Resistance from a drag washer, re: average diameter regardless of surface area

[JasonGotaProblem]

Nice snakehead!

The benefit you are reporting from adding an inner drag washer is puzzling.

Increasing the surface area does not increase the amount of sliding friction. So something else must be going on here.

The formula for sliding friction is simply the coefficient of friction for the two surface multiplied by the load pressing them together. 

The amount of braking work per revolution is a function of distance traveled, which is the "average" circumference of the disk from inner to outer diameter.  With a single drag washer, the larger the hole, the greater the amount of braking work per revolution for the same clamping load.  This is one reason why they put larger rotors and not larger pads in performance car brakes.

This modification appears like you are making the hole smaller.

Here is one theory:  Since the drag washers are not keyed or eared, they are capable of sliding against the face on either side.  If it happens that one drag washer slides on the outward face, and the other on the inward face, you will get two drag surfaces for the same clamping load, and since one has a much smaller diameter, about a 20% improvement seems about right.

But if over time, they eventually both start sliding on the same side, you will see a decrease in drag over the single large washer, as you have decreased the "average" diameter. I guess that you could treat the surfaces with different substances to encourage the desired behavior.

Or if the drag star was bottoming out originally, if any new drag washers are a bit thicker, you may be simply adding more clamping load.

Or a fresh drag washer might increase the coefficient of friction.

Intentionally adding more clamping load can be also an option if the star threads are up for it.  If the star is bottomming out, you can add an extra spacer washer or two, but you may lose the ability to loosen the drag all the way.

Note:  I put "average" in quotes, because there is a proper formula for calculating surface distance traveled per revolution of a disk using inner and outer diameter. I think that I might have put this in another thread, but interested folk can find it by searching the web for velocity calculations for thrust washers.

Hope that you find this useful,

-J

I wanna discuss this further but I don't wanna derail the other thread.

I can't help thinking that the equations I stumbled across when searching the terms suggested above are one of those useful approximation type things that don't capture the full story. Newtons second law wasn't F=MA but that approximation holds up until things get really big or small, etc. I want to dig deeper.

For starters I'll very reasonably assume the laws of physics don't care that I disagree with an equation. Great jump off. But the universe doesn't live on a chalkboard. These frictional forces are acting on real materials that get hot and properties change with added heat. More surface area, and just really.more mass of the drag disk in general probably becomes a consideration.

And let's take the idea to the extreme. A drag disk that resembles the cross-section of a paper towel tube. Really get that average diameter out as close as possible to the outer diameter. This is a thought experiment so we'll briefly ignore the difficulty of giving a woven material of those dimensions any structural integrity. And we'll also briefly ignore the need for far tighter machining tolerances to maintain consistent pressure (I believe we've discussed this element before). Would this really perform better on a drag test? And if so are we talking about max drag at startup? What about smoothness? What about drag performance after something takes a 200yd sprint away from the boat (and gets it hot)?

As I think about this I find I'm actually fairly well situated to test this out. In all but the free time. By that I mean I have a set of hollow punches and sheets of carbon. I'd be interested in testing the rolling resistance of a reel with the entire drag surface area covered with carbon, and then test again at same setting with disks from the same sheet cut to the same OD but a much larger ID.

Would you expect to see increased max drag when cold? What about max rolling resistance after equal # of turns of the star? (I may not be using the right term. By that I mean comparative resistance while line is being pulled out at constant speed)

Oh and one last hail Mary. Say I took a made a drag disk that looked like a bullseye. Picture a bunch of nested thin concentric rings in place of one ring. I am reasonably assuming they all spin in unison and all stuck to the same side. If I calculated the braking force based on the average diameter of each ring, I just struggle to picture how the total braking force wouldn't be the sum of those values. And if so I struggle to see how that number would be increased by removing a couple of the inner rings.

I hope this doesn't read like I'm writing hate mail to the laws of physics but putting Dave's name on the address line. It just seems like a fun topic, and one where the received wisdom seems to contradict logic and some (admittedly less than rigorous) observations so to me, it's worth the challenge.

[jurelometer]

Sliding friction is not a function of surface area.  so it doesn't matter if you add more surface area or how many pieces the surface area comes in.  In very crude (and not 100% scientifically correct) terms, if you increase the surface area for the same compressive force- that force is pressing less per square inch of surface, so you end up back at square one with the same amount of friction.

There are different static and kinetic coefficients of friction for sliding surface pairs.  they both can change with heat.   in the case of greased or ungreased CF, the CoF will decrease with heat, more with grease if I remember correctly.

And before you ask about thermal expansion increasing the clamping load- we covered that here in another thread, and came to the conclusion that it was negligible.

Braking in this case is like braking on a car. The spool is slowed by changing kinetic energy into heat energy.  The amount of energy transformation will be the product of the amount of friction and the distance travelled.  if you dragged a brick a mile down the street it will get hotter than if you just dragged it a few feet.

But the path traveled on your drag disk is circular.  The larger the circle of travel, the more braking work  per revolution.  These circular paths start out smaller  from the inner diameter and gradually get larger toward the outer diameter, so you have to use an equation to sort of average them all out.  This is a classic example of needing a integral equation.  The formula is out there, but I am too lazy too look it up again.  I  think that I already posted it.

By making the center diameter larger, you are increasing the effective circumference.  Orvis (and some others) tried playing this game by putting a very large OD drag disk with a very large ID (looks like a ring) on a fly reel design.  More drag for less clamping pressure.  But since the braking surface is farther from the axis, any angle off axis on the surfaces creates greater travel and therefore more chatter and stickiness.  There are other downsides as well.  I don't think any of these were that successful.

IMHO, the better reel designs go for larger OD drag washers with larger IDs, but not getting crazy about it.

Tribology is often counter-intuitive and there are lots of exceptions to the rules. And I don't claim to really understand any of it very well. But we are dealing with a fairly simple case here and are not trying to get very exact numbers.

-J

[oldmanjoe]

[OhReely]

It's "Hail Mary"

[foakes]

Interesting topic —-

For me, it boils down to using the proper sized reel to match the target species.

From Dave (jurelometer):

"IMHO, the better reel designs go for larger OD drag washers with larger IDs, but not getting crazy about it."

This is why one of the highest quality DAM Quick reels are the  1401 thru 5001 series.  They use a set of very large drag washers both on top and underneath a metal skirted spool.  This evenly "squeezes" the spool to add drag pressure when fighting a fish —- it is very effective.  Add a thin coat of Cal's —- and it is even smoother, and still has complete drag pressure from "0" to lockdown.

A danger in over dragging reels, particularly spinners —- is that the drag might over-reach the physical limitations of the frame and some other components —- and cause a reel failure or stress twisting.

And here again, don't expect the reel to do all of our work —- just as important are proper rod-handling techniques, an awareness of the mechanics of our reels, knowing exactly where and when to make adjustments when fighting a fish without looking, and experience.

Sometimes, we just need to hit the light switch —- and all of the theories & principles of electricity are not needed to do that.

Good topic!

Best, Fred

[jurelometer]

Another way to look at it is that points farther from the center will generate more torque for the same amount of friction.  More of your braking surface farther from the center (larger ID)  means that the frictional resistance will be distributed amongst points on the disk with greater torque. 

Here is a good overview:

https://mechanicsmap.psu.edu/websites/7_friction/7-6_disc_friction/discfriction.html

Things to also consider are vibration dampening for certain materials (e.g., cork)  PV ratings for certain materials (Rulon, Delrin), heat conduction and convection.  These will all be affected by combinations of diameter(s) and surface area.

And the reel has to be designed to be capable of keeping the sliding surfaces aligned at the chosen disk OD.

I don't claim to have anything more than a not-quite-rudimentary grasp on this stuff, so if someone out there has some training in this field, I am all ears.

-J

[oc1]

That makes sense.

But, it seems to me, the difference in a new drag washer(s) and old drag washer(s) is as dramatic as the difference in configurations.  I guess Carbontex is the state of the art, but it doesn't stand up for long.  You can tell the difference after one good fish fight.

[JasonGotaProblem]

I keep wanting to come back to this but it's been a very busy week. And weekend. Didn't wet a single line though.

I will throw this out there though. A lot of what Dave is saying is basic physics and I'm not arguing it. I remember the analogy of a cereal box full of concrete. There's 3 different sized sides to choose from but the weight is the same. It would just be distributed differently depending which you slid it on. But that's basic physics. Get into statics and dynamics and classes and they start talking about where is the mounting point and angles of pull, etc. Also, and this is relevant, on any real world surface I know which side of the box would result in me starting to consider the coefficient of friction of concrete instead of cardboard first.
Even if initially, conceptually, they might read the same on a spring scale while you pull.

I still owe a better response but I needed to get something out there.

[boon]

Also because F= mu N is a hyper-simplification of friction that is taught to 15 year olds. Amontons Second Law applies quite well to a block of wood on a ramp but is not great for many other real-world applications.

[oldmanjoe]

[jurelometer]

The basic law of sliding friction  is useful for a basic understanding, plus it is where you need to start.  The engineers that design race car brakes care about all sorts of fancy stuff around friction, presumably with a focus on very large changes in heat. But they still use a frictional surface concentrated on the perimeter of a large disk because the basic law of friction is still in play.

If we want to get into exact numbers, then I agree it gets  more complicated, but we are not yet in a discussion about exact numbers.  We are discussing simpler stuff, and trying to decide if we can accept the basic law of sliding friction because it seems so counter-intuitive.

If we want to start going down the rabbit hole of more advanced friction, I would start here: 

The reel in the thread that started this debate was using dry drags, but most of us grease the drags.  Under pressure and/or heat, the grease liquifies temporarily and becomes a liquid film.  And the hotter the temps and the greater the pressure, the lower the viscosity.  Now we have friction in a fluid to deal with, which is a function of surface area contact.  But since the carbon fiber surface is so irregular, it probably is not a straightforward fluid surface film situation.  Heck, it might be some sort of quasi- fluid/ non-fluid combo.  I did a bit of reading on fluid friction for lure design purposes, and learned enough to know that if you want to go on this journey, it will be without me. I know my limitations. 

-J

[JasonGotaProblem]

I am doing my best to keep this discussion in the conceptual range, and avoiding it going off the rails. I want it to stay interesting but I recognize it can get tedious fast. I wish we had a subforum for "excessively technical/advanced general topics"

Joe, do you have a decent spring scale I can borrow hiding behind those animated gifs? Mine was at best decent before it got rusty, now it's by no means suitable for anything scientific. I wanna do some testing.

Dave, I wanna circle back to an aspect of the original discussion. Taking as a given that OP really did see an increase in drag by adding the inner washer for the sake of discussion. You suggested that it might be due to it mating with/sticking to the opposite side thus creating an additional drag surface. But that drag surface is at a smaller diameter so my question is why would that matter if a drag disk sliding at a given diameter was happening in one side of the system or the other? And why would it change if the new drag disk started sliding on the same side as the other one?

[JasonGotaProblem]

Also, a case study. If you take a 9/0 senator and swap the stock drags for an aftermarket dura-drag setup with the eared carbon washers you are keeping the same number of stock drags disks but eliminating the outer half of the diameter as an active surface, in exchange for getting both sides of the drag disk to be active. So you are trading diameter for surface area. Those kits see absurd increases in available drag without even cranking the star down all the way.

The plural of anecdote isn't data. But this observation that stands in direct opposition to the principle being evaluated has been done and quantified enough times for it to seem very relevant.

I promise none of this is a "gotcha" type thing and I'm not seeing this as an opportunity to prove anyone wrong. We're all just in pursuit of truth.

[oldmanjoe]

  Yes I do , and you are welcome to use them .
     
Back to this drag set up ..    It`s easy to see why it worked .

[Gfish]

Just read a post by Allan T. mentioning that a digital wt. scale should be more accurate than the spring types. Digital scales can be used to calibrate the spring type.
Does a digital scale require constant calibration? What about putting alota weight force on a digital scale, does anyone know if that messes with calibration?
Looking for a new one after breaking mine...