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



 
 
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  #1  
Old October 21st 17, 03:12 PM posted to rec.bicycles.tech
Joerg[_2_]
external usenet poster
 
Posts: 4,623
Default Why do some forks and frames have brake rotor size limits?

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?

--
Regards, Joerg

http://www.analogconsultants.com/
Ads
  #2  
Old October 21st 17, 09:07 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 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?
  #4  
Old October 22nd 17, 04:07 PM posted to rec.bicycles.tech
Joerg[_2_]
external usenet poster
 
Posts: 4,623
Default Why do some forks and frames have brake rotor size limits?

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

Since my calipers have a large "belly" I can't use the Shimano F203
flat-lined adaptor but must use some that seem only available from
China. Looks like they will adapt from 160mm PM to 203mm though specs
are, as usual, sorely lacking:

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

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.

--
Regards, Joerg

http://www.analogconsultants.com/
  #5  
Old October 22nd 17, 05:06 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 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 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

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.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #6  
Old October 22nd 17, 09:18 PM posted to rec.bicycles.tech
JBeattie
external usenet poster
 
Posts: 2,768
Default Why do some forks and frames have brake rotor size limits?

On Sunday, October 22, 2017 at 9:06:05 AM UTC-7, 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 did the water mist system yesterday -- if mist is two inches of rain in 24 hours and howling winds. It was like riding in a disaster film with the sheets of rain and all the branches and leaves flying around. My lowly 160mm cable discs were screaming, but they stopped just fine. This time of year, braking force is second to traction. The best brakes in the world don't mean a thing when you're sliding around on leaves and needles. Today was wet but no rainfall and oddly warm. I ended up putting my rain jacket in my jersey pocket. Tons of blow-down, and soon will have piles of leaves in the bike lanes. Wanna know what happens to separate cycle tracks when the leaves fall? This: https://bikeportland.org/wp-content/...aves-path1.jpg

-- Jay Beattie.

  #7  
Old October 22nd 17, 10:12 PM posted to rec.bicycles.tech
Ian Field
external usenet poster
 
Posts: 219
Default Why do some forks and frames have brake rotor size limits?



"Joerg" wrote in message
...
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?


Bicycle brake rotors are pretty thin and fragile - the bigger you make it;
the more vulnerable it is.

  #8  
Old October 23rd 17, 01:02 AM posted to rec.bicycles.tech
John B.[_3_]
external usenet poster
 
Posts: 3,557
Default Why do some forks and frames have brake rotor size limits?

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

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.


Years ago I used to see heavy logging trucks using a water spray brake
cooling system in the mountains along the central branch of the Yuba
River in California.

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.
--
Cheers,

John B.

  #9  
Old October 23rd 17, 02:05 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
external usenet poster
 
Posts: 5,394
Default Why do some forks and frames have brake rotor size limits?

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.

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. 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.

--
- Frank Krygowski
  #10  
Old October 23rd 17, 02:36 AM posted to rec.bicycles.tech
AMuzi
external usenet poster
 
Posts: 9,030
Default Why do some forks and frames have brake rotor size limits?

On 10/22/2017 8:05 PM, 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.

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. 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.


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


 




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