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Disc brake rotor size



 
 
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  #1  
Old July 11th 03, 06:58 PM
Michael
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Default Disc brake rotor size

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
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  #2  
Old July 11th 03, 07:25 PM
Mike S.
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Default Disc brake rotor size


"Michael" wrote in message
om...
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


  #3  
Old July 11th 03, 07:39 PM
Doug Huffman
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Default Disc brake rotor size

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" wrote in message
om...
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




  #4  
Old July 11th 03, 08:07 PM
Jose Rizal
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Default Disc brake rotor size

Michael:

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.

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.

http://groups.google.com/groups?q=g:....earthlink.net


  #5  
Old July 11th 03, 08:18 PM
Chris Zacho The Wheelman
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Default Disc brake rotor size

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":
http://www.geocities.com/czcorner

  #6  
Old July 11th 03, 08:35 PM
Jose Rizal
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Default Disc brake rotor size

Doug Huffman:

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.


  #7  
Old July 11th 03, 09:46 PM
Jose Rizal
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Default Disc brake rotor size

Sheldon Brown:

Michael wrote:

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.

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).

  #8  
Old July 12th 03, 05:06 AM
Chris B.
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Default Disc brake rotor size

On Fri, 11 Jul 2003 19:35:18 GMT, Jose Rizal wrote:

Doug Huffman:

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
  #9  
Old July 12th 03, 03:52 PM
Jose Rizal
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Default Disc brake rotor size

Chris B.:

On Fri, 11 Jul 2003 19:35:18 GMT, Jose Rizal wrote:

Doug Huffman:

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.


  #10  
Old July 14th 03, 04:43 AM
Doug Milliken
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Default Disc brake rotor size


On Fri, 11 Jul 2003, Sheldon Brown wrote:
big snip
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.


Actually, the worst case for brake heating is at some intermediate
descending speed -- at very slow speed the power input (force x velocity)
into the brakes is very low, since V is nearly zero. At very high speed
air drag provides all the braking, the F on the brakes is zero.

David Gordon Wilson did a nice little analysis of brake heating in Human
Power some years ago -- back issues available through http://www.ihpva.org

 




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