ijimmy

Trushot , Im sure you are useing deadly sharp broadheads , I'll agree with the others , try some diferant arrows as you are way under spined with your curant setup . Carbons do penitrate better than alluminums , that is unless you are useing the thick walled 15 series or better shafts , in fact I bet 2315's would allmost work for you , or you could try some of easton's new axis arrows , I bet thoose things with a magnus 2 blade infront will go thru solid steel .

Rack-attack

It is also common for outdoor and indoor impacts to be different due to how light gathers through a peep sight. After shooting outdoors then moving into my indoor spot league - I will almost always have to adjust the sight a tad.

Cold/warm will not effect the bows poundage.

Trushot_archer

Gotcha! That makes sense.

Trushot_archer

I thought that as well IJimmy. I did try some 100 grain heads during the pre season to see what, if any, difference there was in spine. Hit the same out to 20 so I opted for the heavier head. Penetration could be affected by that though as you can't tell until you stick somethin'.

Arthur P

I'm not so sure about that. I haven't checked a bow's draw weight in 100 degree heat and then taken it into a walk-in cooler and checked it again after it got cold. I'd bet you'd find a few pounds difference.

For certain, heat and cold will affect the length of the bow's riser, most especially one that's machined out of aluminum billet or plate. The longer the riser, the more effect heat and cold will have on it. When the riser contracts in the cold, the cabling system is under less tension, though the cold might also stiffen the limbs enough to keep draw weight constant. Needs to be tested.

If you tuned the bow at the tail end of summer in hot weather and then don't do anything to tune up for cold weather, the contraction of the riser can be enough to change the bow's tune. It definitely will cause your peep sight to rotate differently. On a super short bow like the Mighty Mite, it could even cause the peep to change position relative to the pins as more string winds around the cam when it's cold. Then you could get into the expansion and contraction of those oversize wheels!

Expansion/contraction is machined aluminum risers' second biggest problem. Bending is their first biggest problem. Again, the longer the riser, the bigger those two problems are.

That's why I feel machined aluminum risers are inferior to forged aluminum or magnesium risers. Honestly, I'd rather have a riser made from resin impregnated laminated wood than either of them... but rather than ramble on, I'll shut up.

Rack-attack

I knew your post was leading to that......LOL

When an entire turn of a limb bolt only changes # by a few pounds - how much "expansion/contraction" would it take in a riser to move the limb pockets enough to throw off tune and send an arrow 1" at 20yds because of increased speed.

I have tested more than a few of my hunting bows in cold and in heat. They draw the same - others i know have done the same with the same results. I will often leave my bow outside on cold mornings to "chill" it then shoot - no change.

I am sure going from 100 deg to 10 deg may change something - But not near enough to cause such problems.

Arthur P

Rack, I don't think you realize how small differences in bow measurements can have a noticeable impact on how the bow shoots.

Aluminum has one of the highest contraction/expansion coefficients of any structural metal. Higher than steel. The changes are small, but maybe I can explain how they might work against someone.

The forumal for calculating expansion/contraction of aluminum is:

L (length in inches) X Delta T (change in temperature) X .0000129

So, using your idea of a 90 degree temp shift and a bow with a 25" riser:

25 X 90 X .0000129 = .029025"

Now, you've also got the aluminum wheels that have also contracted, and the limbs have also contracted some, so you can figure your bow is right at 1/32" shorter than it was when you tuned it up and sighted it in. The string/cable(s) have a very low expansion/contraction coefficient, so they don't hardly change at all. So what's happened is basically that your string and cables are, in effect, 1/32" longer than they were when you tuned the bow. Your cam is slightly out of time. Since the idler wheel is round and concentric and the cam is oblong and eccentric, the string is not going to wrap evenly around both wheels. It's going to pull your peep one way or the other, and the peep isn't even going to rotate properly anyway.

But that's kind of extreme. Lets say the temp shift is only 30 degrees. Like tuning the bow in late summer and hunting with it on a cool autumn morning.

25 x 30 x .0000129 = .009675"

Only 10/1000ths of an inch. Doesn't look like enough to be concerned with, right? However, remember those pesky wheels are also slightly changing diameters and their circumferences have changed a lot more. So you still have those differences in string wrap.

Now a little trig. Assuming your peep is no further than 3" in front of your eye, a shift of only .002" will change your point of aim by 1.88" at 20 yards.

I'm not saying that there will be HUGE changes, but enough to throw you off. It's a good idea to make minor adjustments in the bow's tune, or at least resight the bow, whenever the ambient temperature you intend to use it in changes by 20-30 degrees.

A wood or carbon riser (quite low expansion/contraction coefficients) would go a long way to eliminating the major part of the problem.

AND I HOPE BY THE TIME YOU'VE READ ALL THIS, YOUR BRAIN HURTS AS BAD AS MINE DOES.

Rack-attack

Arthur - you put a good deal of time into that and I thank you.

I do not agree with you on all points and do not have the time now to rebut with an equally thought out answer.

But you do make some good points that I will have to investigate further

I can give you my summation (without the meat to back it)

Its that I have never noticed a difference with temperature changes - and I have shot in all temps. I do not believe these changes you speak of add up to a real world problem.

My interest is peaked and I will put a little more time into this.

JeffB

Arthur,

When you say you prefer a forged alum riser...are you comparing that to a machined solid billet? How do extruded then machined risers play into this? Any better? any worse?

Arthur P

Jeff, an extruded riser blank will have the same problems as risers machined from straight plate or billet. The manufacturing process is very similar. All three types are rolled. The rolling operation on all three types produces a linear grain in the material. For best strength, that grain SHOULD run 90 degrees from the direction of any force is to be applied to the part. In the case of risers, the grain will run lengthwise.

Expansion and contraction mostly follow the grain.

A cast riser has a completely diverse and random grain structure, because it is made from molten metal poured into a mould. Expansion and contraction cannot follow one specific line because there is no specific grain direction. It is equally spread in all directions, so no single direction grows or shrinks more than any other.

A forged riser is made in a similar fashion, only it is injected into the mould under intense pressure. That does form a specific grain pattern. The grain conforms to the outside shape of the part and follows through all the way into the very center of the part. It is not random like a casting, but the expansion/contraction is still distributed throughout the entire profile of the part and not in one single direction.

Another point is that the forged riser is stronger than a riser made from a casting or machined billet/plate/extrusion. It's much more resistant to bending. On the other hand, a forged riser of the same shape and dimensions as a machined one is somewhat heavier because more metal is crammed into the same space and it is more dense.

The problem with castings is the part can have air pockets or sand inclusions (globs of sand that break off from the mould and become imbedded inside the molten metal) which weaken the part. And, due to their random grain structure, they tend to be brittle. Forgings are not.

Forging techniques are so good now that these problems with voids and sand pockets are very much minimized. Another major problem with both castings and forgings is that cracks can develop in the material as the molten metal cools. The cracks can be detected very easily by penetrant inspection. Voids and sand inclusions are usually only found during any machining operations that are done to the part, though there are some expensive tests that can locate them in the blank before they get that far along the manufacturing process.

Another problem with castings and forgings is that the dies used are expensive to develop and make. If you want to change the part, you must first change the dies.

One reason manufacturers have pushed machined risers so heavily is that all they have to do to change a riser design is to reprogram the CNC milling machines. It's much easier and MUCH less expensive to do that than to totally redesign and produce new dies.

Just a side note: Consumer demand is why machined riser bows cost more than forged risers, even though a good forged riser presents fewer problems and is stronger, stiffer and more consistent.

I tried to find some photos on the net to illustrate the differences but wasn't able to. Just think of the internal grain structure of a forging as similar to an onion, kinda the same shape all the way to the core. A casting has a grain structure that resembles sand in a bucket. A hunk of billet, plate or extrusion has a structure like a bunch of arrows, tied up in a bundle.

That's probably more info than you wanted.