[GroundhogCK]

I made an interesting discovery regarding the efficiency of Ross bows yesterday.

My 331 is set up with a 27.5" draw length, and 70 lbs. limbs maxed out.

The experiment started as a result of the fact that I just put together some new Easton XX75 Platinum Plus aluminum arrows (size 2213) for 3d/target shooting with my Ross. The total arrow weight for these is 378.4 grains, which is 5.406 gpp.

Previously, I was shooting 45/70 CarbonTech Cheetah's with a total arrow weight of 345.2 grains, which is just a *hair* (5 grains) under the 5 gpp minimum at 4.929 gpp. This is fine given my short draw length of 27.5" and it's inherrently reduced power stroke.

Here's where it gets interesting... I knew from a previous experiment that the fps generated by shooting my Cheetah's did not seem to change at all regardless of whether I was shooting these arrows tipped with 100 gr. or 125 gr. field points... With both setups, the arrows impacted identically elevation-wise at 40 yards. Further, shooting the Cheetahs with 125 grain tips put my arrow much closer to the 5 gpp minimum, and the arrows seemed to group better. The speed of the Cheetahs is 280 fps.

This kind of stumped me though given that it *should* be physically impossible two arrows are 25 grains different to impact at the same elevation... I figured either it had to do with diminishing efficiency with with the lighter arrow, or possibly had something to do with the difference in FOC %.

Here is where it gets REALLY interesting. Yesterday I printed up a new sight tape for the new Easton XX75's using the On Target Tapes and Charts program. This program had accurately predicted my fps with the Cheetahs, resulting in a dead-accurate tape from the very first one I printed, so I figured the same would hold true for my heavier XX75 arrows. The Tapes and Charts program calculated that my speed with these new arrows would be 264 fps, so I printed up the corresponding tape.

In going to shoot these arrows yesterday, I started noticing that they were all impacting high at the corresponding yardage marks for 20, 30, and 40 yards. To experiment, since I had both tapes attached to my sight at this point, I decided I would try using the old tape for the Cheetah's instead. Low and behold, the 33.2 grain heavier XX75's impacted IDENTICALLY to the Cheetahs!

Now, I realize this should be physically impossible, and I haven't chrono'd the new arrows to verify speed, but accoring to point of impact, the speed with the heavier arrows would appear to very close, if not identical to that of the Cheetahs!

The only two things I can figure again are that this has something to do with picking an arrow weight that maximizes the efficiency of the Ross bow, or the different FOC % of the Aluminum arrows, which is right around 8%, versus the 13+% FOC of the Cheetahs.

If this is the case, it's a no-brainer to shoot the heavier arrows given that the trajectory appears identical, and they're going to have KE given then added weight.

I should also add that this bow is perfectly group tuned and punching bullet holes with bow shafts, no adjustments needed depsite the differing diameter of the shafts.

I realize what I am experiencing isn't physically probable... but does anyone have a better explanation of what might be happening here?

My hypothesis is that it seems that there would appear to be a diminishing level of effiency when shooting progressively lighter arrows out of a Ross bow, which is really pretty cool if you think about it, since max efficiency with heavier arrows = max kinetic energy!

 

[Cougars09]

excellent post

 

[msar]

Neat post, I may have to see if it works for me too!

 

[Guest]

Thanks for posting, good info.
Yes, the Ross cams are very efficient. The review by Norb in Bowhunting World will give you the real world numbers on it.

 

[C-fused]

GCK,

No, you aren't going crazy.

I've noticed the same thing also.

I've taken two different weight arrows (approx. 50 grains difference) and separately tuned and sighted in each out of the same bow. After it was all said and done the pin gaps out to 50 yds. were the same.
Or the time we shot 2315's weighing 505 gr. total along side 3-71's weighing 425 gr. total at 20, 30, and 40 yds. Guess what? They all impacted in the same groups very closely. Goofy I know. 8)
It makes you wonder if ringing all the speed you can get out of your rig is worth it. A balance is probably best.

 

[thespyhunter]

Good info Groundhog. I will have to try this.

 

[GroundhogCK]

Yeah, try it out guys... I was amazed -- it must just be that the cam design is so efficient that there are diminishing returns w/ lighter arrows. I think there is a balance to be had indeed - I wasn't previously a believer in a heavier arrow to achieve more KE, as I always felt the difference of a few pounds of KE was marginal, and negligible at best when considering the alternative of a flatter trajectory. But not it seems that if I can shoot a heavier arrow at the same trajectory, why wouldn't I?

I read that Bowhunting World article -- thanks for posting it. I wish there had been some tests with 70 lbs. at shorter draw settings, as that's what I've got going here. It certainly still helps to highlight the relationship dynamics between draw length, draw weight, and speed though, and the results do indeed indicate a very efficient cam.

I still don't understand how the testor calls the bow "slow" though. We're talking a difference of 10 fps between the Ross and a "speed" bow -- in terms of trajectory, that equates to almost zero gains in range estimation error. If my math is corret, 10 fps would buy you about inch at 40 yards.

I know I sure wouldn't trade my smooth, easy-drawing, forgiving of my adrenaline-charged shooting form during the moment of truth Ross away for an extra 10 fps.

 

[thespyhunter]

I know I sure wouldn't trade my smooth, easy-drawing, forgiving of my adrenaline-charged shooting form during the moment of truth Ross away for an extra 10 fps.

:?:

 

[GroundhogCK]

still helps to highlight the relationship dynamics between draw length, draw weight, and speed though, and the results do indeed indicate a very efficient cam.

I still don't understand how the testor calls the bow "slow" though. We're talking a difference of 10 fps between the Ross and a "speed" bow -- in terms of trajectory, that equates to almost zero gains in range estimation error. If my math is corret, 10 fps would buy you about inch at 40 yards.

Yes!
I get into this all the time with the speed freaks on A/T.
If they took the time to do what you have done they'd know better. It only changes enough to matter past +/-50yrds or so dependng the arrow weight.

That's very interesting. I rarely shoot past 50 yards unless we're out at hunting camp having the annual "how far can out can we put the target and still hit it" contest in September... In other words, trajectory beyond 50 yards is a novelty for me in general.

I wonder why OnTarget Tapes and Charts software accounted for such large difference in estimated speed and corresponding tape trajectory markings with the new arrows... With my first arrows it's estimates were dead nuts from the first tape I printed.

Hmmm... Well, the important thing is I've got an accurate tape, and if I ever need to change it the program is more than adjustable enough to accomodate me.

If any of you guys using slider or click sights on your Ross bows are still using the old school tape with pencil marks on it, you really have to spend the $20 and download Tapes and Charts... It's really great software. Their Shaft Selector program is phenomenal as well, and that's only $10 (you can also get a free 3-day full function trial version of Shaft Selector) -- it's really helped me get my shaft decisions right the first time, and saved me money as a result... from chosing the right shaft model, to length, to optimal tip weight.

I think there's also a bundled package that inlcludes both SS and TAC and some other extras as well. Good stuff.

 

[asylum]

great post