Roler Rockers - Adjustable or Fixed Push Rods

[argos587]

Agreed, staying with N4. They will add more but not too much. Thanks

[aswracing]

Originally Posted by kitabel

The description given for the stock ratio rockers strongly suggests that stock length pushrods are OK for stock lift.
Assuming stock base circle, but increased lift, the pushrods should be shorter by 50% of the added lift.
If the stock lift is .400" (Y/N?), going up to .490" indicates .045" shorter. Is this significant? Probably not.

This is mistaken. These aren't automotive valve trains with a nut to raise or lower the rocker arm. The rocker stays in a fixed location. Correcting the geometry is done with stem protrusion adjustments, not pushrod length.

But in the HD world, the distance between the lifter and the rocker is not adjustable, so rocker geometry is not adjusted with pushrod length. If all you do is add more lobe lift at the same base circle, and do nothing to change stem protrusion or any of the other things that affect the pushrod length you need, then you don't want or need any change to your pushrod length. The lifter will be preloaded the same amount as it was with the lower lift cams. The vast majority of cam swaps on these motors are done this way. It's generally only when head work is involved that geometry gets corrected. Although some guys do it with lash caps.

Here's a drawing I made years ago, that describes the various things that affect the pushrod length you need:

[aswracing]

Relevant to the discussion on W's vs. N4's:



Same bike, same motor/exhaust/air cleaner, same dyno, tested the same day by the same person (me). The only changes were the cams and the EFI was mapped in for each. CR was 10:1, which is the high end of the range for the W's and the low end of the range for the N4's. So in that respect, this test favors the W's.

[aswracing]

Originally Posted by kitabel

How would you know what length to use?

This is the easiest way:

[Hippysmack]

Originally Posted by aswracing

So in that respect, this test favors the W's.

Thanks for posting that.
I've read something along those lines before but didn't save it to memory.

[argos587]

The other concern with push rods relate to valve springs.
Stronger springs may need thicker/stronger push rods. Doubt the springs I'll be using will need stronger rods.

[aswracing]

Originally Posted by argos587

The other concern with push rods relate to valve springs.
Stronger springs may need thicker/stronger push rods. Doubt the springs I'll be using will need stronger rods.

This is exactly correct. The stock pushrods are more than adequate for the stock springs.

So if you're not changing any of the things that necessitate a length change (see the drawing), and you're using the stock springs, there's generally no need to change the pushrods at all. Believe me, I get that question all the time.

A fixed length pushrod is a far superior solution to an adjustable whenever you're using hydraulic lifters, in my opinion. Stronger for a given weight and a lot less to go wrong. It's only when using a solid or travel-limited lifter, that requires a precise pushrod length, when an adjustable makes sense.

A hydraulic lifter is self-adjusting. To the extent the pushrod is a little too long, the lifter shortens itself, and to the extent the pushrod is a little too short, the lifter lengthens itself. Amazing technology that came out in the 1930's. Works really well. All you need is a pushrod length that preloads the lifter in it's self-adjusting range.

[kitabel]

Missing from your remark: any sign that the original hydraulic tappet is used.
Oops.

[Tomcatt]

Originally Posted by kitabel

Missing from your remark: any sign that the original hydraulic tappet is used.
Oops.

You hadn't already dug yourself a deep enough hole?