 May 2008 - Wood Knight Prowler
IT’S ALL ABOUT TORQUE
Part IA: The Wood Knight Prowler silent belt drive system
by Donny Petersen
The title may be a little misleading to the uninitiated, since we automatically think of primary or rear drive belts and pulleys that deliver powertrain torque to the rear tire when we hear the words belt drive system. However, the belt system I’m talking about today connects the pinion shaft and rear camshaft inside the gearcase of a Twin Cam engine.
Knight Prowler products takes its name from the elusive, but magnificent and fierce, black panthers of the Southern US. Bob Wood of Wood Performance Carburetors is all about power, more specifically torque. Bob began his H-D aftermarket career by specializing in carburetors with his popular King Carburetor. He naturally branched out into high-flow air filters, and later into cams.
Lowering The Powerband
Bob’s cam designs are renowned for lowering the powerband into useful rpm ranges. His grinds increase the lift (how much the valves open relative to the duration) and use shorter duration profiles (how long the valves are open) than most other manufacturers. Don’t get me wrong; duration is a good thing. It is a strong component in the camshaft lobe design equation for more power. Many of today’s aftermarket cams, including Screamin’ Eagle’s, generally have longer duration profiles relative to lift, which take away from bottom-end, off-the-line power. These longer duration cams work wonderfully in the higher rpm stratosphere riding, which can challenge the mechanical limitations of pushrod-operated engines. However, more is not always better. So, make no mistake; Bob Wood is totally into power. He is a two-time NHRA Fuel Funny Car national record holder. But too little duration, as exemplified by EPA-dictated stock cams, is also not conducive to maximum power. The difference is that those cam profiles are all about containing emissions. Power is a secondary consideration.
How do we maximize torque? It’s pretty simple; we need maximum cylinder pressure for maximum torque. We get the greatest torque first by closing the intake valves early and second by not opening the exhaust valves too early. We do not want potential compression leaking off because valves are open at the wrong time. Maximum cylinder pressure occurs approximately between 170 degrees and 270 degrees after top dead center (ATDC) of the compression stroke.
Prowler Belt Drive
The Knight Prowler silent advanced belt drive system incorporates a special one-piece, 5mm, steel-billet crankhub with a removable flange. This flange allows for quick installation or service of the timing belt or any other parts related to disassembly. Other highlights include a special, hard, anodized-aluminum 5mm cam gear and steel-billet, adjustable spider gear; a one-piece, billet-aluminum faceplate with Viton seals, and a removable belt guard with a clear Lexan cover so you can view the belt drive in action. The super high-torque, carbon-fiber-reinforced, 5mm belt was designed exclusively for Bob Wood by Goodyear. The silent belt system is available polished or chromed. I have a chromed one, and it has an exquisite, lustrous finish. Minorly bent, out-of-true pinion shafts or scissored flywheels (however you choose to describe this all-too-frequent Twin Cam malady) often generate excessive valve noise and can make parts assembly difficult. The belt drive will alleviate this type of noise and make cam installation easier.
The silent belt system is an innovative gear drive upgrade for the Twin Cam timing chain arrangement. It cures the demonstratively fragile 1999-2006 timing chain spring-loaded adjuster problem. It also replaces the good-looking, but still unproven, newly designed chains and hydraulic adjusters in the 2006 Dyna and all 2007 and later Big Twin models. However, this is where the similarity to other gear-driven cams (manufactured by S&S, Andrews, and Crane to name the majors) ends. When I initially complimented Bob Wood on his superb design, he shrugged my comments off, saying, “There is nothing new here. We’re just applying simple, proven, automotive technology. It’s old, old stuff; other companies have been doing it for 30 years.” Well, it might be old stuff, and it is true that cars have used these technologies for a long time, but what Bob might not get is that no one else thought to use these concepts on a Harley-Davidson. The unassuming-but-talented inventor, Bob takes his gear drive system much further than his predecessors Hmm? Didn’t Bob say, “Nothing new here -- we’re just applying simple proven automotive technology”? Well, yes he does, but sounds to me like he has some skilled inventor traits in him. Nor did anyone else turn these ideas into a beautiful, workable, functional, reliable way to up the performance capabilities of a Harley.
The cam timing is adjustable on Wood’s belt drive system. The camshafts can have the cam timing advanced or retarded a full 10 degrees in about 10 minutes. This will adjust the power and torque curves up or down to suit individual riding styles. Spur or helical gears power Bob Wood’s gear drive cams on the inboard side of the cam support plate. We will discuss these gear types as we proceed.
Spur & Helical Gears
Spur or straight-cut gear drives are nothing new. Their use in Harley camshafts goes back forever. Every Harley used them before the advent of the Twin Cam in 1999, which replaced gear drives with timing chains and sprockets. Spur gears are not in constant contact, which leads to a condition called backlash. According to John Andrews of Andrews Products, “whenever the roller follower on a cam lobe passes the maximum lift point, the forces on the (spur) cam drive gear tooth change direction. If more than 0.002”, backlash is present between the cam and spur pinion gear; this directional change of force will result in an audible click as the backlash moves from the backside of each gear tooth to the front side.” Modern spur gears used by the likes of Andrews, S&S, Wood, and Harley-Davidson are extremely precise. Modern spur gears make the least noise of any version previously used. There really are no further precision gains available under present manufacturing technologies. What is new in Bobby Wood’s design innovation is in using inboard cam support plate helical gears as an option on the camshaft lobes themselves.
Helical-cut gears are much quieter than spur-type gears because there’s continuous contact between mating gears. Helical gears eliminate the clicking to clattering sounds emanating as the result of spur-gear backlash. As Bobby says, “Helical gears have been around for some time and are not new to the automotive industry. Automotive transmissions have been using helical gears for decades for a smoother and quieter transmission. The motorcycle industry is just now beginning to realize the vast improvements gained by using helical gear transmission technology. Helical-designed gears function with less noise than traditional gear designs as they roll smoothly together, not producing the slapping sounds that are common with straight cut gears.
“Another benefit in using helical gears is that they have a larger contact area and oversize or undersize gears are not necessary. Being more complex and difficult to manufacture, helical gears are more expensive. Thousands of phone calls over the years regarding whining or noisy gears prompted Wood Performance to reevaluate the entire noise issue with Twin Cam engines, not to mention the huge problem with slightly bent or out-of-round Twin Cam crankshafts. Complicated? Maybe, but Wood Performance has a simple solution: belt drives on the external gears and helical gears on the inside.” Bob goes on to say that “Knight Prowler helical gears are made only with aircraft-quality 8620 steel. Besides being ground to extremely close tolerances (0.0001”), the helical teeth are ground with a crown, meaning a convex crown of approximately 0.0002” is ground into each tooth on the rear gear, which results in a much smoother and quieter engagement of the teeth. Silence is golden! No expense was spared in the manufacturing of these one-of-a-kind crowned helical gears. PS: We are always looking for a better way! Any input or ideas to make them better is always welcome here!”
Helical gears eliminate clattering backlash. Their smoother operation results from helical gears rolling together as they mesh. It is only recently, with the introduction of the 2006 Dyna six-speed transmission, that the Big Twin begins to take advantage of these and other benefits. The oversize-undersize spur gear fitment procedures common for noise reduction are unnecessary with helical mating gears because of their inherent fuller, constant contact with each other.
Wood Performance presents a viable, reliable, functional alternative to H-D timing chains from an EPA noise-reduction perspective. In my opinion, the combination of quieter inboard-of-the-cam-support-plate helical drive gears and outboard-of-the-cam support plate belt-driven pulleys powered by the pinion shaft can achieve EPA noise-restriction guidelines. Furthermore, the Knight Prowler silent belt system offers performance benefits impossible to achieve with timing chains. The bottom line is that this advanced belt drive system is precise. It is beautiful and trick in every sense of the word. Bobby’s belt-driven cams achieve both form and function and defines Harley cool.
Belt Reliability
You may be asking yourself, “Is the actual belt driving the Knight Prowler cams a fad design or is it reliable over time?” I confess, I wondered much the same. If I did not respect Bobby Wood’s intellect, knowledge base, and design capabilities, I might not have even tried his silent belt system. Don’t get me wrong; I loved the look. The beautiful Knight Prowler chrome gearcase cover, with protective clear plate over the aperture for viewing the cam support plate outboard drive pulleys and belt, in action or at rest, is stunning. There is an optional chrome cover available to conceal this aperture, which is also attractive. Myself, I would never cover the action underneath.
Let us get back to the initial, as it turns out, ill-conceived, belt-reliability concern. Who wants to break a belt in the middle of nowhere on a dark, cold, rainy night; does it get any worse? Even if you’re in the big city, would you find such a belt in stock at the local bike shop? Well, I trusted Bob Wood, and I certainly did not want to risk insulting him by asking him for a spare belt to carry around. I waited until I’d ridden my hopped up 103”, high-compression 2007 FLHXI Street Glide outfitted with a silent belt system with big, .650”-lift, high-torque Wood cams for about 4,000 miles before writing this article.
I do not always follow my own advice, sometimes to the detriment of my wallet. I break in my bikes; I warm up the engine and drivetrain before becoming aggressive, and change the oil probably more than need be. However, after these important considerations, I make my bikes honk. Pretty soon, I was redlining the engine in first gear and winding up the revs when on the highway, just having a ball. So, how did the belt hold up? It still looks new with nary a wear mark. The belt did not stretch. I’ve had zero problems in my experience with the Knight Prowler. In fact, I stopped having any concerns about it a long time ago. There is one caveat, as always. There must be a proper belt installation, with sprockets correctly aligned. But then again, timing chains and their sprockets also need proper installation, as do other manufacturer’s gear drive systems. What makes this belt so special? Carbon fiber and precise manufacturing of component parts are the short answers.
Cam Gear Drive Benefits
Although the silent belt system is half belt drive (located on the outboard side of the cam support plate) and half gear drive (located on the inboard side of the cam support plate), all of the gear drive and Knight Prowler silent belt system benefits apply to it. S&S Cycle was the first to replace the stock Twin Cam timing chains with a gear drive system. This occurred shortly after the introduction of the Twin Cam. Gear driven cams in the Twin Cam engine allow for the utilization of higher seat-pressure valve springs without exacerbating cam-timing wandering. This, in turn, allows for the use of much higher lift cams without loss of power due to imprecise cam timing.
The loose fit of the two timing chains (one to power the rear cam off the pinion shaft and the other to drive the front cam via the rear cam) on their sprockets can result in up to 4 degrees of cam timing inconsistency. The cam timing can continuously shift back and forth through this variance.
Furthermore, in performance applications, these higher seat-pressure valve springs can affect cam-timing wander an additional 4 degrees simply because the strength of the valve springs can overpower the 1999-2006 spring-loaded cam chain tensioners. This effect is lessened on the new hydraulic cam chain adjusters introduced on the 2006 Dynas and all 2007 and later Twin Cams. However, the effect is constantly at work, and will be exacerbated by the hydraulic action of these late-style tensioners as they wear over time. Indisputably, cam timing cannot wander with a properly fitted, gear-drive arrangement or hybrid gear-belt Knight Prowler. A second major benefit of gear-driven cams is that much higher lifts are possible without cam lobe interference from the lobe on the opposing cam.
A gear-driven setup has two mating gears on the outboard side of the cam support plate. The first runs off the end of the rotating pinion shaft. This gear is bolted and slotted to the end of the pinion-shaft and it powers – or, more correctly, transfers power -- to drive the whole valvetrain. This drive gear meshes with a larger gear bolted and splined to the outboard end of the rear camshaft, which sticks through the cam support plate. Thus, the rear cam rotates via these two drive gears. The rear cam’s inboard gear meshes with and rotates the front cam’s inboard gear.
(Too long, so this is continued on a seperate thread) |
Linear Mode
