relationship between TPI and FOC

[joshua blake]

#130434

Hello everyone. First post here so sorry if I overlook some forum decorum. I’ve unintentionally stepped on toes in the past so please just let me know if I am out of line in my questioning. I decided to build up some high FOC arrows for my recurve and while I’m confident they are more than adequate for large elk I ran into some Ed Ashby research where there was a link missing. In a mid 90’s Ashby article there is some extensive writing on TPI (tissue penetration index). The TPI is a calculation based on broadhead dimensions, momentum and shaft diameter in relation to ferrel size (no mention of FOC). Later research seemed to focus more on FOC and I haven’t seen any papers discussing any relationship between to two. For instance is there a graph showing where the effectiveness of FOC and TPI cross to provide the optimum of both. Say I have a relatively poor TPI of 1.0 but a ultra high FOC. What are the effects? Of course this is more curiosity and a desire to optimize my system. I would love to drop the weight of my arrow by 50-100 grains by optimizing the two and gain a hair of speed if possible.

Josh

[Robin Conrads]

#130441

Hi Josh, and welcome to the forums. The only “decorum” required here is mature and respectful conversations, even if you disagree with someone. This forum in particular is dedicated to those who want to discover more about Dr. Ashby’s theories or ask questions about certain related topics, as you have done (so you’re good!), but we ask folks who disagree with his findings or suggestions to visit the Campfire or Bows and Equipment forums, but to leave the folks here to discuss this topic peacefully.

I will message Ed Ashby and see if he can shed a little light on your question.

Robin (aka Webmother)

[joshua blake]

#130447

Wow thanks Robin. I guess there is no better answer than from the man behind the magic. I look forward to seeing what you find.

Josh

[Raymond Coffman]

#130442

Josh / webmom

Thanks Robin for contacting Dr Ashby and Josh for posing the question .

I also, am interested on this issue of how tpi  and foc relate to each other.

Thanks

Scout

[Ed Ashby]

#130585

Hello Josh,

First, let me apologize for not being able to reply sooner but I’ve been away getting huperbaric chamber treatments for the last seven weeks and just occasionally get to come home for a Friday night and Saturday, then it’s back for more treatemnts. I’ve got several more weeks of treatments to go. There’s no internet connection available where I have to stay for the treatments so can’t really keep up with things. Now, on to your question.

The TPI was developed in the mid 1990’s and was derived for the study’s data to that date. That testing was centered around arrows on ‘normal’ to ‘heavy’ weights and having normal to high of FOC. For these arrow setups the TPI works very well for predicting the average outcome penetration in tissues. As later testing progressed into both light/fast arrow setups and into the effects of higher amounts of FOC I found that the TPI did not accurately predict the average outcome penetration. Applying the TPI formula to light, fast arrow setups greatly overestimates the resultant tissue penetration. When applied to Extreme and Ultra-Extreme FOC arrows the TPI greatly underestimates the average outcome penetration in tissues.

Hope that helps answer your question. Hopefully, some day, it will be possible to expand the TPI formula to cover these factors that were overlooked the first time, but much more data, especially with EFOC and UEFOC, needs to be accumulated.

Ed

[Robin Conrads]

#130593

I’m sure I’m not alone when I say how happy I am to see your post. Thanks for answering Josh’s question for all of us.

Best of luck with your treatments. No internet connection sounds kind of peaceful…

[joshua blake]

#130614

Ed thanks so much for taking the time to comment. It sounds like both of these measurements (TPI and FOC) operate independently and applying/understanding the limitations and advantages of each is key. I guess until you can convince more people to let you put holes in their prized caped buffalo we will have to wait to see where the sweet spot between the two is. I guess the nice thing is that two of the three TIP variables are fairly easy to meet by using skinny arrows and sharp single bevel broad heads. The real question is where does FOC begin to significantly cancel out the need for momentum if it does at all. I hope your next visit to the inter webs  finds you in better health.

Josh

[Raymond Coffman]

#130692

Hi Doc

Good to see your post. Thanks for the tpi info –

Great good luck on your treatments, I hope it makes you feel better  –

I think you mentioned someone or group that was continueing the studies? Can we access their data ?

Scout

 

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