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February 29, 2014

Rocker Geometry.

Rocker arms are dirt simple devices on one level, but the geometry of their interaction with other components of the valve train can get quite complex, and has a real bearing on the performance of the engine and longevity of the parts.  I did a fair amount of research on valve train geometry to determine if there were any improvements or adjustments I should make to my valve train, especially since I shaved the head.

In many ways, rocker geometry can be viewed as two separate problems--the geometry of the valve side and that of the pushrod side.  They of course are interrelated, but it simplifies the picture if they are addressed one at a time at least to start.  Usually, the valve side is looked at first, since there are fewer ways to adjust things there.

There are different philosophies on setting valve train geometry, differing mainly in which parameters are being optimized.  One popular approach is known as "Mid-Lift".  In the Mid Lift paradigm, the rockers are aranged such that the angle the rocker tip contact point moves through is centered on the horizontal (this all assumes a vertical valve as we have in the Triumph engines).  Having a symmetrical path of the cotact point arc implies several things:  First, the side-to-side motion of the contact point across the valve stem tip is minimized.  Second, the actual valve lift is maximized.  Third, the force on the valve stem tip is more nearly vertical.

This illustration shows the geometry I found on my head compared to what it would be with a mid lift approach.  The angle the rocker moves through and the radius of its arc are the same for both situations--the arc just occupies a different place in the circle.




Here is the same information shown with the two arcs superimposed:



One little complexity here is that the "rotation center" for the contact arcs is not the rocker shaft axis.  This is because the contact point is actually on the radius of the shoe of the rocker, and the shoe rotates through the same angle as the rocker, and so the contact point moves outward on the shoe surface as the rocker rotates downward.  It turns out (and you can easily prove this if you know your way around geometry and trig) that the rotation center is directly below the rocker shaft axis by the radius of the shoe.
 
I was serious enough about converting my valve gear to mid lift that I calculated how much the rocker shaft would need to be raised (0.224"), and made the appropriate shims for the shaft pedestals.



To check some of the theory, I installed the shims and measured the side motion of the contact point by bluing the tip of a valve stem and rotating the crank a few times.  The first pic shows the pattern for the stock setup, the second  for the mid lift.  As predicted, the mid lift pattern was skinnier and centered slightly farther out than the stock pattern.

 

On some hardcore racing sites, there is a holy war about how to interpret these patterns.  One camp categorically states that a centered pattern indicates proper geometry.  The mid lift folks counter by saying that centering the pattern is not as important as minimizing side motion and maximizing lift.  Triumph Engineers were apparently in the first camp (though, as I understand it, Mid Lift as a design approach may not have been popularized until after the TR6 era).

In the end, I decided not to use the mid lift approach for the following reasons.  First, since the position of the valve doesn't change, raising the rocker shaft raises the pushrod end of the rocker even more, to the point that I'd have to find different push rods, probably custom ones.  Second, I realized that in both the mid lift and stock cases, the rocker shoe does a combination of rolling and sliding across the contact area on the valve tip. In the case of the stock arrangement, most of the motion is rolling, with a little sliding.  With mid lift, there was almost three times as much sliding.  Maybe this is what the Triumph Engineers were optimising for.  Third, the pedistal studs would have been too short, and I'd have to make or buy longer ones.

I think that if someone is building for maximum performance, they might well consider a mid lift approach, especially if they are using roller rockers, since the sliding issue then goes away.

One thing became apparent to me while going through this exercise is that fooling with valve train geometry shouldn't be undertaken unless the side effects are well understood.  For example, it is commonly stated that when a head is shaved, rather than getting shorter pushrods, the rocker pedestals can be shimmed to compensate. The potential danger here is that this kind of change, while possibly correcting pushrod-side geometry, will also affect the valve side, which isn't otherwise affected by head shaving.

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