Dual Disc brakes
 

Dub T asked in another thread why a front disc brake, and I thought it deserved its own topic.

Running disc brakes front and back seems to be somewhat of a trend now.

Bill Mc Cready of Santana is adimantly oppossed to using two disc brakes front and back. his rationale is that disc brakes can fade to the point of run out; if you have discs front and back, and overheat them to that point you are potentially left brakeless.

You've also got an issue with heat from a front disc being transferred to a carbon fork at high enough heat to damage the fork.

His preferred setup is rear disc and rim front. His recommended method for using that setup on long descents is to use the disc to control speed as necessary (supplementing with the front caliper if needed to slow sharply). When the disc begins to fade, use the front rim brake to stop, and let the disc cool off.

His explanation makes some sense.

Of course my personal bias is two rim brakes, no disc, other than for loaded touring, so I'm already biased against discs front or back.

That makes a lot of sense. Disc brakes will develop more heat since they have less leverage over the axle than rim brakes do.

While I don't think disc brakes are quite where they need to be, the, I think I would rather put that much heat energy into a disc rather into a rim that is in contact with my tubes, tires, and/or tire glue. If heat transfer to a carbon fork is a concern, something I've actualy seen is to wrap the fork with that "header tape" used in race cars and hot rods sometimes, used to insulate the forks from the heat. Looks ugly, but may save the fork.

As for the braking technique above, the way I see it starts with the physics and how energy is never created or destroyed, only converted. Using only the rear brake to control the speed of a bike puts all the kinetic energy converted to heat energy into one disc...Wouldn't using both brakes "equally" divide the heat energy put into a single disc in half, theoretically doubling the reduction in kinetic energy before brake fade/failure? At that point, if you are still toasting the discs (both of them), then the whole system is inadequate for the riding the team is doing. Also, if one disc wasn't up to controlling the speed of the bike, just how effective will the other "single" disc be at stopping the team? I'm a pretty firm believer in he reason why cars and semis don't brake with one axle on descents, and the conversion from kinetic to heat (which if I remember right is also kinetic energy...before the physics major riders realize my physics grade) energy is the same regardless of the vehicle...My vote has always been to use both brakes (and the drum if equipped) and if the whole system begins to fade get off and have a nature break and a Power Bar.

LKW

I think his point is to put all the heat on the one disc. Thus if it begins to fade, you've got a front caliper that has little or no heat, and is perfectly adequate to stop the bike while the rear disc cools.

If you're dragging two disc brakes on the straights, you risk fade to the point of inadequate stopping power, and have nothing left in reserve.

Also by putting the heat to the rear disc, you're not risking a front flat from heat from the rim brake.

Here in Ohio we don't worry too much about brake fade. Riding in Georgia, Michigan, Wisconsin and Indiana also hasn't worried us either. I like my rear disc a lot. I realize that modern brakes like the V-brake would work just as well for us, but we moved from an old Motobecane with canti front and center pull rear. With this setup stopping was a hope and a prayer. I was wondering how often and under what circumstances tandems experience fading of disc brakes?

The context of my discussion with Bill was getting ready to come down Haleakala in Maui, a 25 mile descent.

I take Bill's idea of holding a brake in reserve is a real world perspective on how to use a tandem with one disk and one rim brake. None of use want to risk having no brakes while descending on our tandem. Like a front fork failure at speed, it really is disturbing to consider.

It occurs to me that two brakes is not the best design for a tandem that is going on big descents. On a one or a two disk tandem an extra rim brake lever mounted on the drop of the captain's bars and working on a disk brake wheel could serve the same purpose. This is not a drag brake or a brake you plan to use but an emergency brake. If one or both disks fade then the captain moves the to the drops and stops the bike with the cool rim brake. Rims brakes don't weigh much and are easy to add just before and after that big trip to Maui as long as you have the canti or V-Brake studs on the frame or fork.

Radius of brake really does not matter: the overall system has the same energy and thus that energy has to go somewhere. There is the same amount of heat energy created.

There are two things that radius does change:
A) due to leverage it will take more pressure/friction on the disk than on a rim to create the same braking resistance force.
B) the smaller radius results in less mass in the disk than in a rim so it takes less BTUs of heat energy to raise the temperature of the mass.

These factors are what drive disks to larger diameters. The lower pressure benefits since we are using simple human power inputs to generate pressure. The larger circumference increases mass on the order of 3x the square of the radius. Of course more disk mass that helps control heat build up (and radiation) also adds weight...

To me the engineering tradeoffs here are where the limits are and what the failure mode is. I find the thought of blowing a tire off a lot more frightening than fading brakes so I put my $$$ on disk brakes. Especially on the control critical front end of the bike.

I think his objection might be overcome if a vendor were to approach him with a Santana-branded propriety tandem disc fork and brake. Then it would be the best, and all the other tandem makers pitiful fools for not adopting it.

I think the effect of the larger circumference has more to do with the surface area than with the mass. The larger surface area offloads more heat into the atmosphere, rather than storing it as a heat sink.

Swept area increases linearly with radius, assuming the width of the disc remains constant. Mass, neglecting the spindly disc arms, would also increase linearly, assuming constant thickness. The size of the pad obviously remains the same, so there is significantly more area for cooling on a larger disc.

The disc can conceivably run much hotter than a rim before anything bad happens. I'm pretty happy with two discs, having tried them on several steep and fairly long descents with no fade whatsoever (fingers crossed after saying that!).

Must resist commentary, must resist commentary, must resist commentary...... :twitchy:

Let me say this about the premise: Folks have been riding crappy tandems that used to be pretty good tandems with all kinds of brake designs, brake block materials, rims, etc. for decades and prudence has proven to be the key to safe operation.

It's not hard to know when you're pushing the limits of your brakes (outgassing, brake fade and the like), and technique will always trump technology. Moreover, people have proven time and again that they can over cook rim brakes, hub brakes and disc brakes... it really doesn't matter. Then again, there are teams who have made epic descents using a lowly pair of single pull calipers without bursting into flames or driving off the edge of a cliff.

Therefore, given that the average dual mechanical disc systems have greater heat capacity than rim brake systems, it's safe to assume they're certainly "as good as or better" than any tandem with only rim brakes or even a front rim brake plus rear disc. After all, beyond dual discs, there's only the rear drum which trumps all when it comes to heat capacity.

Bill definitely has opinions :50: Interesting he apparently thinks it is safer for a fast moving tandem to blow a front tire from overheating the rim than to experience a progressive disc brake fade as a warning.

If the tubeless hype is true then going tubeless MAY help eliminate the possibility of heating up an inner tube which can cause a heat related blowout. According to Schwalbe their tubeless tires can withstand significantly more heat than a comparable tube type tire. One thing that helps is that you start out with a lower pressure.

Given that due to Santana's steerer size and rake all of Santana's forks including two Santana carbon forks (four if you count 650c sizes) are Santana branded, I don't think fork branding is an issue. I disagree with some of his designs but do not doubt his true belief that he is correct.

A lot tour companies drop of tourists at the entrance of Haleakala so they can cruise down hill on their mountain bikes. They all have dual discs on those heavy clunkers. Just stop often to let the brakes cool.

On another note, try the West Maui loop if you still have time. Its a much nicer ride: gorgeous rural landscape, quaint small towns, beautiful ocean views, sparsely traveled, and plenty of hills. Make sure you bring plenty of water as the shops operate on Hawaiian schedule - "maybe we'll be open tomorrow, depends on the waves". Also, watch out for the oncoming traffic around the blind curves on the single lane stretches.

Mahalo,
CJ

"swept *area*" is the key: area is radius squared * 3.14.

And of course mass is a cubed formula so it too would increase dramatically.

What *does* increase linearly is the brake surface velocity. Larger disc (or rim) swipes faster by the brake pads than a smaller disc. The time between swipes does not change, just the speed of the swipe. Thus the time to cool is the same. But stipulated that larger disc will have more radiation surface to lose heat (as well as more mass to heat slower and then cool slower).

Agreed that disk can likely run hotter before failure. Dual disc brakes was one of my selection parameters when we purchased our tandem.

With different sized discs, wouldn't one fade first (I haven't figured out which one).

We normally ride with just caliper brakes. For Haleakala, Santana insisted that you have a disc or drum brake, so I put on a rear disc.
I originally left the rear caliper on as well, with a bar end lever in the stoker's bull horn, to have an extra third brake if needed. The cable routing setup just looked clunky though and I took it off.

Turned out braking down Haleakala wasn't an issue at all. While there are a couple of tight turns, it's not very steep. Pedaling hard, trying to go fast, we only hit 47mph, and were only on the brakes for turns. Definitely could have done it with just caliper brakes.

If you descend as he suggests, blowing a tire isn't an issue because you use the rear disc to control speed, only limited use of the front rim brake, and the front rim remains cool, and available to stop the bike when the rear fades.

Problem I see with the approach is that you're putting al the heat to the one disc, and making it more likely its going to fade and you're going to have to stop and let it cool off.

We did West Maui loop the day before the Santana trip started. Fantastic ride, just as you describe.

How do you figure that swept area increases with radius squared? If the disc was a plate, sure the area of that plate increases with radius squared, but it's not. The size of the pad remains the same, so the swept area is:

pi*r^2 - pi(r-discwidth)^2 = 2pi*r*discwidth - pi * discwidth^2

r=max dimension of rotor
r-discwidth = inner dimension of rotor, most material (except spindly arms) inside this is missing on a bike disc.

(discwidth = probably the wrong term, but the bit that gets swept by the pads - since the pads don't change for larger brake sizes, this is essentially constant)

Or, it's roughly the circumference of the disc times the width of the disc

The thickness is constant so mass, which is essentially the swept area times the thickness, also increases linearly with radius.

For a car, where the discs are solid plates, the area of the disc does increase with radius^2, but on a large bike disc, with the middle of the plate cut out that's not the case.

The idea of whether effective rotor mass and surface increases linearly with disc diameter or with a squared function was discussed in another thread quite recently. I put forward an argument similar to rahill's above, but focused on it being the circumference that is the only important factor, which is definitely a linear function of the radius/diameter. There were some people in that thread trying to argue that the relationship was non-linear, but none were able to explain why. Any attempts to argue for this must go beyond the simple "area = pi * r^2" argument, because that assumes a solid disc.

No picture of the Tandem in front of the 10,023' Sign? That's mandatory! Also shifting to the 53x11 or whatever the highest one is.

My wife and I have a humble Cannondale 2009 tandem with the stock brick wheels, 203mm G3 rotors and BB7's. The dual discs have been fine to slow us down all the very steep grades where we live and also down longer descents, Mt. Rose (16 miles) and Geiger Grade (8 miles) in Reno. I'd love to try a Tandem with dual calipers and lighter wheels but that's not going to happen anytime soon.

I've got dual discs - I had a rear with a front rim brake. I'll never not have a tandem with dual discs. I've done many long descents which required braking and many short steep and twisty descents that required even more braking. Never had heat issues, never had fade issues.

Reminds me on when Instructing at track events. The beginners often overheat their brakes and instantly think that it's because they are stock and not up to the task of being driven hard on track. In 99% of the cases - it's because they don't know how to brake correctly - braking too early etc.

The thing to do is to consider what you'd want if you were poised atop long, steep and twisty Mix Canyon Rd. I'd want front and rear discs. I think Bill McCready would want dual discs too.

http://imageshack.com/a/img824/3549/fcjw.jpg

Why would't Bill's technique work just as well on a bike with dual discs?

FWIW I ride a bike with dual discs but have retained the option of reinstalling the original V brake on the rear as a third brake if necessary.

Cheers,

Cameron

Pretty similar to Brasstown Bald.

We got down that without stopping with Dura Ace calipers, and a team weight of 350lbs.

http://www.strava.com/segments/614827

With rim brakes we had to stop twice on this. Some really steep sections at bottom that don't show on the graph. I am installing a rear disk and leaving the rear rim as a emergency brake.

http://www.strava.com/segments/613375

Anyone else use their jackets like a parachute on steep hills?

Wondering if a small parachute would work behind the Stoker? Or would it flap to much?

Half humor half serious question.

Has anyone does any serious descending with a kid trailer? At speed does it slow you down or just add to the freight train? What is the max speed that you have gone and felt safe with trailer?

This is where descending style comes into play. Looking at the satellite view of that, I think there would only be 3-4 hard brakes before the switchback at the bottom for us.

The faster you're willing to descend, the less heat is an issue for the brakes.



Unzipping jackets, jerseys ( still attached at the waist so they don't flap wildly) and sitting up will slow your maximum speed a bit.
You need to get in the drops to corner though for added stability.

An actual parachute would be an entanglement risk. Not good in spokes.




We've pulled a burley trailer with a kid in the high 20's, and rolled down some small hills at 30mph or so. I wouldn't want to go much over 10-15mph on something really steep. The lack of trailer brakes could cause a nasty jack knife if you had to hard brake at speed with a trailer downhill.

That would be an example of wher I think you'd want a third brake,preferably a drum, because you'd be dragging to control speed even on straight portions.