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