Single v. twin disks

[Abstruse1]

This question could go on any bike forum, but here I am, so here it is (in way too many words).

Many people think twin disks stop better than single disks. I suspect they're right, but I can't see why. Maybe someone can enlighten me.

Here's the way I see it. Whatever kind of brakes we use, all we have to work with is the force in our grip. Lever length, and cams (in mechanical brakes), or lever length, MC piston diameter, slave piston combined diameter, pad material, pad area, disk material, and the phase of the moon all do the same thing, but they don't multiply our grip strength, they just provide leverage, sacrificing range of motion (at the lever) for increased force (at the braking surface). None of this adds any force to the brakes, except via leading shoes in drum brakes, which is a kind of power assist.

Of course there's a lot more involved than this purely mechanical conversion: cable stretch, cable friction, hydraulic line growth, and caliper/drum distortion due to mechanical forces, for examples. And then there are the effects of heat in hard, sustained braking (as in racing). Also, there's the issue of control -- we not only have to be able to put enough force on the friction surfaces, we need to be able to control that force rather precisely.

So, just considering disk brakes, why would two be better than one? And why are larger disks better (if, in fact, they are)? Is it just the ability to minimize heating via more material? Perhaps other subtleties such as pad behavior (squirm?) under extreme compression? Gas buildup between the disk and pads? Change in the coefficients of friction of the pads and disks under compression? Voodoo?

Of course, the more total pad surface you have, the longer the brakes will last, but that's not braking effectiveness.

So -- what am I missing, guys? Ya gots to 'splains it to me.

The bottom line of all this, other than the fascination of understanding such things, is whether I should go with dual disks on my R75, which adds unsprung weight.

Sorry for the prolixity.

[SteveD]

Better 'n too concise!

Does the answer part lie in your style of riding and where you ride?

Cable v stainless steel hydraulic? SS hydraulic before 2 discs? Cable to 2 discs v ss hydraulic to one disc?

Sorry, such specific questions don't answer a general question, nor did they even attempt to. I don't know the answer.

I suspect an answer or two from the engineer minded airheads will arrive shortly, with varying degrees of prolixness!

[Ken in Oklahoma]

I suspect an answer or two from the engineer minded airheads will arrive shortly, with varying degrees of prolixness!

Well, I'm an engineer, though of the electrical sort. But we all started with basic physics courses. Hopefully I've retained enough that I won't screw this up.

When talking about friction the classic illustration is of a brick being pulled by a horizontal string across a horizontal surface. With mild tension on the string the brick will not slip. However as you increase the tension there comes a point where the brick will slip. That point allows you to calculate the coefficient of friction. And I think the formula goes K=F/N where K is the coefficient of friction, F is the tension on the string, and N is the weight of the brick. That is to say that pulling on the string will result in no movement of the brick initially and as some point tension will become enough to move the brick. Say that a half pound pull on a one pound brick will cause it to move. The coefficient of friction for that brick on the surface that it's lying on would be 0.5

There are some implications from the above. A zero coefficient of friction would be that the brick is sitting on a frictionless surface. (Warm hockey puck on dry ice?) At the other end the maximum coefficient of friction would be 1.0. One pound of force on the surface is all that a one pound brick can muster. And a one pound pull on the string would be sufficient to just move said brick on any surface whatsoever. In other words a one pound pull of the string would be sufficient to pull a one pound brick away from the surface it's lying on.

Coefficient of Friction is a very useful tool to use when designing things that depend upon friction to work, things such as clutches and brakes. The trouble is real world situations can and do go beyond the very useful notion of coefficient of friction. Take cars or motorcycles accelerating on a drag strip. If the maximum theoretical limit was limited to a maximum coefficient of friction of 1.0 then the terminal velocity of a dragster would be far less than what we see today on top fuel dragsters and funny cars. There was a time when I could calculate the terminal velocity of an object after 1470 feet of acceleration at 1G, but that time is not now, and I'm too lazy to look it up on the internet. I believe that nowadays we have top fuel dragsters and funny cars that exceed 3G's of acceleration. Why? It's because the tires are tacky and that cohesion between rubber and the road can be more than my mind to grasp.

The upshot of this long, self-indulgent, treatise is that I have no trouble imagining that dual disks can yield more powerful braking than single disks. The details of how that all works I'm a bit fuzzy on.

How'd I do SteveD on my Prolixness quotient?

Addendum: The surface area of that brick lying on a surface is immaterial in the calculations. I'd say more but I'll cop out professorially saying that the proof can be done by the student. (In other words, I'm too lazy.)

Ken

[Steve in Golden]

Here's an interesting and informative article:

Brake Calipers Explained - Braking The Rules

[Major Softie]

My apologies if Steve's article already points this out - I didn't look.

Brakes turn mechanical energy into heat. The effectiveness of a brake is based on how well it can turn mechanical energy into heat, and how fast it can dissipate that heat. Twin disks can do that twice as well as a single disk. In addition, the torque that a single disk puts on the forks is trying to twist the forks, while a dual disk puts equal torque on each side, and each fork only gets 1/2 as much as the leg of a single disk.

On the down side, twin disks weigh twice as much, and it's unsprung weight - the second worst kind (at least it's not rotating on the wheel).


No free lunch.

[Mal S7]

Brakes are devices for turning kinetic energy into heat energy. Its not so much the number of discs, as the available surface area.

The greater the surface area of disc (and pads) the more energy, and quicker, the energy can be converted. If you had tiny discs brakes where is all that heat supposed to go? Even with a tonne of contact pressure the heat energy has to go somewhere. Sparks?

The discs heat-up, then to keep working, they too need to release heat (mostly by convection to air, some radiant), they can do a better job of this than drums.

I guess if they were made of super-materials they wouldn't have to do this, and so could just get hotter and hotter, but at some point the heat bank would be full and they would become ineffective.

I ran a single disc for many years, it's acceptable except when fully loaded with passenger, The twin ate discs have plenty stopping power.

[barryh]

I think there are two questions:

In terms of how powerful the brakes feel when they are well within their limits, there will not necessarily be a difference between single and twin discs as all else being equal it's down to the ratio of piston area in master cylinder vs caliper cylinder(s)

At the limit of the brakes capability then bigger disks/twin discs etc. must be an improvement.

Given many of us ride well within the limits of our brakes the simplicity and lower unsprung weight of a single disk has some attractions. Twin discs have at times been as much about marketing as anything else.

[Rob]

I always thought the friction coefficient between the front tire and the pavement determined how much braking was available?

[Major Softie]

I always thought the friction coefficient between the front tire and the pavement determined how much braking was available?

As we've mentioned, the "limit of the brakes capability" doesn't just mean the brakes's ability to create friction, it also refers to the limit of the brakes capability to shed heat. That, for instance, is how the limit of a truck's brakes are reached on a long downhill. On a motorcycle, we usually only reach that limit in racing, or if the brakes are barely adequate (as with some older bikes), or perhaps on a long downhill with the additional load of a sidecar and passenger.

[barryh]

I always thought the friction coefficient between the front tire and the pavement determined how much braking was available?

It does and at lower speeds that will be the limiting factor but because the kinetic energy involved goes up with the square of the speed, for Airhead brakes at least there will be some higher speed at which the brakes can't overcome the friction coefficient of the tire and that will transfer the limiting factor back to the brakes.


I love this example of the square law at work and it never fails to sober me up.

If travelling at 60 mph you could come to a complete stop in 150 feet. (Assuming a deceleration rate of 0.8g's) and if a deer was 150 feet ahead when braking began you would not collide with that deer. What if the speed before braking was only 5 mph faster at 65 mph. At what speed would you hit it ?

The answer apparently is not 5 MPH but 25 MPH

http://www.msgroup.org/Tip.aspx?Num=262 ... rms=brakes