roller rockers vs contact tip rockers

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Re: roller rockers vs contact tip rockers

Post by MadBill » Fri Dec 15, 2017 9:19 am

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... 8-[ )
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Re: roller rockers vs contact tip rockers

Post by plovett » Fri Dec 15, 2017 6:19 pm

The rocking chair analogy makes no sense to me. As the rocking chair rotates, a different part of the chair is continuously meeting a different part of the floor. But if a rocker arm rotates along a fixed point on a valve stem, that it totally different. A different part of the rocker arm is continuously meeting the same part of the valve stem. No?

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Re: roller rockers vs contact tip rockers

Post by panic » Fri Dec 15, 2017 6:30 pm

Different chord of the pallet contacts different location on the stem tip, but there is always sliding contact + rolling contact.

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Re: roller rockers vs contact tip rockers

Post by plovett » Fri Dec 15, 2017 6:44 pm

panic wrote:
Fri Dec 15, 2017 6:30 pm
Different chord of the pallet contacts different location on the stem tip, but there is always sliding contact + rolling contact.
That's what I'm thinking.

Rolling contact is sliding contact.

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Re: roller rockers vs contact tip rockers

Post by Walter R. Malik » Fri Dec 15, 2017 7:59 pm

plovett wrote:
Fri Dec 15, 2017 6:19 pm
The rocking chair analogy makes no sense to me. As the rocking chair rotates, a different part of the chair is continuously meeting a different part of the floor. But if a rocker arm rotates along a fixed point on a valve stem, that it totally different. A different part of the rocker arm is continuously meeting the same part of the valve stem. No?

paulie
NO ... the contact point on the valve tip moves from inside to outside, as the valve lift increases; minimizing scrubbing.
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Re: roller rockers vs contact tip rockers

Post by BILL-C » Fri Dec 15, 2017 8:44 pm

Walter is correct. Notice the pear shaped rocker tip on those beautiful Del West rockers on page 5?
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Re: roller rockers vs contact tip rockers

Post by BILL-C » Fri Dec 15, 2017 8:51 pm

What standard are we all using to define " no scrubbing action across valve tip"? .00001"?, .001?, What you can see with your eye? Valve guide clearance? I suspect not everyone is using the same definitions.
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Re: roller rockers vs contact tip rockers

Post by hoodeng » Fri Dec 15, 2017 9:37 pm

I'm not sure at what level i am operating at after seeing some serious science bought into this subject!! [my skill set doesn't look too high though].
I recondition V twin rockers with a simple Trock Cycle tool that i have had since we stopped pedaling generators , it simply allows the rocker to swing through its arc and holds it stable so a grinding operation can take place ,a final polish and they are done , i have seen rockers i faced thirty years ago and there is no detrimental conditions bought about by extended use after using the grinding process . The tool leaves a true 10.5mm radius on the pad with a 1.62:1 ratio rocker . All the old rockers show slide wear on approximately 5mm of the arc with a .500" lift 8mm valve, with a footprint of approx 2-3mm on the stem.
To generate a compound radii as proposed by some would necessitate a plotted rolling jig using two sets [or more] of centers for the shape to be generated [or an NC grinder which is above my pay rate] .
I think i will stay with simplistic.
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Re: roller rockers vs contact tip rockers

Post by midnightbluS10 » Fri Dec 15, 2017 11:46 pm

BOOT wrote:
Tue Dec 05, 2017 3:55 pm
Roller tips sell rockers and until the LS no1 would have bought roller rockers w/o the tip also being roller. Many parts are based on what sells n not what is best for performance, thx to all the bigger is better folks. I read once a manufacturer engineer said roller tips were hot-rodder's pipe dreams. If you look at aftermarket stamped roller tip rockers, you'll see the rocker itself is much more HD than oem usually and that is part of the gain,(ie comp stamped roller tips), the next part is having more consistent ratio or just more ratio(advertised or not). I've also read of upper rpm lose with a ls engine that switch to roller tips and gained it back once retuning to non. I've seen pics of comp cams roller but normal tip rockers, idk if they are special order only or you gotta know someone but I emailed about them with no response awhile back. LOL maybe someone will eventually come out with non-roller tips you can buy or a kit to convert current roller tip sets and be like this is a high rpm only mod :) Or even lightweight roller tips.
Texas Speed has upgraded stock style LS rockers.

https://www.texas-speed.com/p-1088-texa ... ckage.aspx

Same exact OEM-style rockers are on my Vortec V6.
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Re: roller rockers vs contact tip rockers

Post by panic » Sat Dec 16, 2017 10:10 am

The Trock (for Harley-Davidson iron Sportster, shovelhead, and Evo .557" shafts) rocker fixture shown depends entirely on operator skill to follow the existing pallet radius. You'll make an arrow head or a barn door if you're not careful. The Black & Decker, Kwik-Way, Rottler, Sioux, and Snap-On valve regrinding machines frequently have similar fixtures with adjustable conical pilots for different shaft diameters, but the same limitation: they have no fixture, pantograph, template, guide, etc. to restore the original pad radius, and no input the radius if you had that data.

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Re: roller rockers vs contact tip rockers

Post by hoffman900 » Sat Dec 16, 2017 10:19 am

Here is the rocker ratio change over the duration of the valve lift curve from measured results for an application I deal with...

Image

More later...
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Re: roller rockers vs contact tip rockers

Post by hoodeng » Sat Dec 16, 2017 5:03 pm

You are right Panic , one can make a terrible hash of a tired rocker if one is not vigilant to the vagaries of the Trock rocker tool!!! you can not trust the spring to give the required rolling resistance to the cut as it goes through its arc , i found that thumb pressure half on the jig base and half on the rocker at the end will control the cut and also skewing the rocker to the silicon carbide wheel instead of grinding at 90° will broaden the cut and slow it down .
This is probably the time to state that we have to start with parts that are readily retrievable and have a known record of service after re work to start with ,if you start with junk, rework it, you end up with shiny junk that i am sure will come back and leave nasty teeth marks on the buttock region.
There are a number of special tools here used to check the quality of re worked parts , but i would have to say the best quality check on any work is not seeing it again till many years later of satisfactory service, and finding what you have done previously not detrimental to a parts ability to perform adequately .
I have a Repco Syncro-Facer for my valve grinding machine [excellent tool] that came with all the special attachments ,the rocker facing tool was not designed for the Harley type rockers so i made one ,,after many years it is still corroding in the back of the cabinet waiting its chance to turn a good retrievable part into shit.


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Re: roller rockers vs contact tip rockers

Post by panic » Sat Dec 16, 2017 9:03 pm

A warning: a common rumor, widely repeated, purports to answer the question: "what is the pallet radius?".
Answer: "It's the lift".
Not true, although it may be similar in common engines.
A 6" rocker lever with .500" lift moves through an arc of 2.4°.
A 2" rocker with the same lift needs 7.2°
Which one needs a larger pallet radius?

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Re: roller rockers vs contact tip rockers

Post by MadBill » Sat Dec 16, 2017 10:06 pm

A magic pad curvature would be geometry sensitive, e.g. if it was optimized for "90° @ 1/2 lift", it would be progressively less so if paired with a longer or shorter pushrod.
Also of course, the less the lift and the longer the lever, the less lateral motion to deal with, e.g. a Hemi exhaust rocker.
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