Sunday, December 20, 2009

How low can the fork be above your hand?

Several people have expressed concern about hitting your hand if the fork is too low.

I now use just 2.5 cm fork height above the hand in my "W" designs. This results to extremely low strain on the wrist, making the slingshot much more controllable.

I tested the setup with lots of super slowmotion stuff (1200 fps recording speed). The ball does not even come close to hit my hand.

But in order to confirm the field tests, I have started to calculate the setup.

It comes down to the research of Sir Isaac Newton.

He came up with so-called gravity laws.

The earth acceleration is 9,81 m/sec*sec.

Let us assume you have a draw length of 1,20 m (that is quite long). And let us further assume that the ball (15mm diameter, .60") flies with 65 m/sec at the end of the acceleration (V0 = 65 m/s, at the fork). That would mean that the average speed is 32,5 m/sec between pouch and fork.

In this case the ball would need 0,037 seconds (37 milliseconds) to reach your hand.

In this time frame, it would drop 0.5*981 cm * 0,037 * 0,037 = 0,67 cm (under 7mm).

The 15 mm ball passes the fork with its lower rim 1,05 cm over the top of the grip (= your hand).

The shot would have to fly as slow as 40 m/sec (at the end of the acceleration, so V0=40m/s) to make the ball just graze the tip of the grip. That would be a very weak shot.

This calculation does not even assume that the pouch moves towards the fork tips and counterbalances the gravity (a slight upwards vector is applied to the ball). So in reality, the drop is even less substantial.

But of course it would be easy to add another safety margin by making the fork ends higher. It would spoil the effectiveness of the design, though. And then of course the ball may still fall out of your pouch and then hit your foot. Ouch!

So I think it is fair to say that my design is well thought through, supported by the laws of physics and proven by field tests using professional equipment.

Bill Herriman pointed out to me that there is enhanced danger if you do wing shooting (= trying to hit a flying target). That is certainly true, as you may move the slingshot upwards during the shot. This would ruin the safety margin and I do not think that my "W" would be a safe design for wing shooting.


  1. I'm thankfull that in Canada, we're allowed wrist straps, so theres almost no torque on the wrist at all no matter what bands we're shooting with.

  2. Low wrist torque is very good for target shooting. You can shoot more and not get tired. Great designs from Joerg!