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Wheel Timing

By Perry Ratcliff 1999

Good wheel timing is important to consistent shooting.  Have you ever pulled back a cam bow expecting to find a hard wall and discovered that the bow felt mushy at full draw?  This problem is usually associated with wheel timing.  Its not uncommon to receive a brand new bow with wheels that are out of time.  It is up to you to properly adjust the wheel timing and tiller of your bow.   Poor wheel timing can also make it difficult to stay on the spot when aiming.

Quick & Dirty Wheel Timing

You can do a fair job of wheel timing with no special tools as follows:

Slowly draw the bow back while watching the bus cables and cams.
When you pull through the valley into the wall, one bus cable will come to rest on the back side of the cam before the other cable.
If the Back Side of the Top Cam touches the bus cable first, you will need to shorten the bus cable connected to the bottom limb by adding twists to the bus cable.
If the Back Side of the Bottom Cam touches the bus cable first, you will need to shorten the bus cable connected to the top limb by adding twists to the bus cable.
Do not adjust cable lengths by more than one or two turns at a time.  You will be surprised at how quickly the wheel timing will change.
When you have the wheel timing close, make your adjustments by twisting the bus cable in 1/2 turn increments.
Note that shortening bus cables will slightly increase the draw length of the bow.   If the draw length is getting to long, adjust wheel timing by untwisting the opposite bus cable described above.
It was necessary to shorten the bus cable connected to the bottom limb by 5 1/2 turns to bring these wheels into alignment.
Note how the bus cable is in contact with the back side of the top cam in the photo below. Note how the bus cable is well away from the back side of the bottom cam in the photo below.
Top Wheel.jpg (16271 bytes)

Figure 1

Bottom Wheel.jpg (15886 bytes)

Figure 2

 

Precision Wheel Timing

The following technique provides detailed instructions for obtaining perfect wheel timing on two-wheel compounds.  You will require the following tools to perform these adjustments.

Bow Square
Narrow Straight Edge
Ruler or Calipers
Bow Jig, to pull bow back for measurements.
Bow Press, to take tension off of bus cables for adjustment.

Adjust wheel timing as follows:

Remove cable guard slider from bus cables.
Place Bow on Bow Jig and draw the bow back until one of the bus cables comes in contact with the back side of a cam (see Figure 1 above).  Note, it is important that you draw the bow back pulling from the same location as you would if you were shooting the bow.  You may need to draw with an arrow on the string to draw from the correct position on your serving.  I tie nocking points above and below the nock which allows me to time the wheels with no arrow on the string.
Place a bow square on the bus cable as it exits the cam as shown in Figure 3.  Use a pair of calipers or ruler to meaure the distance from the axle to the bow square.   It is very important that the bow square be placed the same distance from the top axle as the bottom axle when making your timing measurments.  As shown in the figure, I chose a distance of 1.5" away from the axle as my reference point. 

Caliper.jpg (18516 bytes)

Figure 3

Next lay a straight edge across two fixed reference points on your cam as shown in Figure 4.  These reference points will be used to determine your wheel timing.   When wheel timing is properly adjusted, the angle represented by these two points will be the same for the top and bottom cams on your bow.

TwoPoint.jpg (20783 bytes)

Figure 4

You are now ready to measure your wheel timing.  Record where the straight edge crosses your bow square.  The initial measurement for the top wheel is 8 3/8" on this bow, as shown in Figure 5.  You may find that the straight edge does not cross the bow square at a measurable location the first time you attempt to take a meaurement.  If this is the case, move the bow square closer or further away from the axle as required to get a good reading.  Make certain that you place the bow square the same distance from the axle for the bottom wheel.  If your wheels are significantly out of time, you may find that the straight edge never crosses the bow square on the second cam.   Adjust the appropriate bus cable in two turn increments until you can obtain measurements for both cams.

Top1.jpg (14671 bytes)

Figure 5, Top Wheel

The measurement for the bottom wheel is 10.5" as shown in Figure 6.  This is nearly a 2" difference in wheel timing when measured by this method.  Small changes in wheel timing show up as large differences in measured timing using this method.  This allows you to make very precise adjustments to wheel timing.

Bottom1.jpg (13119 bytes)

Figure 6, Bottom Wheel

Given these measurements I chose to shorten the bus cable connected to my bottom limb by two turns.  When I rechecked the wheel timing, the top wheel measured 7 1/4" and the bottom wheel measured 7 3/4" for a change of nearly 1 1/2" in wheel timing for two twists of the bus cable (see Figures 7 & 8 below).

Top2.jpg (12156 bytes)

Figure 7, Top Wheel

Bottom2.jpg (11829 bytes)

Figure 8, Bottom Wheel

Given these measurements I chose to lengthen the bus cable connected to my top limb by 1/2 turn.  When I rechecked the wheel timing, both the top and bottom wheels measured 7 1/4" as shown in Figures 9 & 10 below.
Top3.jpg (13077 bytes)

Figure 9, Top Wheel

Bottom3.jpg (12258 bytes)

Figure 10, Bottom Wheel

When you have completed wheel timing using this method, you should find that the bow has a more solid feel against the wall and is easier to aim. 
I would like to thank Don Kudlacek and Roland Haggard for developing this technique and passing their knowledge on to me.  Don is a current IFAA World Champion and Roland is a former IFAA World Champion.  You can't argue with success!   I hope you find it as valuable as I have. 

Good Shooting!