Specific heat absorption capacity of disc brakes

[DasDouble]

Hi,

I have got told by 3 different people now (my father, a preast and another engineer), that my actual disc brakes (203 mm in diameter from Tektro) are undersized for my ~40 - 45kg heavy bike. Compared to scooters and enduros (~120kg; 125cc) which weight about 3 times more, the weight of such a tektro brake (200 gram) is pretty much nothing, as with such weight of material it is quite unrealisitc to transform all the moving energy in heating energy / thermal energy.
All of those 3 people told me that when I have bad luck, the disc brake will overheat and block my (rear- or front-) wheel and / or won´t be able to take any heat anymore which results in a not working brake in a maybe emergency - situation ...

What do you guys think?

Let´s do some little math:

weigt of the disc brake:
0,2kg

specific heat absorption capacity of metal:
460 Joul / kg.

> spec. heat absorpt. capacity of 0,2 kg disc brake:
=92 Joul

Speed: 45km/h (ok, let´s say 60kmh to be realistic):
16,6 m/s.

The deceleration in an emergency brake (half of the speed in meters) => 30 meters:
4,2 m/s²

weigt of bike + rider + gear of the rider:
110kg

But how to go on? Maybe someone is more familiar with this, then I am?

 

[bnoooogers]

The term you want to google is 'brake fade' (the loss of braking power due to heat). I think the main variable is the material composition of the brake pads (and rotor size, but you're already maxed out).

If your brakes were underpowered, they wouldn't be by much. 203mm rotors are mostly used by downhill bikes, and in pro races they routinely hit 60 kph (though they don't slam down to 0 kph with their brakes either).

As for the physics problem you posed, we can do the math but I don't think it will shed any light on this situation because we don't know the temp at which your brakes fade.
kinetic energy = mass*velocity^2 / 2 = 110kg * 16.6m/s^2 /2 = 15,155.8 J
divide by heat capacity of your rotor (92 J/K) and we get a 164 C temperature rise.

This is an unrealistic worst case scenario where no kinetic energy is lost to air drag and no heat is lost to the air or the brake pad or the caliper or whatever.
Add your ambient temp to 164 and thats the rotor temp after a full stop. That doesn't sound very high to me but again, I have no idea what temp causes pads to fade.

 

[vk4bxi]

The other thing that needs to be considered is that the heat is generated in the surface skin of the brake rotor. There is very little time for that heat to progress into the body of the rotor during a few seconds of " oh my god braking", serious brake fade ( or when the brakes lose it ) kicks in about 300C on the surface of the brake pad. So the mass of the rotor is not so interesting for the first couple of seconds, but the surface area of the rotor that is swept by the pad is. After that the cooling of the rotor and its thermal mass are. Of course the pad area and the pressures required to get the friction are important to get the braking effort required to stop the bike. Bottom line, bigger is better if you're going down a mountain but most disc brakes should be good for a one off stop.

Bob

 

[Wheazel]

Do feel any indication of the brakes getting weak?

 

[spinningmagnets]

There are many factors to consider when discussing how to increase braking capability. Whenever disc brakes are mentioned, there are some builders here who feel that standard bicycle rim-brakes are (or should be) acceptable under all circumstances involving a bicycle frame. If you upgrade the rim brake pads to "Kool Stop" pads, that is about as good as rim brakes get.

That being said, downhill bikes (who are notoriously hard on braking systems), use large diameter discs and hydraulic calipers. A member here once wondered why I would want a larger diameter disc (200mm-ish) when I could "lock up" the front tire with a 180mm disc. I feel the added mass absorbs some of the heat generated (I know steel doesn't absorb as fast as aluminum, etc), but...I also feel the longer disc radius (more leverage) allows me to use less pressure to get the same braking power.

Just because I can "lock up" the front tire, doesn't mean I want to do that. I like having a certain amount of sensitivity to my braking ability. maybe it's just me, but that is my opinion. I have a cable-operated 180mm front disc, and I have the parts to upgrade to a hydraulic 200mm disc. I will report back on how badly I screwed up by doing this.

 

[Wheazel]

DasDouble, what do they mean by the brake blocking up?

I have experience from two different setups at both sides of the spectrum if we talk brake force in relation to the requirements.

1. My cargobike with cargo going downhill. It has 26" bike wheel front and 19" moped back.
In the front I currently have a 160mm hydraulic discbrake, and this brake will struggle to stop the bike in a timely manner.
So I use both discs and regen brake in the motor. This provides a safe solution that will stop very fast.
Regardless I will upgrade the brakes to 203mm tektro dorados this spring to have more redundancy.

2. My retired winter trike with two 20" wheels in the front and dual 160mm hydraulic disc brakes. (similar to the brakes on the cargobike)
Can easily lock up the front wheels and have a very good brake feel. A lot of spare brake force under all circumstances.

I don't see the reason for the doomsday talk, you will feel if the brake is fading.
Do you feel any indication of lacking brake force?

 

[Triketech]

I run 160mm rotors on an HPV FS26. I also ride on Big Apples for added grip while drifting through turns, so I'm not exactly an "easy-rider".

From a 35 MPH "panic stop" the rotors reach about 540° F. Above about 700° they'll take an ever so slight warpage which takes some effort.

Originally equipped with BB7 brakes and later upgraded to M785 hydros.

280 Lb total weight.

 

[justin_le]

Let´s do some little math:

weigt of the disc brake: 0,2kg
specific heat absorption capacity of metal: 460 Joul / kg.
> spec. heat absorpt. capacity of 0,2 kg disc brake: =92 Joul
Speed: 45km/h (ok, let´s say 60kmh to be realistic): 16,6 m/s.
The deceleration in an emergency brake (half of the speed in meters) => 30 meters: 4,2 m/s²
weigt of bike + rider + gear of the rider: 110kg

But how to go on?

Nobody seems to have addressed your math/physics question for some reason, even though you came sooo close. What you were missing is E = 1/2 m v^2

You actually don't need to worry too much about how long your emergency stop is. If you are cruising at 60 kph on a 110kg gross weight vehicle, your kinetic energy at that point is
E = 1/2 m v^2 = 1/2 110kg (16.6m/s)^2 = 15266 Joules

If your disk brake rotor has a heat capacity 92 joules / oC, and you do all your braking just with the one disk brake so all this heat is dumped into the rotor, then the rotor temperature would increase by:

15266 Joules / 92 Joule/oC = 165oC.

That's hot, you'd burn your finger touching the brake rotor, but it's not a temperature that would compromise the rotor or the brake pads themselves. In practice, some of your energy will be lost to rolling drag, air drag, to heat on the caliper pads, and some of the generated heat will be shed to passing airflow, so you wouldn't see it _all_ get dumped into the rotor like this, but it gives a good worst case number.

It's not coming to a quick stop that pushes disk rotors to their limit, but long steady descents where the heat just keeps building up. Calculate your Joules from atop a mountain pass and that 15266 number becomes quite small. At this point, the heat capacity of the disk rotor isn't as much a factor as how effectively the rotor can shed heat to the passing airflow.

 

[DasDouble]

Let´s do some little math:

weigt of the disc brake: 0,2kg
specific heat absorption capacity of metal: 460 Joul / kg.
> spec. heat absorpt. capacity of 0,2 kg disc brake: =92 Joul
Speed: 45km/h (ok, let´s say 60kmh to be realistic): 16,6 m/s.
The deceleration in an emergency brake (half of the speed in meters) => 30 meters: 4,2 m/s²
weigt of bike + rider + gear of the rider: 110kg

But how to go on?

Nobody seems to have addressed your math/physics question for some reason, even though you came sooo close. What you were missing is E = 1/2 m v^2

You actually don't need to worry too much about how long your emergency stop is. If you are cruising at 60 kph on a 110kg gross weight vehicle, your kinetic energy at that point is
E = 1/2 m v^2 = 1/2 110kg (16.6m/s)^2 = 15266 Joules

If your disk brake rotor has a heat capacity 92 joules / oC, and you do all your braking just with the one disk brake so all this heat is dumped into the rotor, then the rotor temperature would increase by:

15266 Joules / 92 Joule/oC = 165oC.

That's hot, you'd burn your finger touching the brake rotor, but it's not a temperature that would compromise the rotor or the brake pads themselves. In practice, some of your energy will be lost to rolling drag, air drag, to heat on the caliper pads, and some of the generated heat will be shed to passing airflow, so you wouldn't see it _all_ get dumped into the rotor like this, but it gives a good worst case number.

It's not coming to a quick stop that pushes disk rotors to their limit, but long steady descents where the heat just keeps building up. Calculate your Joules from atop a mountain pass and that 15266 number becomes quite small. At this point, the heat capacity of the disk rotor isn't as much a factor as how effectively the rotor can shed heat to the passing airflow.

Justin I really appreciate your detailed reply, especially that you took a look at my math. Thank you really much, the last sentence was really idea bringing

 

[bnoooogers]

I'll take that as validation of my physics and math and condemnation of my post formatting :wink:

This is an important addition, though:

It's not coming to a quick stop that pushes disk rotors to their limit, but long steady descents where the heat just keeps building up. Calculate your Joules from atop a mountain pass and that 15266 number becomes quite small. At this point, the heat capacity of the disk rotor isn't as much a factor as how effectively the rotor can shed heat to the passing airflow.

Repeated or sustained braking events, without ample time to cool between them, is what causes fade.

 

[spinningmagnets]

Between two downhill riders, one will use more braking (and more often) than the other, as a matter of personal style. If your current set-up is getting hot under your current riding profile, you might be looking for a set-up that can absorb and shed heat faster than your previous kit.

Shimano ICE tech has an aluminum spider at the center of the disc assembly, and black aluminum fins on the caliper:

 

[justin_le]

I'll take that as validation of my physics and math and condemnation of my post formatting :wink:

Oh geez, I swear I read through all the replies without seeing the math addressed at any point, but I must have had a blinder on the middle of your post and just saw the bit about brake fade. Your post formatting wasn't the best but the condemnation should be on my reading thoroughness!

My embarrassment, and thanks for being right on the mark with answering the OPs main question.

 

[Chalo]

My experience with disc brakes on e-pedicabs that weigh as much as 1000+ pounds fully loaded is that three 160mm discs are more than adequate. We started with 203mm discs, but found that their increased tendency to warp outweighed any improvement in braking power. These vehicles are inspected annually for stopping distance.

If three ordinary mechanical discs are good enough for a half ton pedicab, two of them are more than adequate for you.

 

[Ohbse]

Justin and the rest nailed it, for the purposes of fast braking on an infrequent basis DH brakes are more than adequate.

Regarding prolonged descents causing heat accumulation - this is where regen braking really shines! If you've got correctly configured regen then you probably won't be activating your physical brakes until the last few KM/h, at which point the energy is negligible.

I use and can recommend the Shimano ICE rotors, they have held up very well indeed. Combine that with their brake pads with integrated heatsinks and your continuous braking performance is dramatically better than convention 205mm DH brakes.

 

[alexaus]

The Shimano Ice Tech rotors and brakes work really well.
The secret is the stainless steel-aluminium-stainless steel sandwich of the rotors braking tracks, which is riveted to the aluminium spider. The heat capacity is about the same, but the thermal conductivity of the rotor from the braking track where the heat is generated, to the spider and then hub is much higher. After doing a downhill run on the bike, even the hub is warm from the heat conducted to it. Along the way much heat is lost to the atmosphere from the large surface area of the whole thing. And if you need really powerful brakes, they also make "Freeza" rotors which have the aluminium middle layer of the rotor extended towards the center of the rotor to form these big fins, same idea as the finned brake pads. They really dump heat fast! Sadly those freeza ones are only available in center lock and not 6 bolt.

 

[Syonyk]

When you decide you want more fade resistant pads to allow for higher rotor temperatures before fade (which will improve heat flow out of the rotors into the air), the KoolStop electric bike compound pads are awesome. I could reliably fade a brake on a hill I lived on if I dragged the rear going down the hill without any front brake (using normal bike pads), and with the ebike compound, not only did I not fade, I got the rotor hot enough to start bluing. Not recommended temperatures for daily use, obviously, but it demonstrated to me that there's a huge difference with those pads. Well worth hunting them down.

 

[eCue]

Was reading a pdf file how disc size effects braking toque forces titled " Construction of a test bench for bicycle rim and disc brakes "
In it the authors tested the forces that different sized rotors induced https://www.google.ca/url?sa=t&rct...-2010-03.pdf&usg=AOvVaw2olQ4oYoQuRkvWkJihNQRd

The short version

Newtons applied to the rotor under front only emergency braking

160mm = 450
180mm = 550
200mm = 650


My bike has v-brakes on it which we all know barely work in the rain , luckily the fork has a disc mount so will be putting a mechanical caliper and a 180mm disc on it for wet weather stopping power.

 

[Chalo]

My bike has v-brakes on it which we all know barely work in the rain , luckily the fork has a disc mount so will be putting a mechanical caliper and a 180mm disc on it for wet weather stopping power.

You'd think that disc brakes didn't get wet, or something. My first disc brakes (Mountain Cycle Pro-Stop with 9" floating rotors) switched from making deceleration to making noise when they got wet. I have yet to find any discs that don't work differently wet versus dry.

Kool Stop salmon compound rim brake pads work pretty swell in the wet, and they last a long time too. In ancient days, Fibrax leather-based rim brake pads were well regarded for wet braking.

Anyway, if you want brakes that aren't affected by rain, look at drum brakes, not discs. They work about the same in all conditions.

In the pedicab fleet I work with, we started with 203mm rotors on general principle, because these cabs can weigh more than half a ton when loaded. But all the rotors warped chronically. In practice, 160mm rotors provide plenty of stopping power with much more long term reliability.

 

[eCue]

I definitely want to use a disc brake upfront if you use quality parts nothing else has its stopping power

 

[Chalo]

I definitely want to use a disc brake upfront if you use quality parts nothing else has its stopping power

That's a generalization usually made by folks who'll pay $200 for a disc brake, but have never tried a linear-pull brake over $20. It just isn't true overall. In fact, it's usually only possible to get equal braking with a disc if you spend several times more on the disc brake than on the LP brake.

If you're comparing cable actuation to hydraulic actuation, well, there are hydraulic rim brakes. And they're very impressive compared to discs. I don't think the drawbacks of hydraulics make them a good choice for most bikes.

I have all kinds of brakes on my bikes, including discs. Discs can be very good, and they require little skill to set up. They allow rim styles and wheel diameters that otherwise wouldn't be an option. But categorically better stopping power? Nope. Most folks will be thrown off their bikes before they can test the limits of stopping power anyway. They're not heavy enough, and their bikes aren't long enough, to use all the power good brakes have to offer.

I've never found a single other bicycle brake of any kind nearly as powerful as the cable operated rim brake shown below. I made it before V-brakes came to market, but it works on the same general principle.

 

[eCue]

I like those v-brakes they look good for steep off road hills

 

[eCue]

I use brake boosters on my v-brakes they do work well until it rains , then they suck monkey balls.

Like brakes in general roller brakes are not created equal either , Im considering one of the below brakes to use on a 4-speed nexus hub and just be rid of v-brakes all together.

Ive been rocking v-brakes for 20 years enough is enough Im bailing on them.

A old designed improved and refreshed

 

[markz]

pic

2 U-brackets to stiffen it up?

 

[Chalo]

pic

2 U-brackets to stiffen it up?

One of them is the fork arch that keeps the sliders working together.

 

[Chalo]

Roller brakes feel mushy but work well, within their limits. BR-IM70, -IM80, and -IM81 are the ones to go for. They have better heat sinking and more powerful roller cams than other models.

Like other drum brakes, they're impervious to weather and external contamination. Unlike other drum brakes, they use grease so they don't need to be cleaned out, only regreased.