Sheldon Brown said:Quoted message said:Quoted message said:> Could you explain to what "linear pull" refers? That in contrast
> to other brakes.
Quoted message said:Quoted message said:Quoted message said:Actually, the proper distinction is "linear pull" _cantilevers_ as
distinct from traditional center-pull cantilevers.
Quoted message said:Quoted message said:Quoted message said:Center-pull brakes, either calipers or cantilevers are distinctly
non-linear in response.
Quoted message said:Quoted message said:I think you'll find that with the straddle cable for which the
brake is designed gives a linear pull within the limits of cosine
error of less than 5 degrees, which is essentially zero.
Quoted message said:Huh?
The change in angle of the cable to the line of action changes less
than 5 degrees from the normal angle to the lever. That makes its
lever angle change, which is related to the cosine of that change in
angle, less than a percent.
Quoted message said:Quoted message said:I've heard these stories of non-linear straddle cable brakes and
how bad it is, for years.
Quoted message said:The most serious non-linearity arose with the move to "low-profile"
cantis, especially when they are not set up optimally (a _very_
common condition.)
I don't see a diagram of such a brake on your site. However, most of
the non-linear examples that I know of were with the wheel removes and
the brake operated to angles that do not occur in use.
Quoted message said:When "low profile" cantis are set up with the yoke too high and with
the shoes retracted too far into the arms, the "anchor angle" (see
the drawing at: http://sheldonbrown.com/cantilever-geometry )
increases. As the anchor angle approaches 180 degrees, the
mechanical advantage of the cantilever arm approaches zero. When
this happens, the mechanical advantage of the yoke/toggle becomes
irrelevant.
I would have to see such an arrangement to comment on it. The straddle
cable design did not have the problems that were attributed to it. As
I have often explained, brake levers have a 4:1 ME and calipers until
the advent of the dual pivot brake had 1:1. It was the overall 4:1
hand lever to brake pad ME that lay behind the complaints. I used
centerpull brakes when they were the rave and found they performed no
different from side pull brakes except for the pad-climb toward the
tire and the lack of a good QR.
Quoted message said:Quoted message said:Meanwhile all sorts of riders came to the defense of the Campagnolo
Delta brake that is so non linear that it caused crashes and was
discontinued for that reason. It seems linearity is whatever
people want to believe.
Quoted message said:Why bring this red herring into the discussion?
To point out that claims of non-linearity are fashion and introduces by
magazine articles by writers who do not understand the concepts. The
Delta was the darling of the bicycle press for a long time and no one
mentioned its ME range.
Quoted message said:Quoted message said:Quoted message said:I cover this in some detail at:
http://sheldonbrown.com/cantilever-geometry.
Quoted message said:Quoted message said:You might do a sample calculation of the change in mechanical
advantage for such a brake when the brake is applied from a
pad-in-contact situation during braking and for the stroke of 5mm
brake pad wear. Cosine error for that is so small as to make a
negligible change in linearity.
Quoted message said:It isn't clear to me which cosine error you're speaking of here, since a
center-pull canti has two different, interacting leverage systems, the
transverse cable/yoke, and the cantilever arm.
Both the foreshortening of the cable pull and the normal lever length
on which it acts change with the cosine of the angles they sweep.
Quoted message said:Calculating the exact mechanical advantage of a cantilever brake is not
trivial, due to the number of variables involved.
It can be done, as you see from my recent discussion on the dual pivot
brake that analyzed the sweep of the left brake pad. A sketch pad and
calculator will do. I don't have any of the brakes in question so I
can't draw it up to calculate the changes but from your diagrams and
pictures, I can see that there is no significant non-linearity.
Quoted message said:Quoted message said:In contrast the Delta brake had close to a 2:1 change in the course
of pad wear.
Quoted message said:Again that odoriferous fish... Poorly adjusted low-profile cantis
are probably in that range as well.
No they are not. That is my point. It is only the imagination that
makes them so and primarily because people pull them without a rim
between the pads so they sweep an unnatural arc that goes into a
non-linear region.
Quoted message said:Quoted message said:What is bad about cantilever, V, or centerpull brakes is their large
vertical error in rim contact during pad wear. The centerpull
progresses into the tire sidewall while the cantilever can pop under
the rim with wear, its action angle being about 45 degrees to the
plane of the rim.
Quoted message said:This can be a problem. It varies considerably with cantilever stud
placement and rim width. Tends to be particularly bad on mountain bikes
where wide forks move the studs outward, and narrow rims are commonly
used to reduce weight.
Pleas consult your diagram and notice that the brake pads approach the
rim at about a 35 degree angle, the line from P to S and the rim.
This is not a good brake function and it is that way for the brakes I
mentioned above.
Quoted message said:Shimano's "parallel push" linkage and Avid's "Arch Rival" are
"linear-pull" cantis that solve this problem, though at some cost in
added complexity.
Parallel push does not alter the approach angle but merely keeps the
face of the par parallel to the braking surface. It has no useful
effect in braking. The pad still dives the same as the rigid one, its
path governed by the pivot point of the brake arm... which is the
same for either kind of brake.
Quoted message said:Quoted message said:Quoted message said:Linear-pull cantilevers, such as Shimano's "V-Brakes" are, by
comparison basically linear in terms of pad movement vs cable
movement.
Quoted message said:Quoted message said:In comparison to what?
Quoted message said:In comparison to center-pull cantilevers, particularly the
"low-profile" type center-pull cantilevers that immediately preceded
them as standard equipment on most bikes.
As long as the pivot point is the same the dive is the same with
respect to any rim you put in there. V-brakes are only cantilevers
with a different lever extension. Their dive is no different.
Quoted message said:(Aside: Low-profile cantis were introduced in response to a
different problem with the older wide-profile cantis. As
smaller-frame MTBs became the norm, riders were bumping their heels
into the arms of wide-profile cantis.)
Quoted message said:Quoted message said:Quoted message said:Traditional sidepulls are also linear in response, but to refer to
them as "linear pull" brakes would be sort of like calling an
Ashtabula crank a "cotterless" crank: technically correct, but
idiomatically inappropriate.
Quoted message said:Quoted message said:The cotter attached cranks to the spindle while the Ashtabula crank
was one piece. I think the parallel is wanting.
Quoted message said:So does that mean you'd describe a Schwinn Varsity as having
"cotterless cranks"?
Maybe you didn't catch it but the Schwinn Varsity did not have
removable cranks so there is no method of attachment, cotters, square
taper, or otherwise (cotterless).
Quoted message said:Quoted message said:This is about linearity in braking response
Quoted message said:No, it's about the standard, albeit colloquial, terminology used in
the bicycle industry.
Quoted message said:We also commonly use the term "alloy" as a synonym for "alumin(i)um"
and everybody in the biz understands this.
That doesn't mean we have to talk bicycle jive to be understood. It
doesn't hurt use the proper terms and say aluminum or titanium rather
than alloy. I think as sources of information for bicyclists, the
industry could raise the level of technical jargon rather than pander
to the lowest common denominator.
"everybody in the biz understands this" =: "cosi fan tutte" or "they
all do it" so it must be good.
Jobst Brandt
[email hidden]
