How To Check and Set Wheel Timing

Cam timing is critically important issue on aggressive two cam bows such as this hatchet cam model from High Country. (FYI: the camo is Bushlan Brown). How To Check and Set Wheel Timing Use This Step-by-Step Guide and Your Two-Cam Bow will be Perfectly Timed and Ready to Tune. By Bill Winke

I spent several hours leaned over the bow, meticulously setting each adjustment. The rest perfectly centered the arrow, right where it had to be. The nock point was an eighth of an inch above the center of the plunger, exactly where I like it. The nocks of my arrows were rotated so that the fletchings would easily clear the shoot-through rest. Everything was tightened and in place. The bow was ready. "Fast Flight harness systems can lengthen from exposure to heat, high levels of moisture, and from normal stretch associated with breaking-in the bow." The shot through paper should have been a formality. The bow was as good as tuned, I thought. But it wasn't! Three hours later I was still scowling at a tear that I had never seen from one of my bows. I noticed a hammer in the corner; I leered hatefully back toward my bow. No, that would be too easy. The nock of the arrow was passing through the paper fully 5 inches below the point. It was shocking! I could have gotten better flight by throwing the arrow. I moved the nock point more than a half-inch above what I knew was the right position. The arrow was pointing downward at a disgusting angle. Still the ugly low tear. Then it struck me; what if the wheel timing is off? It was off all right - way off. The bow had felt jerky near the completion of the draw, an obvious indication of gross mistiming, but I was inexperienced and the symptoms never translated. Six twists to one of the Fast Flight harnesses brought the bow into perfect time, subsequently producing a tiny shaft sized hole in the paper. My spirits were instantly lifted and I placed the hammer back in the corner . Wheel timing is the one aspect of bow tuning that even many advanced bow mechanics overlook. It is the most insidious deterrent to quick success when tuning any two-cam bow. Even fletching interference, which accounts for most "untunable" bows, is more easily understood. If you can't seem to get rid of a maddening and persistent vertical tear, this feature may offer a few answers.

Understanding Cam Timing

A bow may become mistimed several different ways. We'll look at some of these, but first we need to understand the root of the synch problem. Dissimilar rollover will always boil down to one of the harnesses or cables being longer than the other. The lengths of the harnesses control the position of one wheel relative to the other. Let's look at how this works.

First off, take note of where your harnesses or cables are attached. One end is solidly grounded to the axle, whether directly, or through some sort of yoke system. The other end is attached to the opposite wheel, wrapped partially onto the harness track. When the bow is drawn the harness is rolled onto the same track which in turn draws the other limb toward it. As long as both are working together the bow is in time.

Assume one harness suddenly stretches a quarter of an inch while the other remains the same length. The wheel attached to the longer harness has a little freedom to rotate. Now the cams are in different positions and the bow is out of time. The wheel attached to the longer harness is "ahead" of the other. When the bow is drawn, this wheel will get to its full draw position first – thus the term “ahead”. Make sure you understand that we are talking about the harnesses, not the string. Changes in string length have no affect on wheel timing.

Fast Flight harness systems can lengthen from exposure to heat, high levels of moisture, and from normal stretch associated with breaking-in the bow. Keep your harnesses well waxed to reduce the stretching caused by moisture. I like to leave my new Fast Flight equipped bows in a vehicle during a hot day to accelerate heat induced stretching. Get it out the way in one fell swoop. Once the wax has cooked out and the fibers have stretched, the harnesses change little when exposed to a lesser degree of heat. They are more or less preset.

A break-in period is also required . Usually between 200 and 300 shots will set any synthetic, especially if heat and moisture are also encountered. Allow your bow that many arrows before assuming it is ready to be tuned for the long haul.

In the March issue of Edersbow I went into some detail on the characteristics and applications of several of the modern synthetics. Some offer advantages over Fast Flight when used on the harnesses. For a more detailed analysis of your options, take a look in the magazine archives (go back to the Home Page) for the March issue and look up the article on strings.

Affects of Harness Stretch Most bows require some type of bow press to relax the tension on the strings and harnesses before you can reset timing. Shown here is the . Optional split-limb adapters are available for the Bowmaster as well, . photo by bill winke Harness stretch affects various wheel styles differently. The more aggressive the cam the greater the affect . Obviously, the worse the mistiming the greater the affect on arrow flight, as well. In my opening example I was tuning a hard cam bow. It took only six twists to one of the Fast Flight harnesses to eliminate a 5-inch tear! The steeper the drop in poundage from maximum down to letoff, the more sensitive the bow is to timing problems. Upon release of a mistimed bow, one wheel will rotate out of its letoff position a fraction of a second before the other. This sets up a force imbalance through the string that results in the nock point shifting position away from the wheel that is "ahead". This instantaneous occurrence brings both wheels into balance for the forward travel through the draw cycle. "Steel cable systems can also be of unequal length."  Nock position diagrams show that the majority of the destabilizing energy is applied when the arrow is just starting forward – as it comes out of letoff. If you get a chance to watch Easton’s slow motion arrow flight video you’ll see this affect and how it turns the arrow into spaghetti. Soft cams and round wheel bows are least affected by discrepancies in timing because they simply don't have the radical lever ratios (and abrupt draw weight transitions) of the hard cams. Small amounts of mistiming in these bows will result in less nock point disturbance. The major premise of the single cam bows that are gaining market share is the elimination of wheel timing problems. With only a single cam, these bows will never go out of synch. Steel cable systems can also be of unequal length. They will stretch slightly, as well, but generally their differences will be a result of mediocre quality control on the part of the manufacturer. Just because you have steel, don't assume your timing is perfect.

How To Check Timing

Before you can accurately check your timing, you need a precise reference point. You can mark the wheel with a wax pencil, or the like, to indicate a position relative to the limb at brace. If the wheel is a hard cam this method may not be accurate enough. The most accurate reference point I have found is the relationship between each harness and the groove at the end of its track, with the bow at full draw.

Take a look at your harness track. Follow the groove all the way around, from the point where the harness attaches to its end near the axle. This termination – usually a flat portion of the track - represents the back wall of the draw cycle. Now follow the harness around the groove as you draw the bow. You'll see the harness drop in next to the axle right at the end of the draw. The harnesses of both wheels should drop into their grooves at exactly the same time .

Reset Your Timing

The harness or cable attached to the wheel that has a headstart as viewed at its braced position, or reaches the end of draw first (whichever you prefer) must be shortened. Some bows that were produced in the past had micro-adjustable yoke systems to make timing the wheels a snap, without having to relax the tension on the bow. Golden Eagle still uses a similar system, called Xact Timing, which makes it easy to set timing. If your bow is not so equipped, you will need to twist the longer harness (the one attached to the cam that’s “ahead”) in the direction of its spiral to shorten it. Steel cables can be shortened in this manner, as well.

Using a bow press remove the tension from the string and harnesses and twist the appropriate harness. Click photo for bigger image.
photo by rob eder

To get at the end of the harness, where it attaches to the cam, you will need some means by which to relax the tension of the bow. Most pro shops have bow presses that can be used for this task. There are also several mini presses available to do the same job. They are my choice for convenience, and the best I’ve used so far is the Bowmaster.

Simply remove the harness from its attachment point on the wheel that is "ahead" and give it a couple of twists to shorten it before replacing. Check the timing again. Repeat the procedure until the wheels are precisely timed .

Timing affects arrow flight in a very dramatic fashion – causing tail-high or tail-low tears through paper. Never assume that your timing is acceptable without checking it often, especially if you own an aggressive two-cam bow. If wheel synch should change even slightly during the hunting season your accuracy with broadhead arrows will be destroyed. One of the final steps in becoming proficient with the mechanical aspects of your equipment is to understand, monitor, and correct your wheel timing.

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