On 2017-10-24 14:17, wrote:
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.


It is already a serious problem. The only way to prevent the left side
of the front axle from sloshing partly out of the fork upon heavy
braking is to oil the heck out of all QR moving parts and then close it
as tightly as possible. The left dropout has already wallered out
noticeably. That's just got to stop.


... But I do think that your
brakes are going to get considerably more sensitive.


No problem. The Promax Decipher modulate nicely and right now I have to
reach in hard to slow down on a steep downhill.


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.


It will not reduce pad wear but will reduce rotor wear. However, that's
not my objective. The objective is to reduce heating of the rotor on
long downhill stretches. Not having to pull the lever so hard anymore is
an added benefit. Ok, I could also lose 30lbs instead but we all know
that's not going to happen ...

--
Regards, Joerg

http://www.analogconsultants.com/