A. Clutch basket: Casing that holds clutch internals.
B. Plates: Alternating steel and friction discs.
C. Springs: Apply tension to plates and engage/disengage clutch action.
Barnett's Xavier Romero walked us through a clutch basket's production.
Robert Fisher of Roaring Toyz has built some of the sickest customs around and has blown out a clutch or two in his racing days. He had this to say about today's clutches: "The OEM plates today are amazing. In some cases they're even better than the aftermarket plates. But where the real performance lies is in the springs. When you add power to these motors the clutch springs just can't handle the stress. I typically run stiffer springs with OEM plates in my high-horsepower applications."
Shane McCoy of McCoy Motorsports has been building hot exotic customs for years. With myriad stretched, lowered, big-pony builds that are ridden hard under his belt, McCoy has quite a bit of real-world experience with melting clutches. He says: "With many of the R1's the standard oil configuration just doesn't cool the plates adequately. We route an additional oil line through the clutch cover to keep from melting the plates. Anytime you move into high power you need to give the clutch a working over."
Barnett designs its clutches on the computer to ensure exact specs.
McCoy Motorsports cleverly runs an extra oil line to continuously bathe the clutch plates in cool oil.
Believe it or not, some riders have no idea that pulling that little lever on the left clip-on actually disengages the bike's clutch. Many riders have only one concern when it comes to the inner workings of their machines-that they work. But digging into why it works and how it works opens up the fast paced, better than your buddy, potentially high-priced world of high performance.
What Is It?
A clutch is a mechanical coupling that allows the transmission of rotational energy to be engaged and disengaged, thereby isolating the engine from the rest of the driveline components.
What does that mean? It lets the engine spin while the transmission stands still when the clutch lever is pulled, and forces them to both spin together when the lever is out.
How It Works:
In most motorcycle applications the lever on the left bar acts via cable or hydraulic system to engage or disengage the clutch. Remember, the clutch is engaged (engine and transmission rotating together) when the lever is out. When the lever is pulled toward the rider, the coil springs in the clutch are compressed, allowing the stacked clutch plates to move independently.
The stack of clutch plates are arranged so that plain steel plates and friction plates alternate. One type of plate is connected via splines to the crankshaft while the other plates are connected to an outer basket that drives the input shaft of the transmission.
With the clutch lever pulled in, the springs are compressed-letting the plates spin freely while the engine and transmission are able to spin at different speeds. This makes gearshifts possible while at speed and allows the motor to run freely while the bike is at a standstill.
Key Ingredients And Terms:
Lever: Metal rod, which pivots on a perch located on the left handlebar. Allows rider input to the clutch system.
Clutch Cable Or Hydraulic Lines: These are the mechanisms which translate the rider's input from the lever to the clutch internals..
Friction Plate: These metal plates have multiple sections of friction material attached to them, which grab the steel plates when the clutch is engaged to make the crankshaft and transmission rotate together.
Steel Plate: Smooth metal plates that provide a clean surface for the friction plates to bind to when the clutch is engaged.
Basket: This is the outer housing for the clutch assembly that attaches to the transmission. One set of plates is carried by the basket via keyed grooves or splines.
Springs: Metal coil-type springs that provide pressure to the plates, keeping them in contact when the clutch is engaged.
Slipping The Clutch: Intentionally causing the clutch to be not fully engaged, but not fully disengaged. This allows the engine to rotate faster than the transmission, causing a smoother progression from disengaged to engaged states.
While effective and necessary for riding, this action causes wear of the clutch plates and after extended periods of time causes the clutch to slip on its own and not fully engage when the clutch lever is all the way out.
The Next Step:
Many modifications can be made to the clutch assembly to increase efficiency and performance. Introducing stiffer springs or changing the friction pad compound can greatly increase the amount of torque that the clutch can handle.
SSB headed down to Barnett Performance Products for a factory tour with Xavier Romero, production manager for the 60-year-old company.
Xavier clued us in on the intricacies of the modern clutch and reinforced the idea that altering a stock bike's power and/or drivetrain changes the requirements on the clutch-generally resulting in the need for heavy-duty components.
OEMs design the stock clutch system for the level of power that the bike produces in stock form. Power adders like nitrous, turbos, big-bore kits and superchargers typically make more torque than the stock clutches can handle. In addition, the resistance of the transmission increases when a stretched swingarm and wide-tire kit are installed. In both cases stronger clutches are a must.
In hard riding applications like road racing and serious canyon carving, banging hard downshifts while jamming on the brakes can cause the rear wheel to lock up as the clutch grabs abruptly with the large change in engine rpm. Modern slipper clutches utilize a cam system, and in some cases sophisticated electronics, to allow the clutch to engage more slowly under these harsh conditions.
The science behind these things gets pretty hairy when engineers begin to consider manifold pressures, mass air velocities in the intake tract, relative angles of rotation between the output shaft, clutch basket and carrier and the rear wheel. But rest assured the geniuses that develop these parts have got it on lock. The latest and greatest slipper clutches keep that rear end spinning smoothly under even the harshest downshifts.
Xavier reminded us that, "Even after 60 years of evolution there is still much to be learned about clutches that will make them perform better and last longer."