Recurve limbs tend to confuse beginners, intermediate archers, some accomplished archers, and even some coaches!
Sources of the Confusion
This will be about three-piece takedown recurve bows as they are the most common choice of target archers. The riser has a top limb and a bottom limb attached. These limbs come in three lengths (short, medium, long) which combined with various length risers, you can make the following bows:
Confused yet? (There are risers of other lengths!) Did you note you can make a 68˝ bow three different ways? (Generally, the shorter the limbs, the faster the bow, all other things being equal.)
The limbs then come in typically two pound (2#) increments over a fairly wide range of draw weights: e.g. 14#–48#. So, if you get “long limbs” and put them on a 25˝ riser, to make a 70˝ bow, and those limbs are listed at 32#, will you have a 32# bow?
Recurve limbs have their draw forces measured at 28˝ of draw. (Unless the bow is a youth bow for which it is common to measure the DW at 24˝ or a traditional bow, which are often measured at 26˝). Confused yet? But how many archers have a draw length of exactly 28˝? My guess is not too many. My guess is that most archers will have either longer or shorter draws. If their draw length is longer than 28˝, the draw force “in hand” will be higher than the weight listed on the limbs. If their draw length is less than 28˝, the draw force “in hand” will be lower than the weight listed on the limbs.
Well, if their draw length is exactly 28˝ will that be a 32# bow? Uh, maybe. Making limbs is not a perfect science. If a manufacturer makes a limb that is 31.5# do they discard it? No, they do not. It is “close enough” to 32# to warrant a 32# sticker and it goes in the pile with the rest. Now, don’t go all ballistic on the manufacturers about their sloppy manufacturing tolerances. These are quite reasonable numbers. If they do go “out of tolerance,” the limb is scrapped. And, if we insisted on perfect limb poundages, the price of limbs would skyrocket as so many would have to be rejected as not being “perfect.” (Since they can’t be recycled, so “Make another one, Bill, that one didn’t pass muster.” And if you have to make three to get one perfect one, do not expect them to be cheaper.)
FYI The manufacturers do not measure draw force like you do, they have a machine that clamps the butt of the limbs, fixing those in space. Then they place a force, in the old days this was a weight, on the limb tip and measure how much it bends. These “limb tip deflections” correspond to draw weights of assembled bows (the lower the LTD, the higher the DW).
What You Can Do to Lessen the Confusion?
As a coach, you can help get archers into a proper-sized bow. Youths need to avoid bows with too much mass as their bow shoulders aren’t very strong yet. Shorter archers need shorter bows, etc. Once an archer is fitted with one size of bow then you need to be able to address changes.
If they grow much taller, they may be ready to move up from, say, a 23˝ riser to a 25˝ riser. (Shorter risers have smaller sight windows and if the bow has a low draw weight, too, there may not be enough room in the sight window for all of the aperture positions needed. Longer risers are better for many reasons, but they are also longer and heavier than shorter ones.)
An Aside Bowhunters often use risers that are 20˝ or even 19˝ long. They can get away with such short bows, because their bows have to have a minimum draw weight of 40# (typically) and the shots they are taking are from fairly short range (20-30 yards).
Changing riser lengths is a rare event (buying a new riser of the same length doesn’t pose fitting problems). Changing limbs is much more frequent. Enter the adjustable limb pocket! The first mass produced adjustable limb pocket was introduced by Hoyt archery, and which was so popular, the design was stolen worldwide; we now call it the International Limb Fitting, or ILF. This design was for a limb that pressed into the pocket, with a click stop, and a pocket that allowed the angle the limb made with the bow to be varied a little. Prior to the invention, you screwed the limb bolts in and out to attach and detach the limbs and if you wanted to make a limb angle change, you had to make (saw, carve, whatever) small wedges to slip between the limb and the riser and then screw down the limb bolts trapping them in between. This was more than a little bit of a hit or miss procedure.
With the new ILF design, the limb bolts were locked in place with a lock screw and the limb had a notch in it so it rode up to the bottom of the limb bolt (the butt having a “rocker” designed into it).
When the limb bolt is “backed out,” the limb angles back toward the archer. This increase the brace height a little and lessens how much the limb gets bent at the archer’s full draw. Both of these lessen the amount of energy transferred to the arrows. But you can only do this so much before it becomes dangerous, so typically the draw force can be only reduced about 10% from the printed maximum on the bow. This amount of limb lean is so small that it is hard to see whether a bow’s limbs are “cranked down” or “cranked out” visually while they are being shot.
So, here is our quandary: recurve limbs (once a length is settled on) have their draw weights rated at 28˝ of draw (which your archer doesn’t have), may be slightly less or more due to manufacturer’s tolerances, and can be anywhere from the highest value of draw weight for those limbs to about 10% less than that depending on the limb pocket settings.
If your head is spinning, you are not alone.
Try This Here’s a system that can simplify the situation for you and your archer. To use it you need a reliable draw weight scale (all measures must be made on the same scale). Here’s how to do it:
1. With your archer’s current bow, crank the limb bolts all the way down counting how many turns are being made in the wrench. The reference point for “turns” is the limb bolts all of the way down, so when we get there that will become the new reference point. If it takes three (3) full turns to get them all of the way in, then the limbs were “at” three full turns out from bottom.
2. Measure the draw weight of the bow at your archer’s draw weight. If they use a clicker, put one of their arrows on the bow and pull until the clicker falls off. Easy peasy. Write this number on the limbs with a Sharpie/permanent marker.
3. take 10% off of that full draw weight measure and write that number down next to the first one. That is the draw weight range for your archer’s limbs at that draw length. (Set your archer’s bow back to its original state when done.)
Moving On Up
If they want to increase their draw weight once they are “bottomed out” on their current limbs, they need to buy limbs of the same length, four pounds (4#) heavier. The previous limbs were bottomed out, the new limbs will be backed out, typically maximally. So, if moving from 30# limbs, you move up to 34# limbs and back them off fully (10% of 34 is 3.4 pounds) which gives your archer a net 0.6 pound draw weight increase, which is easily doable and he/she can crank it up from there.
No matter what their “personal draw weight” is, use the ratings on the limbs to make purchases. So, if the limbs were marked 28#, you move up to limbs marked 32#. Whatever their personal draw weight max is, it also will be increased 4# (approximately).
Their personal draw weight, the “weight in hand” is what you need for fitting arrows, etc. The marked draw weight is only used to identify limbs for purchase.
But, Wait, There is More!
Here are two sets of limbs and their maxes (in hand) for that archer:
26# limbs 29.5# max
28# limbs 31.5# max
He also has a pair of 30# limbs, can you estimate what they would measure maxxed out for this archer?
I came up with 33.5#. In each case the difference was about 3.5# and since all of these numbers are fairly close together, that pattern should continue. When the limbs get up over 40# I expect slight differences.
Now, just for fun, take off 10% from each of those max DWs to give a range for each set of limbs.
|26# limbs||29.5# max||26.6# min|
|28# limbs||31.5# max||28.4# min|
|30# limbs||33.5# max||30.4# min|
Can you see that the 26# and 30# limbs cover the same range (26.6#–33.5#) as these three do? There is a small gap from 29.5# to 30.4# when the swap from the 26# limbs to the 30# limbs is made but that is a reasonable “jump.” This is why it is recommended that you buy limbs in 4# increments (another blessing from the ILF system).
Note Realize that often more that 10% can be removed from a set of limbs so that gap is often much smaller.
Safety Note Never exceed the number of turns allowed in the manufacturer’s instructions!
If you try this system, let me know how it works for you or your students.