Disc brake material

[johnrobholmes]

Does anybody have some info or knowledge on pros and cons of different disc material? I can see that aluminum and stainless are popular materials, but nothing is mentioned of grades. My guess is that something like a 303 stainless would have good wear and decent braking, while 6061 would have better braking but faster wear. Maybe another grade is used though?

 

[vanilla ice]

I've never heard of plain aluminum discs. The few I have heard of for motorcycles and cars were plated with some special coating..

 

[johnrobholmes]

I think some trials riders use aluminum discs. Probably hard coated or anodized.

 

[vanilla ice]

Here's a short read I thought was interesting on disc materials for trails bicycles..

http://www.plazmaticbrakes.com/disc_brakes.php

I know for racing motorcycles carbon rotors work well where they are legal. But pricey.

 

[GTA1]

If you don't care about appearance and don't mind some extra weight, nothing really beats cast iron in terms of wear, heat dissipation, and good stopping power.

The catch: no one I know of makes a bicycle one out of cast iron.

Discussions of brakes always need to consider both the rotor and pad material combos.... as there is a wholly different characteristic for each combo.

 

[johnrobholmes]

I would prefer if the rotor was compatible with standard pads available for shimano, hayes, magura, etc. I will probably go with stainless. Gonna get a chainring/ disc brake setup developed and needed to work out the materials for machine time estimation.


Now I gotta figure out how to punch holes along a diameter in solidworks.

 

[John in CR]

John,

If the rotors you are wanting to make are to be attached to any hub motor, then you absolutely don't want to use anything other than stainless steel. The reason being that stainless has a much lower coefficient of thermal conductivity (even than regular steel, and about 15 times less than aluminum), and the last thing you want to do is send extra heat toward a hub motor. eg Putting an aluminum brake rotor on a hub motor wouldn't be much different than attaching a heating element directly to the motor cover. In this application the heat conduction toward the connection to the motor would probably be 50-100 times greater than with stainless, because the spokes on the rotor would need to be a lot thicker with an aluminum rotor. Even cast iron would be a mistake because it's thermal conductivity is about 4 times that of stainless, plus because it's not as strong would have to be thicker too and conduct even more.

Apparently there are some applications where an aluminum brake rotor could be beneficial, like with trials bikes where you don't have much speed to have airflow over the rotors to cool them, so conducting that heat away from the braking surface is an advantage.

John

 

[johnrobholmes]

Good point. These discs will be used on cranks for fixie bikes and on special hubs however. No need to cut my own if it was just for a normal mount, plenty of companies already do that

 

[John in CR]

Good point. These discs will be used on cranks for fixie bikes and on special hubs however. No need to cut my own if it was just for a normal mount, plenty of companies already do that

I see. I would have posted sooner, but then it dawned on me that you might be doing special disks like those HAL has done...extra large that bolt on via the hub motor cover screws.

BTW, what are you doing that cranks need brakes? Sounds interesting.

John

 

[Reid Welch]

If you don't care about appearance and don't mind some extra weight, nothing really beats cast iron in terms of wear, heat dissipation, and good stopping power.

The catch: no one I know of makes a bicycle one out of cast iron.

Discussions of brakes always need to consider both the rotor and pad material combos.... as there is a wholly different characteristic for each combo.

Positively, absolutely correct, Mr. Shean!

Cast iron, for over a century, has been the de-facto standard for braking rotors and drums.
EVERY CAR made today features cast iron rotors or drums.

Pads and lining materials: used to be asbestos-based; are now compounds of mineralized materials such as ceramic-based Kevlar derivatives.

Sad fact: disk brakes are less "stoppy" than rim brakes: less "torque"; because they are small pads working on a small diameter rotating disk,
not a huge-diameter rim.

Aluminum sucks as a braking surface, period. It wears poorly and has no practical reason to be used at a disk rotor, even on a bicycle,
except that it looks pretty, like stainless alloys look pretty. Cast iron for a bike disk brake would look dirty and, and surface-rust,
but it would give totally superior braking power compared against ANY other metal, period.

[youtube]zQ4b36BPbJc[/youtube]
That guy on the right, he is Groucho's uncle.
He never broke down. And he knew just how to brake in comic-timing perfection.

 

[johnrobholmes]

One of my biking buddies is hardcore fixie, and wants a disc brake on his cranks to be different.

 

[John in CR]

One of my biking buddies is hardcore fixie, and wants a disc brake on his cranks to be different.

I think I got it now, a "fixie" means no freewheel. Sometimes my steel trap is slow. :?

John

 

[johnrobholmes]

yes, no freewheel typically no brake.

 

[Link]

John,

If the rotors you are wanting to make are to be attached to any hub motor, then you absolutely don't want to use anything other than stainless steel. The reason being that stainless has a much lower coefficient of thermal conductivity (even than regular steel, and about 15 times less than aluminum), and the last thing you want to do is send extra heat toward a hub motor. eg Putting an aluminum brake rotor on a hub motor wouldn't be much different than attaching a heating element directly to the motor cover. In this application the heat conduction toward the connection to the motor would probably be 50-100 times greater than with stainless, because the spokes on the rotor would need to be a lot thicker with an aluminum rotor. Even cast iron would be a mistake because it's thermal conductivity is about 4 times that of stainless, plus because it's not as strong would have to be thicker too and conduct even more.

Apparently there are some applications where an aluminum brake rotor could be beneficial, like with trials bikes where you don't have much speed to have airflow over the rotors to cool them, so conducting that heat away from the braking surface is an advantage.

John

Wha...? The amount of heat generated in braking is nothing compared to what the hub itself is producing. Only, what, a couple hundred watt seconds at most? Vs a powerful hub which could conceivably generate several times that amount just accelerating to 20mph, I don't think heating the motor would be an area of concern. Besides, there's so much airflow around the disc rotors that even something with the thermal conductivity of copper would probably be cool enough at the center to be touched.

I'd guess you'd want to get heat out of the rotors as fast as possible to help prevent fade. Why do you think carbon is used in really high-end/racing brakes?

Granted, stainless is still probably your best (reasonably priced) option due to the fact that it's much sturdier than aluminum and won't rust or oxidize so easily.

 

[John in CR]

Link you're joking right? Brakes are more powerful than almost all of the hub motors, and it's all turned into heat, unless you want to consider the amount given up as wind resistance, and tire and bearing friction (all also heat dissipation). Where do you think all of that kinetic energy goes when you use brakes unless you have crap brakes or have your bike set up for drag racing then you can stop a lot faster than you can accelerate? Try touching your disk after a medium size hill or just some jackrabbit stop and go riding. Sure they cool off quickly, because that's how they're designed. As designed, due to the low thermal conductivity of stainless steel and thin brake disk spokes, they dissipate all that energy to the air. Change them to give a good thermal route via thicker more conductive metal, and a lot of heat will go directly to the hub motor with heat already being the limiting factor of hub motors. All you'd be doing is reducing maximum performance, and by a significant margin except when braking isn't significant.

There's no escaping the 1st and 2nd Laws of Thermodynamics. It all turns to heat in the end, and heat flows to achieve balance through the easiest means available.

John

 

[Link]

No, I'm not joking.

Take a combined rider and ebike weight of 200kg (assume it's a big guy and a heavy bike, I guess) going 30mph (13.4m/s).

E = 1/2(200)(13.4)²
E = (100)(179.56)
E = 17,900.56 joules (AKA watt-seconds; my estimate was a liiiitle bit off, methinks )

A Wh is 3,600 joules.

17,900 / 3,600 = 4.97Wh. Even if your motor somehow stays at an efficiency of 80% no matter what its RPM is and you have a 1kWh battery, you'd have to come to a complete stop 40 times during your ride to just MATCH what the hub's wasting to heat if you matched the figures in the example.

I'm not going to do the math, but if the hub followed real-life efficiency calculations and you stopped and accelerated to 30mph 40 times during your ride, I wouldn't be surprised to see that your average efficiency dropped to 60%, meaning you'd have to stop 80 times to get the same amount of heat out of the brake as you do the hub. And if you estimate that half of the heat is lost to air (I'm pretty sure I'm being conservative there), that's 160 times you'd have to stop (EDIT: Also you'd run out of battery at 100 stops, LOL). Then maybe you have hills...and an e-brake...and you're not actually going to be stopping from 30mph the second you make it there the whole way...and then wind resistance...etc, etc.



At that point, it might just be more efficient to do the worm all the way to your destination.

 

[fechter]

I've heard of guys making brake discs out of circular saw blades. I think you need a material that is fairly stiff and won't flex too much during braking. I suppose you could leave the teeth on there for that "mean" look.

 

[Drunkskunk]

Link is right, although You would have to wear the furry costume to get the propper efficancy.

I've heard of guys making brake discs out of circular saw blades. I think you need a material that is fairly stiff and won't flex too much during braking. I suppose you could leave the teeth on there for that "mean" look.

Sawblades you say?

I think you just gave me the perfect cure for my warping disc problem.

ok, realy I just want to do it because it would look bad @$$

 

[John in CR]

Link is right...

Maybe we should see if we can get a few more on that bandwagon before the wheels fall off. Suffice it to say that there's a reason bike brake disks are made of stainless and cut with spoke shapes that make them as long as practical. My bikes' brake disks easily see 15-20kw, though not for long periods. My evidence negating any need to crunch the numbers is that I often decelerate over twice as fast as Doc Bass can accelerate on his drag bike, and I'm carrying significantly more weight. Going down hills they see multi KW for much longer periods.

All it would take for aluminum disks to degrade hub motor performance would be a significant downhill section immediately followed by a steep climb. It's not the average over an entire ride, but the short term amounts of heat generated, and aluminum disks would conduct significant heat to the motor cover due to the better thermal pathway caused far greater thermal conductivity and much greater cross sectional area required for the spokes. A hot motor cover that is already hot is going to result in the motor not being able to dissipate the motor generated heat as readily, resulting in hotter windings and decreased performance compared to if a normal stainless disk was used.

John

 

[GTA1]

hat your average efficiency dropped to 60%, meaning you'd have to stop 80 times to get the same amount of heat out of the brake as you do the hub. And if you estimate that half of the heat is lost to air (I'm pretty sure I'm being conservative there), that's 160 times you'd have to stop (EDIT: Also you'd run out of battery at 100 stops, LOL). Then maybe you have hills...and an e-brake...and you're not actually going to be stopping from 30mph the second you make it there the whole way...and then wind resistance...etc, etc.

A very simple way to prevent heat migration from the hub to the disk brake rotor is to insert a thin heat insulator between the two parts.

A fiber washer would do fine.

Slightly thicker than normal bicycle material for the rotor will soak up most of the heat, and very little will be conducted between the rotor's friction surface and the spokes to the hub.

Look at the physical layout: A rotor is flat, exposed to a lot cooling air as it turns.

Between air cooling and radiation, most of the heat will be dissipated.

As a point of reference, many automotive cast iron brake rotors are integrated with the wheel bearing housings, and filled with grease.

I have never seen a regular car so equipped get hot enough to melt the grease in the bearing housing --- even in severe use.

 

[Reid Welch]

yes, no freewheel typically no brake.

Yes, go back to the early to mid 1890s.
No freewheel, fixed gearing, typically pretty low-geared. NO brake, but for the young rider's thickened thighs.

What's new is old new again. "Fixies" forever!

it is all, pure, big bike fixie
[youtube]aZjd9pBmLoU[/youtube]

PS: that final scene is an obviously-staged FUN FINISH for the viewing public.

Note: these bikes weighted, generally, 22lbs, and generally had steel rims,
though wood, laminated veneer of rock maple or the like,
and as light as then-unobtainium aluminum, was available, for rims,
from '95 onward.

BMX boys: eat your hard-ons out. You have nothing on the Iron Men of long ago.
Why, their wheels are fully 28" diameter! Not 26", not 24" and NO aluminum light-o-matics,
not fixie, necessarily, nor any other thing, other than the rare and true precursor of carbon fiber:
freak bikes of BAMBOO tubing with litharge-cemented dropouts, etc.
Litharge cement: stronger than even epoxy of today.
I can tell you how to make it. It is very slow to cure, but very, very sure: can never be un-done at all, short of nitric acid.

Disc brakes. This thread is about disc brakes, not reid-tricks he could never do in real life.

 

[Reid Welch]

No, I'm not joking.

Take a combined rider and ebike weight of 200kg (assume it's a big guy and a heavy bike, I guess) going 30mph (13.4m/s).

E = 1/2(200)(13.4)²
E = (100)(179.56)
E = 17,900.56 joules (AKA watt-seconds; my estimate was a liiiitle bit off, methinks )

A Wh is 3,600 joules.

17,900 / 3,600 = 4.97Wh. Even if your motor somehow stays at an efficiency of 80% no matter what its RPM is and you have a 1kWh battery, you'd have to come to a complete stop 40 times during your ride to just MATCH what the hub's wasting to heat if you matched the figures in the example.

I'm not going to do the math, but if the hub followed real-life efficiency calculations and you stopped and accelerated to 30mph 40 times during your ride, I wouldn't be surprised to see that your average efficiency dropped to 60%, meaning you'd have to stop 80 times to get the same amount of heat out of the brake as you do the hub. And if you estimate that half of the heat is lost to air (I'm pretty sure I'm being conservative there), that's 160 times you'd have to stop (EDIT: Also you'd run out of battery at 100 stops, LOL). Then maybe you have hills...and an e-brake...and you're not actually going to be stopping from 30mph the second you make it there the whole way...and then wind resistance...etc, etc.



At that point, it might just be more efficient to do the worm all the way to your destination.

Apologies, and more, owed, Link!
I did not even fully read your MAIN POST: the opener. I've read it now, though.
I have no intelligent answer for you, of course! Because at root, I am ignorant here this topic.

HOWSOWHATWHEREEVER, I want to make amends; forgive? and I shall study your every word,
and study the topic in general, and then try to opine intelligently, which act also
includes a FREE OFFER to make...happy happy joy joy on your private-feelings parts.
Send me a bus ticket, is all?



Cheers,
bad boy reid, incorrugatable!, this pasteboard card! :wink:

 

[v_tach]

410 or especially 440 stainless (440 is more expensive) works well for rotors. Must be hardened by heat-treating or they'll groove quickly with only a few applications of the pads. I've had several custom designs laser cut when I was having custom parts manufactured for EVO gasoline powered stand-up 2-speed scooters a few years ago.