maddog34

(from FBinNY)" Like you, I (greatly) prefer rim brakes on road bikes, and say so whenever asked."

news flash: Rim Brakes ARE LARGER DISC BRAKES.

now.. what are the differences between "disc" and"rim" brakes?
surface feet per minute.

what will a larger disc do to surface temps,. at the same given road speed and loads?
Create MORE HEAT, but spread it out across a larger swept surface area on the disc....
what about the pads? They receive more heat due to increased surface feet per minute at the contact surfaces....

ever melted a tubular tire/tube on a long, steep, downhill? I have... the rim looked like a moonscape after that too... the aluminum melted and streaked badly... AKA: Galling.
the pads turned SHINY and SMOOTH , AKA: Glazed

My Santana Tandem has TWO Rear brake systems.. a Drum brake (Large contact patches, Low surface speeds, fins on backing plate and outer surface of drum) and Canti Rim Brakes with KoolStop Orange pads... they are adjusted to come on at slightly different points in the lever travel.. the Drum first, Cantis second. Works great.

fooferdoggie 

Here isa pic of the rotor hard to get a good pic but it is a bit discolored. I think this was fine will we were on a long graven decent where I had to keep the speeds around 15 mph because of all the twists and rough roads. then I think my beading process using our driveway was causing issues too like the mechanic said.
the ice-tech and Shimano finned pads were more robust and we used them for 2 years it took a lot longer descents to make them get a little gunked up.
but 25 is as safe a speed as I will go on this road too many cars back out and even at 25 it takes a lot of work to stop fully on that hill. the pads may have gotten a little messed up on the last rotor as I wore it out and more then I realized. I did not realize how fast the tandem went through discs. we only get about 2000 miles fro ma set of pads.

FBinNY 

Not a news flash to me and most long time road riders. We've been saying "why add another disc when already have one", since the advent of bike disc brakes.

However the rest of your post is somewhat lacking.

To start with we have to understand the difference between heat, (calories) and temperature change (degrees).

The total heat produced is a function of the work done, either by conversion of kinetic energy when slowing, or by resisting gravity over time and distance. That in turn is about weight and speed change. So regardless of how you stop, you're converting energy to heat calories. Having no place to go, that flows into the rotor (rim or disc) and to a lesser extent the pads, which being insulators don't take up a large percentage of that energy). That frictional heating of the rotors raises their temperatures, which takes us to part 2.

The relationship between heat produced and temperature change depends on the rotors ability to take up heat. Two factors control that. First the mass, which on rims is roughly 5-8x that of typical steel discs. Then the specific heat (heat added / change in temp) which is roughly double in aluminum vs. steel.

So combining properties one would expect a disc rotor to get roughly 10-15 times hotter for the same conditions.

Lastly, while heating, then rotor (or rim) is also being cooled by the airflow, offsetting some of the temperature rise. This is tricky because, being hotter, discs cool faster, but rims have higher surface area which helps too.le

All things considered, I'd expect that a typical rim brake road bike wins out over a disc brake bike in terms of the ability to manage long descents and/or handle multiple hard stops in a short time.

But all the above doesn't matter if the rider doesn't know how to use brakes effectively, to avoid building up heat without giving the brakes an opportunity to cool.

fooferdoggie 

yes I think the pads maybe got messed up on the last disc. I think they had only a few hundred miles on them but that disc was shot. or its how I bedded them in I should not have used my 5% grade hill I should have done it on the flats like I used too. then they glared over and they needed full baking going down my hill to work overheated the rotor and messed it up. So my bad and now I will bed the pads in like I should have.

Troul 

I would go with a more "solid" surfaced set of rotors & use ceramic pads in the rear with semi metallic in the front.
With ceramic pads, you'll likely notice that they will grab better once they are warmed up during a braking event. I wouldn't ride the rear brake entirely, as the rotor might not stay true for very long, but the pads should not glaze as easily. Cold ceramic pads when engaged, will be slightly more audible compared to resin or semi metallic pads, but not severely loud like how contaminated brakes can get.
TRP might make the thicker rotors in your size too, so consider looking into those when you replace the rotors.
You need as much surface area for the mass & speeds you are moving at, trying to be thrifty with your brakes might end poorly.
Any chance your rims & frame support rim brakes? Could be a failsafe, but might cost more outfitting those compared to buying better disc & brake pads parts.

TiHabanero

Throughout the OP's comments he has consistently noted that the glazing started when he changed to a Sram rotor. Ditch the POS Sram rotor and put a new rotor on that matches the original rotor you took off. Be sure to match brake pads to the original pads that were used as well.

fooferdoggie 

we do have a rim brake and I have my wife use it on longer desents I would switch from front to back to rim but this hill has never needed it. I think the combo of the rotor and the pads and my bedding them in wrong was the problem.

fooferdoggie 

yep tossed it put a magura on and new metallic pads and it works great. it beded in early on the flats and has more stopping power then I got from the sram.

drlogik 

How old are the brake pads? They look like they are deteriorating and small chunks are starting to break off?

mschwett 

good result. but it does seem like you should use the front brake more, brake sooner and shorter and let the rotors cool a little before braking more or dragging them. doesn't seem like a heavy tandem is going to flip you over the bars too easily

fooferdoggie 

I think the old rotors did that. I left them on when I put a new rotor on there.

drlogik 

You used the old pads with a new rotor? That's generally not a good idea.

fooferdoggie 

I know but only had a few hundred on them when I found the old rotor was bad.

t2p

recently had issue with Shimano pads glazing and squealing

Shimano J03A pads - IceTech rotors (standard IceTech - not IceTech Freeza)

light sanding and iso alcohol initially stopped the glazing / squealing - but then it returned

replaced the pads with newer J05A pads - much better

it’s possible I had a bad pair of J03A pads - but have them on another bike and they are fine

fooferdoggie 

so far the new combo is working well. I have tried the hard brake on that hill and it works fine but its only a couple seconds shorter. but there is another 20% grade path we go down only a couple hundred feet but you have to go real slow as there is a cross path at the bottom or this guy. no way you can just slam on the brakes as its too short with a sharp turn at the bottom.

grumpus

It won't, but it will lock up and dump you on the ground in no time at all.