Scrub path length calculation?
#1126177
12/02/11 04:01 PM
12/02/11 04:01 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
OP
master
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OP
master
Joined: Oct 2007
Posts: 4,219
New York
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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°
Boffin Emeritus
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Re: Scrub path length calculation?
[Re: BradH]
#1126180
12/02/11 06:36 PM
12/02/11 06:36 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
You should just post this over on the Advanced Engine Tech forum on Speedtalk.com. This place can barely handle topics like "How much do I torque my rod bolts?" these days...
Look who's talkin
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Re: Scrub path length calculation?
[Re: S/ST 3040]
#1126181
12/02/11 08:38 PM
12/02/11 08:38 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
OP
master
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OP
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Joined: Oct 2007
Posts: 4,219
New York
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I wrote an Excel spreadsheet that does it automatically if you know your rocker length.
Boffin Emeritus
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Re: Scrub path length calculation?
[Re: MR_P_BODY]
#1126185
12/03/11 11:41 AM
12/03/11 11:41 AM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Quote:
Quote:
You should just post this over on the Advanced Engine Tech forum on Speedtalk.com. This place can barely handle topics like "How much do I torque my rod bolts?" these days...
Look who's talkin
From a tech perspective, this forum is a shadow of its former self. I still try to post questions of a reasonably tech-oriented nature (along w/ the occasional one that veers off course ) even knowing the responses are likely to be "less than satisfactory" from a content perspective.
It is what it is... and my s.w.a.g. is the OP's tendency to be "highly formulaic" in his descriptions goes right over the heads of a good percentage of the people who remain on this forum.
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Re: Scrub path length calculation?
[Re: BradH]
#1126186
12/03/11 01:11 PM
12/03/11 01:11 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
OP
master
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OP
master
Joined: Oct 2007
Posts: 4,219
New York
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That's why I mentioned the .xls, I figured there are people who are happy to know that it can be predicted but hate math!
Boffin Emeritus
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Re: Scrub path length calculation?
[Re: polyspheric]
#1126187
12/03/11 04:31 PM
12/03/11 04:31 PM
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Joined: Oct 2006
Posts: 527 alaska,usa
9secondsatellite
mopar
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mopar
Joined: Oct 2006
Posts: 527
alaska,usa
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Quote:
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°
thanks for the post. good info.
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Re: Scrub path length calculation?
[Re: RodStRace]
#1126189
12/03/11 05:22 PM
12/03/11 05:22 PM
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Joined: Mar 2006
Posts: 9,982 MI, usa
dvw
master
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master
Joined: Mar 2006
Posts: 9,982
MI, usa
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Quote:
Interesting math, but how does this translate to heads, rocker arms, pushrods and valves? Let's say that you have trial assembled your combo and done the math provided.
How do you measure scrub? How do you correct it? You can lengthen the valve end with lash caps (requiring shorter pushrods) What else? Machine to move the rocker shaft closer/further from the valve? Try various rockers? Machine the rocker arm pads?
This brings up an interesting point. My heads are -1 indys, T&D individual shaft rockers. I would have thought that the stands would be located thru the previous rocker shaft locations (after milling the pads flat). This is how the previous owner of my heads located them. I found the roller tip to be aprox .065" toward the intake side of the valves. The geometry checker from T&D lined up with-in a few thousands. However I had to mill the head and offset bore the holes that held the stands. Then we installed thread inserts. All looks close now. Doug
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Re: Scrub path length calculation?
[Re: 67_Satellite]
#1126191
12/03/11 07:37 PM
12/03/11 07:37 PM
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Joined: Nov 2003
Posts: 2,131 New Lenox IL
cudadon
top fuel
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top fuel
Joined: Nov 2003
Posts: 2,131
New Lenox IL
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Quote:
I enjoy the 'tech oriented" questions here. If some "ar$eholes"have driven knowledgeable people from this forum, I hope that won't keep the remaining few from sharing their secrets with the rest of us. I enjoy a "mopar only" site and hope to keep it as good as it can be.
Thanks for the info! Don
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Re: Scrub path length calculation?
[Re: RodStRace]
#1126192
12/04/11 02:37 AM
12/04/11 02:37 AM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
Interesting math, but how does this translate to heads, rocker arms, pushrods and valves? Let's say that you have trial assembled your combo and done the math provided.
How do you measure scrub? How do you correct it? You can lengthen the valve end with lash caps (requiring shorter pushrods) What else? Machine to move the rocker shaft closer/further from the valve? Try various rockers? Machine the rocker arm pads?
I didnt do the math to find out the sweep/scrub but with checking and some math I ended up cutting .160 off the pads on my W-5s to run the TD multi shafts to get the minimum sweep which is about .020 now
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