Bowmen of Skelmersdale

Changing Bow Balance Using Distance

What the above principle means for controlling bow balance is that there is an alternative to adding weight. It may be possible to get the same effect by moving the weights already in place. For example, to reduce forward roll without changing overall weight, fit a shorter long rod with the same weight or place v-bar weights on longer rods. The table below summarises the options for changing the balance of the bow.

Dynamic” balance

There is one other point to make about the bow s balance. During the shot, the biggest force acting on the bow is the forward force of the bow hand (about five to ten times the effect of gravity downward). During this time, the balance of the bow affects the way the bow kicks . Briefly, if the centre of gravity is high, the bow will end to kick upward, and vice versa.

This has implications for adding top and bottom stabilisers. Normally, we ask a bow to move straight forward during the shot. A heavy TOP stabiliser will actually tend to make the bow kick UPWARDS initially - even though, after the shot, gravity takes over again and the bow starts to roll forwards once more.

Exercise 1: Set up your bow without stabilisers. Fit a single stabiliser in the bottom limb bush. Holding the bow vertical and lightly at the throat, move the bow sharply forward. The bow will initially tilt away from the vertical. Which way did the bow tip?

Now, move the stabiliser to the top limb bush and repeat the experiment. Did the bow tip the same way?

Exercise 2:Hold the bow pointing vertically upwards, with your hand as nearly as possible in the normal position on the grip. Is the bow balanced on your hand, or does it tend to tilt forward or backward? From this experiment, which way will the bow tend to kick during the shot?

Controlling Torque (rotation)

Weights and distances

Rather in the way that more mass is harder to move in a straight line, mass in a rotating system makes the system harder to rotate. But how much harder depends very strongly on where the weight is. The section on balance talked about moment .

When talking about rotation, the right quantity is called moment of inertia. The moment of inertia of a small weight rotated round an axis some distance away (like a long rod weight swung in the hand) is the mass multiplied by the square of the distance. This boils down to another simple rule of thumb:

“A QUARTER of the weight at TWICE of the distance has the same effect on rotation”

So if you want to cut rotation a lot with minimum weight, use small weights on long rods!

Another consequence of the dependence on distance is that a long, narrow object (like a bow riser) has a rather small moment of inertia about its longest axis and small forces will cause large rotations about that axis.

Exercise: Take an unstabilised bow and, holding by the grip, rotate the bow quickly back and forth. Now fit a light long rod and repeat. What was the difference in the bow s rotation? If possible, compare with the same bow fitted with twin stabilisers and V-bar only.

What bow behaviour needs controlling?

Bow  movement

Timing - When is movement important?

Time and Motion

Controlling bow displacement

Weight, Mass and Inertia

Static and Dynamic properties

The misnamed TFC

Controlling bow balance

Centre of Gravity

Finding the centre of gravity

Changing the balance using weight

Weights and Distances

Rods, weights and risers

Controlling bow vibration

Causes of Bow Vibration

Reducing Vibration - Damping

Bow Resonance

Tuned damping - more on TFC’s