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Wed Feb 10, 2010 2:14 am

I'm going through my first ever rocker geometry check. For starters I read all I could find on the subject, here there and everywhere. What is still bothering me is this question:

At half lift, using stock adjusters, the angle formed by a line through the rocker shaft axis and the valve tip is 90 degrees to the valve stem.

When using swivel feet, won't the height of the foot part add to the length of the valve stem, moving the correct corner point of this 90deg angle to the center of ball within the foot?

My compliments to anyone who understood the question.

Wed Feb 10, 2010 2:21 am

probably.
but at half lift It's better to check that your adjuster is in line with the valve stem.

Thu Feb 11, 2010 3:36 pm

So.. the correct point to aim at could be here

Making the 90deg line meet the tip of the valve resulted in a huge sweep across the tip surface and 0.002 - 0.004 less lift (stock cam 1.1 rockers). I tried without and with lashcaps + shimmage. The adjuster could be made parallel at half lift in both cases.

Here's the new half lift situation (roughly, pic taken before fine adjustments)

Now there's so little movement across the tip it's hard to see. The adjuster angles are pretty much mirror images at full and zero lift. Guess this could be the way to do it.

Thu Feb 11, 2010 4:38 pm

it seems to me that your questions indicate that you know more about correct rocker geometry than 90% of the guys i have seen here answering questions about geometry!

I would agree with your take on the swivel foot situation, and love your pic explaining it!!!

also it looks like your geometry is darn near perfect at 1/2lift

Thu Feb 11, 2010 6:28 pm

I must say bravo!
That image above with the photoshop'd lines explains it all perfect.
Picture is worth a thousand words!

Fri Feb 12, 2010 6:28 am

Not so fast, Modok.

I am just wondering how many shims you had to use to get that position and do NOT lie either? :D .

Is that a stock pushrod?

Fri Feb 12, 2010 7:19 am

nsracing wrote: Not so fast, Modok.

I am just wondering how many shims you had to use to get that position and do NOT lie either? :D .

Is that a stock pushrod?

There's 0.12'' worth of shims. Honestly :D. Using no shims at all was the 2nd option, but this caused the wipe area being wider on valve tip.

You prefer aiming this 90deg "sweet spot" at full lift. Is that because of greater spring pressures of double springs?

I'm using stock springs, and that's a selfmade adjustable stock pushrod. The heads are early 40hp with round bosses.

Fri Feb 12, 2010 8:08 am

It makes more sense to align the adjuster tip and valve stem at full lift when full spring pressure is at its greatest.

This is an idea that was borne from NASCAR. I have set all my high peformance engine builds to that system been few years now.

On ratio rockers, I modified this same system to "slightly-over-straight- up" at full lift to accomodate the swipe pattern a little. Now, the adjusters are in the back on these type of rockers.

YOu gotta do "best fit" sometimes w/ aftermarket stuff. So will compromise on the valve stem/adjuster position to attain the best possible swipe.

Fri Feb 12, 2010 5:37 pm

NASCAR? Af far as I know they don't even idle those things. Why are they concerned about spring pressure at full lift if they mainly run at redline all day?
I think you have some ideas mashed together the wrong way.
Which way the adjuster screw is pointed does not matter to me.
It's about the angles man, the force vectors.
If you have to fudge the angles to get your wipe pattern ect. correct, well then that's just not right.
90deg at half lift is a safe place to be for the valve and pushrod. If anything I think I would want to be a little closer to 90 closed than open on the pushrod side.
And of course NSR believes the opposite!
It's a teeter-totter!

Fri Feb 12, 2010 7:55 pm

Redline? Do you have any idea how much spring pressure they run on them roller rockers?? Try around 750 lbs or more!

If you have tested the pressures on them springs, it is just amazing. I measure springs I install on my RIMAC.

But that idea makes more sense to me than what we practice now w/ the aircooled VWs. With strokers, it is so easy to float the valves so our natural tendency is to put heavier springs.

I like to keep up w/ the trends in racing stuff, machinetools, so on.

Fri Feb 12, 2010 8:10 pm

do i need to keep you two under control!!! :shock: j/k j/k

Fri Feb 12, 2010 10:35 pm

Oh yeah? Well I've tested some 10 or 20 thousand valve springs.
Every Detroit 6v53 engine has 42 springs...................getting tired of those.
So there. Your springs may be bigger but I've got you outnumbered!
(I'm just kidding)

I don't know nuthin bout nascar really, but I'm sure their priorities are quite different.
I'm just trying to keep my valve guides from wearing out and minimize the uh,"general racket" of the valve train.
The first set of swivels I bought came with a set of shims(maybe .060), and the instructions said "clearance rockers 1/8 inch to clear swivel foot". Now they just come in a bag! But I think these instructions still apply, and you might need the shims, though .120 does seem excessive.

Maybe you need stronger camfollower springs.

Fri Feb 12, 2010 11:24 pm

modok wrote: Oh yeah? Well I've tested some 10 or 20 thousand valve springs.
Every Detroit 6v53 engine has 42 springs...................getting tired of those.
So there. Your springs may be bigger but I've got you outnumbered!
(I'm just kidding)

yeah tearing down 3106 cat heads was no fun either. :( especially when the crane wants to stop working mid head removal. :cry: :roll:

Sat Feb 13, 2010 1:15 am

The swivel foot is different than the 911 elephant foot pictured by the OP.

The swivel foot (ford) style does require the rocker arm to be clearenced due to the larger housing for the ball.

The elephant foot (Porsche) has no ball and does not require modification to the rocker arm.

What angle is best for long lasting valve guides, straight up or full lift?

Sat Feb 13, 2010 6:29 pm

I don't know if biasing the angle one way or the other will help guide life, I doubt it. Looks like VW aimed for 90deg at half lift, that's probably as good as anything.
At low rpm max force through the valve train will be around full lift. Here the spring is way stronger than needed and the spring force is the biggest thing.
At high rpm(near valve float) max force will be in the lower half of the lift range. All the force the spring puts out is being used to turn the valve around at max lift.
I don't know enough about cams to tell you what will happen at what rpm, but I am sure it's entirely rpm related.

I just looked at a broken swivel foot, looks like these extend the effective stem height by at least .180". So to maintain stock geometry with a stock cam .180" has to come out of somewhere. A combo of shims, shortened stems and backing the adjuster out a little bit would make sense. If you rely on shims alone the foot might sit too high on the stem tip, and there is only so much that is safe to shorten the stems.
With a hp cam you have more travel, so the extra height is useful.
Say for .460 vs .310 lift you want .080 more stem height to keep the arcs in the same range. So now that's only .100 extra; .060 shim and turn adjuster back .040", there you go close enough.
Of course the whole point of the swivel foot is to make all this a bit less critical, and the more lift the less shim needed to keep things like stock.
Another thing that throws a wrench into it, is the vw style 1.25 rockers don't have the pushrod cup machined at the same depth, I think that is why I had to grind my rockers; to put the pushrod cup back where it should be. I suppose with the swivel feet and stock rockers none or max .060" grinding is needed. True, the .125" is probably for the ford swivels.
ACN says grind .060" and use shims, berg says just use shims, I say do what you can.

Mon Feb 15, 2010 1:28 am

modok wrote:
I just looked at a broken swivel foot, looks like these extend the effective stem height by at least .180". So to maintain stock geometry with a stock cam .180" has to come out of somewhere. A combo of shims, shortened stems and backing the adjuster out a little bit would make sense. If you rely on shims alone the foot might sit too high on the stem tip, and there is only so much that is safe to shorten the stems.

Thanks Modok! That's what I've been looking for in the geometry threads. Just wondering why no one ever mentioned the stem height effect of swivel feet. Perhaps it's just too obvious to some people, it could have helped us first timers though.

Mon Feb 15, 2010 2:49 am

I have checked mine many times for fit ( I use the ball type ajuster's) the cam's I ues are stock or a little hot ( 100 or 110) and thay hit just right on the valve stem. I cut 1/8 inch off the stock push rod and thay come out right on no shim's no grinding. At mid push thay are right in the middle of the stem. It maybe becouse I use stock or mostly stock cam's.

Mon Feb 15, 2010 11:31 am

there is some logic to this so called "NASCAR geometry".. Madoc, you mentioned it's just vectors and you are right. a vector is a force and a direction. the idea it to make the direction (in this case we are mostly concerned with the side loads that are imposed onto the valve stem) closest to zero while the load is the greatest (load is provided by the spring which is of course the greatest at full lift) .. Not sure how to word it better than that (sorry)..

the theory is is to minimize the sideloads on the valve stem to try and save the valve guids. some argue that to do this you should set the stem/rocker angle to 90 deg @ full lift so that as the spring pressure increases the side load decreases (all the way to zero side load when the arm is at 90)

S

Mon Feb 15, 2010 11:39 am

hmmm... thinking out loud, possibly even at 3/4 lift then? as a compromise.

Mon Feb 15, 2010 12:37 pm

1 other argument that could be made for "NASCAR" (I never heard it called that before) geometry is that as the valve approaches full lift, the push-rod gets closer to 90 degrees which affords it more leverages as the force increases. this could arguably reduce the overall compression forces that the push-rod sees.. possibly... I have not run the nubers.. :)

S