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