bow lengths and efficiency for fiber glass

[EwokArcher]

I am very familiar with effeciency and leverage in regards to natural bow materials (real bows :-b ) but delving into fiberglass and it's world of different riser lengths and limb lengths has left me with some questions.

Let's talk about d/r hybrids for starters.

With wood bows it is typically true that as wood bows get shorter they become less efficient not being able to store as much energy as longer versions.
     What is the most efficient length for a.  moderately deflexed/reflexed fiber glass bow?

Is there a standard limb length to riser length ratio? What about standard lengths for risers? I've seen a lot of 16 and 18 inch risers.

What are the trade offs with larger vs smaller risers?

I've seen some 48 inch recurves and a bunch of other very short bows under 54 inches on this site, are these still efficient tools when compared to bows around 60 inches of similar design?

I appreciate the input,
Ty.

[Pago]

I'm not expert but have searched out some of the same information you are touching on in my own attempts to learn.  I don't know much about d/r longbows but I know that the recurve is recognized as the most efficient design.  According to research performed by Hoyt and Hickman (father of scientific archery) the most efficient limb design for any bow incorporates 19.5 to 20" of working limb. It is generally accepted that longer riser lengths increase stability. These are generalities among a long list of generalities which are impacted by an innumerable list of design possibilities which impact the overall efficiency of a bow. Clear as mud? Hope this helps can't wait to see what others have to add.

[EwokArcher]

Very good information. I appreciate it pago.

Here is another question,  when making wood bows it is common practice to have vary narrow tips down to an 1/4 inch to decrease tip mass to improve arrow speed, all of the fiber glass bows I've seen have had pretty much short fat ugly tips, as opposed to the slinder light weight designs I'm used to seeing. What's the reason for this?

[Robertfishes]

This is a tip on a 66" D/R longbow I built . it is 7/16" wide at the string groove.  some guy build them narrower but mine are typically this size.  

[monterey]

Just a guess, but I think stability becomes more and more critical as the D/R or recurve design becomes more radical or pronounced. I have owned and shot.a bunch of old school recurves and the rare cases of limb twisting were with narrow tips.

I have built only straight or slightly reflexed ASL styles and find the narrow tips to be easy to manage as well as efficient.

OTOH, my all time favorite recurve has very narrow tips but was the victim of a ten year old and a car door.

[mikkekeswick]

The same basic principles apply but you just have to realize that you are dealing with a far more elsatic material in glass and a different set of limits. I noticed the tips as well....! Again the same principle as wooden bows here to a point. Glass limbs will be significantly thinner and therefore the width on a recurve is there for stability. You can however use carbon/glass core lams to help with that though. Also bear in mind that the majority of glass bows are mass produced to be handled by the masses so it's a very good idea to 'overbuild' them a little (so they last and can take some abuse!).
I make my longbows between 3/8ths and 1/2 inch at the nocks with no problems. I haven't done too many recurves yet but they have all been a hair over 1/2 inch at the nocks with no real problems regarding stability.