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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
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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
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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
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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
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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
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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 |
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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
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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 |
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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
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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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