Motorcycles are kinda the last bastion of manual transmissions. The obvious difference between automatic and manual transmissions is the clutch. Wait, scratch that, automatics use clutches of a sort too, so it's really more about the fact that the majority of motorcycles use a clutch operated by hand, instead of a computer or series of hydraulic valves. The hand clutch (as we know it) is a unique contraption, mostly because of it's operation and it's operator, you. Humans are a trifle more erratic in their clutching techniques than any automated system could ever be, which plays into the details of design in good motorcycle clutches. Inconsistent engagement and release and the nature of friction in a "wet" clutch design (especially in big V-twins making prodigious torque at relatively low engine speeds) can lead to a whole host of issues in the pursuit of the perfect Harley-Davidson clutch.
We are very aware of the imperfections and the issues that can arise such as clutch slip, or crappy engagement, and "hanging" on quick shifts. But the real killer is when something comes apart just when you need it most. The thing is, the bulk of these issues we bring on ourselves. Aside from sheer abuse, there are two things common to most clutch woes. First is our propensity to hop up our engines. Seems clear that when you make a lot more power you push the capabilities of the clutch right to the edge, and often beyond. Second, most of us don't have a clue what those capabilities are in the first place. Lucky for us, there's a company out there that understands all of this, Barnett Tool and Engineering. And they have been making effective solutions to the problems we've just discussed since the late fifties. Even though Barnett is well-known for its control cables, it's the high-performance clutch components that are a point of pride with the company. Not without reason, since they have really outdone themselves with the Scorpion basket and clutch kit for 2007 and later (six-speed) Big Twins.
1. The stock basket, on the...
1. The stock basket, on the left, is cast; including the studs the pressure plate screws onto (A), in contrast to the Scorpion basket, machined from 7075 billet aluminum and hard anodized, and features replaceable steel studs (A). The Barnett basket retains friction and steel plates that offer substantially more cross-sectional surface area than the stockers. The distinctive red material on those friction plates (B), Barnett refers to as "RQ," which might stand for "race quality," but more importantly are thermally stable blend of fibers and resins, with excellent porosity, compressibility, and durability characteristics. In other words, great material for preventing failures in high-stress, high-heat applications where shift quality is also a major concern. You'll also notice that the steel plates are very smooth. A good thing for reasons that might surprise you but we'll get to that in a minute. In the meantime, notice the difference between the stock pressure plate and the Scorpion version? Beyond the nifty gold color, the Scorpion uses coil springs (C) rather than a diaphragm spring, thus offering a certain tunability that the stock arrangement cannot.
2. The stock setup has a...
2. The stock setup has a relatively light, fixed-rate diaphragm spring where as the Scorpion kit comes with three sets of six coil springs (42 pound gold, 58 pound gold, 82 pound green, and 100 pound red) that can be used as matched sets or in mixed combinations to get grip and pull in balance and just right for any application. That said, one of the biggest rookie mistakes in a clutch set up is to immediately go with the strongest spring available, which is usually not needed and invariably makes clutch pull at the lever too stout for normal humans. If you really need the stiffest springs provided, you'll most likely also need to convert from cable to hydraulic actuation. Speaking of hydraulics, it's during a shift that the greatest demand is placed on a clutch. As the plates slam home, incredible heat (the real enemy) is generated in the pack that doesn't transfer to the entire clutch all that rapidly. Funny to realize that a "wet" clutch is actually oil-cooled, but essentially that's the way it works. That's why pre-soaking the friction plates in primary fluid is crucial. Oil retained in the friction material is squeezed out, taking unwanted surface heat with it. If the plates cannot absorb and discharge oil fast enough, heat builds and glazing (plate imbedding) occurs like brake fade!
3. Given that cooling is...
3. Given that cooling is important, thinner plates will dissipate heat better than thick ones (notice the difference in plate thickness between the two setups-stock left, Barnett right). Aside from a material that's almost impervious to this because it can inhale and exhale oil quickly, the best ally in heat control is smooth steel plates. The smoother the surface the more surface to squeegee fluid and pass the heat on and out. In fact the quickest way to destroy a modern wet clutch is to sand or bead blast the steel plates! The second quickest is using springs that will affect this efficient transfer of heat, by making the strength of your hand the factor that alters the plates ability to breathe oil.