roller rockers vs contact tip rockers

[hoffman900]

Who else has any experience building any max-effort race engines with rules mandated stock style rockers?

I know some SCCA Limited Prep racers who do. They also typically have a max lift rule.

[DaveMcLain]

plovett,
Your thoughts & comments are 100% correct about the movement of the rocker tip across the valve. Both the angle of contact of the rocker tip with the valve tip, AND the distance between the rocker pivot [ trunnion or ball ] to the contact point on the valve tip change as the rocker opens & closes the valve. Result is some amount of sliding action of the rocker tip.

You are both 100% incorrect. The rocker arms contact point at the top of the valve stem can move without creating any sliding action at the valve tip. The way the non roller rocker arm is designed its contact point can change at exactly the same rate as the contact point at the valve would have to change just like it does on the bottom of a rocking chair.

[Walter R. Malik]

plovett,
Your thoughts & comments are 100% correct about the movement of the rocker tip across the valve. Both the angle of contact of the rocker tip with the valve tip, AND the distance between the rocker pivot [ trunnion or ball ] to the contact point on the valve tip change as the rocker opens & closes the valve. Result is some amount of sliding action of the rocker tip.

You are both 100% incorrect. The rocker arms contact point at the top of the valve stem can move without creating any sliding action at the valve tip. The way the non roller rocker arm is designed its contact point can change at exactly the same rate as the contact point at the valve would have to change just like it does on the bottom of a rocking chair.

Rocking Chair ... great simple explanation.

[MadBill]

...

[cjperformance]

plovett,
Your thoughts & comments are 100% correct about the movement of the rocker tip across the valve. Both the angle of contact of the rocker tip with the valve tip, AND the distance between the rocker pivot [ trunnion or ball ] to the contact point on the valve tip change as the rocker opens & closes the valve. Result is some amount of sliding action of the rocker tip.

You are both 100% incorrect. The rocker arms contact point at the top of the valve stem can move without creating any sliding action at the valve tip. The way the non roller rocker arm is designed its contact point can change at exactly the same rate as the contact point at the valve would have to change just like it does on the bottom of a rocking chair.

Rocking Chair ... great simple explanation.

Yes - exactly.
The prifile on the tip is what allows the contact point to rock across the tip without sliding. The tip height at 90 deg to valve stem compared to the rocker fulcrum pivot centerline determines what the valve tip does in relation to tip center to pivot center thru the range if valve lift. The rocker tip profile is then cut to create a non sliding contact point. This makes stock rockers tricky to set up compared to roller tip. The roller tip allows some forgiveness and will cover for (not without long term effect) poor geometry/setup. A non roller tip is not forgiving to poor setup and as has been touched on can be profiled to alter effective average ratios to benefit a control/limited combo but once you start to make this type of tip slide you begin to increase stem loading and will wear stems and guides etc, not to mention tips!

[cjperformance]

...

Nice Bill ! We now need a giant working model of a stock rocker and valve stem with changable tips/profiles to demonstrate how the tip profile can change from sliding inboard to not sliding only rocking to sliding outboard !

[BILL-C]

Maybe some people who are interested in learning something new can actually experiment on a real engine. Grab a bunch of sacrificial rockers,a dial indicator,degree wheel, a grinder, a good set of eyes, and a boat load of patience. It's really easy to find out what makes things worse,so when you do, just go in the opposite direction.

[hoffman900]

This was a really great thread dealing with the radius of the rocker / valve stem contact. http://www.xs650.com/threads/yamaha-xs6 ... try.40042/

[digger]

This was a really great thread dealing with the radius of the rocker / valve stem contact. http://www.xs650.com/threads/yamaha-xs6 ... try.40042/

this says that there is no magic radius that stops the 'scrub'.

[cjperformance]

This was a really great thread dealing with the radius of the rocker / valve stem contact. http://www.xs650.com/threads/yamaha-xs6 ... try.40042/

this says that there is no magic radius that stops the 'scrub'.

No there is no magic "radius" as such, the tip needs a "profile" that has a varying radius to stop scrub. In that article they are dealing with an adjuster that has to rotate and stop at any given location in order to adjust lash. So it must have a radius , not a varying radius profile. You cannot avoid some scrub with a fixed/constant radius, as the article shows.

[Geoff2]

No Dave Mc,
You are the one who is 100% incorrect. Look at any rocker factory type rocker that has mileage on it. If there was NO sliding action of the rocker tip, you would see a thin wear line; a point contact. In actuality, the wear is a band anywhere from 1/8" to about 3/16" wide. That is because the rocker tip is sliding across the valve tip as it opens & closes the valve. The rocking chair analogy is apples to gorillas because you are forgetting TWO important factors not related to rocking chairs. As the valve opens & closes via rocker action, the distance from the valve tip to the trunnion centre changes. It can get larger &/or smaller depending on the geometry, but it WILL the distance will change. The rocker tip moves in an arc; the valve does NOT, it moves in a straight line,.Hence this dimension change results in the sliding action of the rocker tip.
The easiest way to visualise this is with the rocker at 90* to the valve tip, valve on the seat [ closed ]. Say the valve lift is 0.600". At max lift, the valve tip is now 0.600" lower & the distance from the valve tip to the rocker trunnion/pivot has increased...& if it was a non-roller tip it will be sliding across the valve tip; a roller tip rolls.
There is a corollary to this. Tests I have seen show that longer push rods almost always increase valve lift with full roller rockers. A test I saw on a Pontiac V8 used 1.5 & 1.65 rockers. Using 9.205, 9.300 & 9.373"prods resulted in valve lifts of 0.558, 0.573 & 0.574" with 1.65 rockers. An increase occurred with stamped 1.5 & 1.65 rockers. So what is causing the increased lift? I believe the increase in lift occurs as the the valve nears max lift, where the rocker assumes a more extreme angle with longer p'rods, such that the contact point of the rocker tip on the valve is at an increased distance from the trunnion pivot, & this extra distance effectively increases rocker ratio.

[cjperformance]

No Dave Mc,
You are the one who is 100% incorrect. Look at any rocker factory type rocker that has mileage on it. If there was NO sliding action of the rocker tip, you would see a thin wear line; a point contact. In actuality, the wear is a band anywhere from 1/8" to about 3/16" wide. That is because the rocker tip is sliding across the valve tip as it opens & closes the valve. The rocking chair analogy is apples to gorillas because you are forgetting TWO important factors not related to rocking chairs. As the valve opens & closes via rocker action, the distance from the valve tip to the trunnion centre changes. It can get larger &/or smaller depending on the geometry, but it WILL the distance will change. The rocker tip moves in an arc; the valve does NOT, it moves in a straight line,.Hence this dimension change results in the sliding action of the rocker tip.
The easiest way to visualise this is with the rocker at 90* to the valve tip, valve on the seat [ closed ]. Say the valve lift is 0.600". At max lift, the valve tip is now 0.600" lower & the distance from the valve tip to the rocker trunnion/pivot has increased...& if it was a non-roller tip it will be sliding across the valve tip; a roller tip rolls.
There is a corollary to this. Tests I have seen show that longer push rods almost always increase valve lift with full roller rockers. A test I saw on a Pontiac V8 used 1.5 & 1.65 rockers. Using 9.205, 9.300 & 9.373"prods resulted in valve lifts of 0.558, 0.573 & 0.574" with 1.65 rockers. An increase occurred with stamped 1.5 & 1.65 rockers. So what is causing the increased lift? I believe the increase in lift occurs as the the valve nears max lift, where the rocker assumes a more extreme angle with longer p'rods, such that the contact point of the rocker tip on the valve is at an increased distance from the trunnion pivot, & this extra distance effectively increases rocker ratio.

Sorry, you are incorrect. Experiment with tip profiles and you will see what we mean!

[Geoff2]

No Craig,

I am not incorrect. Probably been doing this longer than you have been on the planet.....

How do you explain the wide wear pattern on the tip of stamped rockers, if it is not from sliding action??????? The garden Fairy????

It is all about trigonometry & physics. Do some drawings & you will understand....

[Geoff2]

Just want to expand on something in my 4.35 am post, the rocking chair analogy & why it is NOT an analogy.
The components involved in the RC analogy are the RC & the ground; only one of them moves, relative to the other: the RC. The ground does not move.

In the rocker tip/valve tip scenario, both components move, relative to one another.

[cjperformance]

No Craig,

I am not incorrect. Probably been doing this longer than you have been on the planet.....

How do you explain the wide wear pattern on the tip of stamped rockers, if it is not from sliding action??????? The garden Fairy????

It is all about trigonometry & physics. Do some drawings & you will understand....

Geoff, i have no idea how old you are or how long you have been doing this or in what capacity. Im only 45 and i have been been working on pushrod engines since i was 11, of course i knew jack sh!t then but was/still am keen to learn, so it is extremely possibly/likely that you have been working on engine longer than i have been on the planet. Many here have!

I can say that I know quite a lot of people older with more spanner time than myself that have learnt from me, and i dont say that to bignote. I am also happy to say that i have learnt things from many people younger than myself as i dont close off and see age as something that allows me to think or say that i automatically know more than someone else!

On a properly profiled fixed rocker tip/pad (not a screw adjuster) the wide wear pattern is there beacuse the good garden fairy knows how to plot valve tip and rocker movement and then can wave a wand and come up with a varying radius to stop scrub! Therefore the wide wear pattern is not from scrub !!! Its because a varying radius has a wider area of tip contact because its contact point needs to constantly 'vary'. This also varies the rocker ratio but we can save that complex part for now.

You are exactly correct in saying its all about trigonometry and physics !! Trig is used to plot the contact point thru the arc of the rocker tip and movement of the valve tip. You then use this info to plot the shape that the rocker tip requires so as not to allow tip scrub. Some experimenting with valve tip to rocker fulcrum height during this phase will show you that a different rocker tip shape is required as the valve tip height changes. Simple physics!

So, in this avenue, my garden fairy is pretty well tuned up, i think you better trade your garden gnomes for a fairy.

On the rocking chair,,, imagine riding one with a varying radius on the rocker base, what happens to the pivot center of the chair Geoff ? There is your answer to rocker pad shape!