HillBilly Science, Decoding of Practical Wisdom, It is Momentum and Not Weight Alone Which Chops Wood.

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Some are readers, others are watchers and I'm sure a few are both.  I have two videos and a full blog post today on the same basic theme.  The short video is meant to be a more accessible, shareable, quick download of bullet points and the long version is my nerdy love child.  This is my new strategy if I can pull it off, long geeky video, short shareable quicky video, instagram trailer and blog post.  This project started out when I pulled out Ellsworth Jaeger's book WildWood Wisdom, set it on my kitchen table, turned on a camera and started talked for 15 minutes.  It was supposed to be a super short low effort filler video.  I'm so naive.  Many versions, scripts, shoots, a blog post and 3 or 4 full days of work later, you and I have arrived here on this virtual page.

The short version

The Long Version


The Blog Post

I just dug out Ellsworth Jaeger’s copiously illustrated Wildwood Wisdom from storage.  This book is so much fun to spend a little time with some evening.  It's been a long time since I read any of it and I don't recall it having the best information ever, but it is by far the most fun woodcraft book because it is packed with amusing illustrations.  They alone really are worth the price of admission.  I suggest that anyone who has leanings toward woodcraft/bushcraft type of interests and pursuits get a copy on loan through your local library (if you still have one).  Print copies are cheap used.  https://www.alibris.com/Wildwood-Wisdom-Ellsworth-Jaeger/book/7228375

   Illustration from Ellsworth Jaeger's Wildwood's Wisdom.  I haven't tried it, but I'm very skeptical of the idea of burning out axe handles like this.

Illustration from Ellsworth Jaeger's Wildwood's Wisdom.  I haven't tried it, but I'm very skeptical of the idea of burning out axe handles like this.

Of course I flipped to the axe section straight away.  It is short and contains mostly the usual information.  In it, Mr. Jaeger repeats some common, simple ideas that are somewhat misleading.  To quote him:

“HOW TO CHOP:  In chopping, remember that it is the weight of the ax that really chops, and not the force of the swing.  Too much power behind a blow destroys your aim.  The best way to chop is to swing rhythmically.  Do not use force.”

The first statement is false and the last is pretty useless.  The author takes a stab at some truths, but in a very sloppy way that fails to foster understanding at best and in misleading at worst.   I intend here to try to extract the elements of truth from Mr. Jaeger's assertions and fill in at least what I think are the important missing parts.  I suspect that something approaching truth about how weight (mass) and speed (velocity) play out in actual chopping, and how they vary with varying head weights, could get some people chopping better, quicker.  If I didn't think it were so, I wouldn't bother.

There is no real need to talk about handle length here, so we’ll assume the same length of handle on any imaginary axes.  Head shape does matter though, so we’ll also assume very similar designs and grinds regardless of the axe head weight.  Also, I use repetition and restatements intentionally to drive points home and foster understanding, so get over it ;)

The statements again:

“In chopping, remember that it is the weight of the ax that really chops, and not the force of the swing.  This first statement is incorrect.  We will get back to it, as it is really the stimulus for this conversation. 

"Too much power behind a blow destroys your aim.  Too much force can mess up your aim to be sure, though that is greatly dependent on skill level.  Timbersports competitors hit hard and accurately with heavy axes. Too much is just too much by definition, I'm just saying that what is too much is dependent on context.

"The best way to chop is to swing rhythmically.  Yeah, getting into a comfortable rhythm really does seem to help with performance.

"Do not use force.”   This last statement does not really make sense, but I think he’s just trying to say don’t use excessive force, or his definition of force inherently implies excess.  We'll touch on this in the rest of this discussion, so enough said for now.

The first statement is just not true.  “...It is the weight of the ax that really chops the wood, and not the force of the swing.” Lets look at that, because it can help us understand some basic truths about chopping with an axe.

A pretty simple concept is at play here- MASS combined with VELOCITY equals EMBODIED ENERGY.   In other words, every object has mass and if that object is traveling it embodies kinetic energy.  The faster it is traveling, the more energy it embodies.  (I'm what some might call under educated, so some of these terms may not be exactly standard, but I think the concepts are solid.  I actually consider myself over-"educated", but I managed to escape without becoming overly indoctrinated.)

If we look at this equation in terms of axes, they will do different amounts of work, depending on the speed they are traveling. 

  • If we make any given axe head travel faster, it embodies more energy to do work. 
  • The weight or MASS and the speed, or VELOCITY, can be Added or Subtracted to change the EMBODIED ENERGY, or potential to do work. 
  • A lighter axe has to travel faster to do the same work as a heavy axe travelling at a slower speed.

To understand this, we need to get past the common misconception that the chopper somehow pushes the axe through the wood, and I think that this is part of what Mr. Jaeger was trying to get at.  Depending on the weight of the axe head, it is best to think of chopping as guiding, flinging, throwing or whipping the head of the axe into the work.  By the time bit meets wood, the work is already done, so we are interested in how to embody kinetic energy in the head before it strikes.

Does the weight of the axe do all of the cutting?  No, clearly not.  Stand over a log and drop the axe onto it repeatedly.  An axe dropped won’t do as much work as it would if the axe were swung by a person, even if swung lightly.  The heavier the head though, the more the work appears to be like dropping the axe, and indeed it is.  The lighter the head, the less work it will do if it is just dropped.  But!, while a light axe must be swung faster if it is to carry the same authority as a heavy one, if I cut the weight of my axe in half, my ability to do work with a given amount of energy expended does not decrease by half.

Again, Mass and velocity together make for the embodied energy that does the cutting.  It’s just going to get more complicated from there if we keep digging into physics, but we don't need to, so lets not.  The important part here is that velocity is the significant factor we can change to cut deeper if we are given any particular axe head.  Watch any experienced user work with a light axe and they will be seen to be swinging it with some velocity and not just dropping it.  Conversely, watch any experienced user using a 4 lb axe and it will appear closer to true that they are letting the weight of the head do the work.

I think that Mister Jaeger is trying to say that the combination of weight and speed of the head (momentum) are what do the work and the axeman can chill and let that happen rather than taking a death grip on the handle with intent to bludgeon the work to pieces.  If that is Mr. Jaeger's intent, it is very poorly stated and I think it's likely that he understood the problem physically, but not intellectually. 

For example, I can drop the axe straight down onto wood and it will cut a certain amount, but how much work does an axe head's weight do if you drop it on a vertical surface like a standing tree trunk.  Answer, none, it won't fall onto a vertical surface, it will just fall to the ground.  A still axe has no embodied energy, so someone has to swing it at a vertical target to do any work.  If you chop straight down between your legs, the axe will be pulled by gravity sure enough, but it is not only difficult to add to avoid adding extra velocity to the axe at all as you guide it home, but it would also be silly.  Of course you will add a little more cutting power to the process by swinging the axe, why wouldn't you?  There is simply no happy place in this equation where the weight of the axe does all the work.  A still mass does nothing.  Since the faster it is swung the more work it will do, it is not accurate to say the head's weight does the work and the lighter the axe, the less true that becomes.  This point is not just academic, but technique must vary across a spectrum of head weights if the chopper is to be effective and not wear himself out.

If a small axe has to be swung faster, why would anyone use a light axe, such as the once common pulpwood axes with 2.5 lb heads?  Don’t we have to do more work to swing that light axe harder?  To answer that, let's ask another question, what does harder mean?  A light axe needs more velocity, but does that necessarily equate to more energy expended?  A heavier axe may do more work with less velocity, but it does still require some velocity and the force to create that velocity requires energy input.  An axe head represents an inertia that must be overcome.  Inertia, now there is a thing.

Inertia says that a mass wants to remain still if it's already still and remain moving if it's moving.  Matter it appears doesn't care much for change!  It's like getting a kid out of bed to go to school, then trying to get them to go to bed later when they are running around all hyped up.  A heavier head requires more effort to get it moving.   And of course a light head requires less energy to move, which is good, since it also requires more velocity to do the same amount of work.

    Page from Bernard S. Mason's Woodsmanship

 Page from Bernard S. Mason's Woodsmanship

 

Another issue is that the overall work required to use an axe consists of more than just accelerating the head into the work.  We also have to lift it between strokes.  The heavy axe, again, requires more effort to break it’s dead weight inertia in order lift and swing it.  If there was no cost to dead lifting that heavy axe, we might just all be using super heavy axes that we do indeed just lift and drop.  But of course an axe that requires little authority to lift also has little authority when it falls.  I may have to swing a small axe faster, but it is also easier to swing fast than a heavy axe is, and I don’t have to use as much energy to lift it for every blow.  I can lift a light axe all day long.  Likely there is a sweet spot or range in there for different people that balances these differing energy expenses.  I can do quite a bit of work with the small forest axes at only 1.5 to 1.75 lb on 25 to 26 inch handles, by whipping them into the work in a snappy fashion, but I’d rather not, in most circumstances, because they do require a lot of velocity and regardless of how I generate that, and how much of it, it becomes an issue at some point.  The good side is that I will never tire of lifting it and even if greatly fatigued, I can chop on lightly, whereas eventually a too heavy axe becomes a burden that can no longer be worked with effectively.

The issue of what head weight is best could, and no doubt has been, long argued.  There are strong proponents at both ends that could point to physics and various traditions for proof.  But, the judgement is very subjective, given different bodies and styles, let alone types of work and material cut.  I’m still working my way around handle length and head weights to figure out where I want to be with a multi-purpose axe, and that is a very personal thing.  Obviously, different configurations will be better suited to different tasks as well.  I just wanted to try to explain this though, because I think it will help people understand something important in chopping and can lead us toward better use of both light and heavy axes.

If viewed as a spectrum, as I increasingly do with many things, the extremes of light and heavy axe heads eventually decrease in functionality.  Too heavy is just too heavy for anyone except the mythical lumberjack Paul Bunyan, and too light it just too light.  Sure, you can theoretically swing a very light axe very fast to achieve a high value of Momentum, but it becomes difficult or impossible at some point and the faster the swing, the more likely aim and control will suffer.  In the middle ground are a fair range of weights that can be chosen and adapted to for varied circumstances.

Here is a suggestion.  Stop thinking vaguely in terms of force and power and weight and think more about velocity.  Assuming the same angle of attack, grind etc, velocity is the thing you can change that makes any given weight and configuration cut more or less deeply, and that goes for light or heavy axes.  I am all about velocity, because velocity is something we have some control over with axe in hand.  The pursuit of velocity is also one of the things that will wear you down if you try to generate too much of it, so don't.  It's not about generating as much velocity as possible, but generating enough when it's needed and generating it efficiently.

When you hear people talk about "snap" or "power", or hitting things "hard", they are talking about velocity.  I think most don't know that though.  But, knowing exactly what that factor is that makes an axe of a fixed weight hit the work with more authority can be very enlightening and inform our work and technique for the better.  How to generate that velocity is another topic.

 

PHYSICAL EFFICIENCY

A light axe can be whipped into wood at a high velocity without all that much effort.  This is what I call physical efficiency.  That just means the amount of work actually done for the amount of energy expended by the user.  It is possible for one person to use vastly more energy to do a task than the same task done by someone with a higher physical efficiency.  If someone says that a light axe requires more work to use, they will have a hard time proving it to anyone but themselves outside of obvious extremes, and the opposite is equally true.

In cultivating physical efficiency, one obvious error is that you should not be using muscular effort that is unnecessary.  That means relaxing muscles that you do not need to be using.  Another major factor is that there are many different ways to get the axe to the same spot at the same velocity.  If the axe gets there at the same angle etc, then it should do the same amount of work, regardless of how it got there.  But, how much work and overall movement was done to get it there?  On the way, many joints articulate at different times worked by many muscles.  These orchestrations of motion are remarkable and incalculable.  We don’t think them through as they happen.  It is not a cerebral process, or at least not a conscious one and doesn't need to be. 

Over time, we hopefully get our axes to the target with the required velocity, but do that with less and less energy expended and less and less movement, or at least with the movements that require the least energy for amount of work done.  Some bullet points:

  •  Using muscles unnecessarily is a waste of energy.  Relax.
  • There is more than one way to get the axe where it’s going at a given velocity, but not all will be equally as comfortable or efficient.
  • Chill out.  Unless highly skilled, an aggressive fast pace of work will get you where you are going slower and more fatigued than if you take a more sober approach.

 

IN CLOSING

Many will go into axe work thinking that much force and effort is required.  Experienced axeists are always trying to curb that attitude by saying to relax into an easy rhythm.  One way they try to say this is to "let the weight of the axe do the work" or "Let the axe do the work".  As pointed out above, this is rarely ever actually true and it is easy when thinking in these simplistic terms to be mislead into thinking that a small axe is necessarily more work to use, because it has to be swung "harder".  As I've tried to make obvious though, there are trade offs in either direction and there is no free lunch.  In particular, I think that it is probably more difficult to learn to whip a small axe into the work at a high velocity with very good economy of motion and effort, all while maintaining accuracy.  It is a skill that is hard earned by repetition.  If I were to analyze it more, I might be able to explain and teach the motions involved, but people who are good at it don't think about what they are doing, because they don't need to.  What you can note if you see someone doing it well is that they are fairly relaxed, and the axe accelerates rapidly at the end of the stroke.  Motions are not exaggerated, but pared down to the essentials.  The axe head will be seen to rotate around one or more pivot points, like the elbows, shoulders and especially the wrists in order to create that snap of velocity at the end of the stroke.

Here are what I think are some truisms:

  • Don’t use excessive force, especially when learning. It is likely to throw your aim off, wear you out and it is decidedly unsafe!  However, that said, "excessive" depends on skill and context.
  • Lighter axes require more velocity than heavy axes
  • Heavier axes require more energy to lift than light axes and also require more energy to increase velocity when swinging, but require less velocity.
  • Hitting the target with a given velocity can be achieved with almost endless variation in subtle and not so subtle movements.  How much physical energy you expend to get there is the relevant question, and minimization of that effort is a worthy goal.
  • Whether the axe head is light or heavy, velocity is a useful way to view the generation of cutting power in axes, since it is the factor in the equation that you can actually change with any given axe of a fixed weight.

DANGER, DANGER

The generation of velocity while maintaining accuracy and preventing loss of control can be tricky.  There is hardly a better way for a novice to get into trouble with an axe than by trying to force the use of velocity ahead of skill level.  Don’t push it.  Know that it is a factor, but let ability develop naturally with experience gained.  Don’t expect to do the same work in the same time, or make the axe cut to the same depth with each stroke as someone who is more experienced may be seen to do.  Both excessive force and force poorly applied are a danger to the axe, the user, bystanders and your energy level.  Concentrate on accuracy and becoming comfortable with the movements.  The rest will follow naturally.

     

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      Posted on September 15, 2017 .