Rocker Geometry. Who Agrees?

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Alex W
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Rocker Geometry. Who Agrees?

Postby Alex W » Fri Feb 15, 2008 3:22 pm

Image

I read threw Cranes quick lift information again today and I cant say I entirly agree with it.

Would it not be safe to say that a rockers ratio is computed by dividing d/c? If this is correct "c" will always be the same length since it pivotes at the center of the pushrod ball and "d" will be the only length that changes as the rocker sweeps.

To help visualize the length change in "d" ignore their goofy little arc and actually visualize the contact point with the valve as the rocker would sweep. The more the rocker sweeps the longer the length of the red line "d" grows. Making the ratio increase as the valve is lifted.

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Postby Procision-Auto » Fri Feb 15, 2008 4:22 pm

What about the angle of the pushrod as the rocker pivots?

The pushrod moving away from the "centerline", or resting position.

We could probably do some trig and figure out how much the PR angle
takes away from the valve lift when the cam lobe is at peak lift.

As the pivot point gets closer to "c", the small changes on the pushrod
side are multiplied at the valve side and vice-versa.
Last edited by Procision-Auto on Fri Feb 15, 2008 4:25 pm, edited 1 time in total.

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rockerarms

Postby bigjoe1 » Fri Feb 15, 2008 4:24 pm

I think they got it backwards from what I would like to see. I never use them on anything.

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Postby Alex W » Fri Feb 15, 2008 4:46 pm

Procision-Auto wrote:What about the angle of the pushrod as the rocker pivots?

The pushrod moving away from the "centerline", or resting position.

We could probably do some trig and figure out how much the PR angle
takes away from the valve lift when the cam lobe is at peak lift.

As the pivot point gets closer to "c", the small changes on the pushrod
side are multiplied at the valve side and vice-versa.


I dont believe pushrod angle has any effect on lift. The pushrod is no more than a link between the lifter and the rocker arm. And since both the rocker arm and the lifter are set in one fixed direction of movment (the lifter only moves up and down and the rocker only moves in an arc) we can say that the amount of lifter movement will be the same as the arc length of the cup side of the rocker.

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Postby Procision-Auto » Fri Feb 15, 2008 5:02 pm

I'm going to disagree with you there. Since the pushrod length is fixed,
and the cup distance (to fulcrum) is fixed, the angle of the rod will change
as the rocker pivots.

That ultimately means the lift will be reduced until (unless) the PR tip passes the
maximum arc point of the rocker rotation .
Last edited by Procision-Auto on Fri Feb 15, 2008 5:04 pm, edited 1 time in total.

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Postby kirkwoodken » Fri Feb 15, 2008 5:03 pm

The way it looks to me, if the cam designers design a lobe that is fast opening and slow closing, the best way for their design to be transfered to the valve is by what is refered to as "Mid-Lift" design. I think you should strive for what the designer wants, not a marketing ploy.

This is something I've changed my mind on slightly in recent years, even though I would say try it, if you think it looks good, to satisfy your curiosity. In the old days, like '50's, some stock rocker arms were designed like the Cranes. Early Olds comes to mind.

It would be helpful if one of the magazines would do a comprehensive article on whose rockers come close to "mid lift" and those that don't. Would help the nonprofessional to make a more informed decision. If the mags could actually be objective. Would also be informative if they would provide pivot to valve stem distance.
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Postby Speedbump » Fri Feb 15, 2008 5:11 pm

I've never thought about it and I'm not sure what their quick lift idea is but what is explained in the diagram makes sense to me :? and that's a little scary. If the pushrod cup is below the centerline of the pivot, it has to move laterally away from the center until it reaches the c/l which makes the ratio lower and the leverage higher. After the cup passes c/l, the ratio has to increase since the cup travelin the arc is moving closer to the pivot point. The tip, if it starts above the c/l moves away from the centerline of the pivot and that also increases ratio, but as soon as it passes through the c/l of the arc the ratio starts to decrease. This should happen at 1/2 lift if the geometry is correct, right? So, does Crane move the cup up toward the c/l of the arc and call it quick lift? I find this stuff very interesting and geometry is the only math class I did worth a darn in so I hope I'm seeing this right.
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Postby Alex W » Fri Feb 15, 2008 5:16 pm

Procision-Auto wrote:I'm going to disagree with you there. Since the pushrod length is fixed,
and the cup distance (to fulcrum) is fixed,


This is correct. Would you not call that distance "c"? Since "c" is the radius of the PR cup to the fulcrum(shaft). Now since "c" is a radius of a circle its distance never changes there for lift at the PR side of the rocker will not change either.

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Postby Alex W » Fri Feb 15, 2008 5:21 pm

Speedbump wrote:I've never thought about it and I'm not sure what their quick lift idea is but what is explained in the diagram makes sense to me :? and that's a little scary. If the pushrod cup is below the centerline of the pivot, it has to move laterally away from the center until it reaches the c/l which makes the ratio lower and the leverage higher. After the cup passes c/l, the ratio has to increase since the cup travelin the arc is moving closer to the pivot point. The tip, if it starts above the c/l moves away from the centerline of the pivot and that also increases ratio, but as soon as it passes through the c/l of the arc the ratio starts to decrease. This should happen at 1/2 lift if the geometry is correct, right? So, does Crane move the cup up toward the c/l of the arc and call it quick lift? I find this stuff very interesting and geometry is the only math class I did worth a darn in so I hope I'm seeing this right.


That was my argument in my original post. I think the way they calculated the rocker ratio is incorrect. Go back and look at lines "c" and "d" ( I added them ) and use them to compute the rocker ratio.

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Re: Rocker Geometry. Who Agrees?

Postby randy331 » Fri Feb 15, 2008 5:23 pm

Alex W wrote: The more the rocker sweeps the longer the length of the red line "d" grows. Making the ratio increase as the valve is lifted.


How would the lenght of line D get longer? What am I missing?
It seems to me the only thing that would change on line D would be it's relationship to the valve centerline.
Assuming the diagram is with the lifter on the base circle, the increased ratio right off of the seat would be because of it being at, or near a 90* angle to the valve centerline, while the valve is on the seat. As the rocker swings through it's arc, it would pass the 90* to the valve centerline quicker than (mid lift setup) and therfore an increasing amount of the roller travel will be sideways on the valve tip, instead of inline with the valve travel.
A too long of pushrod would acomplish the same thing.IMO
Wouldn't increasing the rate a valve is opened, also increase the rate it is closed,(symetrical cam) and therefore require an increase in spring on the valve seat to control it?

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Re: rockerarms

Postby af2 » Fri Feb 15, 2008 5:25 pm

bigjoe1 wrote:I think they got it backwards from what I would like to see. I never use them on anything.

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I agree. Why not leave the valve open mid to high lift instead of off the seat?
I would have to cut the pockets in my pistons even more if I were to use them.
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Postby Alex W » Fri Feb 15, 2008 5:49 pm

Before we go any further. Sombody please tell me how you calculate the ratio of a rocker. Is it B/A or d/c?

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Postby Cogburn » Fri Feb 15, 2008 5:51 pm

Effective ratio is b/a.

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Postby Alex W » Fri Feb 15, 2008 6:45 pm

Image

Heres a simple drawing I did of a 1.5 ratio rocker. Notice my dimensions are parallel with the lines. (Not straight with the oriantation of the picture like Crane has).

So the green rocker is on the base circle of the cam. The cam lifts .5054in(Arc length) but you notice the line only moved .5 inches But now on the other side the roller moves .758 inches(arc length) but the valve actually moved .750 inches. So now...

Take 1.5/1.0 & 0.75/0.5 & .7580/.5054 what do you get?
Last edited by Alex W on Fri Feb 15, 2008 6:52 pm, edited 1 time in total.

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Postby Procision-Auto » Fri Feb 15, 2008 6:52 pm

Alex W wrote:This is correct. Would you not call that distance "c"? Since "c" is the radius of the PR cup to the fulcrum(shaft). Now since "c" is a radius of a circle its distance never changes there for lift at the PR side of the rocker will not change either.


I agree with everything except the bold.

If the motion of the PR is moving off the centerline, then the lift must
also reduce.

Here's a hacked up diagram to illustrate what the pushrod is doing.
The distance from fulcrum to c never changes, but relationship between
c and the pushrod centerline does...and that's what is altering the ratio.

Image

A visual of what happens if we install a longer pushrod and machine
the primary side of the rocker to look like this:

Image

Now the pushrod angle will favour a higher rocker ratio as the cam lobe
raises the lifter


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