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Scrub path length calculation? #1126177
12/02/11 04:01 PM
12/02/11 04:01 PM
Joined: Oct 2007
Posts: 4,205
New York
polyspheric Offline OP
master
polyspheric  Offline OP
master

Joined: Oct 2007
Posts: 4,205
New York
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
Re: Scrub path length calculation? [Re: polyspheric] #1126178
12/02/11 04:06 PM
12/02/11 04:06 PM
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
B
BradH Offline
Taking time off to work on my car
BradH  Offline
Taking time off to work on my car
B

Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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...

Re: Scrub path length calculation? [Re: polyspheric] #1126179
12/02/11 05:59 PM
12/02/11 05:59 PM
Joined: Jul 2006
Posts: 4,595
On the south side of Nowhere
S
S/ST 3040 Offline
master
S/ST 3040  Offline
master
S

Joined: Jul 2006
Posts: 4,595
On the south side of Nowhere
I'll hang on to this for further study.

Thanks

Re: Scrub path length calculation? [Re: BradH] #1126180
12/02/11 06:36 PM
12/02/11 06:36 PM
Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY Offline
Master
MR_P_BODY  Offline
Master

Joined: Jun 2003
Posts: 52,972
Romeo MI
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

Re: Scrub path length calculation? [Re: S/ST 3040] #1126181
12/02/11 08:38 PM
12/02/11 08:38 PM
Joined: Oct 2007
Posts: 4,205
New York
polyspheric Offline OP
master
polyspheric  Offline OP
master

Joined: Oct 2007
Posts: 4,205
New York
I wrote an Excel spreadsheet that does it automatically if you know your rocker length.


Boffin Emeritus
Re: Scrub path length calculation? [Re: polyspheric] #1126182
12/02/11 09:55 PM
12/02/11 09:55 PM
Joined: Jan 2011
Posts: 4,785
Utah and Alaska
astjp2 Offline
master
astjp2  Offline
master

Joined: Jan 2011
Posts: 4,785
Utah and Alaska
Look at my exhausts! This just does not look right~!

6947712-exhaust.JPG (90 downloads)
Re: Scrub path length calculation? [Re: astjp2] #1126183
12/02/11 09:57 PM
12/02/11 09:57 PM
Joined: Jan 2011
Posts: 4,785
Utah and Alaska
astjp2 Offline
master
astjp2  Offline
master

Joined: Jan 2011
Posts: 4,785
Utah and Alaska
Here is the intake, looks much better!

6947718-intake.JPG (93 downloads)
Re: Scrub path length calculation? [Re: astjp2] #1126184
12/02/11 11:35 PM
12/02/11 11:35 PM
Joined: Aug 2003
Posts: 43,007
Bend,OR USA
C
Cab_Burge Offline
I Win
Cab_Burge  Offline
I Win
C

Joined: Aug 2003
Posts: 43,007
Bend,OR USA
Those look like stock ductiel iron Hemiroid rockers 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


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Scrub path length calculation? [Re: MR_P_BODY] #1126185
12/03/11 11:41 AM
12/03/11 11:41 AM
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
B
BradH Offline
Taking time off to work on my car
BradH  Offline
Taking time off to work on my car
B

Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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.

Re: Scrub path length calculation? [Re: BradH] #1126186
12/03/11 01:11 PM
12/03/11 01:11 PM
Joined: Oct 2007
Posts: 4,205
New York
polyspheric Offline OP
master
polyspheric  Offline OP
master

Joined: Oct 2007
Posts: 4,205
New York
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
Re: Scrub path length calculation? [Re: polyspheric] #1126187
12/03/11 04:31 PM
12/03/11 04:31 PM
Joined: Oct 2006
Posts: 527
alaska,usa
9
9secondsatellite Offline
mopar
9secondsatellite  Offline
mopar
9

Joined: Oct 2006
Posts: 527
alaska,usa
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.

Re: Scrub path length calculation? [Re: 9secondsatellite] #1126188
12/03/11 05:09 PM
12/03/11 05:09 PM
Joined: Sep 2005
Posts: 12,481
Chino Valley
RodStRace Offline
I Live Here
RodStRace  Offline
I Live Here

Joined: Sep 2005
Posts: 12,481
Chino Valley
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?

Re: Scrub path length calculation? [Re: RodStRace] #1126189
12/03/11 05:22 PM
12/03/11 05:22 PM
Joined: Mar 2006
Posts: 9,791
MI, usa
dvw Offline
master
dvw  Offline
master

Joined: Mar 2006
Posts: 9,791
MI, usa
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

Re: Scrub path length calculation? [Re: BradH] #1126190
12/03/11 05:53 PM
12/03/11 05:53 PM
Joined: Jan 2003
Posts: 1,020
Andrews,In. U.S.of A.
6
67_Satellite Offline
super stock
67_Satellite  Offline
super stock
6

Joined: Jan 2003
Posts: 1,020
Andrews,In. U.S.of A.
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.

Re: Scrub path length calculation? [Re: 67_Satellite] #1126191
12/03/11 07:37 PM
12/03/11 07:37 PM
Joined: Nov 2003
Posts: 2,131
New Lenox IL
cudadon Offline
top fuel
cudadon  Offline
top fuel

Joined: Nov 2003
Posts: 2,131
New Lenox IL
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

6949151-DSC00322.JPG (67 downloads)
Re: Scrub path length calculation? [Re: RodStRace] #1126192
12/04/11 02:37 AM
12/04/11 02:37 AM
Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY Offline
Master
MR_P_BODY  Offline
Master

Joined: Jun 2003
Posts: 52,972
Romeo MI
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

Re: Scrub path length calculation? [Re: Cab_Burge] #1126193
12/04/11 05:29 AM
12/04/11 05:29 AM
Joined: Jan 2011
Posts: 4,785
Utah and Alaska
astjp2 Offline
master
astjp2  Offline
master

Joined: Jan 2011
Posts: 4,785
Utah and Alaska
The lift at the retainers should be around .620, I am getting .542-.553 and most are on the lower side. The lobe lift was .387 and the cam card shows it should have been .400. The intakes are good, its the exhausts that calculated a ration of about 1.39-1.40. It time to get the rockers done, who do I send them to and how much will this extend my budget? Tim

Quote:

Those look like stock ductiel iron Hemiroid rockers 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



Re: Scrub path length calculation? [Re: astjp2] #1126194
12/05/11 02:03 PM
12/05/11 02:03 PM
Joined: Mar 2003
Posts: 1,632
Lubbock,TX
D
DavidDean Offline
top fuel
DavidDean  Offline
top fuel
D

Joined: Mar 2003
Posts: 1,632
Lubbock,TX
I better keep this one but speedtalk might be the better place to post this.

Re: Scrub path length calculation? [Re: BradH] #1126195
12/09/11 04:03 PM
12/09/11 04:03 PM
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
B
BradH Offline
Taking time off to work on my car
BradH  Offline
Taking time off to work on my car
B

Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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...



https://board.moparts.org/ubbthreads/show...e=0#Post6958087


Re: Scrub path length calculation? [Re: BradH] #1126196
12/09/11 04:32 PM
12/09/11 04:32 PM
Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
P
Performance Only Offline
top fuel
Performance Only  Offline
top fuel
P

Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
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...



https://board.moparts.org/ubbthreads/show...e=0#Post6958087






and there you go, the proof.


machine shop owner and engine builder
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