Spine Whack Myth ?

[Robbie Roberson]

Hello fellow thinkers,

First a few questions, does anyone know where this term originated ?

I did a search using "Spine whack" and it gave me twenty pages.......so........there must be something going on here.

Now for the discussion part, I don't think it is a valid test by any means because if I understand this correctly, the knife is upside down when done.

Who pushes a knife blade up against anything upside down from the top ?

Who strikes the blade of a folding knife upside down while doing any task or chore ?

For those "spine whack failure" believers, please help me understand what the advantage is by performing this.

Does anyone else believe in a liner lock stretching or giving in or around the pivot point ?

Now for my theory, I think the liner lock spine whack test is not valid for determining the liner locks ability to hold the blade open because of this;

When the knife is struck upside down near the pivot, I think there is a very slight amount of stretching or movement that occurs in the "pivot pin" area by the downward motion and gravity and therefore causing some sort of liner/tang distortion or mis-alignment of liner, breaking contact and friction and possibly causing a "skating" effect of liner.

This IMO could cause a very slight separation of the blade tang and the liner lock, thus allowing just for a split second the liner lock to not be in contact with blade tang and possibly allowing the angle of the tang to over ride friction and cause the liner to move over (skate) to the unlock position.

Of course all this would happen in a millisecond, the naked eye could never see the movement of the liner, but if a camera could capture the stretching of the pivot area, I think you could see a gap in the liner and tang that could cause failure. Kind of like stepping on ice flat footed for more friction contact compared to heel ot toe contact. If the liner even for a millisecond becomes distorted or out of alignment with the tang, it could skate to the unlock position.

This type of liner lock failure could not occur if the knife were in an upright position and constant pressure were kept on the blade/pivot area (as long as the knife is kept in a upright position, if for a second gravity overrides and the knife is pushed to the side, it could cause the liner to unlock, which I think has happened to some users).

IMO the more pressure applied to the pivot/blade, (while holding knife in up-right position) the more the liner lock should move over farther into the locked position, but a quick snap or hit in an upside down IMO could easily cause the liner to skate over to the unlock position.

Please don't flame me, this is something I have had on my mind ever since I first read the words "spine whack failure".

Thanks for any ideas, opinions or thoughts.

Robbie Roberson

 

[phatch]

Based on those beliefs, you don't need a folder that locks. The lock is all about a blade moving from force on the spine. If that force is unrealistic, so are all locks.

All locks can be tested via a spine whack, not just liner locks.

Phil

 

[RedEdge77]

I also don't think spine whacks are a good test. If you do a spine whack with a swiss army knife it stays up and i hit the back relatively hard.

 

[paulwesley]

spine whack test could be the waste of a good knife!!!

 

[STR]

Spine whacks where you dent wood or table tops constitute gross abuse of the knife. Spine taps on the other hand are a good idea and something any good quality knife should take with no issues at all. If you imagine a thrust movement and sticking the knife into an object, lets say a tree, and then pull back hard to remove the knife. Lets say it comes out of there easier than you expect and you flip or flail your arm back and accidentally hit the wall or another object with the spine of the blade from the momentum of the force you initiated to remove the knife. That is a real world scenario where the spine can be tapped. Also, a knife fight or defense where the blade gets tapped from another object in a defensive situation. So, some tapping should be expected even if just in extreme situations.

Most knives can endure a tap. I test mine on the toe of my boots. This doesn't hurt the boots or the knife and allows me to see if the knife can stand up to a tap. Most all do and there is no further need to keep doing it harder and harder until I get a failure, or the need to move on to a harder surface to really whack it. That is just plain stupid in my opinion.

STR

 

[high]

hi robbie, happy new year , i too a while ago got into this testing with spine whacks. i have come to agree with STR that a reasonable spine whack is o.k.I had taken it to the extreme and caused a chinook and manix to unlock getting into a real flame war. outcome was spyderco replaced the knives and sal looked at the knives himself. I think there was a spring problem or something i will find this out later but i also very likely did those knives no good with the degree of force i hit them with.at any rate i believe that a light test serves to simulate the knife bumping against some hard object and closing . anything more imho is indeed abusing a good tool. high

 

[Robbie Roberson]

Phatch, I think all locks on knives are very much needed. I just don't think a hard spine whack test proves anything other than metal objects can or will flex or give under extreme pressure or forces in some way or another.

I think a gentle tap or gently applied pressure like STR suggest is all thats needed to "test" a liner lock.

Of course not all will fail, just those that may become out of alignment when struck or pushed with enough force to cause the two matching parts in contact to become uneven or not in full contact, then you could have a failure I think.

As far as the lock is concerned, if we had the ability to hold knives exactly perfect in an upright position, only using a stop for the blade tang to come in contact with, then we could all use the friction folder, it has no lock other than some sort of stop located on top of spine that the blade tang comes in contact with.



Robbie Roberson

 

[Cliff Stamp]

First a few questions, does anyone know where this term originated ?

Read the FAQ, Joe Talmadage and A.T. Barr.

Who pushes a knife blade up against anything upside down from the top ?

Who strikes the blade of a folding knife upside down while doing any task or chore ?

There are four considerations, one is just accidents, people have reported turning and popping the spine off a desk and having the blade collapse, the second is for tactical folders used in combat which the user wants to use for back cuts or blocking/parry, the third is heavy utility use such as advocated by Doug Ritter on his Benchmade knife for wood working, and the fourth is various uses which can cause spine loads indirectly.

For example if you just take a knife and stab something, a part of the force will be compressive against the spine, and part of it will be perpendicular and either try to rotate the knife backwards, or act to override the lock depending on the angle the blade makes and the follow through of the stab. For example the UK Pen has no lock and it is trivial even with light stabs to cause the blade to fold for this reason.

Similar when working the knife through very thick material, in a sawing motion, especially when the material is very binding and moving, I experienced this recently when using the UK Pen to cut thick rubber, it was again very easy to have the blade fold and doing it dynamically would be very dangerous. A liner lock which easily failed a spine whack would act similar.

-Cliff

 

[Dirk]

The spine whack test is valid to a point. I agree that and extreme hit on the spine is needed to test a lock. The lock should be tested within the realistic range of uses a folder would go through. There are some folders out there designed to go through more extreme use then others. For me, the test itself isn't about the knife, but about the maker and his/her understanding of how a lock should work. If the lock fails in this test, you should look to see if the lock was built correctly. Not too long ago, a large numbger of knives failed this test. Mainly because makers didn't know the right way to make the lock. Nowadays, most knives will pass this test with no problem. Makers learned from the failure of their knives. Which, is the point.

Anthony Lombardo has developed the ability to get almost any lock/knife to fail the spine whack test. It isn't through a massive hit. I am not sure how he does it. My theory is that he hits the blade in such a way to create a harmonic vibration through the blade, to the lock, causing the blade to bounce off the lock. I don't know if the terminology is correct, but the idea is there. Hit the blade too soft or too hard and it doesn't work. Just right and the blade will slip off the lock.

Personally, I will do a moderate tap for a spine whack and then I will apply pressure to the open blade and see if it pulls away from the blade stop. If it requires little effort to pull away from the stop, I will then do the same thing only watch tot see if the lock is slipping off the blade. Either if the lock is flexing or if it is slipping under minor pressure, I will consider that a failure on my book.

 

[socintel]

I agree with STR, a really hard spine whack is abusive and unneccesary. But I can see where a light spine "tap" could be a good idea. I wouldn't want to use a knife where the lock fails after the spine is lightly bumped. It's really a matter of personal preference, some people just want a knife which could take anything you throw at it, regardless of how unlikely it is to happen.

 

[Habeas Corpus]

Similar when working the knife through very thick material, in a sawing motion, especially when the material is very binding and moving, I experienced this recently when using the UK Pen to cut thick rubber, it was again very easy to have the blade fold and doing it dynamically would be very dangerous. A liner lock which easily failed a spine whack would act similar.

Working a knife through thick and binding material is in my opinion the most common everyday situation where a lock may fail. It's also kind of situation where I feel a good, reliable lock is most important.

BTW, as much as I like the UK Penknife I wouldn't really want to use it all that much for cutting such materials. But I'd much rather use it than some knife with a suspect lock. With the UK Penknife I at least *know* I've got to be careful.

Hans

 

[Habeas Corpus]

spine whack test could be the waste of a good knife!!!

Depends on the test. It also depends on how much you value your fingers. I'd rather do a (reasonable) spinewhack test than pay a visit to the emergency unit.

Been there, done that, got the T-shirt.

Hans

 

[Cliff Stamp]

BTW, as much as I like the UK Penknife I wouldn't really want to use it all that much for cutting such materials. But I'd much rather use it than some knife with a suspect lock. With the UK Penknife I at least *know* I've got to be careful.

The interesting thing is that it actually passes the tests that many locks fail and is actually far safer to use. For example all the posts in the above which say it is abusive to spine whack hard and just do light impacts off the hand or similar, these won't even fold the UK Pen blade back onto the hand. Why would a locking folder be *LESS* stable than a slip joint. How can you possibly argue this makes any sense.

-Cliff

 

[longbow]

Here is the deal as I see it for this test. I belive the lock is released when the knife is spine whacked not through stretching. Just isn't possible. You can't hit it hard enough for the stop pin to bend backwards unless the fit is very poor. What does release it is vibration to the liner lock itself. This can be demonstrated by hitting the back of the knife towards the ricasso and then out towards the end of the blade. In all liklihood if a knife is going to fail at the liner it will do so out towards the end of the point when the knife is given a good wrap. Just sets up more vibration down to the liner. You can even feel it in your hand as you do so. I also don't see how this is gross misuse of a knife. Who knows you may have to use the back of the blade to pound something. You never know. If the lock is made correctly it will perform as it should and you shouldn't have to worry about it failing.

I also belive it is a valid test to perform. I gave a knife to my wife to use of a high end manufacture'r. She is left handed. She was using it to cut something, if I remember it correctly I was putting another gasket around the fridge door and wanted to trim the excess off. She cut the material no problem. Withdrawing the knife she bumped the blade out near the tip on one of the door shelves and the knife release on to here fingers. Nice cuts to say the least.


Now all that being said if I have a liner that fails the wrap out towards the tip what I will do is hold the liner open and then starting at the base of the blade. I will wrap the knife till I am out towards the end still holding the liner against the tang as I do so. This to me tends to set the lock. I have had it work on many knives with suspect liner locks. keepem sharp

 

[Robbie Roberson]

Lots of good ideas and thoughts here.

Longbow, I am not certain what happens if or when a lock fails, it could be movement within the pivot area. Stretching may not be the word I was searching for to describe what I think may happen, it may be closer to flexing or distortion.

When you say you "set" the lock, I think this makes sense because I think if the edges are almost perfectly flat and matching, then you would have more uniform contact friction on each part, possibly resulting in a more secure lock.

I assume that most test are performed by hitting the back of the blade somewhere, which would not effect the stop pin, but I think does what you said, it may cause a vibration.

Dirk said Anthony Lombardo can do it when testing, this is really close to what I think is happening, a sort of vibration or movement that allows the two flat surfaces to separate for a millisecond and skate accross the back of the blade to cause failure.

But my original thoughts were this, does the spine whack test really prove that a lock will fail ? I don't think so.

After reading these threads, I think it says even more that many variables could be involved in lock failure, I think things like cutting through thick binding material such as Habeas Corpus and Cliff Stamp suggest may be a sure way to see if a lock will hold.

I guess to sum it all up in the way I feel, I feel that this "spine whack" test may be causing some uneeded worries. Especially since probably 95% of all the liner locks in the world that will cut something today will be doing so by exerting downward pressure on the blade, which in reality is making the stop pin work harder than the liner lock....

Thanks for all your input.

Robbie Roberson

 

[allenC]

But my original thoughts were this, does the spine whack test really prove that a lock will fail ? I don't think so.

Yes, the spine-whack test really will prove that a lock will fail.
It's hard to believe until it happens.
Before I ever heard of the "spine-whack test", and I was rather new to the forums, I had a locking folder collapse on my hand.

Here's how it happened:
After a long and muddy hike, I took a break and was using my knife to clean some mud from my boots.
I have did this thousands of times when I was in the Army (mostly with my old Schrade LB7), but this time I had a CRKT Grey Ghost Mirage (aluminum scales, liner-lock, AUS-8, a Hammond design--very nice knife).

To loosen the dried mud in the grooves and tread of the boot, I locked the blade open and used the spine to thump the bottom of my boot--just as I have done a thousand times before.
Imagine the look on my face when the blade folded as if it did'nt even have a lock!
Imagine the look on my face when the blade bit in to my hand!

Later, I believed that I had simple failed to fully lock the blade open.
And then I started reading about the "spine-whack test"...
So I inspected my Mirage carefully, locked the blade open, everthing seemed perfect with no blade-play in any direction.
I then did a light spine-tap against the carpeted floor and, click, it folded with virtually no resistance at all--just as it had when I was cleaning my boot.

Then in a fit of suspicion I tested every single locking folder that I owned--none of the others failed.

If you don't believe in the spine-whack test, that's your right, but don't say that you were'nt warned.

Good luck,
Allen.

 

[Cliff Stamp]

Yes, the spine-whack test really will prove that a lock will fail.

The main problem is the name, it makes people think about unconsidering bashing and few realize that it originated with a maker of liner locks. The other issue is that people won't consider that you don't need the exact same loading to have it be an influence, though this is relevant specifically for a lot of promotion of knives.

Same reason that the concrete block test gets the same reaction, a lot of people see it as some kind of arguement that the knife is being promoted for the situations where you need to cut up blocks, they never bother to read the actual reason given by the maker who suggested it (Ray Kirk), which is completely different and has nothing to do with masonary work.

-Cliff

 

[Gryffin]

Hello fellow thinkers,

Hello right back atcha!

For those "spine whack failure" believers, please help me understand what the advantage is by performing this.

It's a pretty well-documented phenomenon: liners locks often fail under dynamic loads ("whacks") producing forces far less intense than the locks can withstand statically. Not all models suffer from it, nor do even all examples of any particular model. But it *does* happen, and since there's always the chance you may strike the back of the blade by accident, it's a good idea to perform such a test on any liner lock you plan to use.

Does anyone else believe in a liner lock stretching or giving in or around the pivot point ?

Can't speak for everyone, but I don't.

Well, technically, everything stretches under stress. The locking liner is a long, thin column structure, but compressing it even a thousandth of an inch isn't easy to do. I don't think that's a significant cause of these mysterious dynamic failures.

This IMO could cause a very slight separation of the blade tang and the liner lock, thus allowing just for a split second the liner lock to not be in contact with blade tang and possibly allowing the angle of the tang to over ride friction and cause the liner to move over (skate) to the unlock position.

I think you may be onto something here...

Between two given surfaces, dynamic (sliding) friction is much, much less than static friction. Perhaps these light, fast spine whacks cause vibrations that move the liner microscopically, possibly even perpendicular to the unlocking direction, but sufficient to change the lock-up from static friction to sliding friction, allowing the lock to disengage under far less load than if the lock-up remained static. Again, the locking liner is a long, narrow column, which might amplify vibrations under the right conditions.

 

[WadeF]

It's a good test to make sure the liner lock has been made properly. Knives that easily fail the spine whack test may easily close on your fingers while doing something routine, like cutting up card board boxes.

Another real world possibility is you are using your knife and accidentally bump the spine of the blade against a ladder, the underside of a table, work bench, or anything else that maybe around you while using the knife. When this happens you are basically performing a spine whack test. If it fails, you maybe injured.

 

[Raoul Duke]

I never realy bought into all this spine wacking, and thought you were all abusing your folders.

I just took one of my EDC's and pinched it in between my thum and finger, held the handle in the middle, put the knife at about 45 deg. and raised the spine about 2-3" above the edge of the coffee table, dropped it down with the weight of the knife behind it and the liner failed first time.

If the spine is horizontal, or the edge leans to the right the lock holds up, If the edge (pointing up) is tapped, ( I prefer to say i'm spine tapping, there is no whack happening here,) It fails every time, If I push the liner over a little to strengthen the lock it doesn't fail first time but fails the second, I dont see repeated taps improving the lock up here, But I do think bending the liner in a little will help.

I am now less confident with liner locks although this is the only one I think I own.