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Discussion Starter #1
I’ve seen many positive things about shimming crossbow risers to fix downward string pressure but I need help with understanding what it actually does. I don’t get how making the riser 1/8” pushed out from the rail just make the string 1/8 inch longer when you pull back. I don’t understand how an added horizontal measurement would make the cams go higher up to release pressure on the rail. Can someone help and explain please? Or can someone send me some videos of doing it and explaining it I can’t seem to find any videos on it. Also I know shimming would probably work but does anyone know if there’s a more simply way such as just adjusting the height of the cams without adding shims. Please help so I can get this thing fixed and go out and hunt more efficiently.
 

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What crossbow do you have? Adding shim between the upper part of the riser where it mates up with the barrel will add string pressure.
 

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What crossbow do you have? Adding shim between the upper part of the riser where it mates up with the barrel will add string pressure.
When you add the shims in that area it lifts the cam end of the limbs slightly, reducing the downward pressure that is
exerted on the bow string in the arrow serving area. This greatly extends the life of the serving in this area.

All the best.
 

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Discussion Starter #4
What crossbow do you have? Adding shim between the upper part of the riser where it mates up with the barrel will add string pressure.
I have a Barnett whitetail hunter STR but I would only add a 1/8” shim so it would be very slight added pressure. Other people have done it and have up to 1000 shots with it shimmed. Worse comes to worse I buy another string in a year or two.
 

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Discussion Starter #5
When you add the shims in that area it lifts the cam end of the limbs slightly, reducing the downward pressure that is
exerted on the bow string in the arrow serving area. This greatly extends the life of the serving in this area.

All the best.
So I saw the picture and am thinking of doing that but does the shims angle or are they just a straight piece of metal that sits between the rail and riser.
 

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Discussion Starter #6
Sorry for all the questions and everything and BR you’ve been helping a lot so thank you. I’m only 14 years old and not very educated in all these things so I just can’t wrap my head around how the shimming lifts the cam end.
 

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So I saw the picture and am thinking of doing that but does the shims angle or are they just a straight piece of metal that sits between the rail and riser.
The side of the shim contacting the rail-stock is left flat. The other side can be given a wee taper toward the bottom.
The taper is really not necessary due to such a very tiny angle at that point. The shim forces the cam end of the limbs
to rise reducing the downward pressure of the bow string.

Feel free to ask questions, that is how a person learns.

All the best.
 

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The actual shims are maybe 3/16 square and sit on top of riser /rail guides and between the rail and riser. The bottoms of the rail/ riser still meet but the shims prevent the two from closing at the top, causing a slight tilt of the riser therefore lifting the string off the rail. My ts 370 has 2500 shots on it. 1200 Of those with shims installed.
 

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When you add the shims in that area it lifts the cam end of the limbs slightly, reducing the downward pressure that is
exerted on the bow string in the arrow serving area. This greatly extends the life of the serving in this area.

All the best.
I guess I'd have to see it. Lowering the riser will lower the limbs. Right? How will that reduce string pressure on the rail?
 

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The actual shims are maybe 3/16 square and sit on top of riser /rail guides and between the rail and riser. The bottoms of the rail/ riser still meet but the shims prevent the two from closing at the top, causing a slight tilt of the riser therefore lifting the string off the rail. My ts 370 has 2500 shots on it. 1200 Of those with shims installed.
I can see tilting the riser helping with the downward pressure.
 

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Discussion Starter #11
The side of the shim contacting the rail-stock is left flat. The other side can be given a wee taper toward the bottom.
The taper is really not necessary due to such a very tiny angle at that point. The shim forces the cam end of the limbs
to rise reducing the downward pressure of the bow string.

Feel free to ask questions, that is how a person learns.

All the best.
The side of the shim contacting the rail-stock is left flat. The other side can be given a wee taper toward the bottom.
The taper is really not necessary due to such a very tiny angle at that point. The shim forces the cam end of the limbs
to rise reducing the downward pressure of the bow string.

Feel free to ask questions, that is how a person learns.

All the best.
Do you know of any videos I can watch to do it so I know I am doing it correctly?
 

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I guess I'd have to see it. Lowering the riser will lower the limbs. Right? How will that reduce string pressure on the rail?
You are not lowering the riser as the riser bolt prevents this. One is just tipping the riser by opening up the top area with
the shims.
As a trial to see what shim thickness is required, using my Barnett Ghost 410 as an example, I cut 1/4" wide by 1 1/4"
long strips from hard card board (a cereal box, shoe box etc. will work). Keep adding these strips until the string is just
lightly touching to a credit card thickness gap when the crossbow is in the uncocked position. Take the total thickness
and this will put one very close to the thickness required for the shims.

Note: one is not cocking the crossbow with the card board shims installed.

All the best.
Take care.
 

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Discussion Starter #13
You are not lowering the riser as the riser bolt prevents this. One is just tipping the riser by opening up the top area with
the shims.
As a trial to see what shim thickness is required, using my Barnett Ghost 410 as an example, I cut 1/4" wide by 1 1/4"
long strips from hard card board (a cereal box, shoe box etc. will work). Keep adding these strips until the string is just
lightly touching to a credit card thickness gap when the crossbow is in the uncocked position. Take the total thickness
and this will put one very close to the thickness required for the shims.

Note: one is not cocking the crossbow with the card board shims installed.

All the best.
Take care.
So my crossbow string is actually pretty fine when it’s uncocked but for some reason when it’s cocked that’s when it has downward pressure. Is this due to an angle from the cams and would the shims still fix this.
 

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Do you know of any videos I can watch to do it so I know I am doing it correctly?
I am not aware of any videos, as I was the one that started the shimming modification.
"Tomontherun" is another member that followed my suggestion. If you viewed the pictures,
it was what he did to his crossbow.
All the best.
 

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Discussion Starter #15
I am not aware of any videos, as I was the one that started the shimming modification.
"Tomontherun" is another member that followed my suggestion. If you viewed the pictures,
it was what he did to his crossbow.
All the best.
Ok I now understand it after reading everything twice. I was thinking that the shim covered the entire gap not just the top. This being said when you tighten it back together will the bottom part that isn’t shimmed automatically touch leaving the top separated making an angle.
 

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So my crossbow string is actually pretty fine when it’s uncocked but for some reason when it’s cocked that’s when it has downward pressure. Is this due to an angle from the cams and would the shims still fix this.
The shims will raise the end of the limbs where the cams are reducing the downward pressure,
there by greatly extending serving life in the arrow-nock area. To further increase serving life,
I added a layer of BCY .030 serving over the top of the original factory bow string serving. I had
to add a bit more clearance to do this. BCY is a very high quality serving material.
As of last night, a Rocky Mountain 405 crossbow I have done this modification on has 970 shots
on the original string and cable system.
Ok I now understand it after reading everything twice. I was thinking that the shim covered the entire gap not just the top. This being said when you tighten it back together will the bottom part that isn’t shimmed automatically touch leaving the top separated making an angle.
Yes.
 

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Discussion Starter #17
187219
From this picture the rail and riser are on the shims straight and if the shims are also straight how does this create an angle?
 

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Discussion Starter #19
You are not lowering the riser as the riser bolt prevents this. One is just tipping the riser by opening up the top area with
the shims.
As a trial to see what shim thickness is required, using my Barnett Ghost 410 as an example, I cut 1/4" wide by 1 1/4"
long strips from hard card board (a cereal box, shoe box etc. will work). Keep adding these strips until the string is just
lightly touching to a credit card thickness gap when the crossbow is in the uncocked position. Take the total thickness
and this will put one very close to the thickness required for the shims.

Note: one is not cocking the crossbow with the card board shims installed.

All the best.
Take care.
Check out the picture and question I sent below.
 
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