roller rockers vs contact tip rockers

[cjperformance]

the difference is in rolling Resistance vs sliding friction

Where is the sliding part?

[digger]

If the contact point moves laterally on the tip then there is sliding

[cjperformance]

If the contact point moves laterally on the tip then there is sliding

On a correctly set up standard type tip rocker operating within design parameters the contact point moves laterally but there is no sliding action.

[digger]

How can it move laterally relative to the tip without Sliding ( except for a roller of course)

[Geoff2]

Paul Kane,

You must have missed the phrase I had in brackets [trunnion centre to roller tip changes...]. Nothing to do with Mid Lift or Non mid lift. What I was referring to if you read my post again is THAT part of the rocker ratio from the Trunnion [ T ] centre to the centre of the roller tip [ or point of contact with valve tip of a non roller tip rocker. The OTHER part of the rocker ratio is the pushrod side of the rocker. So if the T centre to r/tip centre is 1.5" & the T centre to p'rod cup centre is 1", then the nominal ratio is 1.5".
The geometry is quite complex, far from simple, because the angles & distances that control the rocker ratio are changing as the rocker sweeps from start to finish. I call the p'rod side of the rocker the lever arm because it is 'levering' the valve open; the valve side of the rocker is the effort [ E ] arm.
The length of the E arm is not the distance from the centre of the T to the centre of the r/tip axle; the E arm distance is from the centre of the T to the point at which the r/tip makes contact with the valve tip, & will change as the rocker sweeps through it's motion of opening & closing the valve.
Similarly on the p'rod side; the actual contact point of the p'rod & cup changes, changing the length of the lever arm.
Crane had some rockers years ago that created high lift early in the valve opening. I presume this was accomplished by lowering the p'rod cup in the rocker body.

[cjperformance]

How can it move laterally relative to the tip without Sliding ( except for a roller of course)

The radius of the tip is shaped to allow the contact point to move along the valve tip as the valve tip moves from seat to full lift. So the effective pivot center to tip contact point changes thru the lift range but the contacting points do not actually slide against each other.

[BILL-C]

If you have alot of time and patience, you can hand grind the tip of the rockers to have NO sliding action and a very narrow contact patch on tip of valve. This does'nt make the most power though. If you take a more agressive approach, you can make MORE power than with a roller rocker! Does it last forever? No, but long enough to win multiple road race events in a row without excessive wear. Is it very expensive to do and only makes sense to attempt if there are rules requiring stock rockers. It's cheaper to buy multiple sets of jesel or t+d shaft rockers. The time and money we spend on improving the performance of parts dictated by the rules to "save the racers money" is insane.

[Paul Kane]

...with the rocker at 90* to the valve stem tip, the rocker ratio should be the stated ratio...Either side of the 90*, this part of the ratio [trunnion centre to roller tip] changes, & gets larger &/or smaller than it was at 90*.

The "changing rocker arm ratio deal" is essentially true only with a non-Mid-Lift rocker in a non-Mid-Lift setup. When properly installed, a Mid-Lift rocker arm doesn't impose the dramatic ratio changes as does a non-Mid-Lift rocker arm. (However other valve train components can still affect valve timing and rocker arm articulation, such as pushrod flex, etc).

Paul Kane,

You must have missed the phrase I had in brackets [trunnion centre to roller tip changes...]. Nothing to do with Mid Lift or Non mid lift. What I was referring to if you read my post again is THAT part of the rocker ratio from the Trunnion [ T ] centre to the centre of the roller tip or...valve tip of a non roller tip rocker. The OTHER part of the rocker ratio is the pushrod side of the rocker. So if the T centre to r/tip centre is 1.5" & the T centre to p'rod cup centre is 1", then the nominal ratio is 1.5".
The geometry is quite complex...because the angles & distances that control the rocker ratio are changing as the rocker sweeps from start to finish.....Similarly on the p'rod side; the actual contact point of the p'rod & cup changes, changing the length of the lever arm....

I did not miss your specifying "[trunnion centre to roller tip]". It's just that this specific consideration by itself is not enough to draw the conclusion to which you arrived. Rocker arm ratio is essentially figured as trunnion-to-roller axle/trunnion-to-pushrod cup. There cannot be any "ratio"--let alone any "ratio change" by way of rocker arm angle--to speak of based on your description where you are considering only part of the necessary equation (trunnion-to-roller tip side only). It simply cannot be figured, calculated, or even speculated upon.

Incidentally we cannot even speculate on the opening and closing velocities based on radial sweep relative to the valve angle (the side of which you focus upon), because even that requires further input parameters, some of which also rely on pushrod cup positioning (both distance and angle relative to trunnion) on the opposing side of the rocker arm.

And so my response to your post is based specifically on both rocker arm ratio and rocker arm design geometry of the rocker in question, and how these things may or may not actually affect ratio change (and ergo opening and closing velocities, etc). Without the necessary input parameters that you omitted, one cannot make the conclusions to which you arrived.

My point about Mid-Lift vs. non Mid-Lift was included since it is relative to the "changing ratio" subject you touched upon. The "changing ratio" scenario you describe above ("the angles & distances that control the rocker ratio are changing as the rocker sweeps from start to finish") does not influence rocker ratio of a rocker arm with Mid-Lift design geometry (in its correct application) the way it can dramatically alter the ratio of a non-Mid-Lift rocker arm, because with the Mid-Lift rocker the opposing pushrod and roller tip sides are self-compensating (ie, mathematically and geometrically complimentary) to one-another as the rocker arm articulates. And so with a Mid-Lift rocker the issue of changing ratio is as eliminated as can possibly be relative to a non-Mid-Lift rocker arm. But again, we are taking the thread off topic so....


_____________________________________

The "changing rocker arm ratio deal" is essentially true only with a non-Mid-Lift rocker in a non-Mid-Lift setup. When properly installed, a Mid-Lift rocker arm doesn't impose the dramatic ratio changes as does a non-Mid-Lift rocker arm.
^^^^I'm sure that last statement is going to start a wild sidebar debate but those who correctly and thoroughly understand Mid-Lift geometry already know this.^^^^

i was thinking that as i read it !!

Yeah....

[cjperformance]

Maybe no one read it ! , that would normally incite at least 5 pages of sh!t slinging !!

[digger]

How can it move laterally relative to the tip without Sliding ( except for a roller of course)

The radius of the tip is shaped to allow the contact point to move along the valve tip as the valve tip moves from seat to full lift. So the effective pivot center to tip contact point changes thru the lift range but the contacting points do not actually slide against each other.

if it moves along the tip then its actually sliding (even if only slightly and you can see wear marks sometimes). the only way there isnt sliding is if the contact point remained exactly the same throughout the lift.

[cjperformance]

How can it move laterally relative to the tip without Sliding ( except for a roller of course)

The radius of the tip is shaped to allow the contact point to move along the valve tip as the valve tip moves from seat to full lift. So the effective pivot center to tip contact point changes thru the lift range but the contacting points do not actually slide against each other.

if it moves along the tip then its actually sliding (even if only slightly and you can see wear marks sometimes). the only way there isnt sliding is if the contact point remained exactly the same throughout the lift.

No, the tip can be profiled so that its radius rolls/rocks across the valve tip with no sliding action at all. Have a look at the geomerty of a stock rocker and what the valve tip does compared to rocker pivot centerline thru its lift.

[kirkwoodken]

If you use a PR too short, you will end up with a RR rolling from the inside of the valve stem to the outside of the valve stem, just what Comp Cams has said is desirable for years. But that flies in the face of the "mid-lift" people. The Comp method DOES LOOK more like the paddle method in that it would give an increasing ratio as it crosses the end of the valve, with the highest ratio ending up at maximum lift. So, what's better: highest ratio at full lift or mid lift? How long has this argument been going on?

[digger]

The radius of the tip is shaped to allow the contact point to move along the valve tip as the valve tip moves from seat to full lift. So the effective pivot center to tip contact point changes thru the lift range but the contacting points do not actually slide against each other.

if it moves along the tip then its actually sliding (even if only slightly and you can see wear marks sometimes). the only way there isnt sliding is if the contact point remained exactly the same throughout the lift.

No, the tip can be profiled so that its radius rolls/rocks across the valve tip with no sliding action at all. Have a look at the geomerty of a stock rocker and what the valve tip does compared to rocker pivot centerline thru its lift.

Except it doesn't actually roll at all lift increments and this induces higher lateral forces on the guide compared to an actual roller

[cjperformance]

Its more of a rock as the contact point moves across the tip. The contact points constantly change thru the lift arc and within the designed lift range there is less stem loading than with a poorly set roller rocker!

[Geoff2]

CJ,
It is incorrect to say a stock rocker rolls with no sliding action across the [ valve ] tip. On a well used rocker, you will see the wear mark, often 1/8" to 1/4" wide. Since the pad on the rocker is part of an arc of a circle [ & therefore curved ] it would make a very narrow wear mark [ point contact ] on the rocker pad, only if the rocker did NOT slide; but it does slide, hence the wide wear pattern.

P. Kane,
Once again you either didn't read what I said or you misread it. Nowhere did I say that the T to roller tip was the only part of the equation to affect rocker ratio. I specifically said the p'rod side dimension is the other part of the rocker ratio & gave actual numbers as an example, 1.5", 1" for a nominal 1.5 ratio!! The fact is that the dimensions that provide the operating rocker ratio can & does vary through the sweep of the rocker. As an example, a nominal 1.5 ratio rocker might provide an operating or true ratio of 1.45 to 1.6 from valve open to valve close. At some point [ or points ] during the ratio might actually be 1.5.
There is no better example of the variables than the tests done by D. Vizard on rocker ratios, examples given in his BBC book. Two 1.7 alum rockers have the same 0.123" lift @ TDC, but one has 0.607" at full lift, the other 0.623". Another example, s/s 1.7 rockers. Two were very close in lift @ TDC, 0.125" & 0.126", but had 0.619" & 0.601" at max lift; note that the the rocker that had less lift @ TDC had more lift at max lift. It shows the variables involved & the way that the rocker ratio changes, from opening the valve to max lift & closing again.

Incidentally, if you read the original post by the OP, he started out by asking about friction differences, but finished his post asking about differences between roller & FT cams. So not just about friction....