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Wippe is an simple wooden lever that can be used to cock a crossbow of moderate strength. It is basically a much simplified version of the goat's foot lever. Unlike goat's foot lever which pulls the string to the nut, wippe pushes it there. A wippe made of wood is also much easier to manufacture than a goat's foot lever, which can only be made by blacksmith. Cocking a ~300 pound crossbow with a wippe is very smooth and easy with a lever that's only 80cm long. Up to a point lengthening the lever allows even stronger crossbows to be cocked. A wippe-operated crossbow is somewhat slower to cock than a crossbow cocked by hand. However, cocking a bow with a wippe is much faster than with a cranequin or a winch.
While making the wippe parts is easy, designing it so that the lever operates just right with a given crossbow is not. Therefore this article puts much emphasis on wippe's operating principles and the issues that may arise during use.
Parts of a wippeEdit
A wippe consists of four parts:
- Wooden lever
- Steel axle
- Wooden part pivoted to the wooden lever using the steel axle
- Metal hook
In addition the front end of the crossbow has to have a metal loop, around which the lever's metal hook rotates. The core parts of the wippe are shown in the below pictures:
Although the wippe looks deceivingly simple, there are several caveats you may run into while making one. The diagram below shows wippe operation in three distinct phases and illustrates some of these key design principles:
The parts marked with letters are:
- (A) rotating part of the wippe which pushes the bowstring towards the lock
- (B) lower part of the handle
- (C) upper part of the handle
- (D) steel loop around which wippe's hook rotates, attached to the stock using bow irons
- (E) frontmost part of the upper stock
When designing a wippe, there are a number of potential caveats that need to be avoided:
- If the distance between D and E is not sufficiently long, the top of the stock prevents the wippe from moving through it's entire range of motion.
- If the angle between the stock and A is too obtuse, the wippe will be unable to push the bowstring back towards the lock; rather, it just pushes the it against the stock. This can be avoided by keeping the maximum angle clearly below 45 degrees.
- The wippe must be able to push the bowstring past the nut. This allows the bowstring to automatically rotate the nut back to shooting position. If the trigger is spring-loaded or correctly balanced, it will also rotate and lock the nut in place automatically, making cocking the crossbow smooth and easy.
Besides these potential caveats, correctly selecting the length of A and B can be tricky. The problem is that there's no simple formula for determining the dimensions of these parts. Probably the easiest way to find out the correct dimensions is to test different lengths using a CAD program or even cardboard templates. The one thing you definitely want to do is maximize the advantage in leverage the wippe provides. There are two factors you can adjust to do this:
- Make B as short as possible for the chosen draw length.
- Maximize the rotation angle of the wippe handle (BC). Note that if the handle uses an open-ended hook as a pivot point, going way past 90 degrees may not be possible.
Both of these optimizations are intended to allow making C as short as possible, which in turn makes the wippe as light and as manageable as possible.