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Discussion Starter · #1 · (Edited)
And for that matter where the heck has Grumpy been? :confused: (NO offense to other knowledgeable brake posters!)


Anyways, i've got a project i'm thinking about (again) and I need further education if I want to make it work well. I want to make a mechanical disc brake caliper and I want it to be LIGHT. I have two big problems with this project and i'll just have to figure one of them out on my own, but the other - i'm sure I can find an answer here. :)

The problem i'm looking for more information on is with regards to heat dissipation, particularly with the caliper. What I believe I know correctly is that if you have a caliper with greater mass (more weight) you have a caliper that can not only absorb heat better, but also dissipate it better than one that has less mass. Is this correct or just poor assumption on my part?

Also, if it is correct, then should this not also mean that if you make a caliper "too light" that some heat dissipation properties it posesses would also be affected ..... even to the point of poor brake performance?????

I'm not an engineer and don't know everything, but I need input on this particular issue to decide if I want to try to tackle "problem #2".

All/any help that is based on fact (just the facts ma'am) is greatly appreciated.

PATIA!
 

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Depends on what material, and it depends on where the material is removed. A cut-out can help improve heat dissipation because it allows air to pass through it. It can also allow more surface area to be exposed to cool air.

With all that said, I don't think that there's going to be such a HUGE difference.

I may be wrong though, I am not an engineer (though aspiring to be one).
 

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Discussion Starter · #3 ·
XSL_WiLL said:
Depends on what material, and it depends on where the material is removed. A cut-out can help improve heat dissipation because it allows air to pass through it. It can also allow more surface area to be exposed to cool air.

With all that said, I don't think that there's going to be such a HUGE difference.

I may be wrong though, I am not an engineer (though aspiring to be one).
Aspiring or not, Will, I still respect your input of things brake. Besides, you took the initiative and looked further into where I was going to go with things like heat sinks and surface area. From what i've seen in the electronics world I know that these can help and if designed properly can make a substantial difference. I can easily incorporate this into any of my designs if need be. Of course, that's the question - if need be.

Thanks Will. :thumbsup:

If anyone else wants to share, by all means .....
 

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As we know, a brake transforms kenetic energy (motion) to heat. The limit to the amount of heat the brake can handle is defined by the boiling point of the brake fluid and the temp at which the pad material begins to lose it's grip. If the fluid boils, it's no longer an incompressible fluid, you lose compression force of the pads on the rotor, and you get a spongy lever. If the pad material loses it's grip, the brakes "fade". So you want the metal in the caliper and rotor to absorb and then dissipate the heat generated BEFORE it gets to the point of pad fade and/or fluid boil. Different metals have different heat absorbing characteristics but most bike brake calipers are made of aluminum and most bike rotors are steel. So if we assume those materials as constant from one brake design to the next, then it's a given that the amount of heat that can be absorbed is directly proportional to the mass of the caliper and rotor - more metal = greater heat absorption capacity. This means that a heavier brake system (caliper and rotor mass - the mass of the lever, fluid, and brake lines don't come into play in this discussion) can do more braking work than a lighter system before braking performance begins to degrade. Now, heat dissipation is all about surface area - greater surface area = faster rate of dissipation - that's why heat sinks and air cooled engine cylinders have fins. It's also why rotors are drilled - more surface area to contact the surrounding air. So if you take away mass, you can make some of that back with improved heat dissipation. How much of each you need depends on how the bike will be ridden (XC or DH) and requires some computer design work that is probably beyond the home calculator capabilities because it's such a dynamic simulation. Does that give you enough info to decide what you want to try?

(Yeah, I'm a geek engineer! :cool:)
 

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Actually the holes in the middle of the rotor don't do a whole lot. Their primary function is for weight savings, cooling comes second. Most of the cooling is done along the outside edge of the rotor. Wavy rotors or rotors with cutouts along the edge grab pockets of cool air and pull it through the caliper.

Though many calipers are aluminum, the grade of aluminum can make a pretty significant impact.

Btw, he said mechanical disc brake, but the idea behind it stays the same. You want to dissipate the heat before the pad material loses grip.
 

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Discussion Starter · #6 ·
XSL_WiLL said:
Actually the holes in the middle of the rotor don't do a whole lot. Their primary function is for weight savings, cooling comes second. Most of the cooling is done along the outside edge of the rotor. Wavy rotors or rotors with cutouts along the edge grab pockets of cool air and pull it through the caliper.

Though many calipers are aluminum, the grade of aluminum can make a pretty significant impact.

Btw, he said mechanical disc brake, but the idea behind it stays the same. You want to dissipate the heat before the pad material loses grip.
Agreed with the grade of aluminum and it's potential for impact on the system, Will. That much i'm aware of (but thanks for making sure!). What you mix in with a given grade of aluminum can make a profound difference on it's various characteristics. Even then how you produce the part (machined from billet, cast, forged etc) can make an impact.

Now, about the wavy versus drilled rotor deal - mmmmm, not so sure I agree there. Would the holes in a drilled rotor still not allow air to be pulled between the pads? Me thinks so. Outer edge cooling or not some air has to get pulled through there, right? Remember a P.M. conversation you and I had a while back about a pad with a groove through it? Ring a bell? Does that not apply here?

wddamf - yes, it helps immensly. So if what you say is correct, then if one removes caliper material there's potential to still keep "a given amount" of heat dissipation by increasing pad/rotor size, right? Forgetting the rotors for this specific example - larger pads give the potential to increase stopping power by not only increasing pad/rotor contact area, but also by having the natural ability of absorbing/dissipating more heat, yes? Understood with the complexity of the dynamics involved. :madman: That's why i'm posting ..... at least to have a clue. :thumbsup: ;)

Damn, there goes one idea. (shrug) :idea:

Okay, one more Q since i'm thinking about it. While it doesn't pertain to a mechanical brake system, i'm left wondering - with hydraulic brakes there's been talk about calipers with and without heat insulators/isolators in the pad/piston area (specifically, i'm talking about Magura's Marta SL). Okay, one wants a design that keeps the heat generated by the pad and rotor away from the fluid, got it. How is it that an assembly without these can "make due"? Pad material is the only thing i'm seeing ..... maybe tied-in with their specific XC use as well. :confused:
 

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try to make the caliper open-back, or make it with a "window" so air can pass through from front to back. examine any mechanical brake calipers you have, and figure out ways that they can be made better, whether it be material in different places or more surface area (which ultimately may mean fins) or ways to make it stiffer as well. i think wjere you can lose the most weight is the linkage arm that actuates the brake and the piston that pushes the pad. i haven't taken apart a mechanical caliper, but you may be able to take some weight off there. in the end, you may just have to experiment.

like xsl_will, i'm no engineer, but am headed in that direction.
 

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I remember that PM, the holes may have something to do with heat dissipation, but IMO that is not their primary function. They're more there to clean the pads and shave weight.

Think of it this way, a slotted rotor on a car actually runs hotter than a solid rotor. Everytime you brake, those slots are shaving a little bit of pad material off. This gives you fresh pad every time you hit that pedal. Internally vented rotors have the vents along the outside edge. Kind of the same concept. The slots also carry out gases that are produced under hard braking. But for most everyday driving situations, it doesn't make much of a difference.

Back to the bike, while the wheel is spinning, the air is passing over the edge of that rotor, and the rotor is "slicing" into the air. Wouldn't that create turbulent air? And wouldn't that in turn effect how the center cut-outs are pulling in air? That's my logic as far as why the outside edge is most responsible for cooling.

A wave rotor or rotor with cutouts would also prevent the leading edge of the pad from staying one place. And it helps to clean off the pad as well.
 

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Discussion Starter · #9 ·
rm_racer said:
try to make the caliper open-back, or make it with a "window" so air can pass through from front to back. examine any mechanical brake calipers you have, and figure out ways that they can be made better, whether it be material in different places or more surface area (which ultimately may mean fins) or ways to make it stiffer as well. i think wjere you can lose the most weight is the linkage arm that actuates the brake and the piston that pushes the pad. i haven't taken apart a mechanical caliper, but you may be able to take some weight off there. in the end, you may just have to experiment.

like xsl_will, i'm no engineer, but am headed in that direction.
Understood and appreciated. My WW'd Type F BB7 caliper project was a real eye-opener.

Right now I have a design that I could make that would weighs less than HALF of what the BB7 caliper does ..... BUT ..... it's thermal properties SUCK. Another design I have would weigh about 1/2-2/3 of a BB7 ..... BUT ..... with no easy pad adjustments. Ahhh, that evil 10-letter word I "love" so much ..... compromise. :bluefrown:

You know, part of my problem, I feel, is that i'm looking AT their stuff ..... not the so much the actual situation. :madman: I'm not going to say it's right or wrong ..... it's just the way I feel. Call it "gut intuition at play", if you will. Meh, at least i'm not trying to re-invent the wheel.
 

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Discussion Starter · #10 ·
XSL_WiLL said:
Back to the bike, while the wheel is spinning, the air is passing over the edge of that rotor, and the rotor is "slicing" into the air. Wouldn't that create turbulent air? And wouldn't that in turn effect how the center cut-outs are pulling in air? That's my logic as far as why the outside edge is most responsible for cooling.
Yes, I guess it would be more appropriate to say that instead of the drilled rotor pulling IN air, it's doing more to exhaust the aftermath. Agreed?
 

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So if what you say is correct, then if one removes caliper material there's potential to still keep "a given amount" of heat dissipation by increasing pad/rotor size, right?
Kinda. It's a two-fold process. You can remove system mass (heat absorption capacity) if you increase system heat dissipation capability. Keep in mind that increasing pad and rotor size will increase system mass.

Forgetting the rotors for this specific example - larger pads give the potential to increase stopping power by not only increasing pad/rotor contact area, but also by having the natural ability of absorbing/dissipating more heat, yes?
Not really. Larger pads only increase the "stopping power" by the amount of incremental mass gained. They probably deliver a very minimal increase in dissipation. With larger pads comes a reduction in clamping pressure (same lever and piston force dispersed over a larger surface area) and the potential for pad flex so JUST increasing pad size really doesn't increase the actual "stopping power". If you address pad flex (multi-piston caliper or piston size) and increase caliper force (lever and/or master cylinder modifications), then larger pads can deliver a payoff.

Not knowing what you're trying to achieve somewhat limits my input, but something else to keep in mind is that the caliper has to deliver a considerable amount of force to the pads. Structural rigidity of the caliper is a very real design concern. Removing material here will have an effect.

It seems to me that the rotor gets hotter quicker than any other component in the system. I would think that increasing the rate of heat dissipation of the rotor is the "lowest hanging fruit". Maybe incorporate some air scoops in the front section of the caliper, a 'la F1 cars!

Wavy rotors or rotors with cutouts along the edge grab pockets of cool air and pull it through the caliper.
Not trying to be confrontational, but rather genuinely curious - do you have any research to support this idea? I don't see how a thin, flat, rotating disc can move any air at all - with or without cut-outs. But I could be dead wrong.
 

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Anyone even wanna touch on a diagonally (as in reference to the rotor facing, not just slotting) cross drilled rotor as in formula one? Hope's kinda touching on it with the newer rotor system...

Basically, with the right shape, and averall combination of meterials, even down to the adapter(s) and bolts, heat sink will occur.

Hmmm, looking back at your comment about electrinic sinks, I wonder why nobody's done a vaned adapter, with a larger mating surface, and used something like fomblin to aid in the heat transfer...

Not an engineer, switched to business, bike shop business, but know pleanty of engineers, geeks, Lockheed, Boeing peeps,etc. All Silicon Valley customers.

Lemme know the thoughts. I'll pass it along and get someone to play around, and call it a "scientific project". :thumbsup:

Obi..

p.s. Will, i got the brake issue solved, the adapter tolerances were f-in off by .020". Havn't bothered to clean-fix 'em on a cnc, just busted out the veneer caliper and a metal file. :)
 

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Discussion Starter · #13 ·
obionespeedonly said:
Anyone even wanna touch on a diagonally (as in reference to the rotor facing, not just slotting) cross drilled rotor as in formula one? Hope's kinda touching on it with the newer rotor system...

Basically, with the right shape, and averall combination of meterials, even down to the adapter(s) and bolts, heat sink will occur.

Hmmm, looking back at your comment about electrinic sinks, I wonder why nobody's done a vaned adapter, with a larger mating surface, and used something like fomblin to aid in the heat transfer...

Not an engineer, switched to business, bike shop business, but know pleanty of engineers, geeks, Lockheed, Boeing peeps,etc. All Silicon Valley customers.

Lemme know the thoughts. I'll pass it along and get someone to play around, and call it a "scientific project". :thumbsup:

Obi..

p.s. Will, i got the brake issue solved, the adapter tolerances were f-in off by .020". Havn't bothered to clean-fix 'em on a cnc, just busted out the veneer caliper and a metal file. :)
Fomblin is a grease/lubricant. Don't see how it would help ..... okay, unless you want to say that greasing the rotor would help reduce the braking power thus reducing heat generated. :lol: Sorry man, just playing around. :)

As for the heat sinks - IIRC, Magura released their Venti-Disc rotor with "vane technology" they're incorporating said to aid in heat dissipation. Wether or not it works like that is a whole other story - might just be a marketting gimic. If anything i'd like to think that at the very least, there's more mass to absorb/dissipate heat (rather than actually "vent" the rotor). Again, just thinking it.
 

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Discussion Starter · #14 ·
wddamf said:
Kinda. It's a two-fold process. You can remove system mass (heat absorption capacity) if you increase system heat dissipation capability. Keep in mind that increasing pad and rotor size will increase system mass.

Not really. Larger pads only increase the "stopping power" by the amount of incremental mass gained. They probably deliver a very minimal increase in dissipation. With larger pads comes a reduction in clamping pressure (same lever and piston force dispersed over a larger surface area) and the potential for pad flex so JUST increasing pad size really doesn't increase the actual "stopping power". If you address pad flex (multi-piston caliper or piston size) and increase caliper force (lever and/or master cylinder modifications), then larger pads can deliver a payoff.

Not knowing what you're trying to achieve somewhat limits my input, but something else to keep in mind is that the caliper has to deliver a considerable amount of force to the pads. Structural rigidity of the caliper is a very real design concern. Removing material here will have an effect.

It seems to me that the rotor gets hotter quicker than any other component in the system. I would think that increasing the rate of heat dissipation of the rotor is the "lowest hanging fruit". Maybe incorporate some air scoops in the front section of the caliper, a 'la F1 cars.
Gotta keep it short - it's late and i'm tired.

More food for thought with all this above. Aware of pad pressure (big problem in one design, piece of cake in the other). I mentioned that i'm trying to make a mechanical disc brake kit lighter than what's out there already. I've got another idea now that i'll have to "ponder" upon.

Thanks guys. 'Night all. :sleep:
 

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wddamf said:
Not trying to be confrontational, but rather genuinely curious - do you have any research to support this idea? I don't see how a thin, flat, rotating disc can move any air at all - with or without cut-outs. But I could be dead wrong.
Solid evidence? No. But based on your assumption, I suppose you can say that Magura Ventidisc rotors aren't really venting at all either. If those thin rotors can't move any air, then no air will be passing through the hollow section of the rotor.

It may not be moving much air, but it certainly is doing something.

I believe that Galfer also uses this idea as part of their marketing.
 

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AndrewTO said:
As for the heat sinks - IIRC, Magura released their Venti-Disc rotor with "vane technology" they're incorporating said to aid in heat dissipation. Wether or not it works like that is a whole other story - might just be a marketting gimic. If anything i'd like to think that at the very least, there's more mass to absorb/dissipate heat (rather than actually "vent" the rotor). Again, just thinking it.
On this issue, at least in the case of the Hope vented rotors on the Moto brakes, the reviews I have seen from some German magazines say that the while the Motos with the std. 2 piece rotors present some fading, when they switched to the vented ones, the fading went away under the same conditions... so at least it helps a bit.... how significant of an improvement that is... that's a whole other question ;)
 

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Rotors - Relative to the airflow over the rotor created by the forward movement of the bike, I'm questioning how a thin, flat, spinning disc can move any significant amount of air on its own, regardless of grooves, vents, waves, etc. Without something to "grab" the air and push it, I just don't see how it could happen. I think all the holes, vents, slots, etc. aid in:
  • cooling by exposing more surface area
  • constantly relocating the rotor's leading edge on the pad
  • "scraping" the pad clean
  • possibly providing an "escape path" for any gasses created between the pad and rotor interface (a theory I've read)
Don't forget marketing hype and visual appeal (which DOES matter to me! - a uniform circular band of metal would be BORING!!). I'll admit that I could be wrong about the rotor moving air (I'm not an aerodynamic engineer), but I've not seen any research or real world proof that would convince me otherwise. That's my thoughts on rotors.

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Making a mech brake lighter (sorry I missed that earlier!) - I'd Dremel material off the caliper and the adapter (my Avid adapters "look" HEAVY to me - probably the biggest opportunity for savings right there) paying close attention to predicted load applications and stress concentrations. Also consider alternate materials. All the fasteners can go to titanium, and a titanium caliper and adapter would be plenty strong and much lighter. I dunno what the rate of heat absorption change would be but, like I stated earlier, I think the rotor takes most of the heat anyway. F1 cars use carbon fiber rotors and pads because of the light weight and rapid dissipation rate. The calipers are metal - probably Al but I'm not sure. In this application, the rotor is designed to absorb and dissipate almost all of the heat. Alternate rotor materials are obviously an option but probably cost prohibitive for us regular folk! I'd be hesitant to try to lighten a production MTB rotor too much because it has to handle significant loading and you need to maintain a certain amount of braking surface area with the corresponding production pads.

You "could" go with smaller pads and a corresponding smaller rotor contact area (smaller pads will result in higher clamping pressure if used with existing levers and pistons) which should deliver similar braking performance but will run much hotter - then add air scoops to aid in heat dissipation - since there should not be heat generated unless the bike is moving - when the air scoops would become effective. This will be a trial-and-error balancing act but would be a fun experiment if you have the time! I doubt fins on the caliper will deliver much because most of the heat goes into the rotor. Gotta figure out how to cool the rotor - and since I don't believe a thin flat disc can move air, we have to look elsewhere. Ferrari devised a turbine-like device that was inserted into the wheel rim - this spinning "fan" pushed air over the rotor. Maybe a series of vanes could be fixed to the bike hub to move air over the rotor? But even then, I doubt that would best the amount of air moving across the bike due to forward movement.

~~~~~~~~~~~~~~
This is a fun thread!! Thanks everybody for the continued discussion on this one - excellent learning opportunity!
 

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neat experiment idea

I just thought of a neat experiment idea (if somebody has the capability!!)

Select a production vented MTB brake rotor. Build an exact replica but make the brake surface a uniform round band of flat metal (no vents, vanes, waves, slots, etc.). Get a tube with an I.D. just a bit larger than the rotor's O.D. (PVC pipe?). Mount each rotor to a hub connected to a motor with a speed control. Mount a rotor, insert into the end of the tube, and measure the air speed (if any) coming out the opposite side of the tube. Repeat with the other rotor. If there is any air movement at all, I predict it will be minute so the air movement measurement device will need to be appropriate. Any college lab guys out there?! :thumbsup:
 

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Randy,

Thanks - good info! Can you address whether or not a bike rotor's shape can enable it to move air? And/or are all the waves, cutouts, etc. there for increased cooling surface area and pad cleaning?
 
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