Tim McNamara <
[email protected]> wrote in message
news:<
[email protected]>...
> In article <
[email protected]>,
[email protected]
> (Spider) wrote:
>
> > > The standard placement for the rear disk brake does not create problems. As far as design
> > > compromise, all that has to be done is to move the caliper ahead of the fork leg, which will
> > > result in a retention force rather than an ejection force. End of problem. It would cost no
> > > more to make a fork with that mounting than the current design.
> >
> > I agree. All well and good, except: what to do about all the forks already out there? James
> > wants a recall. Not going to happen without much more solid *evidence* (as opposed to force
> > diagrams.)
>
> As far as the forks already out there, it looks like it would be possible to make a bracket which
> would bolt into the existing mounting holes and which would provide new mounting holes in a
> different location. I don't know that this would be possible with every suspension fork, but the
> ones I've looked at would appear to have ample room to accomodate this. This approach would be far
> cheaper and (I think) equally effective to recalling and replacing forks whole.
That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe too
much fork material would have to be machined.
In any case, this doesn't answer my question about the coefficient of friction between dirt and
knobbies. This one figure could significantly change the vertical force on the axle. This is one of
the things that James continually avoids, and I can't figure it out for the life of me.
Just to prove what I initially said about the responses to my questions being condescending
dismissals, here's what I said:
"While I have seen now several dismissals of this question, maybe somebody could now tell me why
none of this matters..."
To which James replied:
"You [sic] inability to work this out for yourself marks you out as a clueless troll."
I'm sorry, Tim, but this is just the thing that I'm talking about. How am I supposed to "work out"
the mu of knobbies on dirt? Since this figure is crucial for the calculation of the *actual* forces
on the QR, one would think that it would not be swept under the rug.
Doing my freshman physics labs, we had a force-measuring device that was essentially a spring with a
known value of k and a calibrated scale that read in Newtons. If I were to goto my local trail, and
with an assistant, drag my bike along with it's brakes locked, hopefully finding the peak of static
friction before it becomes kinetic friction (a lower number,) then we might have some idea of what
mu might be for a few different types of mtb terrain.
Why is this important? IIRC, most mechanical systems are designed to a "sustained use" threshold.
But most of these systems can withstand peak forces higher than that without failure. Thus, if I
ride three times a week on pavement, and use my brakes to the point of endo once or twice a week,
the QR/drop out/brake system should be able to take that without problems. Now, if I constantly
cycled that in a "peak/off/peak/off" manner, I might be able to get unscrewing. But over the course
of this riding season, cycling the brakes a lot, I have yet to see the QR move even a part of a
millimeter.
The experiment that I am performing is actually more controlled than all of the anecdotes that James
has compiled thus far. I closed my Shimano knurled QR in the manner I always have, which is to say
that the lever is on the drive side (opposite the disk and caliper side,) opened to about 95 or 100
degrees, and the nut tighted into the dropout while the fork is under a small amount of compression
(me over the handlebars, compressing it about 10mm). Close the lever over, and make fine register
marks in both sides of the QR and on the drop out, such that I will be able to tell if the QR nut or
lever moves at all, including up and down in the drop out itself (without unscrewing.) Then ride
like I always have.
The marks are in register, just as they have been from the first tightening. I check before and
after every ride, and they haven't moved at all.
So, with all of this, I would imagine that those folks who dismiss out of hand what I am asking
can't really ignore it as a "troll", nor dismiss it with the wildly arrogant claim of "if you
understood it, then you wouldn't be questioning it," etc.
These questions aren't going away, and neither am I.
You stated above that the only use for disks was in mud, and that you didn't see any other reason
for them. Let me list the reasons:
Water (when you need brakes, you need them NOW, not some time after the pads have swept the water
off the braking surface)
Modulation. IME, disk brakes are much easier to "feather" than rim brakes. V-brakes often have that
"on/off" feel to them when they are not in perfect adjustment. And Vs do not keep adjustment long,
especially on a long ride.
Rim trueness. Whack your rim on a rock, it goes out of true, sometimes. With Vs, you either have to
mess around with the barrel adjuster or change the wire clamping position. With disks, if the wheel
turns, you can ride.
Heat dissapation. The rim/tire/tube system can build up heat to the point where tires can blow off
the rim. I have friends to whom this has occurred. Blowing your front tire off at speed going
downhill seems to me to be an excellent way to check on your insurance coverage. Disk systems rarely
build enough heat to compromise performance.
Rim life. Because it's not a wear surface, the sidewall will not become thin from braking.
Rim strength. Theoretically, the rim can be made stronger and/or lighter when it only has to serve
the function of holding the tire/tube in place. This is a question for an engineer.
Ease-of-set-up. My mechanical disk brakes are trivial to set up. The center-pull cantis on my wife's
bike are a complete ***** in comparison.
And, as you say, mud.
Finally, there have been several additional comments about "not using the front brake," said in sort
of a joking or hyperbolic fashion. I'm not exactly sure how these comments do anything but make the
poster of the comments look arrogant, but I'll address it. Washing out the front wheel on a narrow
trail with rocks and trees can get you killed. Worst case, of course. So, most MTBers take pains to
NOT wash out the front from brake lockup and use the rear brake extensively to scrub speed. Weight
shift to keep the rear wheel on the ground and as an effective tool for braking is a skill learned
in the first bit of an MTBer's "career," as it were. Again, this is a subtractive number to James'
hypothesis. I, for one, NEVER use only the front brake when I'm on the dirt.
Spider
One doesn't have to directly experience the problem to know there's a problem.
