6 or 8 inches front disc?

Hi all,

If you are given a choice to choose between a 6" and an 8" disc rotor for
your front disc brake on a single
crown fork, on a full-sus XC/epic bike, which would you go for? why? would
you also opt for the 8" rear?

There are people who would say that a 6" will serve the purpose on an XC,
and is "more than enough" braking power, too much braking power will make
you endo, lost control on loose soil, etc...

There are also people who would say that "more is better", better
modulation, better heat transfer, etc.....

What's your thought and comment?

i was faced with a similar dilemma a while back. /terrible/ brake
squeal, pads getting so hot both they and the disk were turning blue...

the "recommended" solution was 8" rotors, but i was reluctant for the
reasons you state, too much liklihood of endo, etc. effectiveness
wasn't the problem - 6" rotors are already /way/ more effective than any
rim brake solution i've ever tried.

i tried "one last thing" before going down the big rotor road - a
different brand of pad. all problems ended! great braking, excellent
modulation, no squeal, no overheating... i now use e.b.c. pads rather
than shimano and would never bother with a 6" rotor.

jb

Colin wrote:

Hi all,

If you are given a choice to choose between a 6" and an 8" disc rotor for
your front disc brake on a single
crown fork, on a full-sus XC/epic bike, which would you go for? why? would
you also opt for the 8" rear?

There are people who would say that a 6" will serve the purpose on an XC,
and is "more than enough" braking power, too much braking power will make
you endo, lost control on loose soil, etc...

There are also people who would say that "more is better", better
modulation, better heat transfer, etc.....

What's your thought and comment?

Colin:

Hi all,

If you are given a choice to choose between a 6" and an 8" disc rotor for
your front disc brake on a single
crown fork, on a full-sus XC/epic bike, which would you go for? why? would
you also opt for the 8" rear?

There are people who would say that a 6" will serve the purpose on an XC,
and is "more than enough" braking power, too much braking power will make
you endo, lost control on loose soil, etc...

There are also people who would say that "more is better", better
modulation, better heat transfer, etc.....

What's your thought and comment?

Make sure your forks are warrantied for an 8" rotor. Most manufacturers
limit rotor size on their particular forks, and void warranties if
greater than 6" rotors are used.

An 8" rotor is overkill for most off-road applications; only the
steepest downhill courses *may* justify these.

Hmm. Interesting thread. Let me make sure I've got this straight.

The choices are 6" rotors and 8" rotors. The 8" rotors, it appears,
have more stopping power since they are referred to as being
"overkill" amd "more likely to cause endos." Am I with you so far?

Now, if these statements are accurate, then it seems to me that the
post powerful braking system possible for a mountain bike would be the
one that uses 22" rotors: rim brakes!

"Colin" writes:

If you are given a choice to choose between a 6" and an 8" disc rotor for
your front disc brake on a single
crown fork, on a full-sus XC/epic bike, which would you go for? why? would
you also opt for the 8" rear?

There are people who would say that a 6" will serve the purpose on an XC,
and is "more than enough" braking power, too much braking power will make
you endo, lost control on loose soil, etc...

There are also people who would say that "more is better", better
modulation, better heat transfer, etc.....

The advantage of a bigger rotor is primarily larger cooling surface;
the disadvantage is greater weight. Unless you are doing very long
downhill sections at high speed (as in downhill racing) I find it hard
to believe that you need the greater cooling area, and if you're doing
an epic you certainly don't want to carry more weight.

In this case, less is mo less weight to heave up each hill means
more distance covered.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; in faecibus sapiens rheum propagabit

Tim McNamara writes:

Hmm. Interesting thread. Let me make sure I've got this straight.

The choices are 6" rotors and 8" rotors. The 8" rotors, it appears,
have more stopping power since they are referred to as being
"overkill" amd "more likely to cause endos." Am I with you so far?

What gives more stopping power is, ultimately, ability to dissipate
heat, since brakes essentially convert kinetic energy to heat energy
and then dump the heat energy into the environment. Other things being
equal, a larger rotor has a larger radiant surface and is able to dump
more heat.

Now, if these statements are accurate, then it seems to me that the
post powerful braking system possible for a mountain bike would be the
one that uses 22" rotors: rim brakes!

If you dump too much heat into the system which holds your tyres on,
you're going to blow tyres.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; in faecibus sapiens rheum propagabit

Tim McNamara:

Hmm. Interesting thread. Let me make sure I've got this straight.

The choices are 6" rotors and 8" rotors. The 8" rotors, it appears,
have more stopping power since they are referred to as being
"overkill" amd "more likely to cause endos." Am I with you so far?

No, you're not. I don't know where you got the "more likely to cause
endos" bit. "Endos" aren't solely reliant on brakes (let alone the type
of brake) but involve riding position as well, among other things. The
overkill claim comes from the fact that 6" disc rotors provide ample
stopping ability for all but the most demanding incline, eg steep muddy
downhills with a relatively heavy rider/bike *and* very short distance
stopping requirements, such as those in some downhill mountainbiking
courses. However, larger disc rotors exert larger bending moments on
caliper mounts despite the smaller stopping force exerted by the
calipers (compared to smaller disc rotors), and therefore require
relatively stronger mounts than those designed for smaller disc rotors.

Now, if these statements are accurate, then it seems to me that the
post powerful braking system possible for a mountain bike would be the
one that uses 22" rotors: rim brakes!

You're over-simplifying to an erroneous extent. You would know by now
the main mechanical differences between rim brakes and disc brakes; if
not, Google will come up with many threads and pages about that in
cycling-related newsgroups and websites. By now you must also be aware
of the different features of both types of brakes, as well as the pros
and cons of both so there is no need to rehash all of that here.

If the purpose of your post, however, is to try to dismiss bicycle disc
brakes as unnecessary in any case, you need to come up with a lot better
argument than what you've posted, addressing all the relevant issues
that have been discussed and argued for several years now.

Now, if these statements are accurate, then it seems to me that the
post powerful braking system possible for a mountain bike would be the
one that uses 22" rotors: rim brakes!

You're over-simplifying to an erroneous extent.

I think he was being sarcastic... lol

--
Phil, Squid-in-Training

Simon Brooke writes:

Tim McNamara writes:

Hmm. Interesting thread. Let me make sure I've got this
straight.

The choices are 6" rotors and 8" rotors. The 8" rotors, it
appears, have more stopping power since they are referred to as
being "overkill" amd "more likely to cause endos." Am I with you
so far?

What gives more stopping power is, ultimately, ability to dissipate
heat, since brakes essentially convert kinetic energy to heat energy
and then dump the heat energy into the environment. Other things
being equal, a larger rotor has a larger radiant surface and is able
to dump more heat.

Humm. And a rim would have a much larger surface than a 6" or 8"
rotor, which again would indicate that the rim brake would have
greater stopping power than either size disc brake. Thanks for
clarifying that for me.

Now, if these statements are accurate, then it seems to me that
the post powerful braking system possible for a mountain bike
would be the one that uses 22" rotors: rim brakes!

If you dump too much heat into the system which holds your tyres on,
you're going to blow tyres.

I've heard of that happening, although in the 30+ years I've been
riding bikes as an "enthusiast," it's never happened to me. Of
course, I've also never lived in mountainous terrain with the long
steep descents necessary to heat a rim up that much. OTOH in my tour
in the Alps in 2002, I never had a problem with rims heating up.

My wife and I have never ridden our tandem in the mountains; that's a
situation that I could easily see resulting in very warm rims and
might warrant a hub brake- but since my wife hates climbing, it's not
something I'll ever be likely to test.

Tim McNamara wrote in message ...
Hmm. Interesting thread. Let me make sure I've got this straight.

The choices are 6" rotors and 8" rotors. The 8" rotors, it appears,
have more stopping power since they are referred to as being
"overkill" amd "more likely to cause endos." Am I with you so far?

Now, if these statements are accurate, then it seems to me that the
post powerful braking system possible for a mountain bike would be the
one that uses 22" rotors: rim brakes!

You would have a point IF rim brakes generated as much pressure
between the pads as disk brakes do.

The appeal of disk brakes is not that they provide more stopping power
than rim brakes, but rather they the stopping power they provide is
almost constant regaurdless of the conditions. Wet, Dry, Muddy,
Icey...disk brakes work more or less the same in all those conditions.
That is certainly not a claim that can be made by rim brakes.