Front of Centre
Today we are getting technical. REALLY technical. This is the sort of archery subject that gets our Vice Chair excited and animated. Yup, its that bad but it is quite important so we’ll try to be gentle.
Better brace yourself for the incoming facts tsunami. It might get rough…
What is Front of Centre and why is it important?
Front of Centre (FOC) is all about the balance of your arrow and its important because it affects arrow flight and, less importantly for us target archers, penetration. FOC is the measurement of the percentage of the arrow’s weight that is located in the front half of the arrow. The higher the FOC, the more stable and accurate the arrow will be, especially at longer distances. However, too high an FOC will reduce the arrow’s speed and trajectory, making it drop faster. The lower the FOC, the faster and flatter the arrow will fly, but it will also be more prone to erratic flight and less penetration. Therefore, finding the right balance of FOC is crucial for the optimal performance in your pointy bois.
What is the right balance? Well that depends on the type of arrow you are shooting. Its said that for an Aluminium shaft, a FOC of 7-9% is perfectly adequate while an A/C carbon needs to be slightly heavier in the nose at 9-11%. Easton ACE shafts are somewhat notorious given their light weight requiring a very robust 11-16%. These numbers are for guidance only and can be interesting to experiment with if you push the boundaries way past these numbers. Our Vice Chair (yes, the usual suspect) is currently loading up aluminium arrow points with ball bearings. He’s looking for maximum stability in his arrow as fast as possible off the bow and doesn’t care if range is reduced as he only shoots Ali’s indoors at 18m. Why ball bearings? Ball bearings are identical in dimensions and weight – every 5mm BB weighs 8 grains – so the weight of a dozen points can reliably and accurately be made identical.
How do you measure FOC? Well, we’ve noted below how Easton recommend you calculate it. However, if you can’t be bothered getting all numeric, here is an Excel spreadsheet for you: Excel FOC Spreadsheet. Just measure the length of the arrow – groove of the nock to the point (turquoise line below). Then find the balance point of the arrow and measure from the groove of the nock to the balance point (yellow line). Slot the numbers into the spreadsheet and and hey presto .. there’s your FOC.
While porpoising, fishtailing and windmilling (all issues with arrow stability) can have many other causes, a front loaded FOC can help give a nice flight to an arrow. Witness Captain Sausage’s X10’s. An A/C arrow should be around the 9-11% mark. John’s arrows are closer to 14% (13.75%) because the arrows were built with 120 grain points when 110 or even 100 should have been enough. He traded some range and speed for stability. Things that might come back to bite him when he shoots 90m but then everything in archery is a trade-off.
So there you have it. A complex and highly technical part of archery distilled down to four paragraphs, two measurements and a spreadsheet. Wasn’t so bad now was it? We won’t talk about the ball bearing experiment until perhaps later in the year .. assuming they work and don’t end up all over the hall floor giving rise to a keystone cops moment. But that would be a comedy video for a future post! ;o)
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According to Easton Archery, to calculate your arrow’s FOC, follow these steps:
- Divide the arrow’s length (distance from the base of the nock groove to the shaft’s end) by 2.
- Find the balance point. That’s where the arrow balances perfectly. Mark the point, and measure from there to the nock’s throat.
- Subtract the center of the arrow measurement (calculated in Step 1) from the balance point (calculated in Step 2).
- Multiply Step 3’s answer by 100.
- Divide the answer from Step 4 by the arrow’s overall length. That is the arrow’s FOC percentage.
Bow hunters such as those in the USA will load up their FOC to 15% or even 20% (called extreme FOC). This is because they rarely shoot more than 50m when hunting and extra heavy points aid in stability of flight and penetration. Remember, archery problems and their solutions don’t just affect target archers!