Posted By: polyspheric
Scrub path length calculation? - 12/02/11 08:01 PM
I've gotten a few questions about this, and noticed some commentary regarding "how large should this be?" (in addition to where it stops & starts).
Some very large numbers have been reported with no shock & awe.
In general, the scrub should be smaller than you would think, meaning some of those big numbers are really danger signals - the geometry isn't what you want. If you can't find another way, reducing the scrub length is a good direction to go.
All you need to predict what the minimum scrub should be is:
1. set the valve-side lever to mid-lift, yatta yatta
2. what's the effective length of the long lever (shaft center to roller axle or pad radius)
3. what's the net valve lift?
Let "S" = the minimum scrub length
Let "R" = rocker's long lever effective length
Let "L" = net valve lift
The formula is:
S=R-(R^2-(L/2)^2)^.5
Example 1:
R (lever) = 1.75", L (lift) = .600"
S=1.75-(1.75^2-(.600/2)^2)^.5
S=1.75-(3.0625-(.09)^.5
S=1.75-1.724
S=.026"
Example 2:
R (lever) = 1.50", L (lift) = .700"
S=1.50-(1.50^2-(.700/2)^2)^.5
S=1.50-(2.25-.1225)^.5
S=1.50-1.459
S=.041"
As you can see, scrub figures like .080" show something isn't working...
You can also calculate the rocker angle at zero lift and full lift (which will be the same with mid-lift, duh!).
The total arc (1/2 up, 1/2 down) is:
2×arcsin(L/2R)
Example 1 (R (lever) = 1.75", L (lift) = .600"):
L/2R=.600÷3.50=.171
sin=.171
Total arc = 19.74°
Some very large numbers have been reported with no shock & awe.
In general, the scrub should be smaller than you would think, meaning some of those big numbers are really danger signals - the geometry isn't what you want. If you can't find another way, reducing the scrub length is a good direction to go.
All you need to predict what the minimum scrub should be is:
1. set the valve-side lever to mid-lift, yatta yatta
2. what's the effective length of the long lever (shaft center to roller axle or pad radius)
3. what's the net valve lift?
Let "S" = the minimum scrub length
Let "R" = rocker's long lever effective length
Let "L" = net valve lift
The formula is:
S=R-(R^2-(L/2)^2)^.5
Example 1:
R (lever) = 1.75", L (lift) = .600"
S=1.75-(1.75^2-(.600/2)^2)^.5
S=1.75-(3.0625-(.09)^.5
S=1.75-1.724
S=.026"
Example 2:
R (lever) = 1.50", L (lift) = .700"
S=1.50-(1.50^2-(.700/2)^2)^.5
S=1.50-(2.25-.1225)^.5
S=1.50-1.459
S=.041"
As you can see, scrub figures like .080" show something isn't working...
You can also calculate the rocker angle at zero lift and full lift (which will be the same with mid-lift, duh!).
The total arc (1/2 up, 1/2 down) is:
2×arcsin(L/2R)
Example 1 (R (lever) = 1.75", L (lift) = .600"):
L/2R=.600÷3.50=.171
sin=.171
Total arc = 19.74°
Time to send them in for rocker ratio correction, rocker tip correction and blueprinting 
Have you check the net lift at the retainers on all of the valves yet? If not, now is a good time to do that 
) even knowing the responses are likely to be "less than satisfactory" from a content perspective.
Thanks for the info! Don
It time to get the rockers done, who do I send them to and how much will this extend my budget? Tim