Rockers arms
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TECH Resident
Joined: Sep 2019
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From: Melbourne, Australia
Rockers arms I’ve been looking at getting new rockers arms due to needing bigger valve springs .
Question is the lower the MOI the better? Even if the rocker arm itself is heavier?
I have 3 different rocker arms to chose from
Aluminum solid body:
Mass = 123.5g
Moment of Inertia about shaft centerline: 123.5g *in^2 (coincidence that these two values match).
R1S steel:
Mass = 142.7g
MOI = 122.2 g*in^2
R2S steel:
Mass = 131.7g
MOI = 111.3 g*in^2
which one is best
Question is the lower the MOI the better? Even if the rocker arm itself is heavier?
I have 3 different rocker arms to chose from
Aluminum solid body:
Mass = 123.5g
Moment of Inertia about shaft centerline: 123.5g *in^2 (coincidence that these two values match).
R1S steel:
Mass = 142.7g
MOI = 122.2 g*in^2
R2S steel:
Mass = 131.7g
MOI = 111.3 g*in^2
which one is best
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is the lower the MOI the better?
Problem of course is, all else is RARELY equal. Metallurgy, geometry, quality of the bearings, consistency, machining accuracy, ... As well as everything around them including the life they're expected to live. Valve spring pressure, intended RPM, frequency of maintenance, cost ...
MOI is just one data point out of many, not the be-all end-all decision maker.
Thread Starter
TECH Resident
Joined: Sep 2019
Posts: 825
Likes: 262
From: Melbourne, Australia
So in theory the lower MOI the lower the weight acting on the spring, the better it will handle float etc?
The 3 examples I gave are from jesel
their aluminium rocker, steel rocker, and steel pro rocker
The 3 examples I gave are from jesel
their aluminium rocker, steel rocker, and steel pro rocker
Weight alone is not always bad. Additional material can improve stiffness, durability, and fatigue life. The goal is not necessarily the lightest rocker, but the stiffest and most durable rocker with the lowest practical MOI.
The important factors are material properties, stiffness, mass, and MOI. A stronger material such as steel allows for material only where it contributes most to stiffness and strength, potentially reducing both mass and MOI - like those Jessel Pros. Because aluminum is less stiff and less strong than steel, achieving the same stiffness often requires more material, which can increase both weight and MOI. That’s why aluminum rockers are always just full size blocks of aluminum with no profiling or anything (and still sometimes break).
In your application, for the cost, you’re likely fine with the standard steel rocker. If you were racing, in endurance application, had 1” of valve lift etc maybe the pros make sense.
