How does humidity affect drawweight/tiller?

[Far East Archer]

I have been pondering about this for the past 3 months, but never really got anywhere with it. I was wondering, depending on the humidity, how much can draw weight go up/down? Say a bow was made in 80% humidity and then was shipped to a place that was 50% on average. Would the draw weight go up considerably or would the change be so little it wouldn't matter? Also, what types of wood are more affected by climate change, for example, how does bamboo and ipe do in this sort of situation? And are laminated bows less affected to climate change than selfbows?

Last question, can tiller change when climate changes? Sorry for all the questions but I would really appreciate it if someone could answer these for me and anyone else who might be interested in the subject.

-Alex

[DCM]

I think there will be lots of answers, or more accurately responses, to this question but substaintially only speculation and antecdote.  Ironically, it's a thing farily easy to test accurately.

So, here's my speculation and antecdote.  Wood with big pores are the most vulerable, ring porous.  Woods with natural resins tend to react less quickly to changes.  I have witnessed antecdotally, but not measured scientifically, swings of 5# to 7#, or about 10%, on woods like elms and hickories.  I don't personally use oaks but I'd expect they qualify for the same reason.  This with Rh swings of 25%, say from 35% to 60% with acclimation periods on the order of weeks, versus days or hours.  Osage and yew enjoy a reputation for being not as much effected, but this may be due simply to the relative slowness of acclimation or their tendancy to be inherently overbuilt due to their relatively uncommon capaicity for compression.  But there may be inherent qualities which mitigate the effect in these species.  Bamboo is much effected, ipe closer to elm than osage.  I'd presume the area of most bending, under the most stress, would be most effected in terms of tiller, so inner and mid limbs would tend to set and weaken as they tend to carry more of the compression load.

I wish somebody would do some testing, as this aspect of natural bowyery is in my view perhaps the most important.  Another less investigated area, but frequently speculated on is different species capacity for repeated bending, or work fatigue.  We know for sure some metals, spring steel for example (albeit engineered by virtue of grain structure and carbon content) enjoy much higher capacity than others.

[Badger]

David, something I have noticed with my osage bows is that when I take them to the flight shoots they usually end up sitting in my car in very dry hot weather for a few days before the shoot. I meticulously get their weight to just a hair under 50# and without fail after experiencing a few days of the very hot dry weather will pick up a couple of pounds which I have to scrape off. As for tiller, invariably for some reason when they rehydrate to my area the tiller is sometimes way off almost to the point of a hinge, My experience with elm and hickory is pretty much as you describe. As for ipe I don't notice any real difference but I think it is because it tends to change rh rather slowly and tends to remain more stable. Although performance may not be at it's very peak I feel bows are safer at about 7.5% moisture and don't seem to take excessive set their either. Steve

[DCM]

Steve,

My experience with ipe is admittedly limited, one selfbow and perhaps 2 or 3 bbi, which obviously are a different critter.

Your antectdote about osage changing 2# or 3# is interesting but lacks context.  Would you estimate the Rh change at 25%, or less?  I would think less, which might easily put osage in the 10% category with other woods.  Also worth noting, the relationship between moisture content and strenght is known to be non-linear and more pronouned on the higher Rh end.  So, a change from say 25% to 50% would be less dramatic than say 35% to 60%.  Can't imagine osage being very resilient at 25%, despite having folks comment it is.  I have had a few spectacular failures at sustained Rh below 30%.  Obviously much depends upon the specimen, predominantly the condition of the back ring.

[George Tsoukalas]

LOL, DCM...once an engineer... One thing is for sure, FEA,  humidity is the enemy of  selfbows. You will loose  cast in a humid environment. Osage,  black  locust, yew and may be mulberrry would  be least affected  while  hickory, ash, etc  would be the most affected. Jawge

[Badger]

Dave, my relative humidity at myhome is pretty consistent throughtout the year at around 60%, The flight shoots are at bonnevile where I suspect the relative humidity below 25%. But sitting in a hot car in the desert for a couple of days with 10% r/h and temps over 130 degrees can drive the moisture out pretty fast. I think they tend to dry out quicker than rehydrate but I could be wrong about that. I have never really done any real study of moisture affects beyond just noticing things. One thing I have dome numerous times just to check the affects was to use a heat gun to drive moisture out of a bows belly. The affects are pretty dramatic but short lived unless you heat treat. 5# difference with most domestic woods in not uncommon just superdrying, I have never tried it with any of the exotics. Steve

[Pat B]

My limited experience is with 2 sinew backed bows I built. One, a sinew backed hickory(my attempt at the Massey Medicine Bow) when built was 56#@26"...built in the winter when humidity is low in our area. A few months later as humidity increased the draw weight dropped to 45#@26". I sent this bow out to Kenneth in Colorado where the air is dry and in a few weeks it was back up to almost 70#@28". When it came back to the Eastern US the draw weight went back down. Both hickory and sinew/hide glue are hygroscopic, meaning that they readily take on and release moisture. The tiller seemed to be unchanged over these periods of high and low humidity.
   Number 2 was Elkie, the sinew backed osage bow I built to go elk hunting with Kenneth. Again built in winter when dryer and seemed to retain most(lost some weight) of it's weight as humidity increased. When I shipped it to Colorado 2 weeks before our hunt the lost weight had come back and a month after I left the bow with Kenneth after our hunt the weight increased to about 70# at 29" - 30" (Kenneth's draw length). I don't believe the tiller changed on that bow either but Kenneth would have to confirm that.
  I do know from my own experience that humidity does effect draw weight but I haven't noticed any change in tiller. I think with Steve's examples, he is talking about highly specialized bows that are brought to the brink of failure to eek every bit of performance out of them, plus they are woods that are more susceptible to M/C change than say an oily woods like osage or ipe.
  Another example I have was not due to R/H but adding a snake skin backing to an osage bow, using TBII, a water based glue. After a few days of skinning the bow I went out to shoot it and was amazed by how dull and lifeless it felt in hand. A week later it was back up to its past performance...once the moisture added by the glue was eliminated.     Pat

[Far East Archer]

Thanks guys!

Ok, so I guess it is safe to confirm that tiller will not change or not much from climate change unless the design is highly stressed? I plan on shipping out some bows to the states (selling to other archers from here etc) and elsewhere and was wondering what I should do to get minimum weight change. I don't want to make a 50 pounder and have it ending up 60# when he gets it and not able to shoot it. ATM its around 60% RH in my room and I believe thats around average  elsewhere in the world.(unless its somewhere dry like CA or CO) Would it be safe to say I may get only a 5# weight gain in a bamboo backed ipe bow? Or should I be expecting more like 10+ (hope not  ) Also, how much weight would a all bamboo bow gain from such climate change? I suppose the type of finish I use will also play a role in this too?

-Alex

[Pat B]

I would think that an oily, tropical wood like ipe would be less effected by moisture, compared to hickory, ash or maple. Osage is oily and it doesn't seem to change as much as those woods unless it has a sinew back.   Pat