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Re: Rocker arm geometry. Who's right? [Re: hemienvy] #3058116
07/11/22 06:28 AM
07/11/22 06:28 AM
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Originally Posted by hemienvy
He has a couple of facts wrong and because of that his logic conclusions are somewhat inaccurate.

I am splitting hairs with this, but correct is correct. This is not to take away from his otherwise excellent product.


could you explain in detail where he is wrong?


'86 Maple Grove KOS Mopar low qualifier......true street legal with no power adders.

NOS-used when losing since 1940.

Re: Rocker arm geometry. Who's right? [Re: 375inStroke] #3058158
07/11/22 10:08 AM
07/11/22 10:08 AM
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Loudoun County, VA
Brad_Haak Offline
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Originally Posted by hemienvy
There is another rocker arm philosophy to consider.

As the valve opens more, the spring load becomes greater. This argument postulates that, at greater spring loads,
the rocker should approach the valvestem more perpendicularly, so there is less side loading on the valve guides.

To achieve this, the pushrods should be "too short", so that there is more roller scrub (towards the exhaust side)
while the valve is just opening and spring loads are lower.

Geometrically speaking, there is a greater scrub distance than the "mid lift' philosophy. But less guide wear.

Diagram showing low-pivot approach as described above on the left and mid-lift approach on the right

Jesel+Shaft+Rocker+Geometry.png
Re: Rocker arm geometry. Who's right? [Re: 375inStroke] #3058191
07/11/22 11:51 AM
07/11/22 11:51 AM
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powertrip Offline
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" With non-roller rockers, that perpendicular is measured from the rocker arm foot to valve tip contact, through the centerline of the rocker shaft, but with roller rockers, it's taken from the center of the roller rocker tip, through the center of the rocker shaft. Is this correct?"

I do not agree that roller rockers should be measured from the center of the roller tip, it should always be measured from the valve tip contact. The size of the roller wheel does not affect geometry in of itself.

Re: Rocker arm geometry. Who's right? [Re: powertrip] #3058217
07/11/22 12:54 PM
07/11/22 12:54 PM
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Loudoun County, VA
Brad_Haak Offline
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Tech info from Manton Pushrods, FWIW

Screen Shot 2022-07-11 at 12.52.46 PM.png
Re: Rocker arm geometry. Who's right? [Re: Brad_Haak] #3058228
07/11/22 01:23 PM
07/11/22 01:23 PM
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Anchorage, Alaska
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Fascinating, the scenario A is the advice I got. Scenario B is essentially the 90° @ mid-lift in other words


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Re: Rocker arm geometry. Who's right? [Re: Brad_Haak] #3058304
07/11/22 04:13 PM
07/11/22 04:13 PM
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powertrip Offline
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Originally Posted by Brad_Haak
Tech info from Manton Pushrods, FWIW


Appreciate you furthering the discussion. It looks like with the shoe rocker they are trying to minimize "scrub", while with the roller rocker they want to minimize "sweep", both of which I agree with.

Where I differ with some is that you should measure from the center of the roller wheel, why?? All that tells you is the size of the roller wheel, which has no effect on geometry.

The tip of the valve stem is the point of contact, THAT is what matters.

Re: Rocker arm geometry. Who's right? [Re: powertrip] #3058307
07/11/22 04:18 PM
07/11/22 04:18 PM
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Oregon
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That is a confusing point. Some people think the roller diameter is important, others don't. The engineers at Chevy recommend the "low point" or 90 degrees at 2/3 lift, approach for roller rocker arms. I've always thought that the low point design should be used on race engines with heavy springs while the 90 degree at mid-lift works okay for production cars and mild performance engines.

Re: Rocker arm geometry. Who's right? [Re: AndyF] #3058314
07/11/22 04:49 PM
07/11/22 04:49 PM
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Originally Posted by AndyF
That is a confusing point. Some people think the roller diameter is important, others don't. The engineers at Chevy recommend the "low point" or 90 degrees at 2/3 lift, approach for roller rocker arms. I've always thought that the low point design should be used on race engines with heavy springs while the 90 degree at mid-lift works okay for production cars and mild performance engines.


Here is another confusing point: If you set the rocker shaft for "low point" while measuring from the center of the roller wheel, you are actually close to "mid lift" if you measure from the valve stem tip!!

If you want to open another can of worms, there is a debate on what part of the lift curve sees the most stress, hence valvetrain deflection. A clue: it's NOT at full lift where spring forces are highest!

Re: Rocker arm geometry. Who's right? [Re: powertrip] #3058323
07/11/22 05:15 PM
07/11/22 05:15 PM
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Loudoun County, VA
Brad_Haak Offline
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Originally Posted by powertrip
If you want to open another can of worms, there is a debate on what part of the lift curve sees the most stress, hence valvetrain deflection. A clue: it's NOT at full lift where spring forces are highest!

Max velocity is about mid-lift; max acceleration is just off the seat where the valve starts to open most quickly

Screen Shot 2022-07-11 at 5.06.24 PM.png
Re: Rocker arm geometry. Who's right? [Re: Brad_Haak] #3058329
07/11/22 05:26 PM
07/11/22 05:26 PM
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Loudoun County, VA
Brad_Haak Offline
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Might as well throw this into the mix, too ==> HERE THERE BE DRAGONS!

EDIT: When's the last time -- if ever -- that I posted multiple times on a thread and never actually expressed an opinion? All I've done here is provide links and point people to info tied to the questions brought up.

How odd... laugh2

Last edited by Brad_Haak; 07/11/22 06:01 PM.
Re: Rocker arm geometry. Who's right? [Re: powertrip] #3058415
07/11/22 10:25 PM
07/11/22 10:25 PM
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Milwaukee WI
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Originally Posted by powertrip
Originally Posted by Brad_Haak
Tech info from Manton Pushrods, FWIW


Appreciate you furthering the discussion. It looks like with the shoe rocker they are trying to minimize "scrub", while with the roller rocker they want to minimize "sweep", both of which I agree with.

Where I differ with some is that you should measure from the center of the roller wheel, why?? All that tells you is the size of the roller wheel, which has no effect on geometry.

The tip of the valve stem is the point of contact, THAT is what matters.


The center of the roller is the reference point for the center of the arc that the wheel travels through. If you would plot pivot points for mid lift on paper, you would see visually that to achieve true mid lift geometry, the shaft centerline needs to be at a right angle to the roller centerline.


"use it 'till it breaks, replace as needed"
Re: Rocker arm geometry. Who's right? [Re: Brad_Haak] #3058424
07/11/22 10:36 PM
07/11/22 10:36 PM
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Originally Posted by Brad_Haak
Might as well throw this into the mix, too ==> HERE THERE BE DRAGONS!

EDIT: When's the last time -- if ever -- that I posted multiple times on a thread and never actually expressed an opinion? All I've done here is provide links and point people to info tied to the questions brought up.

How odd... laugh2

That link made my head hurt.


In theory, there is no difference between theory and practice. In practice, there is.
Re: Rocker arm geometry. Who's right? [Re: powertrip] #3058512
07/12/22 08:28 AM
07/12/22 08:28 AM
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Originally Posted by powertrip
...If you want to open another can of worms, there is a debate on what part of the lift curve sees the most stress, hence valvetrain deflection. A clue: it's NOT at full lift where spring forces are highest!


...that'd be the 'valve jerk' I believe?

It is the extreme change of mechanical motion that would impart the greatest amount of material stress, therefore I would think the derivate of the acceleration curve would be the answer, thus the 'valve jerk' event.

@Brad_Haak's post with the screenshot shows the relationship of these elements very nicely!!!

Re: Rocker arm geometry. Who's right? [Re: GomangoCuda] #3058633
07/12/22 04:05 PM
07/12/22 04:05 PM
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moparx Offline
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Originally Posted by GomangoCuda
Originally Posted by Brad_Haak
Might as well throw this into the mix, too ==> HERE THERE BE DRAGONS!

EDIT: When's the last time -- if ever -- that I posted multiple times on a thread and never actually expressed an opinion? All I've done here is provide links and point people to info tied to the questions brought up.

How odd... laugh2

That link made my head hurt.



mine too ! panic runaway laugh2
beer

Re: Rocker arm geometry. Who's right? [Re: moparx] #3059291
07/14/22 08:49 PM
07/14/22 08:49 PM
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Washington
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hemienvy Offline
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There are many graphic illustrations of piston motion on the internet, usually with explanations showing some calculus.

There are three piston events that are still not widely understood, however.

When the piston leaves TDC going down, these 3 things ALWAYS occur IN THIS ORDER:

1.) peak piston velocity, when the rod and crank are at a right angle. Crank degrees depends on rod ratio.

2.) The piston is halfway down the bore.This happens before the crank has turned 90-degrees. Crank degrees depends on rod ratio.

3.) the crank has turned 90-degrees. Piston is now more than halfway down the bore. You guessed it, how far down depends on rod ratio.

This is not a matter of opinion, it's a matter of fact. I hope this helps folks.

**************

If the connecting rod was infinitely long, the piston motion would be a perfect sine curve, and all the math would be very easy.
Any rod length less than infinity and the sine curve is "modified", but it's still not that complicated.
Yes, you could say the differences are subtle. "Subtle" is subjective.

Re: Rocker arm geometry. Who's right? [Re: hemienvy] #3059350
07/15/22 08:34 AM
07/15/22 08:34 AM
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Loudoun County, VA
Brad_Haak Offline
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Not sure what piston motion has to do w/ valve train dynamics shruggy

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