Why do some forks and frames have brake rotor size limits?

On 2017-10-31 07:05, wrote:
On Monday, October 30, 2017 at 1:54:27 PM UTC-7, Joerg wrote:
On 2017-10-30 12:35, wrote:
On Monday, October 30, 2017 at 9:39:55 AM UTC-7, Joerg wrote:
On 2017-10-29 12:57, wrote:
On Sunday, October 29, 2017 at 10:02:12 AM UTC-7, Joerg
wrote:
On 2017-10-29 09:45, wrote:
On Saturday, October 28, 2017 at 8:09:18 AM UTC-7, Joerg
wrote:

And grandpa has driven his cars without safety belts
yet survived ...

For people who do not shy away from unpaved roads or
use a lot of singletrack and ride in the rain there is
a much more extreme issue: Wet mud.

There is NO brake the is proof against wet mud. In fact
it is perhaps worse on a disk since the additional
pressure of the pads can turn the silicon present in most
muds into cutting instruments that on rim brakes cuts
into the rubber show rather than the hard and thin disk
pad.


Actually, no. I've had mud literally dripping from the
calipers which had become barely recognizable brownish
blobs. The only thing that happens is that they make an
awful grinding noise just like muddy rim brakes do. With
the two major differences that they still come on full
force immediately and that this grinding does not eat up
aluminum. Aluminum as one of the braking surfaces plain
does not make any sense, certainly not in a muddy
environment.

A downside of bicycle disc brakes is that in contrast to
most motor vehicles the rotors have "vent holes" and weight
weenie spiders. This results in rather fast heat-up and in
"brake mousse" when you plow through thick vegetation on an
overgrown trail. Mashed star-thistle and other weeds get
shredded and a sort of pulp develops which cakes up in the
holes of the rotor. It doesn't diminish the brake force but
lets of a bad stench. One of the reasons why I carry a
Swiss Army knife in a pocket. Not in a pannier, so I can
whip it out in seconds. This also helps in poking out the
giant mud clump that forms behind the BB and can prevent
the rear wheel from turning.

Joerg - that additional noise is wear.


Sure. However, the rotors last thousands of miles, cost around
$20 and take only minutes to change. The pads cost $2/pair for
ceramic-based material (like motorcycles have) and last around
1000mi depending on turf and weather. That is way more hassle
than with a motor vehicle but way less hassle than swapping out
a shot rim.

I started riding again in fall 2013, using an older model MTB
with almost zero miles on it. By the end of 2013 it had around
1000mi on it and the front rim looks horrible.

The problem in our area is this: Mud contains granules of
decomposed granite. Rubber pads need water diverter grooves and
the granules lodge in these grooves. They also pierce the
rubber itself and lodge in there. When pulling the lever that
lets of a horrid grinding noise. Coming down a hill you have to
keep the brake engaged and you can literally hear the rim being
tortured all the way to the bottom of the hill. On flat
surfaces you have to stop and pry out the granules. On a rainy
day that means stopping every few miles. I stand by my opinion
that rim brakes are inadequate for any serious MTB riding.

Well, perhaps it was from older parts but I saw a significant
number of MTB riders walking their bikes back off of hills. Those
were the older cable pull brakes so that might have been when
they were still developing the proper compounds.

https://www.youtube.com/watch?v=Wdb7KEc7xJI


Wow, 2x the stopping distance for the rim brakes. I wouldn't have
imagined it could be that much worse.


This is comparing an 11 speed Ghost with an aluminum el-cheapo
Raleigh. This was a highly unfair test to begin with: On the
climbing test the Raleigh had an eight speed compact that simply
didn't have the correct gearing. And the bicycle size and set-up
was entirely wrong.

On the braking test it was hydraulic disks against what appeared
to the the old-fashioned long throw brakes. There was so much
bend in the brakes that you could bottom out the lever.


I don't think he bottomed out the levers.


On the TT again the problem was that there simply wasn't the
proper gearing. And since the Raleigh wasn't sized correctly it
was nearly impossible to get into an aero position.

Personally in the brake test I believe my Skeleton brakes would
have finished not exactly with the disks but really close. There
is no flex to speak of in my brakes and while I think I could
lock the front wheel and allow the rear wheel to rise off of the
ground there is absolutely no day I would do such a thing.

My bikes are set up similar seating position to the Raleigh. But
I could merely change the stem to gain a position such as the
Ghost. I personally don't give much credit to the aerodynamics of
a frame.


Gearing and such are a different matter but brakes are
safety-relevant. I wouldn't want to compromise there.

Think about this Joerg - if he didn't bottom out the lever wouldn't
he pull harder?


A lever is never supposed to bottom out before the brake force on
respective wheel is maxed. If it did then he'd have faulty brakes and I
am sure he'd not have posted this. The guys look like serious cyclists
who know this.


... As for your opinions on safety I can understand your
position after seeing how much weight you carry.


The difference isn't that large. 20-30lbs in extra bike equipment, water
and whatnot. It's peanuts.

--
Regards, Joerg

http://www.analogconsultants.com/