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Do untuned cams will affect precision?

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I have a Center Point crossbow whose string and cables were recently changed. When the crossbow is uncocked I see the cams synchronized, but when I cocked I see then not tuned. And my bow is not precise. Tell me: Do untuned cams will affect precision? Or maybe my problem are the arrows.
 

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Providing the cams were timed when the new string and cables were installed:
1)the string and or string and cables may have stretched a bit.
2)if you are using a rope cocker, sometimes when one reaches the nearly cocked position, they may pull a bit harder on one side. This will off center the bow string and one cam will be advanced a bit more. Shooting this way the arrow will not hit the aiming point and the farther the shot, the farther off it will be.
In the uncocked position, mark the bowstring on each side of the rail with white out or mistake out. When the crossbow is cocked, these marks should be equidistant from the center of the rail.

Properly tuned, the cams should be in time in the uncocked position and in the cocked position. Some have mentioned if they can not do this they set the cams so that they have a happy medium when both in the uncocked and cocked position.

All the best.
 

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They should be the same both cocked and uncocked. If they cannot be after string adjustments, then the limb(s) are unequally strong.
And there lies the problem.

All my crossbows have the cams in sync ( I believe they can be in the “range of in time" and still may not be in sync) when uncocked. With the variables in limbs, I would think it would be tough to have perfectly matched limb deflections from crossbow manufactures - especially split limb models.

So if the cams can’t be in sync at both uncocked and cocked positions - what’s the better choice? I would prefer uncocked as they would be in sync as the nock end leaves the string with no side torque.

Even if they're out of sync slightly when cocked - they get better the closer the nock gets to the end of the rail during the shot. :confused:
.
 

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And there lies the problem.

All my crossbows have the cams in sync ( I believe they can be in the “range of in time" and still may not be in sync) when uncocked. With the variables in limbs, I would think it would be tough to have perfectly matched limb deflections from crossbow manufactures - especially split limb models.

So if the cams can’t be in sync at both uncocked and cocked positions - what’s the better choice? I would prefer uncocked as they would be in sync as the nock end leaves the string with no side torque.

Even if they're out of sync slightly when cocked - they get better the closer the nock gets to the end of the rail during the shot. :confused:
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I have two split limb RD bows right now that are in sync in both positions. I don't think there is a "choice". Shows uneven force on limbs which effects arrow flight. But my reasoning says it's better to be in sync at the full draw position. Even push at the outset.
 

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I have two split limb RD bows right now that are in sync in both positions.
Are they “exactly identical” in both positions or just close enough?
.
 

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Are they “exactly identical” in both positions or just close enough?
.
Exactly. I check them all the time. I had a problem with erratic accuracy on one of my bows last year. Every time I looked at my cams in the relaxed position, they were in sync. I took the bow to a shop. They tried to twist and adjust the string and cables but no matter what, the limbs at full cock position were out of sync. I had the limbs replaced under warranty.
 
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Exactly. I check them all the time. I had a problem with erratic accuracy on one of my bows last year. Every time I looked at my cams in the relaxed position, they were in sync. I took the bow to a shop. They tried to twist and adjust the string and cables but no matter what, the limbs at full cock position were out of sync. I had the limbs replaced under warranty.
A note on Cam Timing position. In many cases if the cams on a crossbow are in perfect alignment when the crossbow is in the non-cocked position the cams would remain in perfect timing with one another as long as the string was drawn equally on both limbs to the full draw position. If at full draw the cams are out of time it could be a limb problem, but more commonly it's an indication the string is not anchored in its center position. This is very common with hand drawn crossbows since there's no center of the serving markings to show the shooter exactly where he should be anchoring his string each time he draws it back.

That said, I've witnessed and measured a certain degree of uneven drawback even on my ScorpyD crossbow with the Ten Point Accudraw Crank mechanism on it. A visual center serving mark will help and make it very simple to identify this problem.

Let's remember that Timing on a camed crossbow or even a vertical bow is based upon both limbs working equally and in unison. Anything that throws this process of is going to cause varying degree's of inconsistency. Replacing strings and/or cables is not a solution to solve this problem. It's defective limbs, cams out of time or off center anchoring of the string.

Jon Henry
 

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A note on Cam Timing position. In many cases if the cams on a crossbow are in perfect alignment when the crossbow is in the non-cocked position the cams would remain in perfect timing with one another as long as the string was drawn equally on both limbs to the full draw position. If at full draw the cams are out of time it could be a limb problem, but more commonly it's an indication the string is not anchored in its center position.

Jon Henry
I think some people use the terms “in time” and “in sync”as the same thing but I view them as being two different specs.

I never could get my Ventilator cams “in sync” when cocked - or the same distance from the axles to the rail, and this was with solid limbs. My Mission Dagger had split limbs - I believe getting exact matching deflection for 4 limbs would be a difficult task - I don’t believe manf have an inventory of every single limb and match them accordingly when assembling crossbows. In each case it was never more than a fat 1/16” for either.

I have a mark in the center of all my cocking ropes so that I can adjust the handles to put that mark in the center of the cocking groove before pulling. Doing that procedure I don’t think there is that much difference with a sled or hooks, and would seem to make for an even pull.

I’ve often thought the cranks could actually make "uneven pulls” due to how the cork wound individually on the crank’s pulleys. I thought my Mission RSD was a great cocker but I had the covers off and could see the two pulleys could wind differently and one side could pull differently depending how the coils stacked up on the spools. If these cockers had a line winder like a casting reel, the coils would wrap more evenly on the arbors. I’m just not sure how well they do as is.

Awhile back on CT, someone said the sled/hooks will always pull at the center of the string. In the last pic I set up the RSD cocker and before cocking, I pulled on the left cable to move the center mark to the right in the cocking groove. When the string set the latch, the black mark was still at the right side of the cocking groove so that meant there was more string that wound onto the right spool than the left one.

If I could do that deliberately, then I would think it could be done accidentally. I still believe it’s more important to have the cams “in sync” when uncocked as that is when the nock end of the shaft leaves the rail - (in time is just a range and stack up of tolerances to me ).

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I think some people use the terms “in time” and “in sync”as the same thing but I view them as being two different specs.

I never could get my Ventilator cams “in sync” when cocked - or the same distance from the axles to the rail, and this was with solid limbs. My Mission Dagger had split limbs - I believe getting exact matching deflection for 4 limbs would be a difficult task - I don’t believe manf have an inventory of every single limb and match them accordingly when assembling crossbows. In each case it was never more than a fat 1/16” for either.

I have a mark in the center of all my cocking ropes so that I can adjust the handles to put that mark in the center of the cocking groove before pulling. Doing that procedure I don’t think there is that much difference with a sled or hooks, and would seem to make for an even pull.

I’ve often thought the cranks could actually make "uneven pulls” due to how the cork wound individually on the crank’s pulleys. I thought my Mission RSD was a great cocker but I had the covers off and could see the two pulleys could wind differently and one side could pull differently depending how the coils stacked up on the spools. If these cockers had a line winder like a casting reel, the coils would wrap more evenly on the arbors. I’m just not sure how well they do as is.

Awhile back on CT, someone said the sled/hooks will always pull at the center of the string. In the last pic I set up the RSD cocker and before cocking, I pulled on the left cable to move the center mark to the right in the cocking groove. When the string set the latch, the black mark was still at the right side of the cocking groove so that meant there was more string that wound onto the right spool than the left one.

If I could do that deliberately, then I would think it could be done accidentally. I still believe it’s more important to have the cams “in sync” when uncocked as that is when the nock end of the shaft leaves the rail - (in time is just a range and stack up of tolerances to me ).

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Your point is well explained, so thank you for your concise response. Cocking sleds, at least those that I’ve worked with are generally designed to allow string movement as you crank the string back. They do not lock onto a string to prevent the sled from shifting as needed. It’s for this reason I recommend and use a serving mark on my string, so I can visually check where my string is positioned as I place an arrow onto the rail and string.

I also own an older PSE TAC15 with a crank system. PSE bypassed these problems by using a string loop on their crossbows. This ensures perfectly centered latching every time And it also eliminates string serving wear completely. At worst, you would need to change to a new string loop once every 1,200 - 1,500 shots.

musing a string loop on a number of current crossbows would not work without causing major problems with the anti-dry fire or other safety systems, so it’s not a feasible solution in this case.

I agree with the comment that Cam Timing is different than in sync. However Cam Timing at rest is a very specific alignment point that is achieved by losing one cam cable at a time and advancing or retarding the position of that cam by adding or deleting cable twists on that cam cable. By doing so the cam can be moved as little as an 1/8 of an inch at a time, if need be. Using markings on the cam wheels it should be relatively easy to obtain precise alignment of the two cams.

The Timing Of the Cams can be advanced beyond a manufacturers standard number of twists if the owner wishes to experiment with attaining more speed. But if more speed was the goal it’s simpler to move to a lighter arrow and reduce the extra torque on the limbs.

None of this solves the original problem described due to the fact it’s a process of tuning and isolation step to pin point the cause.
 

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I have a Center Point crossbow whose string and cables were recently changed. When the crossbow is uncocked I see the cams synchronized, but when I cocked I see then not tuned. And my bow is not precise. Tell me: Do untuned cams will affect precision? Or maybe my problem are the arrows.
To qualify this question a little better maybe we should should be asking a few questions in order to better understand your issue.
You state "your crossbow is not precise", so to begin with please explain what not precise means to you?
Second, was it ever precise when you shot it or was it suffering similar problems?

You ask if the issue could possibly be the arrows? Are these the same or different arrows from what you were shooting before the string and cables were changed?

The more information yo can help us with the more we're able to analyze and help you identify the problem.

Jon
 

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To qualify this question a little better maybe we should should be asking a few questions in order to better understand your issue.
You state "your crossbow is not precise", so to begin with please explain what not precise means to you?
Second, was it ever precise when you shot it or was it suffering similar problems?

You ask if the issue could possibly be the arrows? Are these the same or different arrows from what you were shooting before the string and cables were changed?

The more information yo can help us with the more we're able to analyze and help you identify the problem.

Jon
In the interest of time I’m going to add a little more information to this thread in hopes it will help other readers understand the crossbow tuning process a little better. And for the record Marty just provided me with some extremely valuable information on strings that help quite a bit.

Each crossbow has a factory ATA Measurement.Knowing this measurement is necessary to tune your crossbow or even just to change your string. It’s the axle to axle measurement as measured from the center of your cam pin to the center of the opposite cam pin.

This distance is achieved thru string twists. Not cable twists, so don’t touch your cables at this point.

You can get pretty close To the correct ATA by simply taking your string length and dividing it in half. So as an example if your crossbow used a 46” string, we’d divide that in half and then apply 23 full twists to a new string. Now mind you we’re talking about FULL TWISTS, not half twists. A full twist is a 360 degree rotation of the string loop.
After completing these twists release the crossbow from the press or whatever you are using to compress your limbs so it can return to normal. Then recheck your ATA measurement to see how close it is to the factory specification. You should repeat this process and either add or remove whole or half twists until you achieve the exact Axle to Axle measurement desired.

Step 2 is Cam Alignment. Cam alignment is achieved by adding or removing Cable Twists one Cam at a time. All cams have a timing mark used by the manufacture of the crossbow to sync the two cams with one another. It’s a good idea to find out where that Timing Mark is and what the exact measurement should be from the inside limb edge to the center of the timing marks.

mince you have this measurement you can take a measurement on each cam From the inside limb edge to the current timing mark position and determine if the timing is dead on or if it needs to be advanced or moved backwards. Once you have this, release the cable tension on only one cam by disconnecting one side of that cable only. The add or remove cable twists (usually not more than 4 or 5, and then reattach that cable. Release the bow press and remeasure the position of that timing mark and compare it to your desired measurement. Continue this process on that cam until you’ve reached the factory recommended measurement from the limb inside edge to the exact center of your cams timing hole.

Once this has been completed perform The exact same process on the opposite cam until both cams have exactly the same alignment and spacing to the inside edge of their respective limbs.

After completion now recheck your ATA measurement. It may require you to add or remove one or two string twists in order to achieve your desired factory ATA again. If not you could be done unless you were looking to advance your crossbows timing. I personally don’t over clock my crossbows, but for those who do, it’s a simple matter of advancing the time equally on both cams to achieve a few extra FPS.
As with all things “over clocking” carries risks along with the rewards. While you gain a little more speed at the same time you’re putting more stress on the limbs. This could lead to limb fracture if you step up the speed to much, so you need to decide if the risk is worth the reward!



BTW - I want to give a big shoutout to Marty for the string twist informations because without his expertise in that area I couldn’t have been as informative in this thread.

Tanks again Marty!
 

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After completion now recheck your ATA measurement. It may require you to add or remove one or two string twists in order to achieve your desired factory ATA again. If not you could be done unless you were looking to advance your crossbows timing. I personally don’t over clock my crossbows, but for those who do, it’s a simple matter of advancing the time equally on both cams to achieve a few extra FPS.

As with all things “over clocking” carries risks along with the rewards. While you gain a little more speed at the same time you’re putting more stress on the limbs. This could lead to limb fracture if you step up the speed to much, so you need to decide if the risk is worth the reward!
Once the cams are in sync, and also in time by twisting/untwisting the cables - adding or removing twists to the string is going to affect the timing.

I start with the string measured to the spec's length, and then twist/untwist the cables to get the timing holes “in sync”. If I need to adjust the timing I’ll add equal amounts of twists/untwists but maintaining in sync being paramount.

If my cams are in sync and an acceptable timing distance, the ATA is going to be whatever it is. Right now my cams are in sync uncocked, the timing holes are a bit hotter at about 1/16” more than the supposedly recommended 3/8”, and the ATA on my DS380 is a bit less than the 17 1/2” recommendation. If I reduce that timing distance back to 3/8” the ATA should get back closer to 17 1/2”.

Last time I checked the axles to the rails were identical uncocked, but I recall a 1/16” difference when cocked (don’t recall which side). Not sure if it’s because one cable has a slightly longer route than the other one or not.

But my main objective is always to have the cams in sync when the string slams home against the stops and the nock end leaves the rail - ymmv.

I’m sure everyone has their priorities for tuning their crossbow, these are mine.
 

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I have two split limb RD bows right now that are in sync in both positions. I don't think there is a "choice". Shows uneven force on limbs which effects arrow flight. But my reasoning says it's better to be in sync at the full draw position. Even push at the outset.
It doesn’t look like Jim Kempf (you do know who he is), Ventilator, and Jerry from Tapp-Nation would agree with that, but it’s your crossbow and you’re entitled to set it up anyway you like. Good luck.


 

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Once the cams are in sync, and also in time by twisting/untwisting the cables - adding or removing twists to the string is going to affect the timing.

I start with the string measured to the spec's length, and then twist/untwist the cables to get the timing holes “in sync”. If I need to adjust the timing I’ll add equal amounts of twists/untwists but maintaining in sync being paramount.

If my cams are in sync and an acceptable timing distance, the ATA is going to be whatever it is. Right now my cams are in sync uncocked, the timing holes are a bit hotter at about 1/16” more than the supposedly recommended 3/8”, and the ATA on my DS380 is a bit less than the 17 1/2” recommendation. If I reduce that timing distance back to 3/8” the ATA should get back closer to 17 1/2”.

Last time I checked the axles to the rails were identical uncocked, but I recall a 1/16” difference when cocked (don’t recall which side). Not sure if it’s because one cable has a slightly longer route than the other one or not.

But my main objective is always to have the cams in sync when the string slams home against the stops and the nock end leaves the rail - ymmv.

I’m sure everyone has their priorities for tuning their crossbow, these are mine.
Hi TP, one of the things that affect the cam variation at full draw is the crossing of the cables through the riser cable clip inside the hollow part on my ScorpyD Orion 150. The crossover point of the cables is not in the dead center of the riser. The crossover point is more on one side than the other as the crossbow is draw or cranked back.
It’s for this reason that the synchronization of the cams at full draw is not usually measured or considered an important factor.
 

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The cables crossed outside and on the right side of my Ventilator as well.

The cables cross in the exact center on my Deathstalker with the newer cable slide design - deep notch and shallow notch. The cable in the shallow notch goes from the right limb puck to the left cam peg and has to be longer than the other cable in order to sync the cams.

But string makers seem to make both cables the same length for the DS (and others) and you have to add twists to one (kinks the heck out of the short unserved section) or take twists out of the other one.

I tried to get a new set of cables that were 1/8” different in lengths but it appeared they were built to the same length and the one shorter one just had the hell twisted out of it - not what I was expecting.

I still have my 2009 PSE Bow Madness bow and it’s cams have adjustments where I can move pegs around to change the draw length and let-off percentage. Seems like having adjustable cams would be a better step towards tuning rather than twisting and kinking really short 17” cables.
 
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The cables crossed outside and on the right side of my Ventilator as well.

The cables cross in the exact center on my Deathstalker with the newer cable slide design - deep notch and shallow notch. The cable in the shallow notch goes from the right limb puck to the left cam peg and has to be longer than the other cable in order to sync the cams.

But string makers seem to make both cables the same length for the DS (and others) and you have to add twists to one (kinks the heck out of the short unserved section) or take twists out of the other one.

I tried to get a new set of cables that were 1/8” different in lengths but it appeared they were built to the same length and the one shorter one just had the hell twisted out of it - not what I was expecting.

I still have my 2009 PSE Bow Madness bow and it’s cams have adjustments where I can move pegs around to change the draw length and let-off percentage. Seems like having adjustable cams would be a better step towards tuning rather than twisting and kinking really short 17” cables.
Boy have we hit on a good topic for a change that I don't think most people are aware of these issues and the problems they can cause. The two things I'd like to add that I learned the hard way are as follows:

Recently, after making ATA (Axle to Axle Adjustments) and re-syncing my cams I decided to go out and test everything. I was interested to shoot at 60 yards to see how much my HHA Speed Dial Setting had changed, if any at all. After two shots I found I was about 6" low, so I moved my HHA Elevation Dial to 65 yards and was dead on. This bothered me because I felt I should now be shoting faster than before the Cam Sync and ATA Changes.

I loaded another arrow and shot a 3rd time. This time my arrow was about 15" left of where I aimed. I new I was on center when I squeezed off the shot, so I was dumbfounded as to what happened. I loaded another arrow, but this time aimed at a dot on the far right side of my target. This shot was the same height as the previous one, and still 15" to the left of where I had aimed it. Now I started carefully checking everything on the crossbow to see what had changed or went wrong. It didn't take long to find that one of the cables had popped loose from the cable crossover clip inside the hollow of the riser. Once I snapped it back into place the windage returned to what it originally had been.

As for the HHA Speed Adjustment change that I previously mentioned I decided to move back into 20 yards after getting my windage adjustment set at 60 yards. The first thing I learned was that my zero had changed significantly. I was 5" to 6" low at 20 yards. I new instantly this was from the Cam and String work I had been doing inside. I made a series of elevation adjustments to my scope to bring my zero back to dead center at 20 yards. I then moved back to 60 yards and found the setting on my HHA Optimizer Speed Dial had reduced back down to a yardage of 57 yards due to the extra speed I picked up on my Cam and String adjustments.

This means changing the yardage tape on my Speed Dial, but more importantly it outlines the importance of checking and adjusting the zero on your scope after making any string or cam adjustments. It's also worth mentioning how much of a windage impact a missing or faulty cable crossover clip can have on ones shooting.

By the time all adjustments were completed and everything was finished, I was back to taking out 3/4" florescent green dots at 60 yards. That's bench rested using a tripod of coarse, but it talks to how effective these crossbows are with weight balanced, spine matched arrows. At some point we all need to change either cables or strings or both, so these things we're talking about should be of interest if people want to keep their crossbows performing well.
 

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I’ve also found that making a cable adjustment to change the timing on one cam will also have some effect on the other cam.

If I add twists to rotate the timing hole away from the limb on my left cam for example, that reduces the ATA. But that reduced ATA affects both limbs and should have some effect (more pull) on the timing of the other cam.

I have measured both timing holes in the past, and then added twists to one cable to move a cam and saw some movement on the other cam.

I do not recall if it was more pronounced when adjusting the longer cable in the shallow slot of the cable slide or the shorter cable in the deep slot, but I think tweaking one versus the other can affect it differently - but I could be wrong.
 

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I’ve also found that making a cable adjustment to change the timing on one cam will also have some effect on the other cam.
If I add twists to rotate the timing hole away from the limb on my left cam for example, that reduces the ATA. But that reduced ATA affects both limbs and should have some effect (more pull) on the timing of the other cam.

I have measured both timing holes in the past, and then added twists to one cable to move a cam and saw some movement on the other cam.

I do not recall if it was more pronounced when adjusting the longer cable in the shallow slot of the cable slide or the shorter cable in the deep slot, but I think tweaking one versus the other can affect it differently - but I could be wrong.
TP,
One of the numerous things I've learned from doing tuning adjustments is any time I make a series of adjustments to string or cams once my adjustments get close I need to shoot the crossbow a few times before making any final adjustments. This is due to exactly what you've outlined.

String or cables bunch up or cams shift position just slightly from tension changes. After a few shots everything settles back into its final position so you can remeasure both ATA and recheck cam synchronization. Make your final small adjustments, shoot again a couple of times and then leave everything alone.

BTW - Don't forget to re-zero your scope at 20 yards when your done.

I'm not totally sure if in all cases I'm picking up a noticeable performance increase in speed, but I do know the tune up improves accuracy consistency when both limbs are working in synconized unison.

Again, I do most of my shooting at longer distances over 50 yards, so deviations are more noticeable.
 
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