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



 
 
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  #11  
Old October 23rd 17, 04:48 AM posted to rec.bicycles.tech
Ralph Barone[_4_]
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Posts: 100
Default Why do some forks and frames have brake rotor size limits?

AMuzi wrote:

snip

Yes, we've previously discussed the advantages of mounting a
disc caliper on the front of the fork. IMHO manufacturers
don't want to dance with troubles from fastener or caliper
mount failure, preferring to load the caliper down against
the blade rather than pulling away from it (home and XMart
installation being what it is).


Thanks Andrew. That explanation for the status quo makes some sense.

Ads
  #12  
Old October 23rd 17, 04:51 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 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."

Ah, diamond disk brakes, surely the next big thing. It would also be
interesting to see if breaking a few carbon fibers causes a hot spot
on the disk or belches black smoke.

Thermal Conductivity in W/(m.K)
Diamond 1000
Silver 406
Copper 385
Aluminum 250
Carbon Steel 54
CF and Epoxy 24
Quartz 3
Pyrex Glass 1
Concrete 0.4 - 0.7
Plywood 0.13
Pine 0.12
Air 0.024

Hmmm... 92.5% sterling silver disk brakes. Not as good as diamond,
but better than CF.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #13  
Old October 23rd 17, 06:48 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 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. :-)


--
Cheers,

John B.

  #14  
Old October 23rd 17, 03:00 PM posted to rec.bicycles.tech
Sir Ridesalot
external usenet poster
 
Posts: 3,582
Default Why do some forks and frames have brake rotor size limits?

On Sunday, October 22, 2017 at 9:36:31 PM UTC-4, AMuzi wrote:
Snipped
Thanks.
In practice, large diameter discs seem to have less pressure
on the fork blade mounts and builders drop the material wall
thickness with big rotors. Consider tandem ten inch discs,
versus the trendy little 140m 'road' discs, where wheel size
is constant.

Yes, we've previously discussed the advantages of mounting a
disc caliper on the front of the fork. IMHO manufacturers
don't want to dance with troubles from fastener or caliper
mount failure, preferring to load the caliper down against
the blade rather than pulling away from it (home and XMart
installation being what it is).

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971


Could also be that mounting behind the front fork leg gives more protection to the unit. After all, the disc brakes were put on MTB bikes before they went onto road or touring bikes and MTB are hitting or coming very close to all sorts of obstructions.

Cheers
  #16  
Old October 23rd 17, 06:09 PM 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 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.

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


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #17  
Old October 24th 17, 10:19 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 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. :-)
--
Cheers,

John B.

  #18  
Old October 24th 17, 03:27 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 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.

  #19  
Old October 24th 17, 06:15 PM 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 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.

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?

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #20  
Old October 24th 17, 07:39 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 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.

--
Regards, Joerg

http://www.analogconsultants.com/
 




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