performance clutch system for manual transmissions that can literally double the torque capacity of your vehicle compared to a single disc system. They work by using two friction discs instead of one, which allows them to handle more torque, more heat and last significantly longer.
How are Dual Disc Clutches designed?
As you can see, Dual disc clutches are similar in design to single disc clutches since they both use flywheels, pressure plates, clutch discs and so on. But the major difference is they use two friction discs instead of one. Which is a hint to the answer we’re after here… So, remember that. The two discs are mounted back-to-back with a floater plate sandwiched in between.
When the clutch is engaged, the pressure plate forces the two discs together, which transmits power from the engine to the transmission.
Alright, well that gives us a little more information. Here we have two discs. So we have doubled the surface area which should double the torque, right? Well, from above we had F=un. Where F=The sliding force provided by the u part which is the coefficient of friction and n being the force pushing them together, also known as the (N)ormal force… hmmm. There’s no surface area even in that equation… What gives? So doubling the surface area isn’t what’s giving us the increase in torque holding capacity.. Or is it?
What are the Advantages of Dual Disc Clutches?
The main advantage of a dual disc clutch is that it can handle more torque than a single disc clutch. Because the two discs provide twice the surface area for friction, which allows them to dissipate heat more effectively. Hmm, well kind of. Doubling the surface area does allow twice the surface to reject the heat and It provides twice the surface area for wear, so you’ll get longer life out of them vs. an equivalent single disc clutch. Dual disc clutches are also less likely to slip than single disc clutches, which makes them ideal for high-performance applications. Yes, that’s true as well, and actually the whole point of going to a dual disc clutch vs. a single, but still not answering why they hold more torque when the basic physics equation isn’t making sense. Yet.
Why dual disc clutches hold more torque.
So the reason the F=un doesn’t seem to work is we’re doubling the surface area which typically reduces the pressure on any given square inch of the friction surface. Let’s say you put 1000 lbs. of force on 10 square inches of disc. Each square inch would now have 1000 lbs/10 in^2 or 100 lbs / in^2 of pressure acting on it. Now let’s double the surface area and try this again. We have 1000 lbs. of force again, but this time 20 square inches of disc. So now we have 1000 lbs / 20 in^2 or 50 lbs / in^2 of pressure… Huh? That just went the wrong way? Well doubling the surface area with half the pressure on each given area ends up canceling each other out and we end up with the same holding capacity… But that would only be if we were doubling the surface area of a single disc or a block sliding across the floor like we did back in physics class…
Here is the trick…
Since the discs are stacked vs. a single surface, each disc gets the FULL 1000 lbs. across its 10 in^2 of surface or now the equivalent of 2000 lbs. of force acting on 20 in^2 which gives us the same exact pressure or 100 lbs / in^2 on each friction surface. Still sounds funny, right? Doubling the pressure and the surface area still gives us the same pressure…
Let’s go back to the sliding block example. Since the discs are stacked and each one has the full 1000lbs acting on it, it’s like having two blocks now equalling twice the weight, or n in the equation with the same friction coefficient, u, which finally doubles the F or Force in the equation…
So, by stacking the discs, allowing the full pressure of the pressure plate to act on each disc and not be divided by them, we’re finally able to double the torque capacity of the system by adding more discs!!!
How about more discs?
Yes, we could do more discs. Like the infamous Ducati dry plate clutch that runs a multi plate clutch. (b-roll to a ducati idling with the clutch rattle sound) Or the Carrera GT that also has a multi-plate clutch disc. (b-roll Carrera GT ripping by) Formula one cars and Top Fuel dragsters have been using this technology for years as well! Formula one uses multi-plate clutches to be able to reduce the overall diameter thus the rotational inertia of the system while Top fuel needs to be able to handle tons of torque and heat during the launch. Even automatic transmissions like in the 996 and 997.1 turbo use multi-plate clutches in the gear stacks. Little tiny ones, but with anywhere from 4 to 10 plates stacked with friction surfaces between each plate.
Won’t the clutch be too stiff with all that pressure?
Well, no… We’ve not increased the pressure plate or the 1000 lbs in these examples at all to increase the torque capacity. In theory, we could use a dual disc with only a 500 lb pressure plate and have a super light clutch pedal but hold the same capacity as a single. But why would we do that? We’re here because our single disc clutch is either slipping under power, our single disc clutch is undrivable due to a heavy pressure plate, or it’s chattery due to a metallic clutch disc being used. So, if we keep the same pressure plate or increase it over the stock pressure plate with the Sachs 764 or 487 to gain not only double, but another 15 to 30% on top of that for a whopping 230 to 260% torque increase between them. All while driving and feeling like a stock clutch!
Conclusion
You may or may not have questioned why dual disc clutches work. But I thought it was fun enough (lol, no pun intended) to bring up since the basic formula at first glance just doesn’t seem to work out. We know they work. We’ve used them in many applications throughout history, but unless you think about it right, it just doesn’t line up. Kind of like a similar argument about wider tires shouldn’t grip any better according to the same formula… But we’ll have to save that for another time.
Youtube Video Link: 
Link to Protomotive Dual Clutch kit: 
Link to Protomotive Dual clutch upgrade: 
Link to Protomotive Dual clutch disc replacement discs: 
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