Mithrandir

I'm going to be building a new road bike in the near future, with a focus on the best braking available. Due to running a triple (I am a clyde, need a triple to do hills), I cannot run pure hydraulic discs, but I will probably use the TRP HY/RD hybrid hydraulic disc brakes.

Now, as a clyde, I'm sure you can appreciate the issues I face with hill descents, so I really want a solution I can rely on for absolute safety. I would like a 200mm rotor in front and 160 in back. My reason is that I feel a 200mm rotor can absorb and dissipate more heat than a 160mm rotor can. That being said, my bike shop feels like a 160 is good enough. In fact they've expressed concern that a 200mm rotor will make it much easier to stop the tire and send me flying over the bars, at least if I run road tires. I can see their reasoning, so now I'm looking for a rotor that will help me dissipate as much heat as humanly possible. What does everyone recommend?

jimc101

Would first check the fork/frame can run a 203mm/180mm combination, as in general road disc bikes are designed around 160mm & 140mm setups. running 200mm rotor on a fork designed for 140mm/160mm rotors could end in failure.

For the inability to use triples, have you looked at SRAM Wi-Fli/ Shimano 11 speed setups, as you can get the range of a triple by using these in a double setup.

For heat dissipation, would look at Shimano IceTech rotors, although these are best used as a complete system, not just as individual parts.

Bezalel

Normally the bigger rotor should go in the rear. Stopping your bike doesn't generate enough heat to affect performance, but riding the brake on a decent will build up a lot of heat. Ultimately the maximum size will be dictated by the frame geometry.

jimc101

Good point, exactly how brakes aren't meant to be used.

nhluhr

Obvious typo because it makes no sense in any way whatsoever to put the bigger rotor on the rear. 100% absolutely use the bigger rotor on front.

As for the best heat dissipating rotor, there is no question - it is the Shimano Ice Tech Freeza rotors. The normal IceTech rotors are a great start since they have an aluminum core to actually conduct heat away from the braking surface and use the entire rotor's surface area to shed heat, however the Freeza rotors also extend that aluminum inner layer to cooling fins to toss heat out by the bucketfulls. Two catches: They are only available in centerlock and they are very expensive. In case anybody decides to post about those other finned rotors (JagWire), please note that tac-welding or riveting an aluminum fin onto an otherwise solid steel rotor is a joke. There is so little surface area for conducting heat into the fins they might as well not even bother.

As for choosing pads to deal with potential heat issues, you'd want a metal pads instead of a resin pad. The metal pads do make more noise though and it's likely the Freeza rotors will pretty much eliminate the issue of heat and resin pads would work fine.

fietsbob

yes, Adding the Aluminum Core that the steel disc attaches to , takes advantage of the faster heat dissipation of aluminum in comparison to steel.
Does not have to be larger https://bike.shimano.com/media/techdo...9830750014.pdf

& Don't Ride the brakes on Hills . use a surge technique .. Decelerate heavier for a moment then let off and let the things cool down ..

Works the same with Rim Brakes ..


Pending release is PaulsComp Cable disc Caliper Both adjusting knobs are large and aluminum.
Is said to be easy to adjust, and Wont be subject to the melting of the plastic Knobs I've seen BB7's do.

cale

I appreciate the benefit of being able to move pads from the rear to the front depending on wear. That's only possible if both brakes are the same type and size. Just a thought.

FBinNY 

I'm a believer of larger rotors in front (or 2 large rotors) for a few reasons. The first is that larger rotors put less braking stress on the fork, with the extreme case of a rim brake (wheel is the rotor) putting the least.

They also dissipate heat better, so the ONLY argument against larger rotors is the weight.

As for over braking and doing an endo, that's a question of brake modulation which is a learned habit. Consider (if you're old enough) a driver who never had power brakes and rents a car with them. The first few stops he'll put himself and passengers through the windshield, but within minutes he adapts and all is good. ANY poperly configured front brake can cause an endo if you squeeze the lever hard enough -- that's the basic proof that the brake is achieving 100% of potential braking power.

As for heat absorption, that's more a question of learning to manage descents and braking. Start by braking less, and managing slightly higher speeds. Since wind drag increases proportional to the square of speed, a few mph higher on a long grade can greatly ease or even eliminate brake use. Increasing your air drag by sitting higher and/or putting one leg at the bottom of the stroke also scrubs speed without creating heat. Lastly, deferring brake use a bit longer while the bicycle accelerates, then doing a short hard brake to check the speed back down, repeated at intervals, is more effective and cooler than a steady dragging of a brake. (I won't get into the details because it get's longer than necessary).

So, half the issue is getting an adequate brake for your weight, but the larger part is learning to manage speed and braking on descents intelligently. If the worst comes to worst on long steep grades (think Pennsylvania) you can do a short term stop to cool brakes like the truckers do.

well biked

I don't know about that from a physics standpoint, but I do know that most forks intended for disc brakes have a maximum safe rotor size as part of their spec. In other words, you can use the smallest rotor you like, but there's a limit as to the largest that is considered safe by the manufacturer.

jimc101

Most forks are spec'd for small rotors, in the MTB world, you need to be looking at Enduro/DH if running over 180mm, in the road world, over 160mm is large, most fork manufactures spec a max size, would be following this for each fork

FBinNY 

That might have more to do with limitations based of the location of the mounting bosses. Or concern that giving a rider more braking leverage than he can manage will cause endos. Otherwise, the general rule is that the closer to the axle the caliper is, the greater the stress on the fork blades. This is the reason that disc brake forks have to built stronger than caliper brake forks.

Regardless, of the brake design, the maximum G-force available isn't determined by the brake, but by the tip/slide limits that depend on front tire traction and the angle from front wheel poin t of contact on the ground to the rider/bike center of gravity. So braking force is capped by outside considerations, but the local stress on the fork varies because of the leverage considerations of where that force is applied between the axle and rim.

StephenH

When I bought my Sojourn, it came with 160mm in front and 140mm in back. One of the shop guys (younger less knowledgeable guy) said I could get adaptors to use larger disks. The adaptor would be a piece that fit between the brake mounts on the bike and the brake itself to move it farther out. I never inquired into this, and don't know if it's feasible or not. I just assumed that as a mountain-bike guy, he knew what he was talking about, though. If you were having a custom frame built, ask the framemaker.

The larger-disk-in-front is based on shaky reasoning. It is generally true that at maximum braking, your braking force comes mainly from the front, as the rear wheel tends to lift off the pavement under that condition. However, the occasions where you actually brake like that are probably few and far between. So the actual amount of braking and heat buildup that DOES get done, can quite likely be split evenly between front and back. So if you are concerned about screeching to a halt in the shortest possible time, while having the lightest bike, it would make sense to use a larger disk on front. If you're concerned about dissipating heat build-up, it would make sense to use two large disks rather than one large and one smaller, and brake both about evenly. And I suspect most riders just sort of grab both brakes at the same time with similar effort anyway. By the way, my tandem came with large disks front and rear. I think they're 180's, not sure.

The pulse-vs-riding-the-brake is built on shaky reasoning. If you do the exact same amount of braking and energy absorption, it's not going to make much difference whether it's completely steady or pulsed. You're still dumping the same amount of heat into those components. With the pulsing, you might get the pad cooled off marginally in between pulses but I wouldn't bet on it. An analogy would be whether you were filling a bucket by running a hose in it at 1-gallon-per-minute rate or dumping a pint in every 7.5 seconds. If the average rate is the same, there's not a lot of difference in the effects produced. FBinNY's comments on the air-braking are right on, but I have doubts that the pulsing does a whole lot. On the other hand, it wouldn't be any worse, either.

With a disk brake, it is possible to have them out of adjustment with less braking available than what there should be, and so proper adjustment and maintenance are going to be critical as well.

As a bigger guy and a tandem rider, I can understand your concern on the brakes. Unfortunately, I don't ride your hills, so I have no clue if those are legitimate concerns or if you're going overboard or what. My guess is that if most guys manage the same hills in a controlled manner with rim brakes, you'd probably be okay with "normal" disk brakes.

I've also noticed that there is a large variation in the speeds people will ride downhill, and one person's "whee! Fun!" descent is another's idea of pure foolhardiness. This makes it hard to judge your braking needs off of what somebody else considers adequate.

well biked

I'm going to have to disagree. With disc brakes only one leg of the fork has the brake attached to it, and therefore the braking forces cause a twisting effect on the fork. The bigger the rotor, the more leverage there is, and the twisting effect on the fork is greater. Some forks are beefier than others, stronger, etc, and can withstand the leverage of bigger rotors. That's why the largest rotor spec is different on different forks.

Rim brakes are different in that the brake is mounted at or near the crown and there's no twisting effect because the brakes are applied to both sides of the rim.

fietsbob

Heat can also be transferred by using Pads with aluminum backing Plates .. now where it is transferred to , may create another problem 'downstream'..


I have seen disc Pads with external finned heat sinks ..

FBinNY 

I agree with you on most of it, but bolded the part where you're thinking backward. It should read "the bigger the rotor, the more leverage, and the twisting effect is reduced.

For any given stopping torque applied to the wheel at the point of contact on the ground (net braking force), the linear force on the brake is inversely proportional to the relative distance from the hub. Ie. if the caliper pads are half way to the rim, the linear force there will be twice the braking force. If the pads are 1/3 of the radius of the rim, the linear force there will be triple. That's compounded by the reality that the larger force is applied farther down on the blade.

So, while we agree that fork twist is an issue along with blade stress, both are greatest when the brake is closest to the hub.

BTW- we don't have to resolve this and agree. Readers can consider your analysis and mine and decide for themselves.

nfmisso

I am always amazed of the lack of basic Physics understanding displayed on this forum.

FBinNY is entirely correct. For those who disagree, make a free body diagram of the system, and you will see that FB is correct, if you don't know what a free body diagram is, you lack the education for this level of discussion, please study.

FB is also correct on "...a short hard brake to check the speed back down, repeated at intervals, is more effective and cooler than a steady dragging of a brake..." You can prove this by looking at the amount of heat getting dumped into the brakes, which drives a temperature increase, and then the cooling effectiveness of the higher air speed and greater average delta T. Heat transfer is a function of the delta T raised to a power - which varies for convection, conduction and radiation modes of heat transfer. In no case is that power 1.0; it is always greater than 1.0.

FBinNY 

Yes, total heat delivered to the rotor is the same, but the difference isn't that the pads get cooled while off, but that they are engaged on the disc less and so don't pick up as much of the heat. Pulsing doesn't keep the disc cooler, but it keeps the back of the pads and hydraulic fluid cooler, and that's the important reason not to just drag the brake.

But my main reason for recommending pulsed braking is to cycle between faster than average speed, and slower. That allows maximum air braking, saving the brakes from heating in the first place. You can let the bike roll faster on straight sections, then have a fresh brake to scrub off speed in advance of the curve, then let the bike speed up after.

Descending is a skill that can be learned, and doing so lessens reliance on the brakes. I've seen people fry their brakes trying to hold a speed only a few mph slower than what would have been terminal velocity anyway. Also people tend to use brakes instead of changing their aero profile.

So it's partly about how you brake, but just as much about how you learn not to.

BTW- on of the key reasons not to drag brakes unnecessarily is safety. If you fry your brakes when you didn't need to, you won't have brakes left for when you do, such as entering a sharp turn, or if you come to a surprise obstruction.



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hueyhoolihan

i always thought that braking was caused by the brake, a transducer, that converted kinetic energy into thermal energy.

fietsbob

Friction creates heat . energy changes states..

StephenH

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There are two effects possible there. One is heat generation at the interface, and the total amount of heat generated for a given amount of braking will be the same. Conservation of energy and all that. With allowance for varied speed, as you noted.
It is possible, but seems unlikely, to have significant heat transfer from one to the other. But note that if this occurred, it would go the other way- the pad is hotter than the rotor, so heat would be transferred from the pad to the cooler rotor, not the other way around.

hueyhoolihan

so, if the brake components don't show uniformity in heat transfer as regards their temperature, they can benefit, from intermittent application where cooling is concerned? if so, pretty simple.

edit: removed "don't not" to "don't". don't not know why i typed it in the first place.

FBinNY 

The rules for brakes are the same for all vehicles and brake types. The rotor (or rim, or drum) is usually a good heat conductor, and the pad is not. So the heat flow from the friction interface is into the rotor (where you want it) until the rotor heats and reduces heat flow in that direction, and "encourages" heat flow into the pad. You pulse to keep the slower to heat pad away from the hotter rotor as much of the time as possible, so the pad doesn't get hot.

You also get better cooling by spiking the rotor to higher temperature, where it's cooling rate is greater. So total heat generated is the same (conservation of energy), but the cooling is maximized.



Bicycles, motorcycles or cars, pulsed interval braking is the best way to manage long grades. Feel free to disagree, or if interested, there is plenty of good, reliable info on the net.

StephenH

Heat flow is always from hotter to colder, not in the direction you want it. A better conductor will conduct more heat, but won't change the direction it's going.
Spiking the rotors to a higher temperature is going to spike the pads to a higher temperature as well, which would be the opposite effect of what was desired.
I've read similar advice for cars, trucks, etc. What I've not seen is any definitive source for that advice. You'll see a lot of stuff floating around the internet that sounds good and gets repeated for that reason alone, but that doesn't always mean it's correct, either. It's kind of like the elephant repellant, where you know it works because there's not any elephants around. Maybe it's one for the Mythbusters to work on.

FBinNY 

Here's a quick experiment. Boil some water then pour into a ceramic cup and a metal one. Pick both up immediately.

It's not about the temperature, it's about the heat flow. If you simultaneously heat a good conductor and an insulator slowly enough they'll both be at the same temp through and through. But if you do it quickly, the insulator won't take up the heat, and will stay much cooler.

If you don't believe this applies to brakes, try this on a bicycle with rim brakes. Get up to a good clip, then make a hard short stop. Now immediately touch the rim and brake shoes, then come back to BF and report.

BTW- heat flux is why we drink coffee and tea from ceramic mugs, not steel ones.

nfmisso

This is just basic Mechanical Engineering, take some Thermodymanics and Heat Transfer courses.