Prometheus
Junior Member
- Messages
- 135
I’ve seen the discussion of friction in tremolo bridges many times before, and in my opinion, they’re often just a little off the mark. So let me put my understanding of it forward, and you can tell me what you think.
Let’s look at the 6-screw trem, and so long as it’s a floatable trem based in essence on Leo’s design, a lot of makers’ offerings can be included here. There’s the bridge itself, with 6 holes. And the screws, which are columns arising vertically from the body, nominally in a straight line. And the surface of the guitar top. There are other factors at play as well, and it’s actually more complicated than it looks. But for right now let’s leave it at that, the simplest model.
When the strings are brought to tension, a considerable force is developed, somewhere around 100 pounds. The mounting screws resist this, at the point where they contact the bridge. So these metal components are pressed together, and attempts to moving them perpendicularly to that force will be resisted by friction.
At first there’s static friction, characterized by the force necessary to begin sliding. And then there’s sliding friction, which is the force necessary to keep the movement going. Sliding friction is at most equal to static friction, and practically always less. Think of moving your kitchen table. You first have to give it a bit of oomph to get it “un-stuck”, but once it’s in motion, the required force is less. This is not because the floor was dirty. It's because there's an extra force holding objects together that's not in play while they're moving. But for both these types of friction, the force required to overcome them is determined by the characteristics of the materials. In the simplest of terms, the harder and smoother the materials are, the less the friction. But of course it always varies with the force between the objects. Have someone sit on that table before trying to slide it, and it’ll take a bigger push.
Luckily, with the mechanical advantage granted by the trem arm, and the fact that we have to overcome spring tension anyway, and the hardness of the materials, the difference between static and sliding friction is imperceptible. But it was worth mentioning. So from here on we're really concerned just with sliding friction.
In addition to the amount of force holding the objects together, the force needed to overcome friction also depends upon the surface area of the contact between the items. In general, the greater the surface area, the less force require to overcome friction. This is because there’s less force per unit of surface area that’s pressing the two objects together. So there’s less friction to overcome. So decreasing the pressing force or increasing the surface area will have roughly the same effect. Here’s an extreme example for you. If there was a nail sticking up out of the floor, and you stepped on it, it would go right through your foot (don’t ask me how I know this). A large force (your weight) in a small area (the point of the nail). But if you had several hundreds of nails sticking up, you could lie down on them and have a nap. Swamis used to amaze people with this trick. Even though each individual point of contact is small, they sum up. Yeah, OK, that's not sliding though, is it? Well, for those of you who’ve moved furniture, you know that a big dresser doesn’t slide across carpet very easily on those pointy little feet. But lie it down on its side and away you go. That’s because those little feet were digging in, because the entire weight was distributed across a couple square inches of contact. Increase that contact surface area to several square feet, and friction plummets.
So, I wonder why people claim that 6-point bridges are bad because they have all those friction points. When in fact, they’re GOOD because of that. If you have only two points, then each of them carries 3 times the force that any of the 6-screws does!! So static friction and sliding friction will be GREATER with a two-point system than a 6-point system. You don’t have to agree with me. Go argue with a physicist or an engineer. Good luck. The laws of physics don’t care what you think, what you think you know, what you’ve heard, or anything else.
Here’s a related note about round and “knife” edges. First, a sharp edge is just a round edge with a small radius. Magnify any sharp edge and eventually it ain’t sharp no more. Secondly, a rounder object (larger radius of curvature) will slide EASIER than a sharp object. This is because it’s not as bothered by all the little surface imperfections. Didja ever wonder why the first bicycles had such big bloody wheels? Because the roads and streets they were riding on were awfully rough, and those things had no shocks. A bigger wheel smoothed out the bumps. A smaller (steel!) wheel would have been murder. Wheels got smaller as rubber technology improved. Now, those are wheels, and they’re turning, and we’re here talking about sliding - but the same principle applies. A sharp edge “feels the bumps” more than a larger radius. A knife edge, given the same downwards force, will slider HARDER than a round edge.
Remember that in all this we have largely been educated by what we’ve been told, what we’ve read, or what we found on the net. But a lot of the information regarding how guitars work and what needs to be improved originated from ... the people selling them. And a lot of that has been geared to people who really don’t have an in-depth knowledge of how all the parts of a guitar work, and how they work together. So if an authoritative voice pipes up and says “Well, there’s a problem with yer widget, ‘cuz it’s flim-flam is all wonky, and that’s really bad fer yer tone! But we’ve got just the thing to fix ya up!”... then millions will believe you. And they’ll all tell everyone the same thing. Eventually, because "everybody knows that", it becomes “the truth”. Except that often, it’s not.
Now, if you’re going to design your bridge so that there’s no sliding, well, then pretty much all of this is moot. All you need then is a good pivot axis. This could be a line, or two points, or two or more aligned points. And sure, a knife edge would be fine. But it’s not going to do much good if you seat it in a rounded support. In other words, a knife edge in a cupped recess is still bad, because that knife edge will want to slide around the bottom of the recess. You’d be much better off with a knife edge in a sharp recess, or a rounded edge in a rounded recess. Incidentally, both of these will have about the same (negligible) friction, but the rounded version will be stronger, and less prone to damage and wear, because again the force is distributed over a greater surface contact area.
So then why does it seem that people are gradually gravitating towards the two-point system? A couple of reasons. First, fewer parts. Always a good thing. Secondly, easier to set up. An even better thing. I know that a good-quality 6-point trem bridge can be installed and set up to be strong and stable. But it’s not easy or simple to do. If or when adjustments are necessary or desired, it’s just as easy (easier) for the average non-luthier player to make it worse rather than better. While the two-point system is not foolproof, it is at least easier to get right.
So, using Occam’s razor (everything else being equal, the simpler solution is the better one), the two-point system has the advantage. But other than its complexity, I maintain that there’s nothing “wrong” with the 6-point system either.
Let’s look at the 6-screw trem, and so long as it’s a floatable trem based in essence on Leo’s design, a lot of makers’ offerings can be included here. There’s the bridge itself, with 6 holes. And the screws, which are columns arising vertically from the body, nominally in a straight line. And the surface of the guitar top. There are other factors at play as well, and it’s actually more complicated than it looks. But for right now let’s leave it at that, the simplest model.
When the strings are brought to tension, a considerable force is developed, somewhere around 100 pounds. The mounting screws resist this, at the point where they contact the bridge. So these metal components are pressed together, and attempts to moving them perpendicularly to that force will be resisted by friction.
At first there’s static friction, characterized by the force necessary to begin sliding. And then there’s sliding friction, which is the force necessary to keep the movement going. Sliding friction is at most equal to static friction, and practically always less. Think of moving your kitchen table. You first have to give it a bit of oomph to get it “un-stuck”, but once it’s in motion, the required force is less. This is not because the floor was dirty. It's because there's an extra force holding objects together that's not in play while they're moving. But for both these types of friction, the force required to overcome them is determined by the characteristics of the materials. In the simplest of terms, the harder and smoother the materials are, the less the friction. But of course it always varies with the force between the objects. Have someone sit on that table before trying to slide it, and it’ll take a bigger push.
Luckily, with the mechanical advantage granted by the trem arm, and the fact that we have to overcome spring tension anyway, and the hardness of the materials, the difference between static and sliding friction is imperceptible. But it was worth mentioning. So from here on we're really concerned just with sliding friction.
In addition to the amount of force holding the objects together, the force needed to overcome friction also depends upon the surface area of the contact between the items. In general, the greater the surface area, the less force require to overcome friction. This is because there’s less force per unit of surface area that’s pressing the two objects together. So there’s less friction to overcome. So decreasing the pressing force or increasing the surface area will have roughly the same effect. Here’s an extreme example for you. If there was a nail sticking up out of the floor, and you stepped on it, it would go right through your foot (don’t ask me how I know this). A large force (your weight) in a small area (the point of the nail). But if you had several hundreds of nails sticking up, you could lie down on them and have a nap. Swamis used to amaze people with this trick. Even though each individual point of contact is small, they sum up. Yeah, OK, that's not sliding though, is it? Well, for those of you who’ve moved furniture, you know that a big dresser doesn’t slide across carpet very easily on those pointy little feet. But lie it down on its side and away you go. That’s because those little feet were digging in, because the entire weight was distributed across a couple square inches of contact. Increase that contact surface area to several square feet, and friction plummets.
So, I wonder why people claim that 6-point bridges are bad because they have all those friction points. When in fact, they’re GOOD because of that. If you have only two points, then each of them carries 3 times the force that any of the 6-screws does!! So static friction and sliding friction will be GREATER with a two-point system than a 6-point system. You don’t have to agree with me. Go argue with a physicist or an engineer. Good luck. The laws of physics don’t care what you think, what you think you know, what you’ve heard, or anything else.
Here’s a related note about round and “knife” edges. First, a sharp edge is just a round edge with a small radius. Magnify any sharp edge and eventually it ain’t sharp no more. Secondly, a rounder object (larger radius of curvature) will slide EASIER than a sharp object. This is because it’s not as bothered by all the little surface imperfections. Didja ever wonder why the first bicycles had such big bloody wheels? Because the roads and streets they were riding on were awfully rough, and those things had no shocks. A bigger wheel smoothed out the bumps. A smaller (steel!) wheel would have been murder. Wheels got smaller as rubber technology improved. Now, those are wheels, and they’re turning, and we’re here talking about sliding - but the same principle applies. A sharp edge “feels the bumps” more than a larger radius. A knife edge, given the same downwards force, will slider HARDER than a round edge.
Remember that in all this we have largely been educated by what we’ve been told, what we’ve read, or what we found on the net. But a lot of the information regarding how guitars work and what needs to be improved originated from ... the people selling them. And a lot of that has been geared to people who really don’t have an in-depth knowledge of how all the parts of a guitar work, and how they work together. So if an authoritative voice pipes up and says “Well, there’s a problem with yer widget, ‘cuz it’s flim-flam is all wonky, and that’s really bad fer yer tone! But we’ve got just the thing to fix ya up!”... then millions will believe you. And they’ll all tell everyone the same thing. Eventually, because "everybody knows that", it becomes “the truth”. Except that often, it’s not.
Now, if you’re going to design your bridge so that there’s no sliding, well, then pretty much all of this is moot. All you need then is a good pivot axis. This could be a line, or two points, or two or more aligned points. And sure, a knife edge would be fine. But it’s not going to do much good if you seat it in a rounded support. In other words, a knife edge in a cupped recess is still bad, because that knife edge will want to slide around the bottom of the recess. You’d be much better off with a knife edge in a sharp recess, or a rounded edge in a rounded recess. Incidentally, both of these will have about the same (negligible) friction, but the rounded version will be stronger, and less prone to damage and wear, because again the force is distributed over a greater surface contact area.
So then why does it seem that people are gradually gravitating towards the two-point system? A couple of reasons. First, fewer parts. Always a good thing. Secondly, easier to set up. An even better thing. I know that a good-quality 6-point trem bridge can be installed and set up to be strong and stable. But it’s not easy or simple to do. If or when adjustments are necessary or desired, it’s just as easy (easier) for the average non-luthier player to make it worse rather than better. While the two-point system is not foolproof, it is at least easier to get right.
So, using Occam’s razor (everything else being equal, the simpler solution is the better one), the two-point system has the advantage. But other than its complexity, I maintain that there’s nothing “wrong” with the 6-point system either.