lower limb taking set: ambient moisture differences

[joachimM]

There's been a lot of talk already on lower bow limbs taking more set in vertically stored bows.
I thought this might have to do with moisture content being higher close to the floor, and the lower limb therefore having a higher MC than the upper limb. Since compression resistance decreases with increasing MC, lower limbs of vertically stored bows might therefore be weaker in compression and take more set when exposed to the same strain.

So I started measuring ambient moisture, on the floor (insulated!) and at 2 m height. I live in a wet climate with a strong marine influence, similar to the UK.
The past few days, ambient moisture on the floor was on average c. 10% higher than higher up (74% compared to 64%). This translates into a marked difference in equilibrium moisture content from 11.7% to 14.1%

So it seems this is an important factor to consider.

[Del the cat]

Interesting... I'll take some measurements on vertically stored timber off cuts.
Del

[Dances with squirrels]

I ain't buying it. I think it's due to other reasons.

I think it's mostly due to not making the bottom limb weak enough. Seriously.

[PEARL DRUMS]

I don't think a 10% swing in ambient moisture would cause a weak limb. If that were the case I would be in big trouble. Our humidity changes by 40-60% throughout a year. I store my bows hanging vertically in my basement from the floor joists, the tips are about 24" off the concrete floor. 

[joachimM]

I ain't buying it. I think it's due to other reasons.

I think it's mostly due to not making the bottom limb weak enough. Seriously.

Why would a bottom limb have to be strained differently from a top limb, especially in symmetrical bows? I always aim for a perfectly symmetrical tiller and don't leave the lower limb stiffer.
Now, if you leave one limb stiffer ("not weak enough"), it means the other (necessarily weaker) limb takes more than half of its share of the stress, and is strained more. More strain, more set. So shouldn't then the weaker (upper) limb take more set?

Pearl Drums: a 10% change in ambient moisture from 65% to 75% at 20°C gives a wood MC shift from 12% to 14% (see for yourself here: http://extension.psu.edu/natural-resources/forests/woodpro/equilibrium-moisture-content-calculator).
That seems like a big deal to me. See line C in this figure (Wood Handbook). For the example in the figure, the compression strength shifts from 55 Mpa to 50 Mpa due to this, which is roughly a 10% change. Any other change in MC of 2% will have a similar effect according to that graph. I'm not saying that, the authors of the wood handbook are.
This was an equilibrium MC difference across 2 m. In a bow, this difference will be more subtle. But maybe enough to make a noticeable difference in set.

Note, I haven't said moisture is the reason, rather a factor to consider.

[Emmet]

Interesting.
 I check moister with a pin type meter and the bottom end of my bows and staves that are stored vertical is always 2-3% high on everyone I check no matter how old. I assumed it was caused by water weight and gravity. I try to keep the ones I'm shooting  horizontal.

[George Tsoukalas]

Interesting but I am more inclined t believe bottom limbs take more stress because they are strained more by the split finger release.
Jawge

[joachimM]

thanks Emmet, that's very valuable information. 2-3% difference is even more than what I'd expect.
By the way, water weight and gravity is not sufficiently strong to overcome the forces of molecular adhesion of water to wood. That's why you'll never see moisture drip out of the bottom end of a vertically stored stave, not even if freshly cut.

Jawge: I'd think that the difference in strain caused by split finger (1 above 2 under) is less than the difference in compression strain caused by the moisture difference (5% for a 2% MC difference). But all this is conjecture, there's no hard facts nor data to support nor reject this hypothesis. It could contribute. I don't know if it does.
By the way, I store most of my bows horizontally, and I've never remarked such difference in set. But my lighter bows (<45#) I shoot with a two-finger split anyway.   

What we do know, however, is that bows shot in too moist conditions (green bowstaves as the extreme example) take excessive set, and we know moisture is the cause. Emmet's findings support that bottom limbs of vertically stored bows have higher moisture content.
1+1=?

Joachim

[willie]

A 10% R.H. difference at that humidity range is certainly possible if the temps near the floor are only a few degrees different (centigrade).  Heating with heaters vs central heat, or poorly circulated areas often exceed this difference from floor to head height.

http://www.coolerado.com/pdfs/Psychrmtrcs/0000Psych11x17US_SI.pdf


read the dry bulb temps (across the bottom) vs the RH lines (sloping lower left to upper right)

[Springbuck]

That is very interesting.  Much more difference than I would have thought.

[George Tsoukalas]

I never store my bows vertically but always get a slight difference in set. Then again my bow for the year is in AC for the summer. Jawge

[bradsmith2010]

I think the split finger,, causes a bit more strain on the lower limb,,

[Badger]

  2% would be very significant. The problem I had with my lower limbs was storing them in a bucket with hardly any air circulation. Since I took them out of the 55 gallon drum I haven't had the same problem. I think I may have been getting more than 5% difference.

[joachimM]

It's very likely that there are multiple causes for this phenomenon, and neither of the proposed reasons being wrong in its own way. Always good to read different opinions and widen my own, sometimes narrow view.

[Dances with squirrels]

Anything is possible I suppose, but I'm not convinced that a 2.4% m.c. difference always happens, or when it does, that it's enough to cause more set in one limb verses another. In order for it to be true, many other things would have to be set perfectly in place. I'm prone to think its more about the way the limbs are stressed relative to each other and the shooter.

I would be a lot more intrigued if someone stored some of their bows with the top limb toward the floor and some of them with the bottom limb toward the floor, and all of the limbs nearer the floor took more set, regardless of whether they were top or bottom. That would make me sit up and take notice.