Disc brake rotor size

"Michael" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

I just noticed that Avid's mechanical disc brakes come with 160, 185 or 203 mm rotors. It's
claimed that the 185mm rotor has 15% more "power", the 203 26% more.

Does a larger rotor really provide more stopping power? Why? I could understand less fade due to
heat, since a larger rotor would dissipate heat better. Does that translate into more "power"?

I assume that dimension is the rotor diameter. But wouldn't a change in rotor diameter change
where the caliper gets positioned, i.e. would require a change to the caliper mounting eyelets?

I'm spec'ing these brakes for a road bike; should I expect to need more stopping power than a
mountain bike for say long mountain downhills?

TIA

I would think the opposite. Mtn bikes "need" large brakes in order to stop NOW. Road bikes usually
don't need the instant deceleration of a mtn bike.

Go with the smallest rotor for the road.

The mounts are adjustable in several directions, so mounting whichever diameter you choose shouldn't
be a problem.

FWIW, I run the small diameter Avid mechanical discs on my XC mtn bike and don't have a
problem stopping.

You're about to unleash a firestorm about road bikes and disc brakes. Get ready for it.

Mike

The power of a brake is its convective heat transfer rate and proportional to Q = UA(Th - Tc) where
Q is the heat transfer rate, U the convective heat transfer coefficient, A the area and a function
of rotor diameter, Th the temperature of the rotor and Tc the cooling air.

I leave it as an exercise for the interested to evaluate the change in the Q per change in
rotor diameter.

"Michael" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

I just noticed that Avid's mechanical disc brakes come with 160, 185 or 203 mm rotors. It's
claimed that the 185mm rotor has 15% more "power", the 203 26% more.

Does a larger rotor really provide more stopping power? Why? I could understand less fade due to
heat, since a larger rotor would dissipate heat better. Does that translate into more "power"?

I assume that dimension is the rotor diameter. But wouldn't a change in rotor diameter change
where the caliper gets positioned, i.e. would require a change to the caliper mounting eyelets?

I'm spec'ing these brakes for a road bike; should I expect to need more stopping power than a
mountain bike for say long mountain downhills?

TIA

Michael:

Quoted message said:

I assume that dimension is the rotor diameter. But wouldn't a change in rotor diameter change
where the caliper gets positioned, i.e. would require a change to the caliper mounting eyelets?

Disc brake manufacturers sell adaptors for different rotor and mounting sizes. These may even be
included with the brakes when you buy them.

Quoted message said:

I'm spec'ing these brakes for a road bike; should I expect to need more stopping power than a
mountain bike for say long mountain downhills?

Be mindful of the fork manufacturer's recommendation. The larger the rotor, the larger the stresses
exerted on the caliper mounts. Suspension fork manufacturers such as Answer and Fox recommend
against rotors > 165mm on their non-DH-specific forks for this reason.

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Doug Huffman:

Quoted message said:

The power of a brake is its convective heat transfer rate and proportional to Q = UA(Th - Tc)
where Q is the heat transfer rate, U the convective heat transfer coefficient, A the area and a
function of rotor diameter, Th the temperature of the rotor and Tc the cooling air.

Incorrect. This is only an indication of the heat generated by the brake, not its stopping power.

For each surface contact on a disk brake, the stopping torque generated with uniform pressure
applied by the pad is

T = (1/3) * F * mu * [(D^3 - d^3)/(D^2 - d^2)]

where F is the force applied on each caliper face, mu is the coefficient of friction, and D and d
are the outer and inner diameters of the rotor annulus.

mu will change as heat is generated on the rotor and pad, but since it's a function of the pad
material, this change can be set aside for comparative purposes.

It's clear that the larger the rotor, the greater the stopping torque.

Well, from a purely mechanical point of view (pardon pun) leverage would be one possible factor.
Hold your wheel in your hands and spin it. now try stopping the spin by grabbing the spokes first
near the hub, then again further out.

May you have the wind at your back. And a really low gear for the hills! Chris

Chris'Z Corner "The Website for the Common Bicyclist": geocities.comczcornerOpen ↗

Michael said:

I just noticed that Avid's mechanical disc brakes come with 160, 185 or 203 mm rotors. It's
claimed that the 185mm rotor has 15% more "power", the 203 26% more.

Does a larger rotor really provide more stopping power? Why? I could understand less fade due to
heat, since a larger rotor would dissipate heat better. Does that translate into more "power"?

I assume that dimension is the rotor diameter. But wouldn't a change in rotor diameter change
where the caliper gets positioned, i.e. would require a change to the caliper mounting eyelets?

Certainly.

Avid's use of the term "power" in this context is sloppy and inaccurate. "Force" would be more
correct, and the force is proportional to the rotor's diameter (assuming the caliper is repositioned
appropriately for the rotor size.)

Braking force is inversely proportional to the tire radius too, so larger wheels would call for
larger rotors to maintain the same amount of braking force.

Quoted message said:

I'm spec'ing these brakes for a road bike; should I expect to need more stopping power than a
mountain bike for say long mountain downhills?

The _length_ of the hill has nothing to do with the braking force needed, but the steepness does.
(Although there is an issue of hand fatigue on long descents with brakes that require a lot of
hand force.)

The length of the hill does get involved with heat dissipation requirements. A larger rotor will be
less prone to overheating on long slow descents.

Sheldon "May The Force Be With You" Brown
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shipped Worldwide captainbike.comcaptainbike.comOpen ↗ sheldonbrown.comsheldonbrown.comOpen ↗

Sheldon Brown:

Quoted message said:

Michael said:

I just noticed that Avid's mechanical disc brakes come with 160, 185 or 203 mm rotors. It's
claimed that the 185mm rotor has 15% more "power", the 203 26% more.

Does a larger rotor really provide more stopping power? Why? I could understand less fade due to
heat, since a larger rotor would dissipate heat better. Does that translate into more "power"?

I assume that dimension is the rotor diameter. But wouldn't a change in rotor diameter change
where the caliper gets positioned, i.e. would require a change to the caliper mounting eyelets?

Certainly.

Avid's use of the term "power" in this context is sloppy and inaccurate. "Force" would be more
correct, and the force is proportional to the rotor's diameter (assuming the caliper is
repositioned appropriately for the rotor size.)

Qualifying the term "force" is also required, since there is the force applied to the pads which is
perpendicular to the plane of the rotor, and there is the force which the pad exerts on the rotor
through friction, which is parallel to the rotor plane. Both are related, of course.

Quoted message said:

Braking force is inversely proportional to the tire radius too, so larger wheels would call for
larger rotors to maintain the same amount of braking force.

Braking torque needs to be examined here as well. The braking torque required is the same for the
same load on any wheel diameter, and since the moment arm on the caliper mounts is increased with an
increase in rotor diameter, the stresses on the mounts increase as well _despite_ the braking force
remaining the same (oe even decreasing).

The units say 'power' and I said proportional. We weren't speaking, that I recall. of torque, force
or drag but of power.

"Jose Rizal" <_@_._> wrote in message
"]news:[email hidden]...

Quoted message said:

Doug Huffman:

Quoted message said:

The power of a brake is its convective heat transfer rate and

proportional

Quoted message said:

Quoted message said:

to Q = UA(Th - Tc) where Q is the heat transfer rate, U the convective

heat

Quoted message said:

Quoted message said:

transfer coefficient, A the area and a function of rotor diameter, Th

the

Quoted message said:

Quoted message said:

temperature of the rotor and Tc the cooling air.

Incorrect. This is only an indication of the heat generated by the brake, not its stopping power.

For each surface contact on a disk brake, the stopping torque generated with uniform pressure
applied by the pad is

T = (1/3) * F * mu * [(D^3 - d^3)/(D^2 - d^2)]

where F is the force applied on each caliper face, mu is the coefficient of friction, and D and d
are the outer and inner diameters of the rotor annulus.

mu will change as heat is generated on the rotor and pad, but since it's a function of the pad
material, this change can be set aside for comparative purposes.

It's clear that the larger the rotor, the greater the stopping torque.

Mike S. said:

I would think the opposite. Mtn bikes "need" large brakes in order to stop NOW. Road bikes usually
don't need the instant deceleration of a mtn bike.

Whether or not road bikes "need" to stop fast, in reality almost any road bike can stop faster than
a mountain bike.

On a road bike, the limit of braking is the pitch over point (point where the rear tire lifts and
the bike starts to go end over end). This occurs at a deceleration rate of about .5 g. Any road bike
with half-way decent brakes can create this amount of braking force.

On an MTB, if the traction is good, the braking is also limited by the pitch over point. But in many
off-road situations, traction is not so good. Often times, the front tire will skid before the pitch
over point, and it is the front tire traction that limits braking.

So, despite the gnarly looking disk brakes that many MTBs use, in reality they can stop no faster,
and in fact usually stop slower, than a road bike.

Mark McMaster [email hidden]

Jose Rizal _@_._ said:

Doug Huffman:

Quoted message said:

The power of a brake is its convective heat transfer rate and proportional to Q = UA(Th - Tc)
where Q is the heat transfer rate, U the convective heat transfer coefficient, A the area and a
function of rotor diameter, Th the temperature of the rotor and Tc the cooling air.

Incorrect. This is only an indication of the heat generated by the brake, not its stopping power.

Maybe this is the bigger issue though. Since the OP wants to use disc brakes for "long mountain
downhills" isn't heat going to be his biggest problem? I just took a few measurements and did a few
calculations and found that even a 205mm disc rotor has only about 50% of the swept area of even a
26" MTB rim. I wonder whether a person would be more likely to have a tire blow off while using a
rim brake or having the rotor warp.

The only real advantage I see to using the disc brake is that it's performance won't decline as much
in the wet. Hardly worth all the disadvantages IMO, especially on a road bike.

Chris Bird

Doug Huffman:

Quoted message said:

The units say 'power' and I said proportional. We weren't speaking, that I recall. of torque,
force or drag but of power.

It is inappropriate to use the term "power" in this case unless it refers to the stopping ability
of brakes.

The equation you outlined, although having the unit of power, in fact defines "heat".

Quoted message said:

"Jose Rizal" <_@_._> wrote in message
"]news:[email hidden]...

Quoted message said:

Doug Huffman:

Quoted message said:

The power of a brake is its convective heat transfer rate and

proportional

Quoted message said:

Quoted message said:

to Q = UA(Th - Tc) where Q is the heat transfer rate, U the convective

heat

Quoted message said:

Quoted message said:

transfer coefficient, A the area and a function of rotor diameter, Th

the

Quoted message said:

Quoted message said:

temperature of the rotor and Tc the cooling air.

Incorrect. This is only an indication of the heat generated by the brake, not its stopping
power.

For each surface contact on a disk brake, the stopping torque generated with uniform pressure
applied by the pad is

T = (1/3) * F * mu * [(D^3 - d^3)/(D^2 - d^2)]

where F is the force applied on each caliper face, mu is the coefficient of friction, and D and
d are the outer and inner diameters of the rotor annulus.

mu will change as heat is generated on the rotor and pad, but since it's a function of the pad
material, this change can be set aside for comparative purposes.

It's clear that the larger the rotor, the greater the stopping torque.

Chris B.:

Quoted message said:

Jose Rizal _@_._ said:

Doug Huffman:

Quoted message said:

The power of a brake is its convective heat transfer rate and proportional to Q = UA(Th - Tc)
where Q is the heat transfer rate, U the convective heat transfer coefficient, A the area and a
function of rotor diameter, Th the temperature of the rotor and Tc the cooling air.

Incorrect. This is only an indication of the heat generated by the brake, not its stopping power.

Maybe this is the bigger issue though. Since the OP wants to use disc brakes for "long mountain
downhills" isn't heat going to be his biggest problem? I just took a few measurements and did a
few calculations and found that even a 205mm disc rotor has only about 50% of the swept area of
even a 26" MTB rim. I wonder whether a person would be more likely to have a tire blow off while
using a rim brake or having the rotor warp.

Heat certainly is a factor on long _steep_ downhills, and the effectiveness of the brake will depend
a lot on the pad material's ability to maintain its properties at elevated temperatures.

it also needs to be pointed out that if you are going to be building this bike with STI/Ergo levers,
then only Avid's ROAD disc brakes will work, and those only come with 160mm rotors. the cable pull
for STI/Ergo is different from MTB v-brake levers. if you will be using MTB style flat bars and
shifters, then you can of course use the MTB systems. you wouldn't need anything larger than 160mm
rotors for a road bike (see the discussion on traction).

"Michael" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

I just noticed that Avid's mechanical disc brakes come with 160, 185 or 203 mm rotors. It's
claimed that the 185mm rotor has 15% more "power", the 203 26% more.

Does a larger rotor really provide more stopping power? Why? I could understand less fade due to
heat, since a larger rotor would dissipate heat better. Does that translate into more "power"?

I assume that dimension is the rotor diameter. But wouldn't a change in rotor diameter change
where the caliper gets positioned, i.e. would require a change to the caliper mounting eyelets?

I'm spec'ing these brakes for a road bike; should I expect to need more stopping power than a
mountain bike for say long mountain downhills?

TIA