I have used an adjustable hammer only once, and that was on my Hayabusa.... The results were somewhat confusing and I didn't find it particularly useful.... I used an MRod adjusting screw threaded into my hammer, and what I found was that over quite a large range of adjustment it made virtually no difference to the velocity.... Only when extended a long ways did it start to reduce the velocity to a significant degree.... I though about why that was occurring, and realized that when you reduced the travel by extending the nose of the hammer, that since the adjuster was sitting on the valve stem when uncocked, the hammer actually moved back, increasing the preload on the spring.... Therefore, even though the travel was reducing, the hammer was seeing a higher average spring force, nearly cancelling the advantage of increasing the hammer travel....

I wanted to increase the travel on the hammer for my new Milleium Pumper project, so this morning I used Lloyd's Hammer Strike spreadsheet to investigate what was happening.... I used a Challenger hammer for the inputs, along with my heavy Disco hammer spring, and came up with the following graphs showing hammer Energy and Momentum.... The adjusting screw is 28 TPI, so each turn of the screw changes the hammer travel by 0.036", and it therefore changes the preload by the same amount.... That information is graphed as "Preload Varies with Stroke".... The second graph was generated with the same hammer travel distances, but is "Preload Constant", set at 10% of the free length.... That value is the same on both graphs when the stroke is 0.58" (the third dot counting from the left).... Both graphs have identical scales, so they are easy to compare, and the lines would cross at that third dot from the left....



The first thing to note is how little the hammer energy and momentum change with the normal setup.... I didn't look at extending the adjuster out a long ways, because I'm after more travel, not less.... but you can see a slight steepening of the trend on the left side as the travel decreases.... That would be consistent with the way my Hayabusa reacted to decreasing the hammer travel.... little change until you went quite a ways, and then decreasing velocity.... Next you will note the drastic difference in the second graph.... Once you hold the spring preload constant, the Energy and Momentum both increase linearly with increases in travel.... I estimated that the spring would become coil bound if the stroke increased beyond about 0.70", so the last (right hand) datapoint is omitted on the second graph....

Ponder this information while I do the next post about how I built a hammer where the travel is adjustable but the preload is constant at whatever you set using the RVA....

Bob