Rocker Arm Design - CHP How It Works

With The Help Of Comp Cams And T&D, We Break Down The Intricacies Of Rocker Arm Design

Stephen Kim Jun 22, 2010 0 Comment(s)
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Overhead cams are a more efficient method of actuating the valves than with a pushrod, but someone in Detroit never got the memo. And thank goodness they didn't. With production pushrod mills turning 7,000 rpm, NASCAR Sprint Cup engines pushing 9,500 rpm, and NHRA Pro Stock motors creeping closer to 11,000 rpm every day, breakthroughs in valvetrain technology have pushed OHV architecture far beyond what was ever imaginable. Furthermore, while street motors will never wind out as high as dedicated race motors, with today's high-flow cylinder heads, turning 7,500 rpm isn't out of the question. While durable valvesprings, and lightweight valves and retainers get much of the credit, these stratospheric rpms wouldn't be possible without commensurate progress in the rocker arm front.

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Stuck between the opposing forces of a valvespring and a pushrod, and responsible for multiplying the motion from the cam lobe onto the valve stem, a rocker arm is one of the most highly stressed components in the entire valvetrain. Current trends such as steeper cam lobe profiles, stiffer valvesprings, and higher rpm only compound the stresses. To find out how engineers build rockers that survive these strenuous conditions, we took an in-depth look at the latest in rocker arm innovations with Chris Mays of Comp Cams and Brad Rounds of T&D Machine Products. Some of the things we discussed included the different aluminum and steel alloys used to manufacture rockers, the effects of weight on rocker performance, how to optimize valvetrain stability, and the benefits of a shaft-mount rocker. Needless to say, rocker arms are no longer simple pieces of stamped steel seesawing on a ball pivot.

Steel Or Aluminum?
Brad Rounds: "There is certainly a tradeoff in strength and durability between aluminum and steel rockers. The vast majority of rockers we manufacture are aluminum, but the NASCAR teams we work with demand the durability of steel. Generally, aluminum rockers are lighter, easier to manufacture, and provide a dampening effect on the valvetrain. However, all aluminum rockers have a finite cycle life. On the other hand, steel rockers are more durable and provide a slight increase in cycle life over aluminum. The tradeoff is that they're heavier, harder on other valvetrain components, and more difficult to manufacture. Although much of what T&D builds for top racing series like NASCAR Sprint Cup is proprietary, and therefore somewhat secret, it can be stated that there has been a strong exodus from aluminum to billet steel in rocker body construction. The reason is quite simple: A rocker arm must last through the high-rpm, fully orchestrated chaos that is now the NASCAR norm.

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"Through years of development and testing, we found the absolute limit of aluminum rocker arms. Some of our customers wanted a rocker that had longer cycle life with higher ratios, and the answer was steel. For us, the progression to steel was actually a natural one. The initial customer for T&D steel rockers was Mercury Marine, a company that uses T&D as an OEM product on many of its offshore powerplants, even in non-race applications. Once the piece was engineered, it did not take long for others to ask for steel. The initial steel offerings were not just an aluminum design milled out of steel. They were designed as steel rockers from the ground up. The current NASCAR Cup rockers have evolved light-years from those initial attempts, not only in design but in material as well. Our proprietary manufacturing process ensures both strength and ductility for maximum reliability. Outside of NASCAR, steel rockers have become very common in blown and nitrous applications, tractor pullers, dirt late-models, and sprint cars."

Chris Mays: "Rocker arms are built from several different types of aluminum and steel alloys, and each has its pros and cons. We feel that our Ultra Gold aluminum rockers are the best aluminum rockers in the industry, using the highest quality aluminum alloy and precision-sorted needle bearings for increased spring pressures and clearance for spring diameters as large as 1.650 inches. Likewise, Comp's Ultra Pro Magnum rockers are a new state-of-the-art design. These rockers are stronger than the aluminum, made of 8650 chrome-moly steel. Their design allows for larger valvespring clearance as well. They also feature a much larger trunion for even greater increases in spring pressure. The best stud-mounted rocker arms we offer are our Hi Tech stainless steel rockers. They have all of the features of the Ultra Pro Magnums plus the benefits in strength of stainless steel."

Weight
Brad Rounds: "Reducing the weight of the rocker arms is always a priority. Where the weight is concentrated in the rocker arms is just as important as the overall weight of the rockers. The lighter the rocker, the less mass the valvespring and pushrod have to accelerate, stop, and accelerate once again in the opposite direction. However, each application dictates how light of a rocker can be used. Removal of weight out by the roller tip always pays the biggest dividends, as long as the rocker is capable of handling the valvetrain loads for that application."

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Chris Mays: "Weight is a major concern for any valvetrain. The key is reducing weight without compromising strength. Comp's Ultra Pro Magnum and Hi-Tech stainless steel rockers address this issue by minimizing weight over the rocker tips in their design. These rockers keep more of the weight centralized over the trunion area where it needs to be instead of over the valve stem, in turn reducing reciprocating weight."

Feedback Loop
Brad Rounds: "Improving the quality of rocker arms is the result of constant dialogue between engine builders and T&D. When the phone rings, it is often an engine builder that would like a rocker arm with a little more ratio, a little less weight, or a cure for some fussy problem. He might also have moved the valve locations substantially and need a completely new rocker configuration. Sometimes a racer wants to add a bigger camshaft to an engine with no valvetrain adjustability at all. That happened with Vipers, the Ford mod engines, and even the GM LS-series small-block. We continually tailor rocker arm assemblies to an engine builder's specific needs. Also, lots of engine builders want something a little bit different from their competition, and we are happy to oblige."

Valvetrain Stability
Chris Mays: "With motors turning higher and higher rpm these days, valvespring technology has improved dramatically. Rocker arms play a big role in overall valvetrain stability, and have evolved right along with the rest of the valvetrain. At higher engine speeds, rocker stability and strength is our primary concern. Rockers do flex, so as engine rpm increases along with the dynamics associated with solid or roller cams, the stronger the rocker the better. This yields more precise valve actuation. Our Hi-Tech stainless steel rockers are the strongest stud-mount rockers we offer. Shaft-mount rockers offer the most stability because by removing the rocker stud they eliminate the highest flex point."

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Rocker Ratio
Chris Mays: "Some engine combinations utilize lots of lobe lift with a relatively conservative rocker arm ratio, while others feature conservative lobe lift and a very aggressive rocker arm ratio. One method isn't necessarily better than the other, and there is a time and place for each. Adding the acceleration speed with the rocker is easier on harmonics and valvetrain stability in relation to rpm. With a higher ratio, the acceleration is in the rocker, so we can make the cam lobe gentler in ramp design. High ratios can be used to open the valve off the seat more quickly, and lower ratios can be used to stabilize a valvetrain that is out of control."

Race Engineering
Brad Rounds: "NASCAR has been the high-water mark in rocker arm R&D for quite some time. If a component can be built to stand up to its rigors, it will more than likely be over-engineered for anything else. The top NASCAR teams continue to experiment to achieve ever-loftier power numbers in hopes of gaining slight advantages over their competition within very tight rules parameters. If they ask for a lighter rocker arm with a little more ratio, we try to give it to them. While extra stiffness and less mass are both desired features, longevity is a key factor. If the rocker arm will last through all the laps of tuning, practice, the race itself, and the post-race celebratory meltdown, T&D makes teams smile. As for whether or not this type of technology will trickle down into the sportsman level, the answer is both yes and no. T&D builds a very high-quality product, so while newer steel rocker arms have trickled down into more grassroots race series, the T&D rocker systems have not become less costly to manufacture. However, many teams have reported that they are getting three to four times the longevity with our steel rockers, so they've actually saved money in the long run."

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Stamped Rockers
Chris Mays: "Stamped steel rocker arms represent the most basic of rocker design, but are still suitable for some engine builds. Many circle track racing classes require stamped steel rockers. Maximum lift and valvespring pressure are the two biggest limitations. A good rule of thumb is that you shouldn't exceed 0.500-inch lift and 350 pounds of open pressure with stamped rockers. In trying to run more aggressive solid cams, this has been a challenge. Comp now offers long-slot nitrided stamped steel rockers for increased lift and spring pressures."

Shaft-Mounting
Brad Rounds: "Stud-mount rockers have been around since the early '60s, and on the surface, look like relative bargains. Although you can pick up a set of stud-mount rockers for $300, factor in the costs of high-tensile strength screw-in studs, guideplates, and stud girdles and the total is closer to $600. That $600 buys a box full of headaches, including 2-hour valve-adjusting sessions and lash settings that move when the stud girdle is reinstalled. Shaft-mount rockers don't make horsepower in and of themselves, but they allow the engine builder to make more power by providing a stable, high-rpm valvetrain platform to work from. For the absolute ultimate in horsepower and high-rpm stability, shaft roller rockers are a must. In addition to reducing flex, they also offer greater flexibility in offsetting the rocker arms for different head and valve configurations. While shaft rockers are usually associated with race motors, today's street motors are turning lots of rpm, and we saw a gap in the market at the sportsman level. This need has now been filled with the release of T&D's new SportComp series shaft roller rockers which list for $800. T&D has streamlined the manufacturing techniques of its most popular small- and big-block Chevrolet rocker sets while maintaining the integrity of its unique bearing and adjuster sizes. The end result is the new T&D SportComp shaft roller rocker, a high-quality shaft roller rocker set priced just slightly more than complete stud-mounted rocker systems. Of course, they are available in standard offsets and ratios."

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Chris Mays: "Shaft-mount rockers have become more affordable in recent years, but stud-mount rockers have proven themselves in high-rpm applications as well. We typically recommend shaft-mount rockers for high-rpm applications that turn in excess of 7,500. Extensive Spintron testing has shown this is the rpm where you will start to have more stud flex with stud-mount rockers. However, with 7/16-inch studs and a stud girdle, you can come pretty close to the performance of a shaft-mount setup with stud-mount rockers."

Roller Tips
Brad Rounds: "For overhead valvetrains, sliding tips do not work very well at all. At valve lifts above 0.550- inch, a plain tip wants to bend the valve stem because the side loads become very high from the sliding motion. Also, with the high spring loads used in racing applications, slider tips don't last long and cause excessive wear on the end of the valve. Even in applications with high valve lifts combined with 5/16-inch or less valve stem diameters, our regular roller tip has a tendency to push the valve stem around, so our needle roller tip is highly recommended. T&D offers a roller tip on a shaft as a standard feature, and a roller tip with needle bearings on a shaft as an option. Both styles of tips benefit from T&D's oiling system, where the rocker pivot is fed directly from the rocker body. T&D also offers a tip axle of a harder material as an option.

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"To fit needle bearings into a roller tip with a given outside diameter means that the inside diameter of the tip must increase. This thinner wall thickness is thought by some to lead to failures, but that hasn't been the case in our experience. Tip failure is usually the product of excessive valve lash and a valvetrain that has gotten out of control. In those cases, the tip would have been destroyed with either a standard roller or needle bearing tips. Certainly the psi loading increases with a needle bearing tip, because at any one time, only two individual needles are taking the load. However, the decrease in friction offsets that. Today, with increased camshaft lift and valvespring rates, as well as decreased lubricating agents and lessened viscosities in race oil, needle bearing tips have become more the standard."

Alignment
Chris Mays: "The best way to check for proper alignment between the rocker arm tip and the valve stem is by finding zero lash. Shortening or lengthening of the pushrod to achieve centering of the roller tip over the valve stem is normally necessary. There are many problems associated with not checking the rocker geometry, the worst being the tip rolling off of the valve stem. This geometry is vital for proper engine dynamics to be achieved. We recommend running 0 to 0.004-inch lash on hydraulic race cams and 0.012- to 0.016-inch lash on solid cams for street applications. Finding zero lash is sometimes tough with a hydraulic lifters, but always check for it on the base circle and tighten the rocker until you feel slight drag when rotating the pushrod in your fingers."

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Design Priorities
Brad Rounds: "Proper valvetrain geometry is always the most important design parameter. We always work toward making the rocker assembly fit the head the best without modifications. This may sound very low tech, but usually it is the greatest challenge. As engines have grown in every dimension, it has become increasingly difficult to get all the pieces together with the proper clearances and the proper geometry. That said, each application sets its own parameters. Some take a fairly standard approach, where a rocker design T&D has already been using can be changed slightly to work just fine in a different application. Others need a clean sheet of paper due to the needs for offsets, ratios, valve angles, and a dozen other variables. We have recently completed the task of developing rocker sets for nearly all the different types of race heads on the market. There are roughly 25 cylinder head manufacturers that we deal with daily. That is, everyone from Dart, Brodix, and Edelbrock to a guy that buys rough castings and builds a few heads for himself. To keep up with demand, those manufacturers take a great deal of T&D's engineering time. Nevertheless, all T&D rocker systems receive the same tireless focus in engineering."

Sources

Comp Cams
Memphis, TN 38118
800-999-0853
http://www.compcams.com
T&D Machine Products
Carson City, NV
775-884-2292
http://www.tdmach.com
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