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.