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How to Adjust the Draw Length on a Vintage Compound Bow

By Charles Moffat - September 2017.

Vintage Cams Vs Modern Cams

Since the compound bow was invented/patented in 1966, the term "vintage compound bow" loosely describes any compound bow that is not a "modern compound bow". However the line between Vintage and Modern is a bit vague and always moving, as time keeps hurtling forwards the definition of what is considered Vintage and what is considered Modern also keeps changing. For the time being however let us consider any bow built between 1970 and 1995 and any bow that uses older cam designs to be Vintage (this means it could be a more recently built bow that uses an older cam design), and any bow built during the last "while" with irregular designed cams that use pins to be Modern.

Below is an example of a Modern irregular cam design. It uses holes with locking bolts to adjust draw length. By irregular what we mean is that the cam is not round, but often is more oval, egg shaped, or sometimes might even look like the Millennium Falcon (like the one below).


Below is another example of a Modern cam which uses locking bolts. To adjust the draw length you lower the poundage of the bow first, then remove the bolt, move it over to desired hole (which is when a manual becomes handy), screw the bolt back in - and then raise the poundage back up to the desired amount. (Changing the bolt's location typically also effects the poundage, so it is best to set the draw length FIRST, then adjust the poundage.)


In this third example you will see bolt holes for both adjusting the draw length and also the draw weight. Definitely one where you would want the owners manual.


Modern cams in all sorts of weird shapes and sizes. Hence why they are so irregular.

In the event that lost your owner's manual and cannot find a copy of it online, here is a rough guide to adjusting it:

  1. Lower the poundage first to the lowest possible setting. This will make it easier to work with.
  2. If it is impossible to lower the poundage on your particular model, then you might need a bow press or to lock the cams in position so that you can relax the tension on the cables.
  3. Mark several chalk lines or marker lines on your cables to keep track of their original positions.
  4. When adjusting the bolts keep track of where the cables are being re-positioned to and whether the lines are getting closer to or further away from the cams.
  5. Test to see if the draw length is now the desired amount. This may require some Trial and Error because you are essentially guessing how much you think you need to adjust it to the desired amount.
  6. Raise the poundage back to its desired amount. Test again to make certain your draw length and draw weight are the desired amount. If not, repeat steps 1 to 6.

Vintage Cam Designs

There is not a lot of variety when it comes to vintage compound bow cam design. Most of them will look almost exactly the same, and within their simplicity lies a simple logic: Less equals More.

A shorter draw length equals more cable on the cam.
Simpler cam design equals more ability to adjust it yourself.
Less gadgetry equals more robust quality.

That last one is basically a reference to the concept that the more complicated you make something, the more easily you can break it. eg. Imagine if you lost the locking bolt for a modern cam. The entire bow could be rendered useless because you are missing that one little part. Furthermore, modern cams are typically made of lightweight alloys - which are more susceptible to damage due to falls, becoming bent, breaking, etc.

A so-called Vintage cam design however is more round and looks like the one shown on the right here, which unfortunately is not adjustable. This sadly is the case with some older compound bows, they have zero draw length adjustability.

Before proceeding to the steps below you first need to:
  1. Lower the poundage.
  2. Lock the cams or place in a bow press.
  3. Your end goal is to provide more slack to the cables so that you can more easily adjust its position.
 So there are basically only two types of Vintage cams.
  1. Those with Adjustable Draw Length.
  2. Those that cannot be adjusted at all.

Of the former, the most common design is for there to be 3 different draw length settings.

  • High - For shorter draw length.
  • Medium - For a regular draw length.
  • Low - For a longer draw length.
To adjust from one to the next you simply move the cable from one "canal" to the next "canal". If your adjustment leaves more cable on the cam, you have shortened the draw length. If your adjustment places less cable on the cam, you now have more draw length.

Less Cable equals More Draw Length
More Cable equals Less Draw Length

The three images below show what happens when you move the cable from one canal to the next.

Shorter Draw Length

Medium / Regular Draw Length

Longer Draw Length

Then just replace the cable on the cams in the correct position, reattach the bowstring to the cable Teardrops, readjust the draw weight/poundage back to its original desired point, and finally either unlock the cams or remove from the bow press (whichever method you used). Then test to make sure the draw length and draw weight is at the desired point. If you made a mistake you may need to go back and repeat the whole process.

Conclusions

This process is actually fairly simple. So easy that if necessary you could adjust the bow in the field with almost no tools (eg. possibly just some Allen keys).

If you have questions about adjusting an unusual style of cam (whether vintage or modern), send me an email via cardiotrek{atsymbol}gmail.com and I will be happy to write another blog post on the topic of adjusting your unusual style of cam. Please include photos of the cam so I can see how it is designed.

If it is just a simple question, leave a comment below.

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