roller rockers vs contact tip rockers

[BILL-C]

Dave Mc,
A roller tip rocker rolls across the valve tip. I don't think there is any dispute about that.
However, a rocker with a pad, such as stock rocker, slides across the valve tip & the best geometry in the world isn't going to change that. We see that with worn rockers that show a WIDE wear mark on the rocker pad. What you are forgetting is that as the rocker moves up & down, the tip swings in & out on an arc, moving closer to, & way from, the valve tip. If the valve tip moved in the same arc, there would be no sliding....but it doesn't move in an arc.

Bill-C.
Yes the trunnion to roller tip distance is fixed. A popular misconception is that this distance forms part of the rocker ratio. It doesn't. The ratio is from the trunnion centre to where the tip makes contact on the valve tip & will change through the rocker travel.

I give up trying to explain this. When your theory doesn't agree with what your eyes are seeing, it's time to update the theory.

[MadBill]

Bill. I think i might have just figured out your age lol. Mark .

Busted!

[turbo2256b]

Maybe my point was missed on slipper tips being more forgiving is due to production tolerances. All the max min of mass production assembly and machining makes roller tips more prone to misalignment to the valve tip. How many have spent hours dialing in roller tip rockers?
Most factory rockers were also designed for around .500 max lift on SBFs and around .550 max lift for BBFs.
Also found on my CAD layouts using factory blueprints found increasing lift in many cases required different distances between tip pivot and push rod locations to maintain valve train geometry.
As stated before canted valve the worst. Especially the ford ones. Part of there issue was intake and exhaust were set at different angles which really needed different rockers for exh and intake. As well as years were aftermarket rockers were really for chevys reboxed as ford.
There were times were I had to order a different cam just to acheave the lift I wanted. Remember wanted a .600 lift and ordered a cam but in the end needed a cam with .625 lift different studs and pushrods to hit the mark.

[Geoff2]

Bill-C,
You give up because you don't understand. You claim that the trunnion to roller TIP is fixed. It is not. The trunnion centre to the centre of the roller IS fixed. There is a difference. The actual, in-operation, rocker ratio on the valve side is the distance from the trunnion centre to the point where the roller tip contacts the valve tip. If you take the the erroneous Miller example earlier of a line drawn from the trunnion centre through the roller tip axle, & this line is at 90* with the valve stem, then the distance from the trunnion centre to the roller tip contact on the valve tip is greater, than the distance from the T centre to r/tip centre. A right angle triangle is formed, & the hypotenuse is the effective rocker ratio, a line from T centre to roller tip contact.

Consider this: longer p'rods generally increase valve lift. How does this happen? It can only happen with an effective increase in rocker ratio.
On the p'rod side, with a longer p'rod, the p'rod is going to push away from T centre, reducing the ratio. On the valve side, the above mentioned distance between the contact point of the roller tip & T centre increases with a longer p'rod, increasing the rocker ratio & valve lift.

[BILL-C]

Sorry Geoff2, but it seems that you don't have actual hands-on experience modifying rocker arms with the intention of changing ratio and sweep pattern. You are trying to use math theory to explain why something can't be done that actually has been done for decades.An experienced machinist can manipulate the shape of a stock rocker tip to change valve action in ways that my high school math teacher could never explain. Put down your text books and get your hands dirty in the shop to find the answers.

[Tuner]

^^^ yup, this ^^^


(in the voice of the old blind monk) "You must not tear the ZigZag paper, Grasshopper."

[Geoff2]

Bill-C,

I have had 'hands on' experience' since 1964. And since that time, the physics & maths hasn't changed & it this which is required to understand what happens...

Perhaps YOU should have listened to your maths teacher more carefully...

Because rocker action is all about maths & trigonometry.

And I was never talking about varying the rocker ratio to change the valve lift. That is a no brainer.

What I WAS/AM talking about is how the rocker ratio of a single rocker changes through the sweep of the rocker arm.

Your claim [ page 4 ] that the rocker tip can be shaped so that there is no sliding action on the valve tip is nonsense & it shows you don't understand the physics involved. Have a look at the pic on page 5. The rocker tip has a large, curved foot so that it can slide on the valve tip as the rocker moves from valve closed-to open-to closed. No amount of massaging rocker length, p'rod length, rocker tip shape etc can eliminate this sliding action. It can be reduced, but not eliminated. There is sliding motion because the rocker tip moves in an arc, & the valve tip does not move through the same arc [ it moves in a straight line ] , so a change in distance occurs, & this change is accommodated by the rolling action of a roller tip rocker, or the sliding action of a fixed tip rocker.

[turbo2256b]

Bill-C,


Your claim [ page 4 ] that the rocker tip can be shaped so that there is no sliding action on the valve tip is nonsense & it shows you don't understand the physics involved. Have a look at the pic on page 5. The rocker tip has a large, curved foot so that it can slide on the valve tip as the rocker moves from valve closed-to open-to closed. No amount of massaging rocker length, p'rod length, rocker tip shape etc can eliminate this sliding action. It can be reduced, but not eliminated. There is sliding motion because the rocker tip moves in an arc, & the valve tip does not move through the same arc [ it moves in a straight line ] , so a change in distance occurs, & this change is accommodated by the rolling action of a roller tip rocker, or the sliding action of a fixed tip rocker.

Cha ching

[BigBlocksOnTop2]

^^^ yup, this ^^^


(in the voice of the old blind monk) "You must not tear the ZigZag paper, Grasshopper."

A tear in the zigzag paper creates an air bleed. Would that be high or low speed?

[hoffman900]

Just about any current engineering undergrad could do the math and write a program to test a myriad of parabolic combinations of rocker tips on the valve stem and its effect on the lift curve.

The whole “yeah, screw dem experts and their math” is incredibly shortsighted and reveals how little you know about the world as a whole.

I’m not picking sides, but the chest pounding displayed on this site sometimes is very knuckledragger-ish

[BILL-C]

Just about any current engineering undergrad could do the math and write a program to test a myriad of parabolic combinations of rocker tips on the valve stem and its effect on the lift curve.

The whole “yeah, screw dem experts and their math” is incredibly shortsighted and reveals how little you know about the world as a whole.

I’m not picking sides, but the chest pounding displayed on this site sometimes is very knuckledragger-ish

Ouch ! That's rough ! Just wondering how you would account for the valvestem clearance in the guide that extrapolates to approx .005 at valve tip and its random movements in your program? Please excuse any typos. my knuckles were dragging all over the keyboard.

[hoffman900]

Just about any current engineering undergrad could do the math and write a program to test a myriad of parabolic combinations of rocker tips on the valve stem and its effect on the lift curve.

The whole “yeah, screw dem experts and their math” is incredibly shortsighted and reveals how little you know about the world as a whole.

I’m not picking sides, but the chest pounding displayed on this site sometimes is very knuckledragger-ish

Ouch ! That's rough ! Just wondering how you would account for the valvestem clearance in the guide that extrapolates to approx .005 at valve tip and its random movements in your program? Please excuse any typos. my knuckles were dragging all over the keyboard.

Well, that would require a more intensive effort, but not only could you model that, but also the effects of heat and the dynamic nature of the rocker arm flexing as well as the effects of cylinder pressure on the whole thing. That’s graduate level work, however.

If you think what’s happening at .5 rpm as you turn it over by hand is anything like what’s happening at 8000rpm, then you’re really discounting what is possible. Factory racing programs, F1,NASCAR, etc are all there. They are not telling a bunch of artisans to just start cutting stuff and see what happens.

[turbo2256b]

Just about any current engineering undergrad could do the math and write a program to test a myriad of parabolic combinations of rocker tips on the valve stem and its effect on the lift curve.

The whole “yeah, screw dem experts and their math” is incredibly shortsighted and reveals how little you know about the world as a whole.

I’m not picking sides, but the chest pounding displayed on this site sometimes is very knuckledragger-ish

Ouch ! That's rough ! Just wondering how you would account for the valvestem clearance in the guide that extrapolates to approx .005 at valve tip and its random movements in your program? Please excuse any typos. my knuckles were dragging all over the keyboard.

My last job got in trouble for finding omissions such as above in programs and showing just how much some of it could affect the out come.

[hoffman900]

Just about any current engineering undergrad could do the math and write a program to test a myriad of parabolic combinations of rocker tips on the valve stem and its effect on the lift curve.

The whole “yeah, screw dem experts and their math” is incredibly shortsighted and reveals how little you know about the world as a whole.

I’m not picking sides, but the chest pounding displayed on this site sometimes is very knuckledragger-ish

Ouch ! That's rough ! Just wondering how you would account for the valvestem clearance in the guide that extrapolates to approx .005 at valve tip and its random movements in your program? Please excuse any typos. my knuckles were dragging all over the keyboard.

My last job got in trouble for finding omissions such as above in programs and showing just how much some of it could affect the out come.

Right. That’s part of the R&D process. I’d hope people check their machinists’ work too. How many rocker arms did it take to figure out the perfect barrel shape radius? How did they know what it was other than “okay, that worked, don’t change the machine’s settings because we’re not sure how or why that worked”? That’s not how engineering and design works.

[MadBill]

It's probably premature to speak up at this point, but: I started out firmly on the 'slide side' of this argument, but then decided to dig deeper and made a large scale (8:1) drawing of a typical rocker and valve tip intersect*. (*I'm sure anyone with even rudimentary computer math skills could dash off a definitive proof/disproof, but that anyone is not me...)
I'm not quite ready yet to stick my head above the parapet waving the rocker flag, but I have to say it now appears to me that a contact pad which presents a large radius to the valve tip at half lift/90° and which becomes tighter as the arm angles up or down from there (Ellipse? Parabola? Other?) may indeed be capable of producing pure rocking contact, theoretically generating no side loading.
(Let the shelling resume... )