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



 
 
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  #21  
Old October 24th 17, 07:44 PM posted to rec.bicycles.tech
Joerg[_2_]
external usenet poster
 
Posts: 4,621
Default Why do some forks and frames have brake rotor size limits?

On 2017-10-22 09:06, Jeff Liebermann wrote:
On Sun, 22 Oct 2017 08:07:48 -0700, Joerg
wrote:
(...)
The rear is native 160mm and currently has a 160mm rotor. I'd like to
upgrade both front and back to 203mm or 8". Mostly to reduce rotor
heating during long descents.


Have you considered adding a water mist brake cooling system instead?
You already have a battery pack for your lighting and a water bottle.
A small automotive windshield pump and IR over-temperature switch
should be easy additions (famous last assumptions) to your existing
machine. If the brake disks are fairly flexible, they should not
shatter when suddenly cooled. If the brake pads are made to operate
in the rain, a water spray mist should not affect their stopping
power. The water mist might also be useful for keeping the disks
clean.


I am carrying enough on the bikes as it is. A bigger rotor should fix
that much better.


I did some quick searching for applicable patents, but found nothing
specific to bicycle brake mist cooling. It MIGHT be patentable. I
did find bicycle misting systems designed to cool the rider, but not
the brakes:
https://patents.google.com/patent/US8714464B2/en
https://patents.google.com/patent/US9296001B2/en
https://patents.google.com/patent/US9186691B2/en


You can buy cooling misters for bike handlebars. I do it the low-tech
way: An cleaned empty yoghurt beker rides in the right pannier. That
gets dunked into rivers et cetera and then dumped over my head. Until
the T-shirt is all soaked. This beker doubles as an emergency bowl for
when I find a dehydrated run-away dog.


I was also thinking that it might be possible to add some vanes to the
spokes near the disk brakes. These would direct additional cooling
air towards the brake disk. However, when I realized that they would
also direct mud and crud onto the disks, I decided that was a rather
bad idea.

Drivel: Here's a patent application for yet another bicycle light.
https://patents.google.com/patent/US20040090040A1/en
The light is simple enough, but the "predicted applications" include
almost every possible bicycle accessory and application. It then
wanders into the realm of barbeques, lava lamps, fireworks, bicycle
calendars, etc.


A light with a pop-out mirror ... great ...

--
Regards, Joerg

http://www.analogconsultants.com/
Ads
  #22  
Old October 24th 17, 07:47 PM posted to rec.bicycles.tech
Joerg[_2_]
external usenet poster
 
Posts: 4,621
Default Why do some forks and frames have brake rotor size limits?

On 2017-10-24 07:27, wrote:
On Tuesday, October 24, 2017 at 2:19:48 AM UTC-7, John B. wrote:
On Mon, 23 Oct 2017 10:09:20 -0700, Jeff Liebermann
wrote:

On Mon, 23 Oct 2017 12:48:29 +0700, John B.
wrote:

On Sun, 22 Oct 2017 20:51:15 -0700, Jeff Liebermann
wrote:

On Mon, 23 Oct 2017 07:02:08 +0700, John B.
wrote:

But re disc brake cooling F1 car brakes appear to work with
the discs red hot. In the 1,000 degree (F) range. And they
use Carbon Fiber discs too :-) And everyone knows that CF
is better.

"Thermal Conductivity of Carbon Fiber, and other Carbon
Based Materials"
http://www.christinedemerchant.com/carbon_characteristics_heat_conductivity.html


"So...Is Carbon Fiber a good heat conductor?
As usual the answer is "it depends." The short answer is NO
not when regular carbon fiber is made up in regular epoxy and
expected to conduct heat across the thickness. IF a highly
carbonized pan fiber with graphite or diamond added, is
measured for heat transmission in the length of the fiber it
is very good and can rival and exceed copper."

On the other hand, they seem to work pretty well :-) See
https://www.youtube.com/watch?v=h5JcHAEmIYM for a visual
indication of heat dissipation. :-)

Impressive. I'll assume it's a carbon-carbon rotor, since all F1
cars seem to using them.

Undoubtedly so. But if the advantage of "carbon" bikes can be
extolled that a carbon-carbon frame must have twice the bragging
rights :-)


http://www.racecar-engineering.com/technology-explained/f1-2014-explained-brake-systems/


(4 pages)
"A typical road car uses a cast iron brake disc with an organic
brake pad. In an F1 car, though, the same material is used for
both disc and pad, and this material is known as carbon-carbon -
a significantly different material to the carbon-fibre
composites used in the rest of the car" In other words, the F1
brakes are NOT made from CF.

Some detail on Formula 1 brakes:
https://www.youtube.com/watch?v=ev6XTdlKElw

Fun destroying brakes:
https://www.youtube.com/watch?v=KslGsXMgmqg The brake starting
at 4:45 sure looks like CF but I'm not sure.

Maybe twin disk brakes would be easier?
http://nuovafaor.it//public/prodotto/75/nccrop/DOPPIO_FRENO_CROSS_ENDURO.jpg


https://i.ytimg.com/vi/Pvwj-WWlKkg/maxresdefault.jpg
https://gzmyu4ma9b-flywheel.netdna-ssl.com/wp-content/uploads/2011/09/Gatorbrake-dual-hydraulic-front-disc-brakes-carbon-rotors01.jpg


https://lh4.googleusercontent.com/-cDfAFWrGR6Q/VHKPsm-f6YI/AAAAAAAAX10/2FCyj87xs0g/s640/14%2520-%25201.jpg
https://www.minibikecraze.co.uk/wp-content/uploads/bs0978.jpg
https://endless-sphere.com/forums/viewtopic.php?f=28&t=56268


Given the coefficient of friction between a 1.25" wide rubber tire
(32mm) and a wet road probably dragging the feet will work. :-)


Joerg's experience is with full suspension MTB's. These things are
incredibly heavy and long wheelbased. He has his judgement of disks
and it is no doubt quite accurate for his experience and riding.

I have disks on a much lighter and shorter wheelbased bike. I know
the failings up close and personal. I simply cannot imagine WHY a
person would want a more complicated system than that offered by the
Campy Skeleton brakes.


The reason can be summed up in one word: Rain :-)

--
Regards, Joerg

http://www.analogconsultants.com/
  #23  
Old October 24th 17, 10:17 PM posted to rec.bicycles.tech
[email protected]
external usenet poster
 
Posts: 3,346
Default Why do some forks and frames have brake rotor size limits?

On Tuesday, October 24, 2017 at 11:39:40 AM UTC-7, Joerg wrote:
On 2017-10-22 18:05, Frank Krygowski wrote:
On 10/22/2017 11:07 AM, Joerg wrote:
On 2017-10-21 17:19, Frank Krygowski wrote:
On 10/21/2017 4:07 PM, wrote:
On Saturday, October 21, 2017 at 7:12:03 AM UTC-7, Joerg wrote:
When researching upgrades to larger rotors I read that there may be
limits for frames and forks. Why?

Explanations were usually scant and contradictory, with some saying it
doesn't matter and some saying it does. After all, when increasing the
rotor diameter by a couple of inches the brake force on the caliper
goes
down by about 30% and then due to it being positioned farther out this
should cantilever back into the same +30% into the frame or fork
bosses
as before. The maximum deceleration achievable on each wheel
remains the
same, until it is very close to locking up. So it should be a wash,
shouldn't it?

Now THAT is something that Frank should be able to answer. I don't
believe that leverage forces are linear are they?

Give me a photo and I'll see what I can do.


This is what I am planning to do:

https://ep1.pinkbike.org/p4pb12868017/p4pb12868017.jpg

The fork has this kind on there right now because the rotor is 180mm
(or in my case 7") and the fork is native 160mm:

https://ep1.pinkbike.org/p4pb12873429/p4pb12873429.jpg


First, to really do a proper job on this I'd need to see a clear side
view of the entire disc brake and rotor (or rotors), plus lower end of
the fork, plus (ideally) the lower portion of the wheel. I haven't given
tremendous attention to disc brakes, because I'm not going to be needing
one. I'm having to make some guesses based on what I can glean from your
photos, plus a few others I found on the web.


This is the front brake:

http://www.analogconsultants.com/ng/...rontBrake1.JPG

The rear brake:

http://www.analogconsultants.com/ng/bike/RearBrake1.JPG

Next is the whole MTB. I'll do a separate post with that because it can
help people with increasing the payload capacity on full-suspension bikes:

http://www.analogconsultants.com/ng/bike/Muddy4.JPG

This is the kind of adapter I am planning to use:

https://erpimgs.idealhere.com/ImageF...963c864243.jpg

It'll move the caliper outwards and also sideways for (hopefully) a
total of 21.5mm increase in distance from the axles. I am not an ME but
my guess is that the load on the swooped upper rear post would increase
by 15-20%. That post has a lot of meat, about 0.400" by 0.400" and the
welds look beefy as well.


But: Since the pads contact the disc at perhaps a 45 degree angle above
the horizontal line through the axle, they put a downward and backward
force on the disc. IOW their force is tangent to the circle that's at
their radius of contact. That means the reaction force on the caliper is
opposite, up and forward. There's a matching force downward and back on
the dropout.

Those two forces form a couple which applies bending moment to the
bottom of the fork blade. Certainly, a steel road fork blade designed
for a caliper brake is likely to be fairly thin and a bit flexible down
there. It's not designed to resist that moment. Brazing mounts onto such
a fork to take a disc brake would be unwise.

But that's addressing disc brake vs. no disc brake (IOW, vs. caliper
brake). What about a larger disc on a fork designed for a disc brake?

ISTM the braking force on the bike is the horizontal component of the
force the caliper applies to the disc. The total force it applies is
upward on an angle. This means a disc is already sort of inefficient (in
some theoretical sense) because of the typical location of the pad and
that aforementioned angle. The total force applied must be much larger
than the required braking force, since a big component is "wasted" upward.

If you move the contact point further outward, ISTM that the angle gets
worse. The force on the disc is even more vertical. For a given braking
force (measured at the tire-to-road point, or at the axle) the pad force
will have to be even higher, since more of it's vector total is wasted
upward.

On a stout mountain bike fork like you showed, I really doubt any of
that will make a difference. The ejecting force (trying to kick the axle
down out of the dropouts) will be higher, but if you're running a
through axle, I doubt you'll have problems.

However, getting back to the caliper itself: It's mounted on two studs.
The discs reaction force on the caliper must be resisted (or transmitted
to the fork) through those two studs. Increasing the standoff distance
will change the nature of those forces, increasing bending stress on the
studs, and perhaps changing the force on the lower stud from
compression+bending to tensile+bending.

Whether any of this will make a difference in your case, I can't tell.
But I doubt it; I think you'll be OK. That's my guess (tm) working
without any good dimensions or other numbers.

I'll note, though, that I still don't understand why front disc calipers
are positioned behind the fork.



That's because that part of engineering is wrong on bicycles but not
much can be done about it by the rider.


... If they were on the front, the force on
the disc would be nearly horizontal, so there would be little or no
wasted vertical component. Application force for a given deceleration
would be lower. Lower application force would cause longer pad life.
There would be no ejection force on the axle, so through axles would be
unnecessary.

But we've talked about this before.


Yup, we have. One of my next mods after the brake upgrade will be to
replace the QR axle with a solid CroMo axle and the old-fashioned big
outer nuts. The QR is too wimpy. It could also fail. Now before anyone
ridicules this as paranoya this is exactly what happened to a friend a
few weeks ago, the QR skewer snapped. Luckily it was the one in the rear
axle but since he is usually pulling a trailer that can also make for an
"interesting" situation.


I don't think that you're going to increase the loading on the fork or swing arm mount enough to worry about. But I do think that your brakes are going to get considerably more sensitive.

This is something you really have to be careful of. While it would reduce the wear on the disk with all of that weight you carry I don't think that it will be a noticable change in disk and pad wear.
  #24  
Old October 24th 17, 10:36 PM posted to rec.bicycles.tech
Joerg[_2_]
external usenet poster
 
Posts: 4,621
Default Why do some forks and frames have brake rotor size limits?

On 2017-10-24 14:17, wrote:
On Tuesday, October 24, 2017 at 11:39:40 AM UTC-7, Joerg wrote:
On 2017-10-22 18:05, Frank Krygowski wrote:
On 10/22/2017 11:07 AM, Joerg wrote:
On 2017-10-21 17:19, Frank Krygowski wrote:
On 10/21/2017 4:07 PM,
wrote:
On Saturday, October 21, 2017 at 7:12:03 AM UTC-7, Joerg
wrote:
When researching upgrades to larger rotors I read that
there may be limits for frames and forks. Why?

Explanations were usually scant and contradictory, with
some saying it doesn't matter and some saying it does.
After all, when increasing the rotor diameter by a couple
of inches the brake force on the caliper goes down by
about 30% and then due to it being positioned farther out
this should cantilever back into the same +30% into the
frame or fork bosses as before. The maximum deceleration
achievable on each wheel remains the same, until it is
very close to locking up. So it should be a wash,
shouldn't it?

Now THAT is something that Frank should be able to answer.
I don't believe that leverage forces are linear are they?

Give me a photo and I'll see what I can do.


This is what I am planning to do:

https://ep1.pinkbike.org/p4pb12868017/p4pb12868017.jpg

The fork has this kind on there right now because the rotor is
180mm (or in my case 7") and the fork is native 160mm:

https://ep1.pinkbike.org/p4pb12873429/p4pb12873429.jpg

First, to really do a proper job on this I'd need to see a clear
side view of the entire disc brake and rotor (or rotors), plus
lower end of the fork, plus (ideally) the lower portion of the
wheel. I haven't given tremendous attention to disc brakes,
because I'm not going to be needing one. I'm having to make some
guesses based on what I can glean from your photos, plus a few
others I found on the web.


This is the front brake:

http://www.analogconsultants.com/ng/...rontBrake1.JPG

The rear brake:

http://www.analogconsultants.com/ng/bike/RearBrake1.JPG

Next is the whole MTB. I'll do a separate post with that because it
can help people with increasing the payload capacity on
full-suspension bikes:

http://www.analogconsultants.com/ng/bike/Muddy4.JPG

This is the kind of adapter I am planning to use:

https://erpimgs.idealhere.com/ImageF...963c864243.jpg



It'll move the caliper outwards and also sideways for (hopefully) a
total of 21.5mm increase in distance from the axles. I am not an ME
but my guess is that the load on the swooped upper rear post would
increase by 15-20%. That post has a lot of meat, about 0.400" by
0.400" and the welds look beefy as well.


But: Since the pads contact the disc at perhaps a 45 degree angle
above the horizontal line through the axle, they put a downward
and backward force on the disc. IOW their force is tangent to the
circle that's at their radius of contact. That means the reaction
force on the caliper is opposite, up and forward. There's a
matching force downward and back on the dropout.

Those two forces form a couple which applies bending moment to
the bottom of the fork blade. Certainly, a steel road fork blade
designed for a caliper brake is likely to be fairly thin and a
bit flexible down there. It's not designed to resist that moment.
Brazing mounts onto such a fork to take a disc brake would be
unwise.

But that's addressing disc brake vs. no disc brake (IOW, vs.
caliper brake). What about a larger disc on a fork designed for a
disc brake?

ISTM the braking force on the bike is the horizontal component of
the force the caliper applies to the disc. The total force it
applies is upward on an angle. This means a disc is already sort
of inefficient (in some theoretical sense) because of the typical
location of the pad and that aforementioned angle. The total
force applied must be much larger than the required braking
force, since a big component is "wasted" upward.

If you move the contact point further outward, ISTM that the
angle gets worse. The force on the disc is even more vertical.
For a given braking force (measured at the tire-to-road point, or
at the axle) the pad force will have to be even higher, since
more of it's vector total is wasted upward.

On a stout mountain bike fork like you showed, I really doubt any
of that will make a difference. The ejecting force (trying to
kick the axle down out of the dropouts) will be higher, but if
you're running a through axle, I doubt you'll have problems.

However, getting back to the caliper itself: It's mounted on two
studs. The discs reaction force on the caliper must be resisted
(or transmitted to the fork) through those two studs. Increasing
the standoff distance will change the nature of those forces,
increasing bending stress on the studs, and perhaps changing the
force on the lower stud from compression+bending to
tensile+bending.

Whether any of this will make a difference in your case, I can't
tell. But I doubt it; I think you'll be OK. That's my guess (tm)
working without any good dimensions or other numbers.

I'll note, though, that I still don't understand why front disc
calipers are positioned behind the fork.



That's because that part of engineering is wrong on bicycles but
not much can be done about it by the rider.


... If they were on the front, the force on the disc would be
nearly horizontal, so there would be little or no wasted vertical
component. Application force for a given deceleration would be
lower. Lower application force would cause longer pad life. There
would be no ejection force on the axle, so through axles would
be unnecessary.

But we've talked about this before.


Yup, we have. One of my next mods after the brake upgrade will be
to replace the QR axle with a solid CroMo axle and the
old-fashioned big outer nuts. The QR is too wimpy. It could also
fail. Now before anyone ridicules this as paranoya this is exactly
what happened to a friend a few weeks ago, the QR skewer snapped.
Luckily it was the one in the rear axle but since he is usually
pulling a trailer that can also make for an "interesting"
situation.


I don't think that you're going to increase the loading on the fork
or swing arm mount enough to worry about.



It is already a serious problem. The only way to prevent the left side
of the front axle from sloshing partly out of the fork upon heavy
braking is to oil the heck out of all QR moving parts and then close it
as tightly as possible. The left dropout has already wallered out
noticeably. That's just got to stop.


... But I do think that your
brakes are going to get considerably more sensitive.


No problem. The Promax Decipher modulate nicely and right now I have to
reach in hard to slow down on a steep downhill.


This is something you really have to be careful of. While it would
reduce the wear on the disk with all of that weight you carry I don't
think that it will be a noticable change in disk and pad wear.


It will not reduce pad wear but will reduce rotor wear. However, that's
not my objective. The objective is to reduce heating of the rotor on
long downhill stretches. Not having to pull the lever so hard anymore is
an added benefit. Ok, I could also lose 30lbs instead but we all know
that's not going to happen ...

--
Regards, Joerg

http://www.analogconsultants.com/
  #25  
Old October 25th 17, 12:58 AM posted to rec.bicycles.tech
John B.[_3_]
external usenet poster
 
Posts: 3,556
Default Why do some forks and frames have brake rotor size limits?

On Tue, 24 Oct 2017 10:15:53 -0700, Jeff Liebermann
wrote:

On Tue, 24 Oct 2017 16:19:42 +0700, John B.
wrote:

On Mon, 23 Oct 2017 10:09:20 -0700, Jeff Liebermann
wrote:
Impressive. I'll assume it's a carbon-carbon rotor, since all F1 cars
seem to using them.


Undoubtedly so. But if the advantage of "carbon" bikes can be extolled
that a carbon-carbon frame must have twice the bragging rights :-)


I don't think it would be a good idea to brag about having a bicycle
made from the same stuff that caused the Challenger space shuttle
disaster. The leading edges of the wings were made of carbon-carbon.
When the wings were hit by ice during takeoff, it punched some rather
large holes in the carbon-carbon.


?????????????
"Disintegration of the vehicle began after an O-ring seal in its right
solid rocket booster (SRB) failed at liftoff. The O-ring was not
designed to fly under unusually cold conditions as in this launch. Its
failure caused a breach in the SRB joint it sealed, allowing
pressurized burning gas from within the solid rocket motor to reach
the outside and impinge upon the adjacent SRB aft field joint
attachment hardware and external fuel tank. This led to the separation
of the right-hand SRB's aft field joint attachment and the structural
failure of the external tank. Aerodynamic forces broke up the
orbiter."

Given the coefficient of friction between a 1.25" wide rubber tire
(32mm) and a wet road probably dragging the feet will work. :-)


Good point. Perhaps an anchor and rope thrown overboard might be more
suitable for stopping the bicycle on a wet road?

--
Cheers,

John B.

  #26  
Old October 25th 17, 01:13 AM posted to rec.bicycles.tech
Jeff Liebermann
external usenet poster
 
Posts: 2,569
Default Why do some forks and frames have brake rotor size limits?

On Sun, 22 Oct 2017 09:06:07 -0700, Jeff Liebermann
wrote:

Have you considered adding a water mist brake cooling system instead?


Curses, it's not going to work. The problem is that a very fine water
mist sprayed at a red hot brake disk will vaporize (evaporate) the
tiny water droplets before they hit the red hot brake disk. That's
nice for evaporative cooling the air around the brakes, but does
nothing to cool the actual brake disk. To do that would require
larger droplets, a higher velocity droplet spray, or both. Dumping
liquid water on the brakes would also work, but that's like having
your own private rain storm, which was the original problem.

Grumble...


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #27  
Old October 25th 17, 01:21 AM posted to rec.bicycles.tech
John B.[_3_]
external usenet poster
 
Posts: 3,556
Default Why do some forks and frames have brake rotor size limits?

On Tue, 24 Oct 2017 11:47:12 -0700, Joerg
wrote:

On 2017-10-24 07:27, wrote:
On Tuesday, October 24, 2017 at 2:19:48 AM UTC-7, John B. wrote:
On Mon, 23 Oct 2017 10:09:20 -0700, Jeff Liebermann
wrote:

On Mon, 23 Oct 2017 12:48:29 +0700, John B.
wrote:

On Sun, 22 Oct 2017 20:51:15 -0700, Jeff Liebermann
wrote:

On Mon, 23 Oct 2017 07:02:08 +0700, John B.
wrote:

But re disc brake cooling F1 car brakes appear to work with
the discs red hot. In the 1,000 degree (F) range. And they
use Carbon Fiber discs too :-) And everyone knows that CF
is better.

"Thermal Conductivity of Carbon Fiber, and other Carbon
Based Materials"
http://www.christinedemerchant.com/carbon_characteristics_heat_conductivity.html


"So...Is Carbon Fiber a good heat conductor?
As usual the answer is "it depends." The short answer is NO
not when regular carbon fiber is made up in regular epoxy and
expected to conduct heat across the thickness. IF a highly
carbonized pan fiber with graphite or diamond added, is
measured for heat transmission in the length of the fiber it
is very good and can rival and exceed copper."

On the other hand, they seem to work pretty well :-) See
https://www.youtube.com/watch?v=h5JcHAEmIYM for a visual
indication of heat dissipation. :-)

Impressive. I'll assume it's a carbon-carbon rotor, since all F1
cars seem to using them.

Undoubtedly so. But if the advantage of "carbon" bikes can be
extolled that a carbon-carbon frame must have twice the bragging
rights :-)


http://www.racecar-engineering.com/technology-explained/f1-2014-explained-brake-systems/


(4 pages)
"A typical road car uses a cast iron brake disc with an organic
brake pad. In an F1 car, though, the same material is used for
both disc and pad, and this material is known as carbon-carbon -
a significantly different material to the carbon-fibre
composites used in the rest of the car" In other words, the F1
brakes are NOT made from CF.

Some detail on Formula 1 brakes:
https://www.youtube.com/watch?v=ev6XTdlKElw

Fun destroying brakes:
https://www.youtube.com/watch?v=KslGsXMgmqg The brake starting
at 4:45 sure looks like CF but I'm not sure.

Maybe twin disk brakes would be easier?
http://nuovafaor.it//public/prodotto/75/nccrop/DOPPIO_FRENO_CROSS_ENDURO.jpg


https://i.ytimg.com/vi/Pvwj-WWlKkg/maxresdefault.jpg
https://gzmyu4ma9b-flywheel.netdna-ssl.com/wp-content/uploads/2011/09/Gatorbrake-dual-hydraulic-front-disc-brakes-carbon-rotors01.jpg


https://lh4.googleusercontent.com/-cDfAFWrGR6Q/VHKPsm-f6YI/AAAAAAAAX10/2FCyj87xs0g/s640/14%2520-%25201.jpg
https://www.minibikecraze.co.uk/wp-content/uploads/bs0978.jpg
https://endless-sphere.com/forums/viewtopic.php?f=28&t=56268

Given the coefficient of friction between a 1.25" wide rubber tire
(32mm) and a wet road probably dragging the feet will work. :-)


Joerg's experience is with full suspension MTB's. These things are
incredibly heavy and long wheelbased. He has his judgement of disks
and it is no doubt quite accurate for his experience and riding.

I have disks on a much lighter and shorter wheelbased bike. I know
the failings up close and personal. I simply cannot imagine WHY a
person would want a more complicated system than that offered by the
Campy Skeleton brakes.


The reason can be summed up in one word: Rain :-)


But last Sunday I started out my "weekend" ride in the rain. It had
been raining nearly all night and the roads had a lot of water on them
- note we have been having floods here in Bangkok lately - but it
appeared that the rain was ending so off I went.

Unfortunately my weather forecasting facility wasn't working very well
and I rode 20 Km of a 30 Km ride in light rain and flooded roads in
many places. I was splashing through water in some places and cars
were splashing through (and splashing me) in others.

Of course, Sunday is much lighter traffic then on work days but still,
Bangkok is rated as one of the cities with the most chaotic traffic
in the world, and I did have to stop suddenly several time, on flooded
roads with wet wheels and brakes.

My brakes worked just as they do in the dry. Back brake stops me
somewhat slowly and front brake stops rather suddenly, both brakes
together provides best stopping. No long wait after grabbing a brake
lever although I did think of you with your stopping problems and I
have the feeling that the brake lever pressure might be a tiny bit
more to stop in the rain but if it was it was so little that it
couldn't be quantified.

But of course I am using quality brake pads. Why it costs me US$12.12
a wheel just for pads alone.... but they do last a year or more.
--
Cheers,

John B.

  #28  
Old October 25th 17, 01:50 AM posted to rec.bicycles.tech
John B.[_3_]
external usenet poster
 
Posts: 3,556
Default Why do some forks and frames have brake rotor size limits?

On Tue, 24 Oct 2017 11:39:39 -0700, Joerg
wrote:

On 2017-10-22 18:05, Frank Krygowski wrote:
On 10/22/2017 11:07 AM, Joerg wrote:
On 2017-10-21 17:19, Frank Krygowski wrote:
On 10/21/2017 4:07 PM, wrote:
On Saturday, October 21, 2017 at 7:12:03 AM UTC-7, Joerg wrote:
When researching upgrades to larger rotors I read that there may be
limits for frames and forks. Why?

Explanations were usually scant and contradictory, with some saying it
doesn't matter and some saying it does. After all, when increasing the
rotor diameter by a couple of inches the brake force on the caliper
goes
down by about 30% and then due to it being positioned farther out this
should cantilever back into the same +30% into the frame or fork
bosses
as before. The maximum deceleration achievable on each wheel
remains the
same, until it is very close to locking up. So it should be a wash,
shouldn't it?

Now THAT is something that Frank should be able to answer. I don't
believe that leverage forces are linear are they?

Give me a photo and I'll see what I can do.


This is what I am planning to do:

https://ep1.pinkbike.org/p4pb12868017/p4pb12868017.jpg

The fork has this kind on there right now because the rotor is 180mm
(or in my case 7") and the fork is native 160mm:

https://ep1.pinkbike.org/p4pb12873429/p4pb12873429.jpg


First, to really do a proper job on this I'd need to see a clear side
view of the entire disc brake and rotor (or rotors), plus lower end of
the fork, plus (ideally) the lower portion of the wheel. I haven't given
tremendous attention to disc brakes, because I'm not going to be needing
one. I'm having to make some guesses based on what I can glean from your
photos, plus a few others I found on the web.


This is the front brake:

http://www.analogconsultants.com/ng/...rontBrake1.JPG

The rear brake:

http://www.analogconsultants.com/ng/bike/RearBrake1.JPG

Next is the whole MTB. I'll do a separate post with that because it can
help people with increasing the payload capacity on full-suspension bikes:

http://www.analogconsultants.com/ng/bike/Muddy4.JPG

This is the kind of adapter I am planning to use:

https://erpimgs.idealhere.com/ImageF...963c864243.jpg

It'll move the caliper outwards and also sideways for (hopefully) a
total of 21.5mm increase in distance from the axles. I am not an ME but
my guess is that the load on the swooped upper rear post would increase
by 15-20%. That post has a lot of meat, about 0.400" by 0.400" and the
welds look beefy as well.


I think that you are talking about the difference between a 160mm disc
and a 203mm disc. The radius of the 203mm disk is 101.3mm and the
radius of the 150mm disk is 80mm. Thus the moment arm of the larger
discs is ~26.6% longer then the smaller.

But this is a two way street as any force applied to the wheel will be
multiplied 1.26 times with the larger disc and of course the effect of
any force applied to the rim of the larger disc will be multiplied
1.26 times at the center of the wheel with the larger discs :-)

A 6061 - T4 aluminum, a common aluminum alloy, has a yield strength of
21,000 psi. 0.4" x 0.4" is 0.16 square inches so the theoretical load
that will cause it to bend the mount bracket might be as high as about
3,360 lbs, or 1.68 tons :-)
--
Cheers,

John B.

  #29  
Old October 25th 17, 04:11 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Posts: 5,394
Default Why do some forks and frames have brake rotor size limits?

On 10/24/2017 8:13 PM, Jeff Liebermann wrote:
On Sun, 22 Oct 2017 09:06:07 -0700, Jeff Liebermann
wrote:

Have you considered adding a water mist brake cooling system instead?


Curses, it's not going to work. The problem is that a very fine water
mist sprayed at a red hot brake disk will vaporize (evaporate) the
tiny water droplets before they hit the red hot brake disk. That's
nice for evaporative cooling the air around the brakes, but does
nothing to cool the actual brake disk. To do that would require
larger droplets, a higher velocity droplet spray, or both. Dumping
liquid water on the brakes would also work, but that's like having
your own private rain storm, which was the original problem.

Grumble...


No problem. You pump it into the right end of a hollow axle, and provide
channels connecting that with the inner hollows in a double-sided disc
similar to an automotive disc, like this:
http://www.autopartsapi.com/eEuropar...169e817e2b.jpg

The water flows out radially, carrying heat with it but leaving the
braking surface dry.

You keep forgetting that for Joerg, weight doesn't matter! ;-)


--
- Frank Krygowski
  #30  
Old October 25th 17, 03:47 PM posted to rec.bicycles.tech
Joerg[_2_]
external usenet poster
 
Posts: 4,621
Default Why do some forks and frames have brake rotor size limits?

On 2017-10-24 20:11, Frank Krygowski wrote:
On 10/24/2017 8:13 PM, Jeff Liebermann wrote:
On Sun, 22 Oct 2017 09:06:07 -0700, Jeff Liebermann
wrote:

Have you considered adding a water mist brake cooling system instead?


Curses, it's not going to work. The problem is that a very fine water
mist sprayed at a red hot brake disk will vaporize (evaporate) the
tiny water droplets before they hit the red hot brake disk. That's
nice for evaporative cooling the air around the brakes, but does
nothing to cool the actual brake disk. To do that would require
larger droplets, a higher velocity droplet spray, or both. Dumping
liquid water on the brakes would also work, but that's like having
your own private rain storm, which was the original problem.

Grumble...



Water does work. I have gone through creeks at the end of a long
downhill and ... phsssss ... cooled it off. A spritz from the water
bottle can also help as it's usually only the front rotor that heats up
a bit much. However, out in the boonies on a hot day one does not want
to spritz away too much of that stuff.


No problem. You pump it into the right end of a hollow axle, and provide
channels connecting that with the inner hollows in a double-sided disc
similar to an automotive disc, like this:
http://www.autopartsapi.com/eEuropar...169e817e2b.jpg


The water flows out radially, carrying heat with it but leaving the
braking surface dry.


Wouldn't that scald the left leg of the rider?


You keep forgetting that for Joerg, weight doesn't matter! ;-)


Within reason it doesn't. If larger rotors cause a few ounces of weight
increase I couldn't care less. Just like the ruggedizing of the MTB
rear-end has probably increased the weight by 2lbs and I am happy as a
fish in the water because now things are rock-solid.

So, Frank, any words of wisdom from the ME when looking at the caliper
mount photos? Assume a worst case vehicle weight of 300lbs (rider + bike
+ cargo), 20mph, steep downhill, full emergency stop on grippy rock.

--
Regards, Joerg

http://www.analogconsultants.com/
 




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