On Tue, 24 Oct 2017 11:39:39 -0700, 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.

I think that you are talking about the difference between a 160mm disc
and a 203mm disc. The radius of the 203mm disk is 101.3mm and the
radius of the 150mm disk is 80mm. Thus the moment arm of the larger
discs is ~26.6% longer then the smaller.

But this is a two way street as any force applied to the wheel will be
multiplied 1.26 times with the larger disc and of course the effect of
any force applied to the rim of the larger disc will be multiplied
1.26 times at the center of the wheel with the larger discs :-)

A 6061 - T4 aluminum, a common aluminum alloy, has a yield strength of
21,000 psi. 0.4" x 0.4" is 0.16 square inches so the theoretical load
that will cause it to bend the mount bracket might be as high as about
3,360 lbs, or 1.68 tons :-)
--
Cheers,

John B.