That doesn't really apply in my situation. I've ridden all sorts of rim brakes for decades, in various states of function. And I've ridden my disc bike when the rear pad was contaminated. Even contaminated and screeching, the disc brake still only operated as badly as a crappy rim brake. I could stop the bike with just the contaminated rear brake. As a matter of fact, the day I tried my first disc bike (a Domane 4.5 that I subsequently bought!) I had been trying all sorts of high-end bikes (which I couldn't afford) including the salesman's personal bike which he races, and the first time I braked hard on the disc bike it blew them all away. I couldn't believe the braking force I could generate with just my index fingers. Would have required a handful of fingers to get that much braking force with rim brakes.

Both my daughter's bike and her husband's bike have disc brakes; I rode hers brand-new from the LBS and while it braked great, I didn't experience the wet-my-pants "OMG the brakes are so friggin' good" sensation. But then, perhaps I have a high tolerance for crappy brakes: having ridden a bunch of less-than-stellar motorcycles over the years, nowadays my criteria is "will the thing slow down if I apply brakes hard enough" :)

As an engineer though, and having worked on accelerating and decelerating (braking) machinery which is driven by many hundreds of horsepower, the subject of braking can be very fascinating--especially when the human element is involved. Human-initiated braking systems are usually not linear, partly by the inherent design of the mechanism, and partly by intent. Human muscle groups don't work linearly and human perception isn't linear. Some braking systems take this into account; for example, how about a car's brake pedal, activated by the foot. Let's say the pedal travels 10 cm total with a force of 100 lbs (I hate to mix metric units with imperial but it fits better here). If the brake system was perfectly linear, a brake pedal of travel of 1 cm (10%) would require 10 lbs of force and exert 10% of the maximum braking capability. A brake pedal travel of 2 cm (20%) would require 20 lbs of actuation force and yield 20% of the maximum braking capacity, etc. That first 10% might be perceived as being too "grabby" in a passenger luxury sedan so perhaps the designer may tone it back a little, perhaps through the design of the actuating mechanism or through a "snubber" in the brake line. In a sports car though having a little more braking force on initial application may be perceived as a good thing, the driver may think the brakes are more "snappy" and the designer may tweak the system accordingly.

For a bike disc brake system, it's actuated by the hand lever, which pushes on a piston in the master cylinder. This piston displaces hydraulic fluid and generates pressure in the hydraulic system. The hydraulic pressure in turn moves a larger piston, and thus yields a mechanical advantage (small master cylinder piston, large slave cylinder piston). The braking force from a given hydraulic fluid pressure is actually fairly linear: if 500 psi is full braking torque on the wheel, then 50 psi would be about 10% braking torque on the wheel. However, the hand lever's interaction on the hydraulic fluid's pressure isn't linear: the lever is moving about a pivot, pushing on the piston at different angles, and is thus non-linear. Also, the hand's squeeze force is likely non-linear as well. So if the brake lever moves 30 mm at 30 lbs of hand squeeze to achieve full brake lock-up, a 10 mm lever movement probably won't be 10 lbs and probably won't be 1/3 of full braking torque. However, it might be perceived as operating quite linearly because of the human body's perception of force! (That's perfectly acceptable,by the way).

A rim brake also operates non-linearly: the lever's pull on the cable isn't perfectly linear, the cable's actuation of the brake mechanism won't be exactly linear either. We could carefully plot the geometry of the system at various braking points and determine how non-linear it is. I could certainly envision how a very perceptive driver could feel the difference in non-linearity between a rim system and a disc system, even though both systems could be quite capable of achieving the same wheel lock-up at the same amount of brake lever squeeze. So if a rider prefers a disc brake feel, great; if a rider doesn't notice, that's okay too.

So Cal. Yes, commute in the rain, slush, snow, sleet and flying cat weather. The rims get this nasty, black worn rim junk mixed with mud and sand.

I hear that! That's what I meant when I said that bikes were deceptively simple and elegant. Yea, boiled to it's essence, a carbon frame is really just one big piece of plastic. But the technology to produce that plastic and then form it into the right shapes to get the desired effect? Not so simple.

Let's take context into account. The calipers had been set up very loosely before, and now I've come close to maxing out their braking power. I could move them out a touch for a bit of a compromise: more braking power, but less than I currently have now.

Also, they "grabbed" because I pulled the levers harder than I normally would, instinctually, given the situation.

Nonetheless, it proves your initial assertion false:

"That little initial millisecond it takes the rim brakes on my other bike to activate and grab--would have resulted in me hitting his car and flying over, and being very badly injured. This one experience convinced me of the complete superiority of disc brakes."

On aluminum rims, with tightly adjusted rim brakes, there is no "initial millisecond."

Also, there is no evidence that hydraulics stop any faster than rim brakes in dry conditions. This is a figment of your imagination.

Even with crappy carbon rims, stopping distances in dry weather is not very different.

Your hyperbole, fear mongering and lack of evidence are your undoing.

Uh, dude? Did you take your meds today? "undoing"? "fear mongering"? I was describing my experience. If that upset you, you might want to have a look at that.

Also, in week and a half, I will have been riding bicycles for A HALF CENTURY. I know rim brakes intimately, and they cannot, in my experience compare to discs. You can disagree, but the rest of your remarkable hostile crap does not apply.

Read your own post again. It's full of factual errors, hyperbole, and unsubstantiated claims.

1. you claim rim brake pads sit "much farther away". This is false. I set my pads between 1 to 1.5 mm away. If you think half a mm is "much farther away," you are delusional.

2. you claim rim brakes take a few milliseconds longer to "grab." This is false. Rim brakes stop just as well as hydraulics in dry conditions.

3. you engage in fear mongering by saying rim brakes can lead to "death." This is demonstrably false. In dry conditions, rim brakes stop just as quickly as hydraulics. Hydraulics work better in wet, but most riders are sensible enough to brake a little sooner and ride a little slower in the wet.

4. Also, stop projecting. You are the one who is or should be taking meds if you are pounding on a car roof and swearing profanity at a driver.

Perhaps you should use better judgement when "bombing down a hill" behind a car. Better judgement trumps relying on hydraulic brakes 100% of the time.

5. The number of years you have been cycling is in on way shape or form admissible as "evidence" of the alleged superiority of disc brakes.

6. mechanical discs, hydraulics, low end rim brakes, high end rim brakes, carbon rims vs. all rims introduce a whole host of variables which prevent making absolute statements about braking superiority or lack thereof.

7. finally, you have no evidence on braking distances.

"You know, I hear this all the time, as though nothing unexpected ever happens to ordinary riders; as though cars and other riders and dogs and road conditions are always predictable. Those of us who ride in traffic, especially in urban and suburban areas, can't always "plan" when we're going to have emergency brake to save our lives. It's actually not about being a pro or racer.

Thing is, disc brakes require less time and effort and hand strength to reach full effective braking force in all conditions due to the fact that the pads sit .5mm or less away from the rotors, whereas rim brake pads sit farther--usually much, much farther--away from the rims, and can take milliseconds longer to begin to "grab". (Hydraulics also respond much quicker than cables.) This may not seem like a lot, but at speed, in an emergency, those milliseconds could be the difference between a close call and death or injury. Additionally, disc brakes are much easier to modulate than rim brakes--at least hydraulics are--allowing incredible precision in applying just the right amount of braking force.

Happens, naturally, that I have a story regarding this very thing: the quicker reaction time of discs in saving my life. Bombing down a steep hill, doing something under 30mph, some ******* decided to U-turn, with no signal, right in front of me. Say less than 50'. With disc brakes, I was able to feather and hold the brakes right at the point before they locked. My tires were *literally* squealing on the pavement. I stopped so close that I had to turn my front wheel to the right so as not to hit it against the side of his car. I was close enough to reach out and pound on his roof with my fist as I yelled curses at him. ONE FOOT--That little initial millisecond it takes the rim brakes on my other bike to activate and grab--would have resulted in me hitting his car and flying over, and being very badly injured. This one experience convinced me of the complete superiority of disc brakes--between the stopping power and the fast response and ability to finely modulate the deceleration. I, personally, will never ride anything else again.[/I]

Here we go again: exaggeration, hyperbole, and a lack of evidence.

Rim brakes and hydraulics have the same stopping distances in the dry. Hydraulics have an advantage in the wet. However, in order to garner this advantage, you have to take on a whole host of DISADVANTAGES: this includes:

-higher cost,

-aero penalty,

-weight penalty,

-buying a new frame and fork

-the safety risk of slicing yourself with an exposed pair of hot discs

Here is my personal experience: once I re-adjusted my rim brakes, the performance just blew me away.

You've done a very good job of creating an adversarial interaction where there was none, and found ways to take offense and attack me for it. I feel bad for you because this means SO MUCH to you that you're about to stroke out over a stranger on the internet relating HIS experience. I really feel sorry for you, man.

You're overreacting to a ridiculous degree.

This is all about the facts. The evidence is as follows:

-hydro's and rim brakes with aluminum surface stop equally well in dry conditions.

-hydro's stop better in wet conditions.

Instead of facts, you come up with unfounded, catastrophic scenarios about rim brakes leading to death. It's totally preposterous. It's very funny, but still, I think we're all better off talking facts rather than fear mongering.

Adhere to the facts instead of resorting to bombastic, exaggerated catastrophizing and you will see the facts for yourself.

This argument is an amazing waste of time and energy!

We have both options. Pick what you want to pick, takes some Geritol, some of your BP meds, and go ride the bike you picked....

I can't think of a reason not to use disc brakes unless your frame isn't compatible.

I don't think so. Disk brakes increase durability, performance, and convenience, whereas clipless increases performance at a sacrifice in convenience. In my experience, a set of rims with rim brakes lasts about three months, or one season, in winter use on salted and sanded roads. Disks are one way of getting rid of rim brakes.

Personally, I use drums -- all weather utility with minimum maintenance.

I'm currently riding a rim brake bike down the same hill I've ridden my mountain bike. This hill has intersections and 4 way stops with traffic entering and leaving. With disc brakes I worried less about stopping and more about how fast i could get thru the intersections regardless of traffic. However on rim brakes Im constantly worried about my speed getting too high and whether or not someone will pull out in front of me before i have a chance to slow down, not stop.
To me its a no brainer and a safety concern because if you don't have disc brakes you're looking for a crash. Disc brakes are a god send.

Disc brakes are the new helmets. Whether or not they're better or even if they work at all is irrelevant. The real fear by the detractors is that they'll eventually be forced to adopt them.

Drums are generally pretty straightforward to retrofit onto bike frames designed for caliper brakes.

I've had mountain bikes with v-brakes and with discs and have discs on my commuter/tourer, and there's no comparison for me. Hydraulic discs stop me much more quickly, stay dry and effective when I cross waterways, and the pads are far easier to change than bolt-on brake pads. They also don't wear out the walls of expensive rims, none of which are cheap to replace. I've heard that mechanical ones are better for touring in case something breaks while you're out in the boonies, but in thousands of miles on hydraulic discs I've yet to have a system leak or become otherwise non-functional. Like clipless pedals (SPD on my mountain and touring bikes, Look Keo on my road bike) I swear by them.

Actually, standard pedals are more efficient than clipless pedals.

Also, dual pivots stop just as effectively as hydraulics in dry conditions.

I can see people preferring hydraulics if they do a lot of wet weather riding.

For dry weather conditions, hydraulics offer no advantage in stopping power, but exact penalties in weight, cost, and aerodynamics. Not to mention you need to buy a new frame and/or bike if you currently have a bike with rim brakes. That's a rather expensive upgrade. That's $2.5K minimum vs. an $80 upgrade to a pair of ultegra 6800 dual pivots.

Rim brakes are not less convenient to use or maintain.

I commuted daily for 5+ years with a road bike in dry conditions and I didn't come anywhere close to wearing out the rims.

Rim brake performance was excellent (dual pivots).

People who have both rim brakes with alu rims report their hydraulics do not offer shorter stopping distances in dry conditions.

I have no idea why you insist on arguing with all of us. I gave my opinion of how hydraulic discs on my heavy loaded bikes and clipless pedals perform for me; it was not a technical argument. If you prefer calipers and standard pedals, so be it. I personally find climbing easier when I can spin my feet rather than only alternately pushing down, and I like being securely fastened to the bike at higher speeds or when the surface is bumpy. My preference, OK?

I personally can understand that someone prefers the feel of hydraulic, or has the need to ride in the rain or mud, in which case the hydraulic discs are an advantage. And I personally can understand that someone prefers the clipless pedals. No argument from me there!

I think American Euchre's point though is that people keep claiming the hydraulic discs stop faster than rim brakes. Not so. In dry conditions, with properly maintained and properly setup brakes, the well-understood principlesof vehicle dynamics show that it doesn't matter whether the brakes are drum, or hydraulic disc, or mechanical rim brakes: if they are capable of achieving wheel lock-up (and indeed they are), then all three of them will offer exactly the same stopping distance. Now the lever feel may be different for all three, but a locked-up tire is a locked-up tire, regardless how it got there.

Again, I personally have no dispute with anybody who says they prefer system A or B or C, as long as they preface their statement with the acknowledgement that it's their preference. And if their riding conditions warrant the tradeoffs of hydraulic discs, then that's a sound decision. But the use of non-scientific claims to support their decision bothers me, as an engineer.

I also would offer my opinion that if someone feels that riding down a hill at high speed in traffic would be unsafe with rim brakes, but they are now perfectly confident at the same high speeds downhill in traffic with the hydraulic disc brakes, they might just be a tad overconfident...

Here's something else to consider: a locked wheel is not the fastest way to stop because the coefficient of kinetic friction is lower than of static friction. The fastest stop comes from applying the maximum braking force possible without locking the wheel, or threshold braking. If a brake is easier to modulate, this is easier to acheive and the real world stopping distance will be shorter.

Theoretically, yes: the tire should have 10 to 15% slip for maximum coefficient of friction under dry conditions, that gets a COF of about 0.7 give or take. Sliding (100% lockup and full slip) drops slightly to 0.6 or 0.5 depending on the tire compound. The difficulty is modulating to achieve that slip exactly. Even with a very responsive brake lever and a skilled operator, modulating can quickly go down the curve into the far-too-little-braking category. The COF curves below 10% slip are very steep and immediately drop way below 0.5.

In other words, locking up the brakes do indeed drop off the COF to less-than-maximum--but not much! Whereas letting off the brake just a smidge too much will immediately drop the brake's COF way, way, way below the slightly-reduced COF at full lock-up. So in real-world panic stops on dry clean pavement, there's a much better chance of getting stopped by staying on that brake HARD rather than backing off and trying to modulate.

That's the beauty of a well-designed 4 wheel ABS in a car. The wheel speed sensors and modulating valves can react far more quickly and far more effectively than a human pumping the brakes.

edit: Oh, and when I wrote "staying on that brake HARD" I'm referring to the front brake. That's where the stopping power is.

I'm curious. Of those people who say rim brakes stop just as well as discs (in dry,) how much do they weigh? IOW, do discs come into their own for heavier riders? I'm a clyde, and my experience is that no rim brake can stop me as well as a mechanical disc, even in dry. Maybe all the different brands of pads I've used have all been sub-standard? If so, I need to find out what magic pad I should be using.

I personally have been in a situation of being on tour (no choice but to ride in the rain,) going down a 13% grade in the rain, and not being able to control my speed with rim brakes. I started at 25 mph and reached 46 mph, all while my (rim) brakes were on as hard as I could squeeze.

Good point. I've always thought it interesting that the same brake system is used for all road bikes, whether used by Clydesdales or nymphs.

You should try Kool-stops ... IMHO, they feel firmer and stop better than the standard Shimano pads.

But they wouldn't help you descending in the rain on a loaded touring bike ... that is a place where disc brakes would be a clear choice.

Oh, I vary between 155-165.