roller rockers vs contact tip rockers

[MadBill]

Most of the roller tip advantages involve the lack of valve stem side loading or being "forgiving" enough that the geometry does not need to be exactly correct.

The slipper tip is more forgiving geometry wise. One of the reasons roller tips were not used in production engines often.
Have done many valve train stack ups on cad to design rocker arm/ valve train set ups. Found lots of interesting things about rocker geometry as well as bucket tappet and roller follower issues.

If you say so ... I will respectfully disagree especially when it comes to sideways forces.

I too am skeptical about the 'forgiving' slipper tip, especial with high valve lifts, where the rocker's greater angular sweep would surely be more of a challenge to accommodate in a generic part.

[turbo2256b]

The slipper tip is more forgiving geometry wise. One of the reasons roller tips were not used in production engines often.
Have done many valve train stack ups on cad to design rocker arm/ valve train set ups. Found lots of interesting things about rocker geometry as well as bucket tappet and roller follower issues.

If you say so ... I will respectfully disagree especially when it comes to sideways forces.

I too am skeptical about the 'forgiving' slipper tip, especial with high valve lifts, where the rocker's greater angular sweep would surely be more of a challenge to accommodate in a generic part.

Issue with slipper tips at lest with factory style lifters and higher lift cams is they were not designed for a lot more lift. Clearance issues with the retainer, to short of slipper pad, slot in rocker not long enough hits stud.
Back in the day had big issues with roller rockers realy made for BBCs being sold as ford rockers being used for canted valve ford heads best was using factory lifters with the roller fulcrum kit.

[Geoff2]

The Miller analogy is only correct [ in terms of no side loading on the valve/guide ] when a line drawn from the trunnion centre to the roller contact point on the valve stem tip is at right angles to the valve stem. If the geometry is reasonably correct, this will occur at one point through the sweep of the rocker arm, from zero lift to max lift. Either side of the this point, the roller tip will exerting a side force on the valve stem against the valve guide. This part of the rocker ratio [ from trunnion centre to contact point on the valve stem tip ] also varies & will be at it's minimum with the above mentioned rt angle [ 90 degrees ] & will be greater either side of it [ increase in rocker ratio, all else being equal ].

[Geoff2]

Please disregard the last sentence in my previous post. It s incorrect. What I meant to say: with the rocker at 90* to the valve stem tip, the rocker ratio should be the stated ratio, all else being equal. 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*.

[BILL-C]

I checked the actual rocker ratio on a bucket full of good used stock sb ford 289 rockers for a FIA spec build years ago. The ratios varied from 1.52 to 1.65. Blueprint spec is 1.6. Aftermarket stock replacement versions varied worse, ranging from 1.48 to 1.7 with the same part number. Hand contouring the valve tip end can really have a huge affect on valve action and power curve of engine. VERY time consuming though. Surprisingly little valve guide wear results, even with radical changes that should wipe out the guides instantly if you go by commonly believes theories.

[DaveMcLain]

Side loading if your meaning forces not in line with rocker movement aligned with valve push rod reduction is best alleviated in rocker shaft designs inwhich twisting of the rocker is reduced about as much as possible.

Canted valve setups can be one of the worst to figure out as its kinda a 3D motion not a 2D motion.

Some rockers like fords rail rockers were designed to limit rocker twist some of todays roller tips do something like the rail rockers with washers either side of the roller tip

A properly designed canted valve setup should have the pushrod in plane with the rest of the valve train and should not in theory have any more rocker twist than an inline valve setup.

[MadBill]

I checked the actual rocker ratio on a bucket full of good used stock sb ford 289 rockers for a FIA spec build years ago. The ratios varied from 1.52 to 1.65. Blueprint spec is 1.6. Aftermarket stock replacement versions varied worse, ranging from 1.48 to 1.7 with the same part number. Hand contouring the valve tip end can really have a huge affect on valve action and power curve of engine. VERY time consuming though. Surprisingly little valve guide wear results, even with radical changes that should wipe out the guides instantly if you go by commonly believes theories.

The ratio can vary widely through the arc, due to both the valve and the pushrod end geometry and their interacting . DV tested a couple of dozen p.n.s and reported in his BBC book. One advertised as 1.7:1 started at 1.01:1, was 1.11 @ TDC and maxxed out at 1.679. Another advertised as 1.8 varied from 1.88 to 1.932.

[panic]

A properly designed canted valve setup should have the pushrod in plane with the rest of the valve train

Both the tappet and the valve stem?

[DaveMcLain]

A properly designed canted valve setup should have the pushrod in plane with the rest of the valve train

Both the tappet and the valve stem?

From the pushrod seat in the lifter up through the rocker and to the valve stem, yes. If not then the rocker also gets side to side movement where it tries to twist around the stud.

This is why when you retrofit a hydraulic roller into an engine like a Ford big block or Cleveland or a Big Chevy it can be a problem.

[HeinzE]

Several posts here have suggested that the roller tip rocker always excerts its force into the valve tip in a straight line where a contact tip will act with side load, that being the big difference. After studying a few drawings of both types and the geometry involved I have begun to question that and in fact believe that both act virtually the same and both excert force in a straight line. What seemed to me to be overlooked was the fact that the contact pad radius is a small part of a lage circle whose center is the same distance from the contact pad as the length of the distance from the center of the rocker shaft to the center of the pad. This circle would be the same size as what you would get if you took the rocker arm, either roller tip or contact type and spun it on its shaft and scribed a line off the center point of either. If you take that circle and atach an imaginary plumb bob at the center and swing the rocker up or down you will see that the plumb bob remains vertical but moves along the circle of the pad. The direction of plumb bob...the line of force...doesn't change, rather the arc of the pad sweeps along it. The center of the pad circle will arc in the same circle size as the rocker tip does around it's center and the entire circle will move back and forth depending on if it is approaching or moving away from one of the 90deg points of this circle. This is what we see at the valve tip when checking the rocker geometry and is the reason it is always wider than the point of contact made by either a roller or contact pad. If our rocker geometry is correct, at mid lift we want to see a line drawn from either the tip of the valve or the center of the roller axle through the center of the rocker shaft, depending of which style rocker we have. So at the begining of valve lift the contact point of either the roller or contact pad will begin moving forward until it reaches mid lift and then moves back again as it goes to full lift, repeating this in reverse as the valve closes. And to minimize applying the load offcenter we want this contact line to be centered on the valve tip. The contact tip rocker is in reality a roller rocker but with a roller size described by the radius of the pad arc and doesn't roll but slides it's circumference along instead. The center of the pad radius circle acts the same as the roller radius and it's axle, both direct the force straight on to the valve tip and both contact points move back and forth the distance of the width of the circle segment defined by the number of degrees of travel along their circumference.

Karl

[Paul Kane]

The Miller analogy is only correct [in terms of no side loading on the valve/guide] when a line drawn from the trunnion centre to the roller contact point on the valve stem tip is at right angles to the valve stem.

The inquiry by the OP was about "roller tip friction reduction" and my post is intended to correct that presumption and state the real intended purpose of the roller tip, which is to aim forces in-line with the valve stem throughout the rocker arm's radial articulation. Second, as was indeed specified in said post, the reason for the roller tip on the rocker arm is to minimize "for-and-aft" rocker arm articulation side-loading specifically (nobody said, "eliminate," the roller wheel "minimizes"). Therefore, Miller's analogy is correct as described...and I think it's pretty clever analogy.

If the geometry is reasonably correct, this will occur at one point through the sweep of the rocker arm, from zero lift to max lift. Either side of the this point, the roller tip will exerting a side force on the valve stem against the valve guide.

If you are referring to compounded valve train geometry (such as in the TFS Twisted Wedge head for the small block Ford for example), all bets are off and valve side loading (outside of fore-and-aft rocker arm articulation) is always present whenever the valve is being actuated, not only sometimes. There isn't a stud rocker arm on the planet that can correct such engineering oversights. But in a properly engineered valve train the roller tip does minimize fore-and-aft side-loading upon a valve guide far greater than a shoe-tipped rocker arm could ever hope.

...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).

[^^^^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.^^^^]

[HeinzE]

Yes, the rocker geometry is only correct at mid lift, assuming it is set up properly to begin with...but as convention for this discussion we can assume it is. The only "side" load imparted into the valve stem is by the off center motion of the contact points as the rocker swings through it's arc. This is the same in both roller and contact tip rockers. But in both instances the force into the valve tip is coming straight on...in the roller tip because the line of force stays perpendicular to the rocker axle center and the roller contact point and in the contact tip because the line of force stays perpendicular to the center of the circle described by the arc of the pad and the sliding contact point along the pads surface. Both types push straight into the valve. The contact pad is just a small part of a circle and as the rocker swings up or down the location on the valve tip where the line of force contacts moves left or right depending on if it is opening the valve or closing it, but it's because the entire circle is moving left to right. It has to follow the path of the rocker as it swings through it's own arc. Lock a roller tip on it's axle and the width of the contact line would still be the same as when it is free to move.
In that situation it becomes a different animal, however, since the rollers' diameter would be significantly smaller than a proper contact pad and would begin to impart actual side load into the valve stem.

Karl

[F-BIRD'88]

Good walk through explaination HeinzE Karl.
Your effort and contribution is appreciated.

[cjperformance]

The Miller analogy is only correct [in terms of no side loading on the valve/guide] when a line drawn from the trunnion centre to the roller contact point on the valve stem tip is at right angles to the valve stem.

The inquiry by the OP was about "roller tip friction reduction" and my post is intended to correct that presumption and state the real intended purpose of the roller tip, which is to aim forces in-line with the valve stem throughout the rocker arm's radial articulation. Second, as was indeed specified in said post, the reason for the roller tip on the rocker arm is to minimize "for-and-aft" rocker arm articulation side-loading specifically (nobody said, "eliminate," the roller wheel "minimizes"). Therefore, Miller's analogy is correct as described...and I think it's pretty clever analogy.

If the geometry is reasonably correct, this will occur at one point through the sweep of the rocker arm, from zero lift to max lift. Either side of the this point, the roller tip will exerting a side force on the valve stem against the valve guide.

If you are referring to compounded valve train geometry (such as in the TFS Twisted Wedge head for the small block Ford for example), all bets are off and valve side loading (outside of fore-and-aft rocker arm articulation) is always present whenever the valve is being actuated, not only sometimes. There isn't a stud rocker arm on the planet that can correct such engineering oversights. But in a properly engineered valve train the roller tip does minimize fore-and-aft side-loading upon a valve guide far greater than a shoe-tipped rocker arm could ever hope.

...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).

[^^^^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 !!

[digger]

the difference is in rolling Resistance vs sliding friction