Specific heat absorption capacity of disc brakes

[Voltron]

From Hayes tech manual...


"The reason for the spoke design is that there are two sources of stresses in the rotor. The first is mechanical stresses due to torque and the second is thermal stresses within the rotor. As the braking surface heats up, it expands. The inner portion of the rotor near the hub is comparatively much cooler. With the outer braking surface expanding with higher temperature and the temperature of the center remaining largely unchanged a thermal stress is imparted on the spokes. The spoke design is specified such that the mechanical stresses and the thermal stresses occur in opposite orientations, attempting to cancel each other out and lowering the total stress in spokes as opposed to adding together. The result is the “sweeping forward” spoke pattern."

It's worth remembering a major design goal for normal bike gear is to save weight... And if you're going for minimal weight, legs forward maybe is better for regular leg powered riding. Once you don't care about saving grams, and loads and weights go up then overbuilt to where it doesn't matter about the legs is prob better.
Also, not always the case, but some of the rotor failures go back to a loose or missing rotor bolt...

[speedmd]

The spoke design is specified such that the mechanical stresses and the thermal stresses occur in opposite orientations, attempting to cancel each other out and lowering the total stress in spokes as opposed to adding together.

The heat stress tensile loads the spokes and mechanical loads are in compression. That should certainly help them keep their shape better. Still would like to see some test results showing the difference in failures.

[liveforphysics]

Experimentally, the difference in spoke leading or spoke trailing is whether or not the rotor wads up against caliper and fork leg or not after a few times braking from 80-100mph.

Feel free to drink bicycle part marketing kool-aid, but perhaps wear gear if you're going to stress something that has a buckling moment failure mode.

Things buckle because when forces aren't aligned perfectly, whatever direction something begins to deform has the vehicles braking forces vector into the bend as a positive feedback and it goes from flat to wadded up like a lace doily. When the braking forces are in tension, up until the outside of the rotor braking surface suddenly collapses all the spoke forces get realigned by vehicle breaking forces as things flex and deform under stress.

I only ran bicycle brake rotors for the first race or two before going to motorcycle brakes, because even while they lived longer with spokes in tension they still boil/gas pads, boil fluid, and still thermally fail rotors by the whole disc braking area reaching a temp where it has inadequate strength.

[speedmd]

Agree Luke, that moto brakes are the way to go on high speed setups. What we are seeing in the fails is not tensile fail, but what looks more to me like compression buckling of the brake pad contact area of the rotor. High heat would certainly accelerate this as would stacking spoke loads to it. IMO tension would not cause spoke buckling and would show as a snap - separation of the area. Have not found any photos of what looks like spoke compression buckling causing the failures. These cheap straight gauge cookie cut out rotors are a borderline design IMO. Much better to go with the alloy center hub and a thicker brake disc in the pad contact area.

[Chalo]

I only ran bicycle brake rotors for the first race or two before going to motorcycle brakes, because even while they lived longer with spokes in tension they still boil/gas pads, boil fluid, and still thermally fail rotors by the whole disc braking area reaching a temp where it has inadequate strength.

Bingo. If you only need high instantaneous power, bicycle discs can be reasonably okay. But if you need to sink a lot of total energy, you're much better off using a rim brake-- because the rim can be a kg or more of metal that has much higher specific heat than a stainless brake rotor, plus a much larger surface area from which to discharge heat. German brake tests have proved this, and they were using ordinary bicycle rims rather than the bigger heavier sort that we prefer around here.

Motorcycle disc brakes are coupled to motorcycle hubs and forks, which all do the job nicely but at a huge surcharge in weight and crudeness compared to bicycle components.

[speedmd]

Rim brakes are great for slow mo, but will never come close in ultimate performance. Not even!

[wturber]

Rim brakes are great for slow mo, but will never come close in ultimate performance. Not even!

I note that the diameter of the front disc brakes in the photo are approaching the same as the motorcycle's rims. There are also two of them. So as the need for high performance increases, does disc brake sizing tend to mimic what we already have with rim brakes? It sure seems so. Note that the less important rear brake is smaller (I can't tell if there are two of them). And yes, I understand that at some point it is desirable to move the heat generated by braking away from the pneumatic tire.

[Chalo]

If we're going to compare our electric bikes to motorcycles operating at multiple orders of magnitude higher energy levels, why stop there? Wing spoilers could be the next big deal we can't live without. Maybe even retro-rockets.

In the world we inhabit, bicycle disc brakes can't dissipate as much gross energy as bicycle rim brakes. That's all I'm saying. If we want to talk about truck brakes or whatever, then the results change.

[Voltron]

Mmmm... Downforce wings...

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images (1).jpeg (16.55 KiB) Viewed 629 times

[speedmd]

In the world we inhabit, bicycle disc brakes can't dissipate as much gross energy as bicycle rim brakes.

Tiny 140 -160 mm ones are certainly at a great dis-advantage. No argument. Super thin ones are also poor examples of their potential. Those twin 8" rotors your playing with, it gets close. When comparing what we could expect from the options, lets look at several of the factors as I see it and compare.

Limits of Pad force, in rims, at least standard ones, can take no where near the amount that a disc can take. It is quantum force difference.

Friction Coefficients for comparison sake we can assume we can roughly equate.

Exposed surface area, the rim certainly has an advantage, but only has one side exposed to the out side air. Interesting thermal model, having a high pressure tire insulating the interior region. Certainly limited.

The photo of the doctor laying heavy on the brakes show a important contrast and most likely why even small rotors perform as well as they do. It is the tremendous delta T they can operate at that rim brakes just will never match because of tire constraints.

I asked every brake rep at last months show about dual rotor setups. Most thought I was kidding. I don't see any interest on their side to explore the real potential in a light weight larger (10 - 12") or multi rotor setups. Hanging a huge diameter brake ring off the rim would be something someone may have to try on their own but would certainly be a interesting comparison.

[Chalo]

In the world we inhabit, bicycle disc brakes can't dissipate as much gross energy as bicycle rim brakes.

Tiny 140 -160 mm ones are certainly at a great dis-advantage. No argument. Super thin ones are also poor examples of their potential. Those twin 8" rotors your playing with, it gets close.

Twin 8" rotors I think would exceed most rims' heat rejection rates, while completely eclipsing their instantaneous stopping power. Big rotor discs are already stronger on a momentary basis and reasonably close in heat handling.

The only catch there is that all the dual rotor bike wheels I've seen push the spoke flanges way too close together. You get stronger brakes at the cost of a weaker wheel. 135mm fatbike front hubs and forks might give us a toehold to fixing the problem (which probably isn't a real problem for pedal cyclists). The kinds of doods who want fatbikes for the street would probably turn over their hard-earned shekels for dual front discs that are equally unnecessary.

Well, it looks like somebody already thought of that:

[Voltron]

Nice that they're running one rotor each way... that's a great way to test the spoke direction theories.

[speedmd]

spoke flanges way too close together. You get stronger brakes at the cost of a weaker wheel

Closer flanges should make a stronger wheel vertically and should also be stiffer-stronger under brake and drive loads. Laterally certainly not as good. Interested in how the dual 8's work out.

[amberwolf]

Wing spoilers could be the next big deal we can't live without.

I'm ready.

[Lebowski]

[DasDouble]

Rim brakes are great for slow mo, but will never come close in ultimate performance. Not even!

I note that the diameter of the front disc brakes in the photo are approaching the same as the motorcycle's rims. There are also two of them. So as the need for high performance increases, does disc brake sizing tend to mimic what we already have with rim brakes? It sure seems so. Note that the less important rear brake is smaller (I can't tell if there are two of them). And yes, I understand that at some point it is desirable to move the heat generated by braking away from the pneumatic tire.

The disc can take more energy without getting as warm as rim brakes. Plus You have better airflow on disc brakes + you have two brake shoes per disc brake. Two disc-brakes are already double that amount of brake shoes. And I also think the space of contact between disc brake and a brake shoe is also bigger then the contact area of rim brakes.

Edit: Of course its better to have bigger disc brakes because then the torque is bigger you can stop your wheel with + you can controll it better by that without locking it that easy.

[Chalo]

The disc can take more energy without getting as warm as rim brakes.

Um, no. Heating is a function of the amount of energy in, divided by the mass being heated and its specific heat. Rims are heavier than discs, and aluminum has a much higher specific heat than stainless steel, so for any amount of braking energy the discs will get way hotter.

Plus You have better airflow on disc brakes

No. How do you reckon this? Rims have the same linear velocity as discs, but higher angular velocity. And they have much more area exposed to air. Sometimes radically more.

+ you have two brake shoes per disc brake. Two disc-brakes are already double that amount of brake shoes.

Have you counted the number of brake pads on any rim brake lately? Sure, you can use two discs on the same wheel (at great extra expense), but you can also use two rim brakes on the same wheel (at minimal added expense). Apples to apples, that's the same number of pads.

And I also think the space of contact between disc brake and a brake shoe is also bigger then the contact area of rim brakes.

Not on most bicycle disc brakes. Those pads are pretty small.

Of course its better to have bigger disc brakes because then the torque is bigger you can stop your wheel with + you can controll it better by that without locking it that easy.

Sure, that's one of the chief benefits of rim brakes. They get superior mechanical advantage by applying friction near the outer radius of the wheel. You can't have disc rotors as big as the rim.

[eCue]

I did find the TRP 160 disc to be adequate until yesterday. I was riding home from the store with 50 lb in a pack when I noticed its weakness. With 275lb its Pretty weak. 200mm disc here I come

[wturber]

The disc can take more energy without getting as warm as rim brakes. Plus You have better airflow on disc brakes + you have two brake shoes per disc brake. Two disc-brakes are already double that amount of brake shoes. And I also think the space of contact between disc brake and a brake shoe is also bigger then the contact area of rim brakes.

Ummmm .... what Chalo wrote. ;^)

There are certainly some advantages to disc brakes. But the same is true for some rim brakes. The problem that I see is that a LOT of people don't really understand what those advantages and trade-offs are.

I can tell you the one thing I really dislike about my disc brakes - and that's the frickin' noise they make. It can be downright annoying. I guess I need to look into finding a different pad material.

[John in CR]

2 words...Regenerative Braking.

Make your mechanical brakes just low speed helpers and high speed safety backup, and the problems and maintenance all but disappear.

[Chalo]

2 words...Regenerative Braking.

Make your mechanical brakes just low speed helpers and high speed safety backup, and the problems and maintenance all but disappear.

That's a fine approach if you use a direct drive motor. But for an e-bicycle, as opposed to an e-motorcycle, fixed gearing has more drawbacks than advantages. Coasting is the soul of bicycling. Dragging a big heavy lossy motor means never being able to coast freely, the way a bicycle should.

At pleasant, safe, and quiet speeds, it's possible to spend most of your time coasting. But not if you're working against the drag of a direct drive motor. Giving up that benefit for regenerative or plug braking, when you could be using quiet, controllable, cheap, lightweight rim brakes instead, seems like a net loss.

It's different for motorcycles. But where I live, in a developed country, motorcycles have a bunch of licensing and insurance and right-of-way requirements attached that I don't have to fool with as a bicyclist. What kind of bicyclist would I be if I didn't go for minimum drag and friction?

If a pound of effective brakes saves me a ton of hassle, I'll take them.

[amberwolf]

There are certainly some advantages to disc brakes. But the same is true for some rim brakes. The problem that I see is that a LOT of people don't really understand what those advantages and trade-offs are.

Though if they're googling they'll find a lot of (heated) discussions here on ES about it over the years.

Sure, you can use two discs on the same wheel (at great extra expense), but you can also use two rim brakes on the same wheel (at minimal added expense).

Like this, for the ultra-cheap method:


Though either a fork built specifically for this (bosses front and rear) or bolt/clamp-on units built specifically for it would give a better mounting angle for hte rear pads and arms, it still works well enough.

(if it was a steel fork, I'd've welded the stuff on instead of clamping it, and it'd probably be even stiffer--that's likely to happen on the next iteration).

[wturber]

That's a fine approach if you use a direct drive motor. But for an e-bicycle, as opposed to an e-motorcycle, fixed gearing has more drawbacks than advantages. Coasting is the soul of bicycling. Dragging a big heavy lossy motor means never being able to coast freely, the way a bicycle should.

That is certainly a matter of preference, use profile and some debate. For my daily commuter, I'm super happy with my direct drive and don't find the coasting issue to matter much at all. I do plenty of coasting down hills and generally find it hard to coast going up hills. Somehow that never works out for me. I also don't really like spending lots of time not pedaling. Saving every tiny bit of energy is certainly a laudable goal on a regular bicycle. But given that the electricity for my 30 mile commute costs me about 10 cents, I don't see the general need to treat energy conservation with the same level of importance as on a regular human-only powered bicycle.

OTOH, if I every do electrify a road bike, I will be more inclined to lean toward that ethic and will use a small, lightweight geared hub. But that will be so that the bike will continue to feel like a road bike - something I don't find nearly so important with my commuting bike.

[DasDouble]

Alright, I see the point but then why do brands like brembo lead the market with their disc brakes? I mean there must be a reason for it, right?

[eCue]

Brembo makes some sic calipers

excerpt from their site

For drivers who demand the utmost from their car

Brembo kits consist of cross-drilled or slotted ventilated, one-piece or composite discs and aluminium calipers with 4, 6 or 8 pistons; a complete set of high performance pads; metal-braided hoses and high quality hardware.

More braking torque and thermal capacity on the track

Using radial-mounted fixed calipers together with oversized discs gives the system greater braking torque and superior thermal capacity. The result is a braking action that is noticeably more immediate and decisive even in normal road usage, and is extraordinarily powerful and consistent under the repeated loads of track use.