Why are disc brakes different sizes?

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

Quoted message said:

Hi,
now that I have finished installing the kitchen, I can get away with
upgrading to some Hope Mono Minis.

Noticed that the discs come in 3 different sizes; 140, 160, & 180.
Anyone know why and what could the advantage be between the sizes?

Thanks

Benefit is stopping power & heat resistance...

Consider:

1. Rider/bike weight - Heaver needs bigger disks to help stop more weight
due to the reasons below:
2. Descent length - if you're doing lots of hill descents then you have a
problem with heat build up. Larger disks have a larger surface area so are
able to go longer without brake fade, which does happen!
3. Modulation (I think it's called that) - Basically the larger the braking
surface the more your brakes will grab potentially. This is because you've
got more leverage on the wheel. Bigger callipers will also affect this (you
can get 6 pot callipers now).
4. It's easier to lock a rear wheel than a front - so normally people run
smaller disks on the rear as they give a more balanced feel.

So...
Brake callipers affect the "grab" of the brake - more/bigger pots will tend
to slam on rather than pull on slowly. They'll have a negligible affect on
heat as it's the pad/disk that suffers with heat, although fluid can boil -
had that once, basically the brakes pull themselves on, but it's still due
to too much heat between pad/disk.

Brake pads also affect stopping. Cheaper pads may last longer but they may
overheat and not work so well with heat build up. Generally the pads that
come with a set of disks aren't the best (lucky you if they are!).

Disk size - Really down to preference, weight isn't really an issue unless
you're really trying to run light. Stopping and modulation is more
important.

General rules (for me anyhow) Front/Rear :
XC hardtail - 165/145
XC Full Sus - 185/165
DH Rig - 205/185 (although a lot of people run 205/205).

I've got cable disks on my XC hardtail, hope minis on my XC full sus and
Hope M4/Mini on my DH bike (gives me major grunt on the front).

Given the budget I'd probably do hydraulic on the XC hardtail, but really
the deore disks are OK for pretty much everything - just not as positive.
Avid do better cable systems.

All in all, never underestimate how hard you can pull on a lever if you're
going to crash - and if the brakes are too big you'll be locking the front
up, which doesn't bode well if you're already going to fast!

AndyC

in message <[email hidden]>, Rob

(') said:

now that I have finished installing the kitchen, I can get away with
upgrading to some Hope Mono Minis.

Noticed that the discs come in 3 different sizes; 140, 160, & 180.
Anyone know why and what could the advantage be between the sizes?

The larger the disc the larger the radiant area and consequently the
faster it can dump heat into the environment. A brake is a device for
turning momentum into heat, so the faster it can dissipate heat the
more braking power it offers. Also, of course, the larger the disc the
greater the leverage, but that has a downside - many forks are not
rated for the stresses imposed by 180mm disks. Larger disks are also
marginally heavier.

Remember that your rim brakes are effectively 622mm disks - although, of
course, in this case overheating the rotor can cause your tyres to
blow!

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Diplomacy, American: see Intelligence, Military

Simon Brooke said:

Also, of course, the larger the disc the
greater the leverage, but that has a downside - many forks are not
rated for the stresses imposed by 180mm disks.

?

Surely, greater leverage with smaller disks, all things being equal.

in message <[email hidden]>, Al C-F

(') said:

Simon Brooke said:

Also, of course, the larger the disc the
greater the leverage, but that has a downside - many forks are not
rated for the stresses imposed by 180mm disks.

?

Surely, greater leverage with smaller disks, all things being equal.

The arm of the lever is the diameter of the disk.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; An enamorata is for life, not just for weekends.

Simon Brooke said:

in message <[email hidden]>, Al C-F

(') said:

Simon Brooke said:

Also, of course, the larger the disc the
greater the leverage, but that has a downside - many forks are not
rated for the stresses imposed by 180mm disks.

Surely, greater leverage with smaller disks, all things being equal.

The arm of the lever is the diameter of the disk.

And teh force on teh lever is the inverse of teh diameter, if teh
retarding couple is teh same, and teh couple required is proportional
to teh retarding force desired and teh diameter of teh wheel, and teh
retarding force is limited by road surface slip or bike geometry
(over-the-bars).

So teh maximum leverage is teh same regardless of disk size (until you
get such small disks that you can't generate the limiting case
retarding force).

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

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

Quoted message said:

On Mon, 16 May 2005 23:28:01 +0100, Simon Brooke <[email hidden]>

Quoted message said:

in message <[email hidden]>, Al C-F

(') said:

On Mon, 16 May 2005 17:41:18 +0100, Simon Brooke
<[email hidden]> wrote:

>Also, of course, the larger the disc the
>greater the leverage, but that has a downside - many forks are not
>rated for the stresses imposed by 180mm disks.

Surely, greater leverage with smaller disks, all things being equal.

The arm of the lever is the diameter of the disk.

And teh force on teh lever is the inverse of teh diameter, if teh
retarding couple is teh same, and teh couple required is proportional
to teh retarding force desired and teh diameter of teh wheel, and teh
retarding force is limited by road surface slip or bike geometry
(over-the-bars).

So teh maximum leverage is teh same regardless of disk size (until you
get such small disks that you can't generate the limiting case
retarding force).

regards, Ian SMith

How come bigger disks work better then?

AndyC said:

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

Quoted message said:

On Mon, 16 May 2005 23:28:01 +0100, Simon Brooke <[email hidden]>

Quoted message said:

in message <[email hidden]>, Al C-F
([email hidden]'😉 wrote:

> On Mon, 16 May 2005 17:41:18 +0100, Simon Brooke
> <[email hidden]> wrote:
>
>>Also, of course, the larger the disc the
>>greater the leverage, but that has a downside - many forks are not
>>rated for the stresses imposed by 180mm disks.
>
> Surely, greater leverage with smaller disks, all things being equal.

The arm of the lever is the diameter of the disk.

And teh force on teh lever is the inverse of teh diameter, if teh
retarding couple is teh same, and teh couple required is proportional
to teh retarding force desired and teh diameter of teh wheel, and teh
retarding force is limited by road surface slip or bike geometry
(over-the-bars).

So teh maximum leverage is teh same regardless of disk size (until you
get such small disks that you can't generate the limiting case
retarding force).

How come bigger disks work better then?

1: define better

2: the question was not about performance of teh disk, but strength of
teh fork. Whether the disk works better or not is irrelevant to
whether the fork can resist the resulting forces.

regards, Ian SMith
--
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|o o|
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"Ian Smith" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

AndyC said:

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

Quoted message said:

On Mon, 16 May 2005 23:28:01 +0100, Simon Brooke <[email hidden]>
wrote:
> in message <[email hidden]>, Al C-F
> ([email hidden]'😉 wrote:
>
> > On Mon, 16 May 2005 17:41:18 +0100, Simon Brooke
> > <[email hidden]> wrote:
> >
> >>Also, of course, the larger the disc the
> >>greater the leverage, but that has a downside - many forks are not
> >>rated for the stresses imposed by 180mm disks.
> >
> > Surely, greater leverage with smaller disks, all things being equal.
>
> The arm of the lever is the diameter of the disk.

And teh force on teh lever is the inverse of teh diameter, if teh
retarding couple is teh same, and teh couple required is proportional
to teh retarding force desired and teh diameter of teh wheel, and teh
retarding force is limited by road surface slip or bike geometry
(over-the-bars).

So teh maximum leverage is teh same regardless of disk size (until you
get such small disks that you can't generate the limiting case
retarding force).

How come bigger disks work better then?

1: define better

2: the question was not about performance of teh disk, but strength of
teh fork. Whether the disk works better or not is irrelevant to
whether the fork can resist the resulting forces.

regards, Ian SMith

Well, better would be um, they stop the bike better (more leverage I guess).
I'm thinking along the lines of why do DH bikes run bigger brakes, as it's
not just heat dissipation, short tracks also need bigger brakes. And what's
more it allows single finger braking, although that's also a function of
multi-pot callipers I suppose.

And can't most single crown forks cope with brakes up to 185mm, and triple
clamp (i.e. double crown) go with 205mm?
Mind you, having said this I think that Mazzochi's 66 series and the Manitou
Shermons are covered for 205mm forks.

I think the only time I've heard of brakes being too big is when the
leverage of the disk/brake on the wheel pulls the wheel out of it's QR
retaining slot (not nice). Hence the reason that Marzzochi don't warranty or
advise putting 205mm disks onto z130/z150 forks. They wouldn't do that if
there was no difference between a 205mm and a 185mm rotor's leverage would
they?

Andy

in message <[email hidden]>, AndyC

(') said:

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

Quoted message said:

On Tue, 17 May 2005 08:44:47 +0100, AndyC <[email hidden]>

Quoted message said:

"Ian Smith" <[email hidden]> wrote in message
news:[email hidden]...
> On Mon, 16 May 2005 23:28:01 +0100, Simon Brooke
> <[email hidden]> wrote:
>> in message <[email hidden]>, Al C-F
>> ([email hidden]'😉 wrote:
>>
>> > On Mon, 16 May 2005 17:41:18 +0100, Simon Brooke
>> > <[email hidden]> wrote:
>> >
>> >>Also, of course, the larger the disc the
>> >>greater the leverage, but that has a downside - many forks are
>> >>not rated for the stresses imposed by 180mm disks.
>> >
>> > Surely, greater leverage with smaller disks, all things being
>> > equal.
>>
>> The arm of the lever is the diameter of the disk.
>
> And teh force on teh lever is the inverse of teh diameter, if teh
> retarding couple is teh same, and teh couple required is
> proportional to teh retarding force desired and teh diameter of
> teh wheel, and teh retarding force is limited by road surface slip
> or bike geometry (over-the-bars).
>
> So teh maximum leverage is teh same regardless of disk size (until
> you get such small disks that you can't generate the limiting case
> retarding force).

How come bigger disks work better then?

1: define better

Well, better would be um, they stop the bike better (more leverage I
guess). I'm thinking along the lines of why do DH bikes run bigger
brakes, as it's not just heat dissipation, short tracks also need
bigger brakes. And what's more it allows single finger braking,
although that's also a function of multi-pot callipers I suppose.

I think it /is/ just heat dissipation. Disks heat rapidly and the amount
of retardation is limited by their ability to dump that heat into the
environment.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

Wise man with foot in mouth use opportunity to clean toes.
;; the Worlock

Simon Brooke said:

in message <[email hidden]>, Al C-F

(') said:

Simon Brooke said:

Also, of course, the larger the disc the
greater the leverage, but that has a downside - many forks are not
rated for the stresses imposed by 180mm disks.

?

Surely, greater leverage with smaller disks, all things being equal.

The arm of the lever is the diameter of the disk.

Apologies for not being clear - we might be at cross purposes.

Moments around the hub:

One side:
radial force on the disc at the radius of the disc from the hub

Other side:
radial force on the wheel at the diameter of the wheel.

A small disc has less mechanical advantage than a large one, or, the
wheel has a greater mechanical advantage (leverage) working against a
smaller disc.

OK. No clearer. I'll give up.