HIT impact on dynamic spine?

[Christopher.Reed]

In thinking through this years arrow build, I am still struggling to find an answer to this simple question.

For clarity; as a HIT system adds significant rigidity to the front of an arrow (3.5” in my circumstance w/ Snyder core) what is that impact on the dynamic spine?

While it seems patently obvious that there would be an impact, I can’t seem to find a reliable resource that defines it.

As a long draw/high poundage guy (31.5”/80lb) I want to squeeze as much dynamic spine out of an arrow as possible lest I start lobbing logs, but I think the question has broader ramifications for all bow hunters and am very curious to hear our experts insight.


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[Mighty Mouse]

It will stiffen the dynamic spine, but how much is anyone's guess. Not enough that I would try to get away with 300 spine arrows if I were in your shoes.

If you're worried about total arrow weight getting too high, choose a lighter HIT and head, skip the collar, and put them on a midweight 250 spine shaft. There are several 250 spine 4mm shafts out there around 10 gpi. You could easily keep TAW under 500 gr if you want to. You may actually need a higher gpi shaft to slow your arrows down enough to get fixed heads to fly well. At 31.5"/80# you won't be "lobbing" anything...more like shooting frickin' laser beams.

 

[thunderchicken21]

See page 24 here about the Aeroconcept system (ACS 1.0): http://org.firenock.com/pdf/2022_Catalog_S.pdf

Lot's of other good information in that catalog throughout if you are a real techy kind of person.

 

[Christopher.Reed]

It will stiffen the dynamic spine, but how much is anyone's guess. Not enough that I would try to get away with 300 spine arrows if I were in your shoes.

If you're worried about total arrow weight getting too high, choose a lighter HIT and head, skip the collar, and put them on a midweight 250 spine shaft. There are several 250 spine 4mm shafts out there around 10 gpi. You could easily keep TAW under 500 gr if you want to. You may actually need a higher gpi shaft to slow your arrows down enough to get fixed heads to fly well. At 31.5"/80# you won't be "lobbing" anything...more like shooting frickin' laser beams.

I always appreciate your insights and thank you for your input!

The question arose from the performance of last years arrows through my new bow. The arrows in question are 563g total arrow weight (225g up front) and was optimal for my incorrect DL (30.5) and 70lb bow. Those arrows still fly flawlessly and it made me wonder why as on paper, they are pretty under-spined.

It’s seems like the HIT system would effectively shorten the static spine from 31” to 27.5” and allow me to stick with a 250 spine but I am certainly no physicist or engineer. I do however have an insatiable curiosity to understand the “why” of things and might have missed my calling .


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[Christopher.Reed]

See page 24 here about the Aeroconcept system (ACS 1.0): http://org.firenock.com/pdf/2022_Catalog_S.pdf

Lot's of other good information in that catalog throughout if you are a real techy kind of person.

Thank you for sharing! Diving in now.


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[Mighty Mouse]

The question arose from the performance of last years arrows through my new bow. The arrows in question are 563g total arrow weight (225g up front) and was optimal for my incorrect DL (30.5) and 70lb bow. Those arrows still fly flawlessly and it made me wonder why as on paper, they are pretty under-spined.

Spine charts/calculators just provide a rough starting point, not a hard-and-fast requirement for good arrow flight. I've been able to get arrows significantly weaker and stiffer than the "on paper" recommendations to fly well. The only way to truly know is to try it.

It’s seems like the HIT system would effectively shorten the static spine from 31” to 27.5” and allow me to stick with a 250 spine but I am certainly no physicist or engineer. I do however have an insatiable curiosity to understand the “why” of things and might have missed my calling

I don't think it's quite as simple as subtracting the insert length off the total arrow length. The segment of the shaft with the HIT inside may not be bending, but the weight of that forward segment is still acting on the rearward portion of the shaft and affecting the dynamic flexure of the rearward portion. I would guess dynamic behavior of a 31" shaft with a 3.5" HIT is equivalent to a shaft slightly shorter than 31" with a standard length insert (but not a full 3.5" shorter).

If you keep front end weight moderate, I think 250 spine will work fine for you. Below is OT2Go/qSpine output for a 30.5" C2C 250 spine 4mm Axis with 150 gr on front and 30 gr on rear assuming typical specs of a longer modern bow (335 fps IBO, 7" BH, 34" ATA, 85% LO). If you wanted more TAW and the same dynamic spine, you could use a denser shaft like a Day Six (12.6 gpi) and get around 560 gr/295 fps.

 

[5MilesBack]

The bow either tunes with them or it doesn't. I've never had a problem getting my bows tuned with any spine arrows I've used. Last year I was shooting 75lbs at just under 33" draw with 250's and 200gr up front. No problems. I've also tuned to 200's and even 300's without any trouble. This year I'll be at whatever poundage my new to me 70lb maxes out at, and don't anticipate any issues tuning whether I use my 300's or 250's.

 

[Christopher.Reed]

Spine charts/calculators just provide a rough starting point, not a hard-and-fast requirement for good arrow flight. I've been able to get arrows significantly weaker and stiffer than the "on paper" recommendations to fly well. The only way to truly know is to try it.


I don't think it's quite as simple as subtracting the insert length off the total arrow length. The segment of the shaft with the HIT inside may not be bending, but the weight of that forward segment is still acting on the rearward portion of the shaft and affecting the dynamic flexure of the rearward portion. I would guess dynamic behavior of a 31" shaft with a 3.5" HIT is equivalent to a shaft slightly shorter than 31" with a standard length insert (but not a full 3.5" shorter).

If you keep front end weight moderate, I think 250 spine will work fine for you. Below is OT2Go/qSpine output for a 30.5" C2C 250 spine 4mm Axis with 150 gr on front and 30 gr on rear assuming typical specs of a longer modern bow (335 fps IBO, 7" BH, 34" ATA, 85% LO). If you wanted more TAW and the same dynamic spine, you could use a denser shaft like a Day Six (12.6 gpi) and get around 560 gr/295 fps.
View attachment 537387

While it is certainly anecdotal and not precise by any means, I took the liberty of plugging my current arrow into QSpine with the HIT length removed from the C2C length and this was the result.

With the performance I am seeing, that seems significantly more accurate than the actual specs as listed below:

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[Christopher.Reed]

I should also add that there would obviously be a weight difference of 3.5” of GPI so it’s certainly not a perfect comparison, it just seems like a more accurate depiction of real world performance.


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[nphunter]

I should also add that there would obviously be a weight difference of 3.5” of GPI so it’s certainly not a perfect comparison, it just seems like a more accurate depiction of real world performance.


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I think that is a good approach. I’d like to drop a spine because I’m in the same boat with long hits and my arrows show super stiff if I remove length. I’d like some lighter spine arrow and build them exactly the same and see which ones group better down range.