falling 101
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GASoline71
Mr. Nice Guy
When "engineers" become fallers...
However... you will notice when the wedge does some weird stuff to the hinge when driving it into a sloped backcut.
Good luck... your gonna need it...
Gary
Unknown Cutter
ArboristSite Lurker
i have a question about sloped backcut for you math guys
it seems to me that if the backcut is angled up to the notch it would be more effective than if the cut angled down?
i dont use this cut but the discussion got me to wondering
it seems to me that if the backcut is angled up to the notch it would be more effective than if the cut angled down?
i dont use this cut but the discussion got me to wondering
dont matter to me if the glass is half empty or half full, aslong as its booze
Terrific discussion, techdave! It's great to see some really thoughtful analysis instead of the loud posturing so common around here. I'm working to extend some of your remarks, but I keep finding serious errors in my thinking. In the meantime, I wanted to publicly applaud your high-quality post.
Unknown Cutter, if by effective you mean effective at pushing the tree over, then mathematically the angle of the back cut per se isn't an issue. But as others have pointed out, the greater the distance from the backcut opening to the hinge, the more effective the wedge, and that distance is greater the greater the angle of the backcut (for a cylindrical tree). But there are complicated practical issues for real trees, and techdave has done a great job at laying out a bunch of them.
TexasTreemonkey
ArboristSite Operative
Lets just take a vote. I for one believe if there is not a foriegn obstacle (a fence) behind the stump and you can get a straight shot dont do an angled cut. period. 1 vote for straight.
juststumps
ArboristSite Guru
we are not saying the same thing.....the only mechanical advantage you are getting is in driving the wedge.....driving a 1" wedge into a 17" cut is easier than driving a 1" wedge into a 12" cut...
you would have to use a 1.5" wedge in the longer cut,, to get the same amount of work,, and then there goes you advantage...
i don't conclude at all...
you might be mixing the properties of a lever up with those of an inclined plane
the wedge doesn"t care... its the hinge that does...
juststumps
ArboristSite Guru
only if you want to do it wrong,, and leave four foot tall stumps,, while having to fight the weight of your saw cutting up hill.....
juststumps
ArboristSite Guru
WRONG WRONG WRONG
you are really confusing the properties of levers and incline planes now...
techdave
ArboristSite Operative
Forces and backcuts...+thanks Moray
HI moray, thanks for the compliment. Wish I could draw it and scan it!
Hi you all, the wedging creates forces acting on the stump and bole sides of the backcut. It also creates a moment aka torque on the stump and bole sides of the backcut.
The moment created is simply the force pushing apart the two sides of the backcut, times the distance from the place that force is applied to the hinge.
Orientation doesnt matter, as someone pointed out with their steering wheel analysis.
But FORCE, that is another matter. In a conventional (flat) backcut the forces generated as the wedge goes in are ONE a bigger one trying to move the two sides of the backcut apart. this is the useful effect we are movign the tip of the tree with to get it to go over in the desired direction.
--- and TWO a lesser force from friction trying to push the stump over forwards, and by pushing the bottom of the bole forwrds cause rotation of the tree backwards. BUT neither of these things happen in a typcial cut, as the stump to be is too firmly rooted to rotate under the axe or hammer blows. And if the base dont move, and the hinge dont fail the only movement of the bole will be to roatate as the wedging TORQUE is trying to do.
(IF IF IF the hinge were to suddenly fail during wedging the butt of the bole might mvoe forwards and cause the treeto fall backwards. this is similar to the effect people can get if they push too low on the tree with a loader or dozer.)
Back to force ONE above, if you diagram it out you will see the force from the wedge that is trying to open the sloped backcut has a large component pusing forwards on the bole of the tree trying to shear the hinge and move the bole off the stump. Possibly before the tree has committed to going the pro0per direction~!
BAsically as far as force and moment go, the angled backcut creates a greater rotational moment trying to tip the tree (acting on what could be a MUCH weaker aspect of the wood than in a conventional backcut), AND a MUCH higher shear force on the hinge, equal to the force pushing forwards on the base of the bole.
ALL of this, still dont see any good reason to slope backcut, and many reasons not to slope it.
All of this to reanalyze a cut most all of us agree is not a good idea!! DOH
ps to jump a tree off the stump, anything that breaks the hinge while the tree has got some of its rotation as forward velocity will cause the tree to jump forwards.
If the hinge breaks to late, the center of mass of the tree is moving almost straight down, aka just before the tree hits the ground the hinge breaks.
A dutchman can be used intentionally for this purpose. A steeply sloped STOPE can help get the butt down on the ground quickly if the hinge has been broken somewhat early.
HI moray, thanks for the compliment. Wish I could draw it and scan it!
Hi you all, the wedging creates forces acting on the stump and bole sides of the backcut. It also creates a moment aka torque on the stump and bole sides of the backcut.
The moment created is simply the force pushing apart the two sides of the backcut, times the distance from the place that force is applied to the hinge.
Orientation doesnt matter, as someone pointed out with their steering wheel analysis.
But FORCE, that is another matter. In a conventional (flat) backcut the forces generated as the wedge goes in are ONE a bigger one trying to move the two sides of the backcut apart. this is the useful effect we are movign the tip of the tree with to get it to go over in the desired direction.
--- and TWO a lesser force from friction trying to push the stump over forwards, and by pushing the bottom of the bole forwrds cause rotation of the tree backwards. BUT neither of these things happen in a typcial cut, as the stump to be is too firmly rooted to rotate under the axe or hammer blows. And if the base dont move, and the hinge dont fail the only movement of the bole will be to roatate as the wedging TORQUE is trying to do.
(IF IF IF the hinge were to suddenly fail during wedging the butt of the bole might mvoe forwards and cause the treeto fall backwards. this is similar to the effect people can get if they push too low on the tree with a loader or dozer.)
Back to force ONE above, if you diagram it out you will see the force from the wedge that is trying to open the sloped backcut has a large component pusing forwards on the bole of the tree trying to shear the hinge and move the bole off the stump. Possibly before the tree has committed to going the pro0per direction~!
BAsically as far as force and moment go, the angled backcut creates a greater rotational moment trying to tip the tree (acting on what could be a MUCH weaker aspect of the wood than in a conventional backcut), AND a MUCH higher shear force on the hinge, equal to the force pushing forwards on the base of the bole.
ALL of this, still dont see any good reason to slope backcut, and many reasons not to slope it.
All of this to reanalyze a cut most all of us agree is not a good idea!! DOH
ps to jump a tree off the stump, anything that breaks the hinge while the tree has got some of its rotation as forward velocity will cause the tree to jump forwards.
If the hinge breaks to late, the center of mass of the tree is moving almost straight down, aka just before the tree hits the ground the hinge breaks.
A dutchman can be used intentionally for this purpose. A steeply sloped STOPE can help get the butt down on the ground quickly if the hinge has been broken somewhat early.
Well I apologize for my comments not having a scientific tone but I do have experience with both types of back cuts and to state it correctly, in my opinion, the sloping back cut does not provide an advantage in getting the tree to fall. I did not take notes of my experiements and maybe it was just luck. I know what works well and I have never had an experienced sawyer tell me to use a sloping back cut. I suppose if you are cutting some trees on a river bank or steep incline you may be able to use one. For that matter, you could use one on any tree. It is your choice and you are free to do what you want.
I really wish I didn't read this thread to begin with because I doubt if the people who want a scientific explanation are worth their salt with a saw. Here is an idea, go get a government grant to figure out the forces and what not but at the end of the day, I doubt if you get all of us guys using a flat back cut to switch to a sloping back cut.
Merry Christmas
I really wish I didn't read this thread to begin with because I doubt if the people who want a scientific explanation are worth their salt with a saw. Here is an idea, go get a government grant to figure out the forces and what not but at the end of the day, I doubt if you get all of us guys using a flat back cut to switch to a sloping back cut.
Merry Christmas
ropensaddle
Feel Lucky
The standard or common notch and back cut,the lumbermans notch and back cut and slight variations in notch width is all I use and a flat backcut is laid into them after kerfing for reference.
TheKid
ArboristSite Operative
wow, this post has gathered steam! great! where's clearance? I don't know if this has been discussed yet, but the reason I was told not to execute the sloping back cut was that (esp if you cut the hinge accidentally) you effectively create a ramp for the butt to slide down the stump inspiring it to fall OPPOSITE the face cut.
another thought
TD, in trying to extend what you have so clearly presented, I began to realize how complicated this whole thing gets when you leave the simple world of classical physics and start thinking about all the possible ways real wood is going to screw up the simple equations.
You rightly mention the friction between the wedge and the two faces of the kerf. We should add to your enumeration of the assumptions of the classical anaysis the assumption of a frictionless wedge. The frictional force on a real wedge is actually going to be enormous given the tremendous weight of the tree normal to the wedge, and we are certainly going to have to account for it in a more complete analysis.
In the ideal case of a frictionless wedge, the only forces between the tree and the wedge will act perpendicular to the wedge faces.
The frictional forces of the real wedge act parallel to the wedge faces. For the upper face in the flat backcut the friction, as you drive in the wedge, acts to push the bole forward (as you noted). Isn't this just the "shear" force that everyone is so worried about that tends to break the hinge? Now it may be larger in a slanted backcut, but it seems it will be present in a flat backcut as well.
There seems to be universal agreement here that wedging a slanted backcut is much more likely to break the hinge, and maybe this is true, but I think someone needs to show that it is.
Here is a contrary thought that shows why I retain a bit of skepticism about this claim. Forget the wedge for a moment, but hang on to the hammer. Swing hard and hit the bole right above the backcut, trying to drive the bole off the stump. Now go up 30 feet and do the same swing striking directly above the first contact. The force vector is identical in both cases--the dreaded horizontal "shear" force that wants to break the hinge. But does anyone believe those two hammer blows are equally likely to be successful? It seems that distance from the hinge is a factor we must consider.
This is an excellent problem, and there is certainly much more to it than has been discussed so far. I look forward to hearing what others have to say.
TD, in trying to extend what you have so clearly presented, I began to realize how complicated this whole thing gets when you leave the simple world of classical physics and start thinking about all the possible ways real wood is going to screw up the simple equations.
You rightly mention the friction between the wedge and the two faces of the kerf. We should add to your enumeration of the assumptions of the classical anaysis the assumption of a frictionless wedge. The frictional force on a real wedge is actually going to be enormous given the tremendous weight of the tree normal to the wedge, and we are certainly going to have to account for it in a more complete analysis.
In the ideal case of a frictionless wedge, the only forces between the tree and the wedge will act perpendicular to the wedge faces.
The frictional forces of the real wedge act parallel to the wedge faces. For the upper face in the flat backcut the friction, as you drive in the wedge, acts to push the bole forward (as you noted). Isn't this just the "shear" force that everyone is so worried about that tends to break the hinge? Now it may be larger in a slanted backcut, but it seems it will be present in a flat backcut as well.
There seems to be universal agreement here that wedging a slanted backcut is much more likely to break the hinge, and maybe this is true, but I think someone needs to show that it is.
Here is a contrary thought that shows why I retain a bit of skepticism about this claim. Forget the wedge for a moment, but hang on to the hammer. Swing hard and hit the bole right above the backcut, trying to drive the bole off the stump. Now go up 30 feet and do the same swing striking directly above the first contact. The force vector is identical in both cases--the dreaded horizontal "shear" force that wants to break the hinge. But does anyone believe those two hammer blows are equally likely to be successful? It seems that distance from the hinge is a factor we must consider.
This is an excellent problem, and there is certainly much more to it than has been discussed so far. I look forward to hearing what others have to say.
With all due respect, juststumps, these two statements flatly contradict each other.
And I don't think I am confusing a lever with an inclined plane; I am merely accounting for the fact that the scenario we are discussing includes both a lever and an inclined plane.
The inclined plane, of course, is the wedge.
The lever is the bole of the tree. The fulcrum for the lever is the hinge. We are applying force to the lever with the wedge, and the lever arm is the distance from the hinge to the contact area on the wedge. The load we are trying to move with the lever is the entire bole of the tree. The longer the lever arm, the more overall mechanical advantage for the entire system, which is why, as I have said, the slanted back cut has a theoretical advantage. Do you agree?
Mitchell
ArboristSite Operative
Dolmar
Do not think it has been mentioned yet, but I seem to remember the Dolmar product brochure for 2007 has a guy cutting a slanted back cut? A European technique perhaps?
I know the discussion is theoretical however, practically speaking A tree [if its cut right] would not be wedged off its hinge with out difficulty whatever the cut.
Jumping trees= just cut the hinge as they are tipping.
Do not think it has been mentioned yet, but I seem to remember the Dolmar product brochure for 2007 has a guy cutting a slanted back cut? A European technique perhaps?
I know the discussion is theoretical however, practically speaking A tree [if its cut right] would not be wedged off its hinge with out difficulty whatever the cut.
Jumping trees= just cut the hinge as they are tipping.
clearance
Addicted to ArboristSite
Here I am. It is wrong, don't do it. The backcut should be level with the flat cut of the undercut, but slightly higher. Thats all, end of story.
I tried playing with the math involved with the jumping tree a couple of years ago, but when it got too hairy, I did an experiment with my camera instead. I stood up a wooden yardstick (the tree) on a flat deck railing and used a brick next to the base to keep the butt from moving away from the fall. Then I repeatedly let it fall, flat side down, toward the railing, snapping still shots at various stages of the fall. Results: the butt would first start moving away from the brick (jumping) when the yardstick was only about 15 degrees above horizontal. The typical jump distance was about 1 inch.
smokechase II
Addicted to ArboristSite
Wedging
Originally posted by juststumps
...driving a 1" wedge into a 17" cut is easier than driving a 1" wedge into a 12" cut...
Then Moray's response: "With all due respect, juststumps, these two statements flatly contradict each other."
**********************
Moray; I believe Juststumps is correct.
All other things being equal, face - hinge - wind - lean - wedges etc., the closer a wedge is to the hinge the harder it is to drive.
If Juststumps is driving wedges at the very back center of the back-cut in both instances the longer distance is a longer lever.
What we have here is a failure to communicate.... err
What we have here is an incline plane lifting a lever.
The incline plane requires less force to drive if it is working with a longer lever.
The bottom of the butt log in the back-cut is a lever.
Besides the sloping back-cut being more work to make, if wedged its tendency is to push the tree off the stump (break the hinge) rather than pivot at the hinge.
Originally posted by juststumps
...driving a 1" wedge into a 17" cut is easier than driving a 1" wedge into a 12" cut...
Then Moray's response: "With all due respect, juststumps, these two statements flatly contradict each other."
**********************
Moray; I believe Juststumps is correct.
All other things being equal, face - hinge - wind - lean - wedges etc., the closer a wedge is to the hinge the harder it is to drive.
If Juststumps is driving wedges at the very back center of the back-cut in both instances the longer distance is a longer lever.
What we have here is a failure to communicate.... err
What we have here is an incline plane lifting a lever.
The incline plane requires less force to drive if it is working with a longer lever.
The bottom of the butt log in the back-cut is a lever.
Besides the sloping back-cut being more work to make, if wedged its tendency is to push the tree off the stump (break the hinge) rather than pivot at the hinge.
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