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What is the reason the braking surface of a disc has so many holes?

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What is the reason the braking surface of a disc has so many holes?

Old 04-29-19, 08:28 AM
  #26  
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Originally Posted by PepeM View Post
How thick are bicycle discs?

If we assume that the holes are circular, for the surface area of a drilled disc to be larger than that of a solid disc, the following would need to be true:

surface area of exposed material due to hole > surface area of the material that would be there if there was no hole

2*pi*r*th > 2*pi*r*r

th > r

Is that usually the case? What am I doing wrong?
You're not wrong.

Whoever said that the drilled rotors have a larger surface area was wrong. It's a smaller surface area.

But that still helps with heat

First off less surface area rubbing on the brake pad means less friction overall and less heat generated in the first place. Since the smaller surface area is still plenty enough to stop the bike quickly and thus doesn't dramatically increase the amount of time friction is applied to the rotor, the drilled rotors build up less heat in the first place.

Heat does disperse more from an irregular surface than from a flat one though. But the holes aren't adding much to that. The wavy edge around the outside of the rotor does more for heat disbursement than the holes buy giving the edge flowing into the wind more surface area than it would have if it was a true circle. But even then I'd imagine that's immeasurably minimal and more for aesthetics.

The holes' bigger purpose is to give dirt a place to escape rather than be stuck in the pad like a wedge.
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Old 04-29-19, 08:31 AM
  #27  
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Originally Posted by PepeM View Post


How do those guys at Moto GP manage to not die while using discs without holes all over them?

Behind the brakes: stopping power in MotoGP? | MotoGP?
It's a SIGNIFICANTLY better brake rotor, for starters.

Bicycle brake rotors are a cut piece of sheet metal. Motorcycle racing rotors are a finely crafted piece of precision machinery made of metals that are designed to dissipate heat better.

There is also the 100+mph wind buzzing past the motorcycle brake rotor that helps keep it cool. The 10mph wins on a bicycle isn't doing much.

Most importantly...brakes? I thought the point of racing bikes was to win, not slow down and look cool blasting 'Low Rider'. Get off the brakes and hit the throttle!
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Old 04-29-19, 08:39 AM
  #28  
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Originally Posted by PepeM View Post
How thick are bicycle discs?

If we assume that the holes are circular, for the surface area of a drilled disc to be larger than that of a solid disc, the following would need to be true:

surface area of exposed material due to hole > surface area of the material that would be there if there was no hole

2*pi*r*th > 2*pi*r*r

th > r

Is that usually the case? What am I doing wrong?
You didn't say there was going to be math required!
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Old 04-29-19, 08:40 AM
  #29  
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Originally Posted by Skipjacks View Post
Motorcycle racing rotors are a finely crafted piece of precision machinery made of metals that are designed to dissipate heat better.
I think my Gillette razors are too. I will check the packaging when I get home.
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Old 04-29-19, 08:46 AM
  #30  
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Are they cross-drilled?
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Old 04-29-19, 08:47 AM
  #31  
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Wild guess:

Solid pedalbike rotors would work just fine.

Someone go test my claim and report back.
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Old 04-29-19, 10:02 AM
  #32  
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Originally Posted by Koyote View Post
On this topic, google works just fine.

My point is that many questions have already been asked and answered about a thousand times. And I wouldn't go with this response if the OP didn't already have a history of this sort of thing.

We're talking about an OP who recently started a thread asking for advice on how to take a crap in the woods...Apparently he keeps falling into his own s**t:
Any advice on taking a crap in the wild or dense forest?
Originally Posted by KoyoteJust a question: if you think the question was stupid and beneath you (which seems a reasonable conclusion from your smartass response), then why did you bother looking up the info for me? You could have just moved on. Or do you enjoy acting this way?


LOL.... Enough said...
.
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Old 04-29-19, 10:11 AM
  #33  
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You're all wrong. It's to reduce windage in crosswinds. Safety first.
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Old 04-29-19, 10:21 AM
  #34  
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Originally Posted by 308jerry View Post
Originally Posted by KoyoteJust a question: if you think the question was stupid and beneath you (which seems a reasonable conclusion from your smartass response), then why did you bother looking up the info for me? You could have just moved on. Or do you enjoy acting this way?


LOL.... Enough said...
.
Huh? I "looked up" information for you? I don't recall doing that. Please explain, because I am confused.
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Old 04-29-19, 11:42 AM
  #35  
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Originally Posted by indyfabz View Post
That thread wasn't even original. I have been around long enough to have seen at least one other thread about "making" in the woods. Wasn't all that long ago either. Maybe within the last couple of years.

So you're saying that it was the number two number two thread?
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Old 04-29-19, 11:45 AM
  #36  
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Originally Posted by aclinjury View Post
Honest question: is it that hard to just not reply?
Surely you're not original in this either?
Honest question--is it that hard not to reply to a reply that you don't think is necessary?
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Old 04-29-19, 12:06 PM
  #37  
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Originally Posted by PepeM View Post
How thick are bicycle discs?

If we assume that the holes are circular, for the surface area of a drilled disc to be larger than that of a solid disc, the following would need to be true:

surface area of exposed material due to hole > surface area of the material that would be there if there was no hole

2*pi*r*th > 2*pi*r*r

th > r

Is that usually the case? What am I doing wrong?
Your formula is correct (yay, math!). However, the heat transfer coefficient (call it "h") is affected, too. Here's an approximate equation:

Heat transfer rate = h * Area * (Rotor Temperature - Air Temperature).

The accurate set of equations are way too much for a forum post. But this equation can be used to understand the issue. The heat transfer coefficient h is affected by the flow of air over the rotor, and I suspect that the holes increase turbulence and hence increase h more than they decrease surface area.

Last edited by WizardOfBoz; 04-29-19 at 12:13 PM.
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Old 04-29-19, 12:13 PM
  #38  
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Originally Posted by PepeM View Post


How do those guys at Moto GP manage to not die while using discs without holes all over them?

Behind the brakes: stopping power in MotoGP? | MotoGP?
That's a carbon brake. Different beast.
Formula 1 and Airliners also use Carbon Brakes. They are actually fantastic brakes, and have much more stopping power in any condition than metal.
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Old 04-29-19, 12:16 PM
  #39  
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Originally Posted by Koyote View Post
We're talking about an OP who recently started a thread asking for advice on how to take a crap in the woods...Apparently he keeps falling into his own s**t:
Any advice on taking a crap in the wild or dense forest?
His threads are quite effective at garnering the attention he craves. For the life of me I don't know why people stand in line to feed a troll, but they do.
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Old 04-29-19, 12:22 PM
  #40  
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Originally Posted by Skipjacks View Post
Whoever said that the drilled rotors have a larger surface area was wrong. It's a smaller surface area.
More surface area for cooling, less for braking. Not that either is likely to make a significant difference on bikes.
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Old 04-29-19, 12:31 PM
  #41  
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Originally Posted by Skipjacks View Post
You're not wrong.

Whoever said that the drilled rotors have a larger surface area was wrong. It's a smaller surface area.

But that still helps with heat

First off less surface area rubbing on the brake pad means less friction overall and less heat generated in the first place. Since the smaller surface area is still plenty enough to stop the bike quickly and thus doesn't dramatically increase the amount of time friction is applied to the rotor, the drilled rotors build up less heat in the first place.

Heat does disperse more from an irregular surface than from a flat one though. But the holes aren't adding much to that. The wavy edge around the outside of the rotor does more for heat disbursement than the holes buy giving the edge flowing into the wind more surface area than it would have if it was a true circle. But even then I'd imagine that's immeasurably minimal and more for aesthetics.

The holes' bigger purpose is to give dirt a place to escape rather than be stuck in the pad like a wedge.
Originally Posted by Skipjacks View Post
It's a SIGNIFICANTLY better brake rotor, for starters.

Bicycle brake rotors are a cut piece of sheet metal. Motorcycle racing rotors are a finely crafted piece of precision machinery made of metals that are designed to dissipate heat better.

There is also the 100+mph wind buzzing past the motorcycle brake rotor that helps keep it cool. The 10mph wins on a bicycle isn't doing much.

Most importantly...brakes? I thought the point of racing bikes was to win, not slow down and look cool blasting 'Low Rider'. Get off the brakes and hit the throttle!
Originally Posted by WizardOfBoz View Post
Your formula is correct (yay, math!). However, the heat transfer coefficient (call it "h") is affected, too. Here's an approximate equation:

Heat transfer rate = h * Area * (Rotor Temperature - Air Temperature).

The accurate set of equations are way too much for a forum post. But this equation can be used to understand the issue. The heat transfer coefficient h is affected by the flow of air over the rotor, and I suspect that the holes increase turbulence and hence increase h more than they decrease surface area.
Originally Posted by ironnerd View Post
That's a carbon brake. Different beast.
Formula 1 and Airliners also use Carbon Brakes. They are actually fantastic brakes, and have much more stopping power in any condition than metal.
Main point is: "Moar surface area" is not the answer to "why do rotors have holes?"
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Old 04-29-19, 12:48 PM
  #42  
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Originally Posted by ThermionicScott View Post
More surface area for cooling, less for braking. Not that either is likely to make a significant difference on bikes.
Drilling holes in the rotor does not give it more surface area.

Assuming the rotor is 1/16th inch thick and you drill a hole that is 1/4 inch wide, the surface area lost will be exactly half of the original surface area of the lost piece.

The 1/4 inch 'hole' have a surface area of 1/10th of 1 inch (double pi r^2 since the missing hole would have 2 sides, thus doubling it's surface area)

So the rotor loses .1 square inch of surface area from the disc...but adds some back as the sides of the new hole

The new hole is .25 inches across with a dept of 1/16th inch. So the sidewall of the hole is .05 inches. (2 pi r = C, C*1/16th equals surface area of the circumference wall)

My 1/4 inch hole and 1/16th inch rotor make it exactly double. A thicker rotor would have more surface area on the new side of the hole, but would have to be 1/8th inch thick to even equal the surface area lost from the hole itself.

MATH FIGHT!!!!!
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Old 04-29-19, 12:52 PM
  #43  
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Originally Posted by PepeM View Post
Main point is: "Moar surface area" is not the answer to "why do rotors have holes?"
Right. It's not the reason why.

It helps with cooling a little due to less friction on the rotor.

But the reason they do it is dirt dispersal.

On your car the brake pad is much closer to the rotor than on a bike and the car rotor is going much faster. So if debris gets in front of the brake pad it will eithe rnot be able to get under it or will be pushed all the way out due to the increased speed it went in.

On a bike a small piece of debris could lodge in between the pad and the rotor and get stuck, hindering brake performance. The holes in the rotor give that debris some place to escape.

Which is why I said all that in my first answer above....
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Old 04-29-19, 01:22 PM
  #44  
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When spun fast enough, the holes have a small time warping capability which will make your times appear to be faster than they are. It's a holistic, quantum-gaussian effect, not really well-understood by mainstream physicists, but a fellow at the bus stop explained it to me quite convincingly so I'm sure it's right.
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Old 04-29-19, 01:28 PM
  #45  
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Originally Posted by livedarklions View Post
When spun fast enough, the holes have a small time warping capability which will make your times appear to be faster than they are. It's a holistic, quantum-gaussian effect, not really well-understood by mainstream physicists, but a fellow at the bus stop explained it to me quite convincingly so I'm sure it's right.
They are speed holes.

They make the bike go faster.



(To anyone who gets this....high five)
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Old 04-29-19, 01:47 PM
  #46  
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Originally Posted by Skipjacks View Post
Drilling holes in the rotor does not give it more surface area.

Assuming the rotor is 1/16th inch thick and you drill a hole that is 1/4 inch wide, the surface area lost will be exactly half of the original surface area of the lost piece.

The 1/4 inch 'hole' have a surface area of 1/10th of 1 inch (double pi r^2 since the missing hole would have 2 sides, thus doubling it's surface area)

So the rotor loses .1 square inch of surface area from the disc...but adds some back as the sides of the new hole

The new hole is .25 inches across with a dept of 1/16th inch. So the sidewall of the hole is .05 inches. (2 pi r = C, C*1/16th equals surface area of the circumference wall)

My 1/4 inch hole and 1/16th inch rotor make it exactly double. A thicker rotor would have more surface area on the new side of the hole, but would have to be 1/8th inch thick to even equal the surface area lost from the hole itself.

MATH FIGHT!!!!!
Hmm, I was just a math minor in college, so I'll defer to you on that.

Isn't one other reason for rotor holes to increase turbulence for cooling?
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Old 04-29-19, 01:52 PM
  #47  
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Originally Posted by ThermionicScott View Post
Isn't one other reason for rotor holes to increase turbulence for cooling?
I think they might also be to give the forward facing edge more air resistance....to help warm the rotors during cold days.
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Old 04-29-19, 01:53 PM
  #48  
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If we all just went to google we would hardly need this forum. Then we would just sink even further in our own little internet silos.

Originally Posted by Koyote View Post
Honest question: have you ever heard of google?
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Old 04-29-19, 01:57 PM
  #49  
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This article explains a lot about the subject

https://www.good-guys.com/hotnews/tr...-brake-rotors/
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Old 04-30-19, 03:13 AM
  #50  
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The holes are primarily for gas dissipation.

When the disc and pad get hot, gases are produced between the surfaces, reducing friction. You're squeezing a bed of air. Holes (and grooves) give a pocket for these gases to escape, maintaining frictional contact.
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