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What is the purpose of downward string pressure on the rail instead of having it ride in the center of the arrow and not touch the rail?
 

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Many bows ( my Mission Dagger as an example) have trigger claws that drop downward, the string coming out over the top of the dropping claws. I believe the down pressure helps keep the string from jumping over the nock. The Mission has, what I consider, a ridiculous amount of down pressure, but due to the fixed design of the cams/spacers, there is nothing that can be done about it. That string does hit the nock in the center, but the down pressure causes the center serving to flat spot pretty quickly. I know Barnett and perhaps some others have the arrow/nock sit high on the rail, and the string hits a moon nock below center. Not sure why they do that ( other than trying to prevent partial dry fires), but they’ve been that way for years.
 

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Deck pressure is also there to lower noise. What Joe says is likely the most important reason.
 

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Many bows ( my Mission Dagger as an example) have trigger claws that drop downward, the string coming out over the top of the dropping claws. I believe the down pressure helps keep the string from jumping over the nock. The Mission has, what I consider, a ridiculous amount of down pressure, but due to the fixed design of the cams/spacers, there is nothing that can be done about it. That string does hit the nock in the center, but the down pressure causes the center serving to flat spot pretty quickly. I know Barnett and perhaps some others have the arrow/nock sit high on the rail, and the string hits a moon nock below center. Not sure why they do that ( other than trying to prevent partial dry fires), but they’ve been that way for years.
Copy that on premature center serving wear. Not a fan of downward pressure bowstrings.
 
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Illiminating the downward bow string pressure greatly increases the life of the serving in the arrow latch area
and will increase the life of the bowstring.
1)I removed the factory serving and installed a base layer as tight as possible in the arrow-latch area.
2)I added a second layer of BCY .030 serving in the arrow-latch area slightly narrower than the base layer.
3)To all the serving ends, I added a wee drop of Gorilla Glue for added separation prevention insurance.

Note all my crossbows: Barnett's of several series, Carbon Express 390 Pile Driver and the rebranded Rocky
Mountain 405 and 415 have the above modifications. All crossbows listed have shims added to do away with
the downward bowstring pressure. All crossbows that used the cable slide or Barnett Teflon strip, this has been
removed and the cables have been served in the cable slide area with one layer of BCY .030 serving.

Teflon Silicone Grease is lightly applied prior to shooting my sessions of 12 arrows to the cables, bowstring, servings
and the rail. This is a high quality lubricant that does not dry or or gum up, but stays moist.

My best results are as follows:
1)serving the BCY .030 over the factory serving in the arrow-latch area was 1400 shots when separation started.
Removing my serving, I found the factory base layer had separated. When this happens the top layer has no choice
but to start to separate.
2)replacing the base layer with Brownell which I happened to have, as tight as possible, then adding the top layer
of BCY .030, my best results were just over 5000 shots in the arrow-latch area. This test bow string which is an
original has just over 11,000 shots and is still in good shooting condition that just requires a new top layer of BCY .030
serving in the arrow-latch area. This 5000 plus shot serving took a real test as I swapped cams from the Carbon Express
390 Pile Driver crossbow and this boosted the crossbow up to 440 feet per second testing the 380 grain arrow-point
combination from the original advertised speed of 405 feet per second.

The mentioned crossbows all have all front mounted risers, some with two bolts (Barnett's) while the others have three bolts.
I have only tried these modifications on crossbows shooting 1/2 moon nocks.

The crossbow I am presently testing is a Rocky Mountain 405 rated at 405 feet per second shooting a 380 grain
arrow-point combination with stock cams. Test, shooting that arrow, I have recorded 423 feet per second. The crossbow is
advertised as having 200 pound draw weight limbs but the limb label shows 210 pound draw weight. Using the new, original
factory supplied bow string and cables, but reserved in the arrow-latch area, cable slide removed and cables served in that
area, I have just over 600 shots.

Wishing you all the best.
Take care.
 
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