Crossbow Building Wiki


Measuring stretch on the bow's back and compression on the belly helps avoid bow breakage due to overdrawing, as well as to get as much performance out of a bow as possible without compromising safety. The technique described here measures the average stress of a limb or the entire bow. Therefore, if there are any weak spots, the bow will fail at smaller amount of stretch or compression than a properly tillered bow would. This method has also been described briefly by Richard Middleton (2007: 43-44).

With this method it's possible to measure the percentage of stretch (or compression) with +-10% accuracy. So, for example, if real stretch is 0,5%, the measurements would vary between 0,45% and 0,55%. This kind of accuracy requires taking 3-4 separate measurements and counting the average. If some of the results deviate noticeably (e.g. 30%) from the average, they are probably flawed.

According to Baker (2000d: 107) wood can compress about 1% before failing. Middleton (2007: 43-44) claims that yew in general can stretch 1% safely, and that some pieces can take almost 2% stretch without failing. This method is also suitable for determining the maximum stretch and compression values for any given material.

Process step by step[]

Start by making a small, but distinctive mark near the nock:

Measuring bow stretch and compression - 01

Next take a long strip of thick paper and tape it as close to the center of the bow as possible. Make sure the top edge of the paper is parallel to the top edge of the bow:

Measuring bow stretch and compression - 02

Next cock the bow and pull the strip of paper so that it tightens and follows the curvature of the bow precisely. Take care not to stretch the paper too much, or the results will be flawed. Make a mark to the paper at the place where it meets the mark on the bow:

Measuring bow stretch and compression - 03

Now uncock the bow and remove the bowstring, then repeat the above procedure:

Measuring bow stretch and compression - 04

Once both measurements (cocked and at rest) have been taken, place the paper on a hard surface (unlike here) and put a steel ruler on top of it:

Measuring bow stretch and compression - 05

Carefully make a mark near both earlier measurements:

Measuring bow stretch and compression - 06

The idea is that we use a pair of vernier calipers for measuring the last section with precision:

Measuring bow stretch and compression - 09

Push the pointy edge of the calipers firmly against the mark you made using the steel ruler. This will make a small hole in the paper, which ensures the calipers don't move sideways when you take the measurements. First take the shorter measurement:

Measuring bow stretch and compression - 07

Then move on to the longer one:

Measuring bow stretch and compression - 08

Finally, add these measurements to the base measurement you took with the steel ruler. Then simply count the amount of stretch or compression:

B = Base measurement (taken with the steel ruler)
S = Shorter measurement (taken with calipers)
L = Longer measurement (taken with calipers

Stretch (%)     = ((B+L)/(B+S) - 1.0) * 100
Compression (%) = 100 - ((B+S)/(B+L) * 100)