Advanced Clutch Tech With Mantic
Melbourne, Australia–based Mantic Engineering is a new name on the Corvette-clutch scene, but parent company Clutch Industries Pty Ltd has been building clutches since 1951. CI has been an OEM supplier to Holden (GM), Ford, Nissan, and Toyota. With that kind of pedigree, it should come as no surprise that Mantic prides itself on building high-performance clutches with OEM-style engineering, R&D, and quality.
In addition to a full line of single- and multi-disc clutch systems, Mantic is known for innovative clutch features. One example is company’s the ER2 Street Series covers: They use a patented Groove Design that is CNC machined on the pressure plate’s friction surface, which Mantic says increases the torque-drive capability by more than 8 percent over a non-grooved cover.
Because Mantic does extensive testing—both in the real world and on the only clutch dyno in Australia—we wanted to show you some of the terms, formulas, and calculations used in the company’s clutch design and testing. Some of it is pretty advanced, but it provides a solid understanding of the forces at work during clutch operation.
Let’s start with some basic terminology.
Clamp Load: This is the load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel.
Coefficient of Friction (μ): This is the measured resistance that occurs between two surfaces.
Mean Effective Radius: This is the effective radius of the friction surface, measured from the inside to the outside of the friction area; it’s used to calculate torque capacity.
Torque Capacity: This the amount of torque that can be safely transmitted through a clutch. It is calculated by multiplying the load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel, multiplied by the coefficient of friction, multiplied by the radius of gyration, and multiplied by the number of clutch discs.
Torque capacity is affected by 4 factors:
1. By decreasing the diameter, the torque capacity is reduced.
2. By decreasing the clamp load, the torque capacity is reduced.
3. By adding a second clutch disc, the torque capacity is doubled.
4. By increasing the coefficient of friction, the torque capacity is increased.
It is therefore possible for the two smaller discs in a dual-disc clutch to provide a greater torque capacity than the one big disc in a single-disc unit.
Mass Moment of Inertia (MMOI) measures the ability of the clutch and flywheel assembly to resist changes in rotational speed about a specific axis. (The symbol “I” is used to refer to the moment of inertia when making calculations.) This term is of particular interest with clutches and flywheels because to accelerate a vehicle, it’s necessary to overcome the vehicle’s resistance to acceleration, or MMOI.
The larger the MMOI number, the smaller the angular acceleration about that axis is for a given torque. Therefore, a higher MMOI number makes the flywheel accelerate slower at that torque amount. A lower MMOI, meanwhile, allows for faster gear shifts and improved engine response. Reducing the MMOI has much the same effect as adding power to the engine, enabling it to accelerate more quickly.
Decreased system weight also makes for an incremental improvement in the vehicle’s power-to-weight ratio. A dual-disc clutch system with smaller-diameter discs will actually be lighter than a single-disc system, in spite of its intermediate plate and extra clutch. This is because the effect of the weight decreases dramatically as the diameter gets smaller: Weight is proportional to the radius squared. If the radius is halved, the weight is decreased by a factor of 4.
There are two advantages to lowering the MMOI of a clutch assembly:
1. There is less inertia—and therefore less power—required to spin the clutch assembly. The net effect is that the vehicle is able to accelerate faster.
The MMOI of a non-point object is calculated by the following formula:
I = k x R2 x M (measured in kg / m²)
M is the mass
R is the radius of the object from the center of mass
k is a dimensionless constant called the inertia constant that varies with the geometry of the object in consideration. For example, k = 1 for a thin-walled cylinder around its center, or k = ½ for a solid disc around its center.
2. The clutch disc(s) will not “spin on” for as long; this allows gear changes to happen more quickly. Again, the results are faster acceleration, and less time when there is no power being transmitted to the wheels.
It is important to note that the MMOI is proportional to the radius squared. So a small change in the radius or diameter of a clutch has a dramatic effect on the MMOI. For example, an increase in diameter of 40 percent—say, from 200mm to 280mm—approximately equates to a doubling of the MMOI, or a doubling of the resistance to changing the rotation rate. In layman’s terms, that means twice the power is required to accelerate the clutch.