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

On 2017-11-11 11:55, jbeattie wrote:
On Saturday, November 11, 2017 at 10:47:10 AM UTC-8, Joerg wrote:
On 2017-11-11 08:59, jbeattie wrote:
On Saturday, November 11, 2017 at 8:34:13 AM UTC-8, Frank
Krygowski wrote:
On 11/11/2017 10:37 AM, Joerg wrote:
On 2017-11-11 07:21, jbeattie wrote:

... I don't think most people need discs with giant rotors,
but Joerg is a special case.


All one has to do is go down French Creek Road and then
Holly Drive out here on a loaded bike and the li'l 6-incher
in back starts to smell. Can't use the front too much because
it's loose gravel. Similar for some of the long hills in the
Sierra east of here. The 8" rotors I mounted yesterday will
allow me to roll down many of the long downslopes without a
cooling-off break or spritzing water onto the rotors.

A friend had 8" on the front and still lost the front brake
at the last curve on a long hillside. One of those "Oh
s..t!" experiences.

Hmm. So even 8" isn't sufficient. Good to know.

My brakes are about 25" diameter. I'm keeping them! ;-)

In Joerg's neighborhood but much further into the Sierra, I did
Monitor Pass, Ebbetts, Carson and Luther multiple times on loaded
and unloaded bikes with either single pivot Campy NR or Mafac
cantis. Tioga, too. Ebbets has some 20% pitches. I never once
had brake problems.


Very different scenario because there you can just "let'er rip"
most of the time. I described a road that has a long downhill
stretch and is gravel. You have to hold the brakes the whole time
because there is forest left and right. Sure enough about 70% down
a Shepherd and a smaller dogs came out of the woodwork and were
totally unfazed but my brightly lit aluminum horse. They sauntered
across. At 40mph that would have been a nasty crash. At 15mph I
could even greet the dogs properly. There are also diagonal ruts
and washouts that you won't see in time at high speed. A reminder
could be seen in one curve where a car driver must have thought
differently, flew off the road and chopped some trees. The wreckage
was already hauled off but there still was a torn-off front axle
and a ripped out coil spring way out in the bushes. Ghastly.

Actually, you can't let 'er rip down Ebbetts. My son, who is about
200lbs, 6'5" and basically all leg muscle, descends Guardsman's Pass
on CF rims and caliper brakes.
https://www.youtube.com/watch?time_c...&v=hRM3bFXlyNk


Right, and three guys crashed. That is not my definition of a safe ride.


https://www.youtube.com/watch?v=NKdVMYo1H38 Basically the whole world
descends much hairier descents than French Grade Road on pretty
mundane equipment.


And crashes, hard. What I do not understand why the riders braked so
late when they were obviously far off the ideal line for that curve and
then did not drop behind their seats. No wonder the rear wheel comes
off, you've got to move the CG as far back and as low as possible.
Staying on or above the seat that does not work. Maybe some of the road
bikers would need MTB training. "Belly riding" is an almost instinctive
and instant reaction when a crisis situation like in the video comes up
for a mountain biker.


... Having forest to the left and right is basically
SOP around here. I could commute to work on Fire Lane 5 if I were so
inclined. https://www.youtube.com/watch?v=XbWCFwIxgnw&t=62s (on a CX
bike). It gets nasty at 3:00. Anyway, get to the bottom, turn right
and you're practically down town. I've done that on a SuperSix.


That's singletrack, yet even there it can happen:

https://www.youtube.com/watch?v=a07sV453Qaw

I had a close one with a buck. Thank God for disc brakes which came on
hard and instantly.

Here I was talking about a gravel road wit a long descent. There are
houses left and right in the trees and people out there let their
animals run free. A collision with an animal at 40mph can end badly.


Speaking of climbing, my son's friend Luke did an Everest the other
day -- almost 34,000 feet in 120 miles in 11 hours. 100 times up an
11% neighborhood road in the upper Avenues in SLC. The guy is a
phenom.


He sure is. Like this long distance bike commuter:

https://www.youtube.com/watch?v=G52mQpoheL8

I couldn't do that anymore.


... He's on track for over a million feet of climbing this year,
and he was off his bike due to a serious injury for a month. His last
Everest was up every canyon in SLC, including the Midway side of
Guardsman Pass, which is one of the most difficult climbs/descents in
the US. All on caliper brakes. Yes, it's not Cameron Park, but still
some serious climbs.


On wide open roads I don't have a problem with rim brakes (unless it rains).

A problem arises on gravel or dirt roads with long continuous descents.
Nearly all the braking happens in back then and almost the whole time.
Bikes don't have jake brakes.

--
Regards, Joerg

http://www.analogconsultants.com/