Rocker Geometry. Who Agrees?

[Procision-Auto]

That would be fine if the lift allows the pushrod to pass through the max
arc point.

You need to account for the angle of the rod between the lifter seat and
the rocker seat. The PR is not moving in a linear fashion.

[Alex W]

I think we are on the same page but be careful how your measuring. What are the A and B dimensions refrencing off of? Take the same picture and rotate it say 10deg then what would you do? Rotate the dimensions so they are square with the bottom of the picture again?

[xenginebuilder]

Couple of points to consider:
The rocker ratio at the rocker based on physical rocker dimensions is almost worthless to discuss because the "ratio" you care about is the ratio of lifter motion to valve motion. You can have an extreme ratio per se at the rocker and still have a low relative ratio of valve motion. For example a Chrysler A block with the 118deg bank angle has a very steep angle between the lifter to pushrod, the pushrod itself is not traveling the same distance as the lifter, so tip travel at the rocker is reduced. However the rockers built for that engine are designed with that in mind, and develop enough motion to bring the valve movement back in line with the intended "ratio" of the rocker, so that if you multiply the rated lobe lift x the published rocker ratio you get the proper valve lift. The physical rocker ratio at the rocker might need to be 1.9 to one to achieve that. All pushrod V-8's have more or less the same issues.

Most of the various rocker "schemes" that purport to have correct ratio or "quick" ratio etc. can only be set up for one specific geometry, eg each rocker arm that is supposed to do a certain thing has to be individually manufactured for the valve stem height, valve angle, pivot height, pushrod height and angle. Since every engine uses different dimensions, and and each set of heads might be set up differently, to get a single rocker design to cover all eventualities is a bit of a problem.

The rocker "ratio" constantly changes at each lift point, no matter who makes the rocker. The published ratio is only the theoretical ratio at max, you could try 6 different brand of 1.6 ratio rockers and find that the actual net valve lift changes +- .030" on the same engine, and different again on a different engine.

I would say that Crane's claims are valid, but:
Assuming you know a lot about the profile of your cam, and that for some reason rather than change the cam, you need or want more valve action at the beginning, and you can measure the valve motion accurately enough to tell the difference, then a rocker specifically designed to do that might be useful. If you have a cam that already has very aggressive opening or closing rates at the seat, you very likely will induce some valve spring issues. This is one of those products that needs to be evaluated in conjunction with your cam designer, to get a reliable result.

My personal preference is for a rocker to have the least amount of radical movement possible for a given "ratio" target, and make decisions about camshaft aggressiveness at the cam.

[Alex W]

I see no way lift can be lost because of pushrod angle. Would you agree that the pushrod is fixed to the lifter via the ball socket? So any motion imputed into the PR has to go some where and that some where is the .5054 arc in my photo above. If all .5054 lift at the cam is not going into the .5054 arc where is that lost motion.( We are not considering PR flex and valve lash).

[Procision-Auto]

Alex,

Here is a drawing using two points on the arc (red & green), and a
a virtual point (blue) with a linear relation to the red dot.

Notice how much higher the blue dot would be if the rocker did not arc out:



I guess what Crane is trying to explain is that the relation between
primary side and secondary side of the rocker is not always 1:1 as
the cam lobe raises the valve.

[Alex W]

Alex,

Here is a scale drawing using two points on the arc (red & green), and a
a virtual point (blue) with a linear relation to the red dot.

Notice how much higher the blue dot would be if the rocker did not arc out:



I guess what Crane is trying to explain is that the relation between
primary side and secondary side of the rocker is not always 1:1 as
the cam lobe raises the valve.

That makes perfect sence but you are still taking you lift measurments in a vertical direction. Whats the measurment between the center of your of your circle and the red dot. And the distance between the cicle center and the blue dot.

I think this would all make more sence if you look at me original post and use my "d"/"c"=rocker ratio.

Just to add your cam lift is the length of the arc between your red and green dot.

[Procision-Auto]

There are at least two ways to interpret what you're asking:

1. Does the value on the primary side always equal the d/c ratio
at the roller tip with respect to the PR groove.

2. Will rocker ratio vary between the cam lobe lift and actual valve lift
throughout the cycle.

I now believe you are referring to #1, however the Crane site is describing
point #2 (as my replies do).

Are we on the same wave now?

[Alex W]

Alright!!! I think we are getting somewere here. Heres the answers
1. No the d/c ratio will not be the same!
2. The rocker ratio will vary between the cam lift and valve lift but not at all like crane describes!

To understand this you will have to completly ignore cranes way of thinking and start using my d/c ratio.

So we can say that "c" is a constant length, since "c" is the distance from the center of the rocker shaft to the center of the pushrod ball measured in a linear fashion( no horizontal, or vertical dimensions) Agreed?

Now "d" is where things get interesting. If you noticed closley in my first picture that the length of "d" is actually measured from the center of the rocker shaft to the CONTACT point of the roller to the valve . So using these idea here is a drawing of the same 1.5 rocker set up at mid lift I used earlier with "c" = 1".

Notice how the length of "d"(dotted lines) grows as the roller is swept threw its range of motion.

So we can now say that since "c" is a constant and "d" is always growing that all rockers will have a increaseing ratio a lift increases.

I should also add that setting up a rocker with the midlift theory does minimize the amount of roller travel across the valve stem.

[MadBill]

But in this drawing, the valve lift per degree of arm rotation is proportional to the length of a line extended at right angles to the valve stem axis from the roller's contact point to the pivot, not proportional to the effective arm length from the pivot to the roller contact point... (In other words, the adjacent side of the right angle triangle formed, not the hypotenuse)

[Alex W]

But in this drawing, the valve lift per degree of arm rotation is proportional to the length of a line extended at right angles to the valve stem axis from the roller's contact point to the pivot, not proportional to the effective arm length from the pivot to the roller contact point... (In other words, the adjacent side of the right angle triangle formed, not the hypotenuse)

I am not sure I follow you on that one? The drawing shows the rocker at three lifts- yellow(closed), red(mid-lift), green(open). The dotted lines are drawn from the shaft center line to the rollers contact point. It might be hard to see but the dimensions are showing the lengths of the dotted lines.

[brket racer]

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

JOE SHERMAN RACING ENGINES

So what do you prefer Joe?

[bigjoe1]

Anything more than hipo street, I use the Jesel shafts. I always use more ratio on the intake side. so this gives you a big choice of ratios.For street, I use Comp stainless or aluminum.Last time I used Crane was when I won the Engine Masters contest, and they were one of last sponsors to pay me my money(about a year to wait)

JOE SHERMAN RACING ENGINES

[EDC]

My personal preference is for a rocker to have the least amount of radical movement possible for a given "ratio" target, and make decisions about camshaft aggressiveness at the cam.

That's all I've been worrying about. Mostly because the max lift rule classes I deal with. Minimal wasted motion and lift loss at the valve.

[MadBill]

But in this drawing, the valve lift per degree of arm rotation is proportional to the length of a line extended at right angles to the valve stem axis from the roller's contact point to the pivot, not proportional to the effective arm length from the pivot to the roller contact point... (In other words, the adjacent side of the right angle triangle formed, not the hypotenuse)

I am not sure I follow you on that one? The drawing shows the rocker at three lifts- yellow(closed), red(mid-lift), green(open). The dotted lines are drawn from the shaft center line to the rollers contact point. It might be hard to see but the dimensions are showing the lengths of the dotted lines.

Understand first we're talking just of the effective motion of the valve end of the rocker. The pushrod end geometry operates semi-independently, based on the angles formed between the pushrod and that end of the rocker.
I'll try to illustrate differently:
1. As the angle that the rocker pivot-to-roller contact point centerline forms with the axis of the valve stem increases beyond 90°, the valve lift per degree of rocker rotation decreases. To take a ridiculous extreme, imagine the valve stem is 4" in diameter. Now visualize the rocker arcing further and further, as if attempting to lift the valve say 2". Eventually the arm has rotated almost 90° and is practically parallel with the valve stem. At this point, the motion of further rotation will become almost entirely 'sweep' across the stem with virtually no further lift, so the effective rocker ratio at the valve end is down to ~zero. The same phenomenon occurs less dramatically at lower angles.

[1989TransAm]

So MadBill if I follow you correctly the effective maximum rocker arm ratio will be when the rocker arm is perpendicular or ninety degrees to the valve stem. That would seem to support the mid-lift theory.

The narrower the sweep on the valve stem the more lift on the valve. With a wider sweep on the valve stem, lift would be lost because there would be more horizontal motion thus taking away the perpendicular motion.

I might have carried your explanation to far.