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Long-reach dual-pivot brakes: Different mechanical ratios across different makes?



 
 
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  #1  
Old August 10th 05, 10:25 AM
Mykal
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Posts: n/a
Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?

Now comparing two makes of long-reach (57 mm) dual-pivot brake calipers, I
notice the two have a differing geometry that may translate to significant
differences in braking performance because of different levels of mechanical
advantage.

The two long-reach calipers that I am comparing are the Shimano R600 and the
Tektro 521AG. (See http://sheldonbrown.com/harris/brakes.html)

Now using a short-reach caliper (RX100) for a reference, I see the long reach
brakes are bigger, as expected, but there appears to be significant
differences in mechanical proportions between the Shimano and Tektro
long-reach calipers -- the Shimano R600 has the same mechanical proportions
as the short-reach brake, but the Tektro has 5.3 mm less leverage on the
cable side of the pivots, as measured from the center pivot to the offset
pivot. Thus, the R600 appears to present the same mechanical advantage as
standard short-reach calipers, while the Tektro 521AG appears to have an
advantage reduced by about 15%, according to calculations based on following
data.

Data obtained by eyeballing and use of dial-caliper:

Overall reach:
RX100 = 49.5 mm
521AG = 56.4 mm
R600 = 57.4 mm

Center-pivot to offset-pivot, center-to-center distance:
RX100 = 29.9 mm
423A = 29.9 mm
R600 = 35.2 mm

Looking at dimensions related to the leverage imposed by the brake arms on
the pivots, I note the outboard (brake side) distance to the pivot between
the long reach brakes is comparably equal, while the inboard distance (cable
side) varies by over 5 mm. Notably, between the Tektro long-reach brake and
Shimano short-reach brake, the inboard leverage is the same. Thus, the Tektro
brake appears to be essentially a short-reach brake that's been modified to
work in long-reach applications, but without preserving the 5.6:1 mechanical
advantage presently associated with dual-pivot brakes.

If the Tektro provides 85% of the 5.6:1 mechanical advantage (29.9 / 35.2 =
84.9), then it presents a mechanical advantage of around 4.8:1, about the
midpoint between the 5.6:1 advantage of conventional dual-pivot brakes and
the 4.0:1 advantage of vintage single-pivot brakes. (Source for standards:
http://draco.acs.uci.edu/rbfaq/FAQ/8f.15.html)

Now I'm wondering about what may be an optimal selection of brakes for the
bike I'm assembling. Use one model? Use the other? Use a mix? Or use neither?
(I must use "long-reach" calipers; but issues of mechanical advantage and
other points are in consideration.)

Comments?


--
Mykal Crooks
Seattle, WA


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  #2  
Old August 10th 05, 01:33 PM
Qui si parla Campagnolo
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Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?


Mykal wrote:
Numbers snipped

Now I'm wondering about what may be an optimal selection of brakes for the
bike I'm assembling. Use one model? Use the other? Use a mix? Or use neither?
(I must use "long-reach" calipers; but issues of mechanical advantage and
other points are in consideration.)

Comments?


Think you are overthinking this, I have installed both and both work
well when it comes time to effectivley stop a bicycle and it's rider.

Perhaps the R660(?) shimano long reachers are a bit more refined
looking and expensive but either will work just fine.

  #3  
Old August 10th 05, 02:22 PM
Basjan
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Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?


"Mykal" wrote in message
...
Now comparing two makes of long-reach (57 mm) dual-pivot brake calipers,
I
notice the two have a differing geometry that may translate to significant
differences in braking performance because of different levels of
mechanical
advantage.

The two long-reach calipers that I am comparing are the Shimano R600 and
the
Tektro 521AG. (See http://sheldonbrown.com/harris/brakes.html)

Now using a short-reach caliper (RX100) for a reference, I see the long
reach
brakes are bigger, as expected, but there appears to be significant
differences in mechanical proportions between the Shimano and Tektro
long-reach calipers -- the Shimano R600 has the same mechanical
proportions
as the short-reach brake, but the Tektro has 5.3 mm less leverage on the
cable side of the pivots, as measured from the center pivot to the offset
pivot. Thus, the R600 appears to present the same mechanical advantage as
standard short-reach calipers, while the Tektro 521AG appears to have an
advantage reduced by about 15%, according to calculations based on
following
data.

Data obtained by eyeballing and use of dial-caliper:

Overall reach:
RX100 = 49.5 mm
521AG = 56.4 mm
R600 = 57.4 mm

Center-pivot to offset-pivot, center-to-center distance:
RX100 = 29.9 mm
423A = 29.9 mm
R600 = 35.2 mm

Looking at dimensions related to the leverage imposed by the brake arms on
the pivots, I note the outboard (brake side) distance to the pivot
between
the long reach brakes is comparably equal, while the inboard distance
(cable
side) varies by over 5 mm. Notably, between the Tektro long-reach brake
and
Shimano short-reach brake, the inboard leverage is the same. Thus, the
Tektro
brake appears to be essentially a short-reach brake that's been modified
to
work in long-reach applications, but without preserving the 5.6:1
mechanical
advantage presently associated with dual-pivot brakes.

If the Tektro provides 85% of the 5.6:1 mechanical advantage (29.9 / 35.2
=
84.9), then it presents a mechanical advantage of around 4.8:1, about the
midpoint between the 5.6:1 advantage of conventional dual-pivot brakes and
the 4.0:1 advantage of vintage single-pivot brakes. (Source for standards:
http://draco.acs.uci.edu/rbfaq/FAQ/8f.15.html)

Now I'm wondering about what may be an optimal selection of brakes for the
bike I'm assembling. Use one model? Use the other? Use a mix? Or use
neither?
(I must use "long-reach" calipers; but issues of mechanical advantage and
other points are in consideration.)

Comments?


--
Mykal Crooks
Seattle, WA


I am ashamed to admit i never even contemplated this issue (even if just out
of curiosity) when I bought the Tektros. But the $33 price for a set did
not make me think any further! I agree with Peter, I think the Tektros will
be just fine...

Jan


  #4  
Old August 10th 05, 05:03 PM
Hank Wirtz
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Posts: n/a
Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?


"Mykal" wrote in message
...
Now comparing two makes of long-reach (57 mm) dual-pivot brake calipers,
I
notice the two have a differing geometry that may translate to significant
differences in braking performance because of different levels of
mechanical
advantage.

The two long-reach calipers that I am comparing are the Shimano R600 and
the
Tektro 521AG. (See http://sheldonbrown.com/harris/brakes.html)


Big-ass SNIP


Comments?


My comment is that you put WAY too much thought into this.

I just put a set of 521AGs on my old Peugeot PX10 that I'm modernizing. I'd
argue that their level of polish is nicer than the Shimanos. The rubber
o-ring on the adjuster, the cartridge shoes and the metal QR help, too. They
work well with Veloce Ergopower levers with interrupter levers in between.

They replaced a mid-80s set of short reach Dia-Compe Royal Compe IIs with
drop bolts. I am now able to run 28mm tires with fenders, where I could only
run 23s with fenders (or 25s without) before.

And if you're like me, ashamed of the Tektro logo on an otherwise Campy
bike, well, that's nothing a Dremel with a felt wheel couldn't fix. If a set
of logoless 521AGs is good enough for Giant's OCR line, sitting in on
Ultegra and 105 groups, then they're good enough for my old French bike.

I say go for the Tektros. The set goes for a third of what the R600s do.

-Hank


  #5  
Old August 10th 05, 06:01 PM
David White
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Posts: n/a
Default Long-reach dual-pivot brakes: Different mechanical ratios acrossdifferent makes?

I may be having a slight disagreement with others. I have retro-fitted a number
of older bikes with long reach dual pivots. I got several front calipers from
Harris. They were the A550 model because I needed the "normal" nut attachment
and no other caliper seems available in this configuration.

Anyway, since the A550 are sold individually, I needed to get rear calipers and
the front caliper from a recessed fitting set works great with a normal nutted
rear brake bridge.

A buddy lent me some Tektros he had tried and disliked opting instead for the
R600 model. I also purchased some set of Nashbar long reach calipers for the
low price of $20/set. The Tektro and the Nashbars look very similar but not
exactly matching. Both seemsd fine in terms of design, finish, etc.

Using the exact same model brake pads that I have been using on my old Campy
and Suntour calipers, here is what I think:

I found the A550 to feel significantly more stiff and responsive. I found both
the Tektro and Nashbar calipers to feel too spongy and soft - requiring more
hand pressure on the levers to get the same response than the old single pivots
that I was replacing. I ultimately put back the single pivot calipers on the
rear and returned the Tektros and Nashbar calipers.

Now the Nashbar and Tektros were in the rear position only and the extra
cable/housing may have something to contribute to the feel that I did not like.
However, the same cable and housing was used with the Campy/Suntour rear
calipers as with the dual pivots so I think this was a pretty good test.

I will also point out that the friend who gave me the Tektro set also felt them
to be too soft and spongy (he told me this later when I asked) and that was why
he went to the Shimano R600 model on his bike.

I also have a single A550 rear caliper that I have yet to try. Based on the
feel of the front, I think it may feel better to me (less soft/spongy).

The price on the Tektros and Nashbar calipers can't be beat. But I'd rather pay
a bit more and get something that feels like an improvement over the single
pivots I already have.

Just my $0.02.

Mykal wrote:

Now comparing two makes of long-reach (57 mm) dual-pivot brake calipers, I
notice the two have a differing geometry that may translate to significant
differences in braking performance because of different levels of mechanical
advantage.

The two long-reach calipers that I am comparing are the Shimano R600 and the
Tektro 521AG. (See http://sheldonbrown.com/harris/brakes.html)

Now using a short-reach caliper (RX100) for a reference, I see the long reach
brakes are bigger, as expected, but there appears to be significant
differences in mechanical proportions between the Shimano and Tektro
long-reach calipers -- the Shimano R600 has the same mechanical proportions
as the short-reach brake, but the Tektro has 5.3 mm less leverage on the
cable side of the pivots, as measured from the center pivot to the offset
pivot. Thus, the R600 appears to present the same mechanical advantage as
standard short-reach calipers, while the Tektro 521AG appears to have an
advantage reduced by about 15%, according to calculations based on following
data.

Data obtained by eyeballing and use of dial-caliper:

Overall reach:
RX100 = 49.5 mm
521AG = 56.4 mm
R600 = 57.4 mm

Center-pivot to offset-pivot, center-to-center distance:
RX100 = 29.9 mm
423A = 29.9 mm
R600 = 35.2 mm

Looking at dimensions related to the leverage imposed by the brake arms on
the pivots, I note the outboard (brake side) distance to the pivot between
the long reach brakes is comparably equal, while the inboard distance (cable
side) varies by over 5 mm. Notably, between the Tektro long-reach brake and
Shimano short-reach brake, the inboard leverage is the same. Thus, the Tektro
brake appears to be essentially a short-reach brake that's been modified to
work in long-reach applications, but without preserving the 5.6:1 mechanical
advantage presently associated with dual-pivot brakes.

If the Tektro provides 85% of the 5.6:1 mechanical advantage (29.9 / 35.2 =
84.9), then it presents a mechanical advantage of around 4.8:1, about the
midpoint between the 5.6:1 advantage of conventional dual-pivot brakes and
the 4.0:1 advantage of vintage single-pivot brakes. (Source for standards:
http://draco.acs.uci.edu/rbfaq/FAQ/8f.15.html)

Now I'm wondering about what may be an optimal selection of brakes for the
bike I'm assembling. Use one model? Use the other? Use a mix? Or use neither?
(I must use "long-reach" calipers; but issues of mechanical advantage and
other points are in consideration.)

Comments?


  #6  
Old August 10th 05, 09:06 PM
Luns Tee
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Posts: n/a
Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?

In article ,
Mykal wrote:
Overall reach:
RX100 = 49.5 mm
521AG = 56.4 mm
R600 = 57.4 mm

Center-pivot to offset-pivot, center-to-center distance:
RX100 = 29.9 mm
423A = 29.9 mm
R600 = 35.2 mm


If the Tektro provides 85% of the 5.6:1 mechanical advantage (29.9 / 35.2 =
84.9), then it presents a mechanical advantage of around 4.8:1


The pivot-to-pivot distance does not enter into calculating the
mechanical advantage. Consider the limit as this distance approaches
zero - your calculation would indicate an infinite MA, but clearly, the
single-pivot sidepull brake that the limit represents does not have such
an MA.

The MA of the caliper is the average of individual MAs for each
arm, the contact point between the two arms being positioned so that the
two contribute equally (both pads move equal distances with cable pull).
The MA of an individual arm is the ratio of the horizontal distance from
the pivot to the cable, to the vertical distance from the pivot to the
brake track.

For the 'Y' arm of the caliper, the MA is exactly what it is if
a single-pivot caliper were built using that arm. In fact, for the
short-reach brake I'd measured (Shimano 105 circa '99), it is indeed
right around 4.0.
The other arm of the caliper has a higher MA, partly from the
greater distance to the cable, but also from the reduced vertical
distance to the brake track.

That the caliper's MA should be the average of each arm's is
easily seen by considering what the MA means - how much cable is pulled
to change the distance between brake pads by a unit distance. Each pad
individually moves half of the total pad distance change, and the
cable pull to move each arm is just this half distance times the arm's
respective MA. Add these together for the two arms, and you have the average.

But actually, rather than measuring the four dimensions it takes
to get the MA this way, you can arrive at the same result with only two
measurements. Look for the point between the two pivots where the two
arms contact each other (shimano calipers have the so-called centering
screw here), and project this point onto a straight line between the two
pivots.
This point is actually a sort of pivot between the two arms,
and you can treat the brake as though it were a single-pivot brake whose
centre is at this point. If you measure the horizontal distance from
here to the cable and the vertical distance from here down to the
brake track, the ratio of these is the MA of the caliper.

-Luns
  #7  
Old August 10th 05, 09:46 PM
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Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?

I might also add that replacement of the Nashbar pads at least (in my
case with Koolstops) should be considered mandatory. The OEM pads were
very hard and ineffective in the wet.

  #8  
Old August 11th 05, 06:06 PM
Mykal
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Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?


"Luns Tee" wrote in message
...
In article ,
Mykal wrote:
Overall reach:
RX100 = 49.5 mm
521AG = 56.4 mm
R600 = 57.4 mm

Center-pivot to offset-pivot, center-to-center distance:
RX100 = 29.9 mm
423A = 29.9 mm
R600 = 35.2 mm


If the Tektro provides 85% of the 5.6:1 mechanical advantage (29.9 / 35.2

=
84.9), then it presents a mechanical advantage of around 4.8:1


The pivot-to-pivot distance does not enter into calculating the
mechanical advantage. Consider the limit as this distance approaches
zero - your calculation would indicate an infinite MA, but clearly, the
single-pivot sidepull brake that the limit represents does not have such
an MA.


Yes, I agree the pivot-to-pivot distance doesn't necessarily affect the
mechanical advantage. But with the brakes I am evaluating, it happens to be a
relection of the length of the portion of the arm that is inboard of the
fulcrum -- the arms are shorter on the cable side of the pivots, but the same
longer length on the brake side of the pivots as the the R600 brake.
Moreover, the proportions of the R600 are the same as the smaller,
conventional caliper brake of which the RX100 is an example. Thus, the R600
and RX100 appear to present the same mechanical advantage (MA) -- around
5.6:1.

With the Tektro 521AG, on the otherhand, the mechanical advantage appeared to
be 4.8:1, by guestimation based on the assumption that the R600 has a 5.6:1
MA and that because each arm of the 521AG has 15% less leverage.

Regarding consideration of "the limit as this [pivot-to-pivot] distance
approaches zero" -- the way I'm looking at it, the MA does not approach
infinity, but approaches zero as the portion of the arm on the inboard side
of the pivot approaches zero. I'm looking at it the arm and pivot as a lever
and fulcrum, and thinking that the greater the portion of lever on the
actuator side of the fulcrum, the greater the amount of leverage (MA) exerted
on the actuated body. Coversely the lessser the length on the inboard side
(as evidenced by a reduced pivot-to-pivot distance with a constant
pivot-to-brakepad distance), the lesser the MA. When pivot-to-pivot equals
zero, then there is no lever on the cable side of the fulcrum; thus, no MA.

The MA of the caliper is the average of individual MAs for each
arm, the contact point between the two arms being positioned so that the
two contribute equally (both pads move equal distances with cable pull).
The MA of an individual arm is the ratio of the horizontal distance from
the pivot to the cable, to the vertical distance from the pivot to the
brake track.

For the 'Y' arm of the caliper, the MA is exactly what it is if
a single-pivot caliper were built using that arm. In fact, for the
short-reach brake I'd measured (Shimano 105 circa '99), it is indeed
right around 4.0.
The other arm of the caliper has a higher MA, partly from the
greater distance to the cable, but also from the reduced vertical
distance to the brake track.

That the caliper's MA should be the average of each arm's is
easily seen by considering what the MA means - how much cable is pulled
to change the distance between brake pads by a unit distance. Each pad
individually moves half of the total pad distance change, and the
cable pull to move each arm is just this half distance times the arm's
respective MA. Add these together for the two arms, and you have the

average.

But actually, rather than measuring the four dimensions it takes
to get the MA this way, you can arrive at the same result with only two
measurements. Look for the point between the two pivots where the two
arms contact each other (shimano calipers have the so-called centering
screw here), and project this point onto a straight line between the two
pivots.
This point is actually a sort of pivot between the two arms,
and you can treat the brake as though it were a single-pivot brake whose
centre is at this point. If you measure the horizontal distance from
here to the cable and the vertical distance from here down to the
brake track, the ratio of these is the MA of the caliper.


I think it not unlikely that my calculation of MA for the Tektro brakes is in
error, as you suggest by describing measurements to take to provide data that
will support a more comprehensive conclusion. My conclusion is dependent upon
unverified assumptions -- that the RX600 has a 5.6:1 MA, for example -- but I
still think the Tektro has a significantly less MA in comparision to the
RX600. Move the fulcrum, and you alter the MA, correct?

-Luns



  #9  
Old August 11th 05, 06:32 PM
Mykal
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Posts: n/a
Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?


"Hank Wirtz" wrote in message
...

My comment is that you put WAY too much thought into this.


This project has turned into a WAY too much thought project -- once embarked
on such a path, I'm gone for a long trip. This issue regarding on LR brake
over the other is admittedly trivial by comparison to what got me started
into this level of obsession this time...

I just put a set of 521AGs on my old Peugeot PX10 that I'm modernizing. I'd
argue that their level of polish is nicer than the Shimanos. The rubber
o-ring on the adjuster, the cartridge shoes and the metal QR help, too.

They
work well with Veloce Ergopower levers with interrupter levers in between.


Polish is not an issue for me -- both brakes look just fine -- I'm evaluating
function only. Regarding the brake shoes, that's not an issue either. Either
brake will be upgraded with Kool Stop salmon pads mounted in "Dura" shoe
holders. The brake levers are beat-up 105 STIs, so neither cosmetics nor
brand prestige is a factor.


They replaced a mid-80s set of short reach Dia-Compe Royal Compe IIs with
drop bolts. I am now able to run 28mm tires with fenders, where I could

only
run 23s with fenders (or 25s without) before.


My bike came with disc brakes, which I'm dumping because they are heavier,
cost more to maintain, and provide road performance that is inferior to rim
brakes. For rim brakes, the bike requires longer reach calipers, a welcome
restriction because it means I can run wider tires and install fenders.

snip


I say go for the Tektros. The set goes for a third of what the R600s do.


I already own a pair of Tektros; like everybody says, the price is right. But
I have one R600 also.

I might put the R600 on the front (will have to swap in a longer mounting
bolt) and put a Tektro on the rear. If the Tektro does indeed have less MA,
as I think it does, then it may actually be better suited for rear wheel
duty. (?)

--
Mykal Crooks
Seattle, WA


  #10  
Old August 12th 05, 09:08 AM
Luns Tee
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Posts: n/a
Default Long-reach dual-pivot brakes: Different mechanical ratios across different makes?

In article ,
Mykal wrote:
Regarding consideration of "the limit as this [pivot-to-pivot] distance
approaches zero" -- the way I'm looking at it, the MA does not approach
infinity, but approaches zero as the portion of the arm on the inboard side
of the pivot approaches zero.


Regarding zero vs. infinity, you're right - I misread which
numbers you were assigning to which caliper: your second chart used a
different designation for the Tektro caliper than your first and I
ended up reading the R600 number as being for the Tektro. Still, a
double-pivot brake with zero distance between pivots is a single-pivot
brake, but single pivot brakes do not have the zero MA that your
reasoning would suggest.

I'm looking at it the arm and pivot as a lever
and fulcrum, and thinking that the greater the portion of lever on the
actuator side of the fulcrum, the greater the amount of leverage (MA) exerted
on the actuated body.


Yes.

Coversely the lessser the length on the inboard side
(as evidenced by a reduced pivot-to-pivot distance with a constant
pivot-to-brakepad distance), the lesser the MA. When pivot-to-pivot equals
zero, then there is no lever on the cable side of the fulcrum; thus, no MA.


No. The arm acting on one pivot does not end at the other pivot:
it ends at the cable. The dimensions that matter are the distances from
each pivot _to the cable_, not the distance from one pivot to the other
pivot. The two pivots may be closer to each other on the Tektro caliper,
but both arms can be made longer, putting the cable farther to the side,
to increase the MA. Whether this is the case or not, I don't know - you
haven't given any measurements involving the cable's location.

Move the fulcrum, and you alter the MA, correct?


Not if one (or both) endpoints are moved to compensate.

-Luns
 




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