valve rocker arm tip profiling

[SchmidtMotorWorks]

There are two ways to go about it.

The ideal way is like the Blair example posted above.
The problems with that method are:
1. Requires a good parametric CAD system to develop efficiently, still takes a couple hours to find a solution.
2. Requires the rocker pivot to be higher than most push-rod engines (near the perpendicular of the valve tip)
3. Requires special equipment to grind.

The other way is to just dress whatever radius cleans up.

There are some simple machines made for doing that.

https://www.youtube.com/watch?v=He1uhpV4mIw

[wyrmrider]

The Blair method gives terrible rocker geometry, as does the Lunati and others
then there are the ohc rocker pads where the contact moves from the open to closing point of the contacting cam- on these the tips are the easy part
Racer Brown spent a lot of time on the Datsun
Others on the Ford Pinto and similar
never had the luxury of a cad system- must be nice

[stealth]

There are a few schools of thought about "mid-lift" geometry and rockers.

Have a look at Jesel's web site. They suggest having the rocker roll (or scrub In this case) the longest distance before mid-lift to reduce friction when the spring pressure is the greatest at Mid to full lift, with very little roll/scrub after mid-lift.

Seems to be a good idea....

[Art Smith]

John-

thanks for the link to your U-tube video; a classic example of a picture is worth a thousand words. have you experimented with tip radius??

Art Smith
artesmith@earthlink.net

[pdq67]

Are you limited to cam lobe geometry or max. valve lift?

I ask because if not, then I would look at maximizing lobe design.

I think back years ago to the cams that had way long advertised durations and .050" numbers, but low lift because the valve springs weren't up to speed to do better lift-wise.

Might consider this??

Kind a like the Chevy -929 family of roundy-round cams here, the .390"/.410" lift jobbers that run something like 235 duration or so at .050" lift.

pdq67

[Art Smith]

pdq67-

the camshaft profile is a given AND maximum lift is also specified..... the challenge is connecting the dots between zero and maximum lift in such a manner that maximizes the area under the lift curve. I understand for the forum it's a very strange place to look for power, but it's all the rules permit!!

Art Smith
artesmith@earthlink.net

[pdq67]

Art,

Did what I said above have any merit even though it can't be used?

If the racing organization has set the cam spec's, then I would look to head porting to cheat IF they haven't also set that in stone!

Carnut1 can probably help you here big-time as far as head port and chamber work goes, imho...

Oh, and btw, my new EQ 50 cc chamber, 200 intake port heads are way nice!

pdq67

[SchmidtMotorWorks]

John-

thanks for the link to your U-tube video; a classic example of a picture is worth a thousand words. have you experimented with tip radius??

Art Smith
artesmith@earthlink.net

Yes, If you want to change the radius the best way I have found to do it is to dress the radius on a surface grinder wheel. Use a wheel about 100 grit and then polish with cratex.

[wyrmrider]

art
not a good idea to put your e-mail in the public forum-- there are robot data miners that look for them
what exactly did they spec for the cam just duration at what lift and lift or do they specify the curve and check with a cam doc
back when we had to run the exact stock cam in stock drag classes we retarded a bunch, lower gears, are revd the snot out of them
did you say what motor we are working with here and what class
do they limit springs and check the rockers too
are we talking about pushrod rockers? stud or shaft
if shaft you can offset the bushings
are the rockers adjustable?
mech or hyd lifters

[panic]

The video of the 911 rocker doesn't really help.
1. as stated in the video, it's not a pantograph that replicates a known and correct radius - the new surface contour depends entirely on operator skill.
2. the 911 pallet follows a convex cam lobe, not a flat valve stem tip, the shape is completely wrong.
3. that fixture is part of a Black & Decker, Kwik-Way, Rottler, Sioux, Snap-On, etc. valve refacing machine found in many old shops

You will also read comments such as "the pallet radius design remove all scrubbing and sliding contact; the radius rolls across the stem tip". This is impossible, disregard entirely. AFAIK there are no OEM mid-lift rockers, they all have some anti-scrub built into the pivot height.
Michael Turkish has some math on this

[pdq67]

Art,

You never answered my question?

Can you cheat by porting? Or is port volume locked down?

pdq67

[Vincenzo]

The Blair method gives terrible rocker geometry,

My understanding from your post is that you have never used the Blair method, and are making an assumption from the descriptive illustrations.

Having myself used the method many times, I can vouch for it's effectiveness, but care is needed to lay out exactly what is required, and it most often is used in conjunction with a new valve lift design, so all works together in harmony.

Most effective when used on pushrod motorcycle engines such as Norton Commando etc, where roller tipped rocker arms are seldom to be found

[Walter R. Malik]

I don't know how you should get there but, the rocker arm tip radius should be such that the rocker tip will roll across the the valve stem tip from inside toward the outside as the lift becomes greater with as little scrubbing as possible.

[wyrmrider]

I've read all of Blair's stuff and there's lots of it
if it works for you- key thing you said and I said is that cam has to match whatever method you use
for me Blair ends up with the rocker coming off the valve at high lifts
am I missing something?

[wyrmrider]

Went back and looked at Blair's pics again
I just do not agree with his premise that all motion should be down from tangent
that gives a long shoe on one side
I like half up half down which evens out the extensions, as hinted at in one of his later pics