Stock Rubbing Rocker Arms and Performance Roller Rocker Arms
Cam manufacturers refer to their camshafts by the valve lift not the camshaft’s lobe lift. The actual camshaft lobe lift is multiplied by the rocker arm ratio to obtain the valve lift. Valve lift is almost always greater than cam lobe lift. The exception is the 1936 to 1947 Knuckledhead. The Knucklehead’s rocker arm has a 1:1 ratio, whereby cam lobe lift and actual valve lift are the same. Valve lift is how far the valve extends into the combustion chamber..
- The upper-balled end of the pushrod fits into a mating pocket in the end of the rocker arm, which swivels up-and-down a short distance on a rocker arm shaft. This up-and-down distance is equal to the cam’s lobe lift. In the case of an Evolution 1988 to 1991 carbureted L-camshaft, the cam lobe lift is .3046”. Therefore, the rocker arm oscillates up-and-down, slightly less than ⅓”. The valve lift becomes .495” because the Evolution rocker arm ratio of 1.625 acts as a multiplier (1.625 x .3046 = .495”). Therefore, the valve will extend almost ½” off its valve seat into the combustion chamber.
- The major mechanical reason for a rocker arm is it connects the pushrod on the opposing right side of the head to where the valve is locates on the other (left) side.
- The approximately 3 ⅝-inch long rocker arm transfers the dictates of the rotating cam lobe to the top portion of the valve stem encased and supported by dual cylindrical valve springs.
- The stock rocker arms and some performance ones have a steel pad that sits over and contacts the top of the valve stem.
- The cam lobe, lifter (tappet), pushrod actuated rocker arm pad rubs along and down on the top of the angled valve stem, pushing it to open.
- This creates friction both on top of the stem but more so on the valve stem’s side as the pad pushes the valve stem down. Correspondingly, this process also exerts pressure against the angled valve guide.
- Hence, there is a slight frictional loss of horsepower as the engine wastes energy working against itself.
- Furthermore, there are very real wear issues of valve against valve guide affecting parts longevity.
- On a roller rocker arm, the rocker arm pad is replaced with a roller.
- A roller rocker, rolls effortlessly on the top of the valve stem as it pushes the valve open. This minimizes side-force against the valve guide. Parts longevity increases and frictional loss of horsepower reduces.
- Higher lift performance cams exacerbate the valve-to-valve guide wear pressure and friction.
- Therefore, in a more perfect world, efficiency would be gained if the valve and valve guide were not angled. This would eliminate side load. However, a single camshaft dictates angularity because the heads and their valves are wider. The 1929 to 1973 Flathead engines have four separate camshafts with one cam lobe each that locate directly under the valve stem. This eliminates friction loss and wear issues but the inefficient combustion chamber design negates the positives of zero angularity.
- In a perfect world (VRod)), the camshaft is in the head over the valve stem eliminating the inefficiencies of a valve train and an angular valve with direct actuation of cam lobe against the top of the valve stem.
Valve Spring Purpose
The function of the valve spring(s) is to close the valve back onto its valve seat after the elliptical cam lobe rotates past its highest point. Valve opening compresses the valve springs. The springs decompress thus pushing the valve closed as well as assisting the rest of the valve train follow the descending cam lobe.
- Therefore, the valve spring(s) act to close the valve by decompressing.
- Valve springs also have design properties that combat harmonics that can easily occur with the rapid and changing movements of the valve being forced open by the ascending cam lobe and being closed by the decompressing springs as the cam lobe lift decreases.
Why does Valve Lift Increase over the Cam Lobe Actual Lift?
Now I can explain why the cam lobe lift is less than the valve lift. If the rocker arm ratio were 1:1; that is, the extension of the rocker arm that cups over the upper balled-pushrod was on the same plane as the rocker arm extension on the other side that actuates the valve, then cam lobe lift and valve lift would be equal. This is the case with the 1936 to 1947 Knuckleheads. Actually, the valve lift would be a little less than the cam lobe lift because of lash (spacing between valve train parts) and flex (pushrods).
However, on every Big Twin from 1948 to present, one side of the rocker arm offsets to the other so a multiplication of the cam lift transfers to the valve stem thus increasing valve-opening lift.
The stock Twin Cam’s rocker arm ratio is 1.625:1. Sometimes, when I replace the stock rocker arms with roller ones, I take the opportunity to use rocker arms with an increased rocker arm ratio from the stock 1.625 multiple to increase the valve lift (torque), relative to cam duration (horsepower).
Torque gets the beast going and horsepower keeps her going.
Use Roller Rocker Arms above Evolution .585 Valve Lifts
Performance 101 dictates the use of roller rocker arms with Evolution performance cams especially above .585” valve lift both for preventing frictional loss of horsepower and valve train longevity.
If I have to replace worn stock rubbing rocker arms in a stock engine or one with a mild performance cam, I will generally use roller rockers because they will assist even a stock or near stock engine efficiency.
Increasing Lift Relative to Duration: Longer-Ratio (High Ratio) Rocker Arms
I find that many cam profiles move the power band up too high in the rpm range for my personal liking.
High rpm riding is fun but not always practical.
I want grunt power in the lower rpm ranges, where I use my bike the most.
Do not worry, there will still be lots of power in the top end especially have if there are higher flow aftermarket heads or if the stock heads are ported and polished.
The trick is to increase the rocker arm ratio from the stock 1.625 for 1984 to 2000 Evolutions and 1999-to-present Twin Cams. The rocker arm ratios for earlier Harley-Davidson’s are 1.420 for 1965 to 1984 Shovelheads, 1.500 for 1948-1965 Panheads, and 1.000 direct ratio for 1936 to 1947 Knuckleheads.
Two increased rocker arm ratios that come to mind are a milder Ultima 1.675 and 1.690, Crane Cams 1.725, and JIMS 1.745 ratios.
Increasing the rocker arm ratio increases the lift at the valve without changing the camshaft. This typically adds .015” to almost .040” valve lift to the engine, depending on the cam lobe lift of the camshaft. After installing longer-ratio rockers, always check that the engine has adequate (.040” minimum; .060” if conservative) piston-to-valve clearance before running the engine.
Let’s take a 1988 to 1991 (L-carbureted) cam lobe lift of .3046” whose valve lift becomes .495” after multiplying by the stock 1.625 rocker arm ratios.
Stock Rocker Arm Ratio: 1.625
Evolution L-cam valve lift is (.3046” cam lobe lift x rocker arm ratio of 1.625 = .49475”) .495” valve lift.
Following are a sample of performance roller rocker arms with longer (high) ratios that will increase valve lift relative to camshaft duration.
Ultima: 1.675 Ratio
Evolution L-cam valve lift is (.3046” cam lobe lift x rocker arm ratio of 1.675 = .510205”) .510” valve lift.
There is an increase in valve lift of .015”.
Ultima: 1.690 Ratio
Evolution L-cam valve lift is (.3046” cam lobe lift x rocker arm ratio of 1.690 = .514774”) .515” valve lift.
There is an increase in valve lift of .020”.
Crane: 1.725 Ratio
Evolution L-cam valve lift is (.3046” cam lobe lift x rocker arm ratio of 1.725 = .525435”) .525” valve lift.
There is an increase in valve lift of .030”.
JIMS: 1.745 Ratio
Evolution L-cam valve lift is (.3046” cam lobe lift x rocker arm ratio of 1.745 = .531527”) .532” valve lift.
There is an increase in valve lift of .037”.
On longer-ratio rocker arms, the cam lobe lift stays the same while valve lift progressively increases when using a 1.675, 1.690, 1.725, or 1.745 rocker arm ratio. Cam duration also remains the same. Therefore valve lift increases relative to duration, lowering the power band into more friendly territory.
Longer-ratio rocker arms typically move the power band down increasing engine torque and horsepower primarily in the midrange. Higher lift always assists in takeoff torque.
Performance rocker arms have roller ends instead of a metal rubbing pad to actuate the valve stems. They may also have altered rocker arm ratios such as an Ultima roller rocker arm with an increase of .050” ratio to 1.675 rocker arm ratios. Other examples are a Crane roller rocker arm with a ratio of 1.725, an increase of .100” ratio over stock and JIMS roller rocker arm with a ratio of 1.745, an increase of .120” ratio over stock. I use increased rocker ratio arms effectively to enhance lift when unsatisfied with too long, for my purposes, duration figures of a particular cam.
Enhancing a stock cam’s valve lift by using increased ratio rocker arms is one way of lowering the power band into more useable territory and gaining extra low-end torque. Therefore, I use increases in rocker arm ratios to boost torque and lower the power band of a particular camshaft.
It is important that the valve springs can handle the increase in valve lift without the springs coil binding or the upper valve collar hitting the top of the valve guide or the valve extending too far into the combustion chamber and hitting its mating piston valve pocket.
JIMS 1.745 Roller Rocker Arms Picture
JIMS roller rocker arms are cast from 4340 chromemoly steel and then heat treated for maximum strength. The stock rocker arm ratio for Evolution and Twin Cam is 1.625. The lift of the camshaft’s lobe multiplies by 1.625 to further open the valve into the combustion chamber. Therefore, an increased rocker arm ratio to 1.745 will open the valve even further.
I find that many performance camshafts have very long durations for a street bike. Duration is how long the valve stays open for. Long duration cams move the engine power band into higher rpm areas, which many riders only use occasionally.
Higher cam lift will lower the power band into more usable rpm range. Lift is associated with low end torque whereas duration increases top end horsepower. Torque gets the beast going and horsepower keeps her going.
Using a longer-ratio (higher ratio) rocker arm will increase the valve lift into the chamber without affecting duration. This will move the engine power band down to midrange rpm. JIMS roller rocker arms are:
- Designed to reduce friction (valve-to-valve guide side loading) in the valve train to reduce frictional loss of horsepower and increase parts longevity.
- This marginally increases horsepower and provide more horsepower with less heat in the top end.
- The roller rockers feature a 660 bronze bushing fit to .0007”-.0012”, for the best wear resistance and oil control.
- The roller that replaces the stock rubbing pad is made from bearing grade material which reduces valve guide wear and valve tip galling.
- Another feature is a segmented parabolic pushrod cup which, reduces friction at pushrod ends. This also puts the load on the outer perimeter of the pushrod ends.
- There is full oiling of the roller tip, valve springs and pushrod ball end.
- The durable 52100 bearing steel roller tip gives the longest possible life
Installation requires a thinking engine builder. Modification to the heads and rocker boxes may be necessary to install rocker arms with a longer-ratio with specified clearance. Why?… the rocker arms will oscillate, in this particular case 120” higher and lower than the stock 1.625 rocker arms (1.745 – 1.625 = .120”).