#1
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Centerpull brakes
Though I've been critical of Grant Peterson's effort to revive
centerpull brakes, I have to admit that the latest Rivendell Reader (#30) gave me reasons to pause. First of all, in the letters section, a reader (Thomas Papetti) points out that dual-pivot sidepulls won't track an out-of-true rim the way all other brake types will. I'm not sure how much that counts in favor of centerpulls (Papetti doesn't discuss them at all). Even though most riders would probably use long-reach dual pivots in lieu of centerpulls, long-reach single-pivot sidepulls and cantilevers are still available if tracking is a significant worry. Then in an article, Jan Heine makes several claims for the superiority of sidepulls. He says, "The long lever on the cable side [above the pivot, if I understand him correctly] and the short lever on the pad side [below the pivot] gives centerpulls a huge mechanical advantage over sidepulls. Just like a bolt-cutter, where long arms and short jaws allow you to cut through thick steel bolts." Later on, he adds that only with the advent of dual-pivot brakes did sidepulls catch up to centerpulls in this respect. That doesn't seem to jibe with Jobst's discussion in the FAQ, which seems to suggest that single-pivot sidepulls and centerpulls had the same mechanical advantage. He argues that only the part of the arm below the pivot needs to be beefy, because that's the only place where flex matters. "As a result, the arms above the pivots can be incredibly skinny, as they don't need to resist flex. That is where the weight savings of a centerpull originate." He doesn't say what the weight savings is. He writes, "And since the pivot on centerpull brakes is on the fork leg, adding reach does not increase the flex or change the mechanical advantage. So there is not incentive to go to short-reach brakes." I think he means that since the distance between the pad and the pivot point always remains the same, adding lever length above the pivot doesn't effect the mechanical advantage. That sounds dubious to me -- doesn't changing the length of a cantilever brake's straddle cable effect mechanical advantage even if all else remains the same? Furthermore, from the photos of old centerpulls in the Reader, it doesn't seem as if the distance from pivot to pad was always kept constant (some long-reach brakes seem to achieve that reach in part by increasing the lever below the pivot). He notes that centerpulls don't stick out to the side like centerpulls. It seems to me that that problem can be cured by using v-brakes, though that introduces problems of its own (e.g., mating them to road levers). He says that even the standard-reach (what would be called long-reach by most) dual-pivot brakes offered by Shimano don't leave much room for fenders. Is that true? Just how big a tire would you have to run before you would crowd out the dual pivots? (Elsewhere in the Reader, Grant says that centerpulls will clear a 40mm tire or a 38mm tire with fenders; what's the limit with the current long-reach dual pivots?) Furthermore, why not long-reach single-pivots, which are still available? Probably his most controversial claim is that single-pivot sidepulls "offered inferior performance" and became widespread mostly because of Campagnolo's advertising and gruppo packaging. Contrast that with Jobst's claim in the FAQ that, "The centerpull brake of the 1950's, was popular for nearly a decade, in spite of being entirely without merit, being worse in all respects than the side pull brake with which it competed." In the past, I've been pretty fed up with Grant's talk of reviving centerpulls, because it so often appeared side-by-side with complaints about Rivendell's financial straits and lack of popularity. I do notice a more refreshing tone in this issue: he mentions a bike planned for next year that will have braze-on pivots for centerpulls and adds, "But we don't expect anybody to follow that lead., and we fully expect to sell fewer bikes because of it. Centerpulls, for as much sense as they make, will still scare off most customers." |
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#3
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Centerpull brakes
Simon Brooke writes:
I know that V brakes have largely taken over from cantis, but personally I'm not yet persuaded that they offer a real practical benefit for most uses - I think they're more of a style statement, and the special levers mean that they're not so easy to interchange with other components. But I can't now see any reason to prefer old-style crossed lever centrepulls to cantilevers. I haven't used sidepulls since my mid-70s Schwinn Continental was stolen in 1976. As I recall they were Dia-Compes. What I recall about them is that they stopped the bike fine. I've never seen weight comparisons, which by rights ought to include all the relevant hardware such as hangers, straddle wires, etc. Also, it's never seemed that there were cantilevers of the same quality as a Campy, Dura Ace or Superbe sidepull. From a design perspective, it seems to me that (1) centerpulls are much more complicated than either single- or dual-pivot sidepulls; and (2) centerpulls are more prone to flex under the force of the rim acting on the brake pads, mainly due to flex in the hanger with the center bolt and pivots. I've never tried centerpulls mounted on braze-ons, which might reduce or eliminate the flex problem. A number of people seem to claim this is the case, including Tony Oliver, Jan Heine and Grant Petersen. They've used 'em and I haven't, so at this point I have to take their word for it. V-brakes were created to benefit the mountain bike industry. The issue was cable routing and simplifying basic setup, which created an economies-of-scale cost savings for manufacturers like Giant, Cannondale, Trek, etc. The need for a cable hanger was problematic for rear suspension and, to a lesser extent, front suspension. Plus they were new and different, and a less-than-astute bike magazine industry mistakenly took V-brakes as a consumer-side improvement rather than a supply-side improvement. There may be some benefits for consumers, of course. It's harder to set up V-brakes wrongly than is the case with cantilevers. The lever feel is different because there is less friction loss with the longer cable pull. The mechanical advantage is slightly higher, trading off modulation for reduced hand grip. There are downsides, including very close tolerances between the rim and the brake, which is a problem if the wheel gets bent or a spoke breaks. And the pads are much thinner, wearing out faster and needing to be replaced more often (also, the thinner pads are much less compressible, which adds to the "power brake" feeling of V-brakes; this is mistakenly perceived as "more stopping power"). Every decent brake- whether sidepull, centerpull, cantilever, V-brake, disc brake, drum brake, what have you- provides enough stopping power to make the wheel skid. In that regard, none of these brakes provide more stopping power than the others. The question of choice really rests on other factors (lever feel, cable routing, clearance, etc). |
#4
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Centerpull brakes
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#5
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Centerpull brakes
Gary Young writes:
Though I've been critical of Grant Peterson's effort to revive centerpull brakes, I have to admit that the latest Rivendell Reader (#30) gave me reasons to pause. I think two things should be noted about all brakes before the dual pivot era, and that is they all had a 1:1 ratio in the "caliper" and a 4:1 ratio in the hand lever, all brakes being interchangeable under any hand lever. This includes sidepull, centerpull and cantilever. The second feature is that sidepull brakes have the pad pivot above and only slightly (rim half width) offset from the braking surface so that there is essentially no position change as the pad sweeps through its wear life (cosine error). Centerpull and cantilever brakes approach the rim at nearly a 45 degree angle and have large vertical change throughout pad wear life, so much so that cantilever brakes have dived under the rim leaving the bicycle with no brake at all. The cantilever dives under but had the advantage of endless mud or radial tire clearance, the centerpull goes into the tire as it wears and offers no advantages whatsoever. I believe that is why it died so quickly as it should have. It was sold on the premise that it had a higher mechanical advantage, something that at first inspection it appears to have through its long levers. They are twice as long as the pad arms... but there are two of them, each receiving half the force. First of all, in the letters section, a reader (Thomas Papetti) points out that dual-pivot sidepulls won't track an out-of-true rim the way all other brake types will. I'm not sure how much that counts in favor of centerpulls (Papetti doesn't discuss them at all). Even though most riders would probably use long-reach dual pivots in lieu of centerpulls, long-reach single-pivot sidepulls and cantilevers are still available if tracking is a significant worry. Then in an article, Jan Heine makes several claims for the superiority of sidepulls. He says, "The long lever on the cable side [above the pivot, if I understand him correctly] and the short lever on the pad side [below the pivot] gives centerpulls a huge mechanical advantage over sidepulls. Just like a bolt-cutter, where long arms and short jaws allow you to cut through thick steel bolts." Later on, he adds that only with the advent of dual-pivot brakes did sidepulls catch up to centerpulls in this respect. That doesn't seem to jibe with Jobst's discussion in the FAQ, which seems to suggest that single-pivot sidepulls and centerpulls had the same mechanical advantage. They had to have the same ME or you couldn't operate them with the same pad clearance. The purpose of the dual pivot is to allow half the pad clearance of former brakes and this required accurate centering. This is necessary to offer the higher mechanical advantage today's avocational riders need to stop their bicycles. Just recall the story bicycle shops had to come up with to explain why riders could not stop their Campagnolo Record equipped bicycles... "These are racing brakes. Racers only need to modulate speed, not stop." and the like. If you believe that you deserve to be led around by the nose. The faster you go the harder you must brake. Descending a mountain pass with straights and hairpin turns requires standing the bicycle on its front wheel into every turn. This is done with two fingers by racers using 4:1 brakes. He argues that only the part of the arm below the pivot needs to be beefy, because that's the only place where flex matters. "As a result, the arms above the pivots can be incredibly skinny, as they don't need to resist flex. That is where the weight savings of a centerpull originate." He doesn't say what the weight savings is. Flex in any part of the system eats up hand lever stroke. Who invents this cock and bull stuff anyway? The lower arm must be stronger because it is loaded in torsion from brake pad drag. He writes, "And since the pivot on centerpull brakes is on the fork leg, adding reach does not increase the flex or change the mechanical advantage. So there is not incentive to go to short-reach brakes." I think he means that since the distance between the pad and the pivot point always remains the same, adding lever length above the pivot doesn't effect the mechanical advantage. That sounds dubious to me -- doesn't changing the length of a cantilever brake's straddle cable effect mechanical advantage even if all else remains the same? Furthermore, from the photos of old centerpulls in the Reader, it doesn't seem as if the distance from pivot to pad was always kept constant (some long-reach brakes seem to achieve that reach in part by increasing the lever below the pivot). These are not people to be believed. They have no idea what they are talking about and do it profusely. It reminds me of the kooks I see at InterBike every year with a new crank mechanism that will make you go faster, not to mention how much mechanism and weight it adds to the bicycle. He notes that centerpulls don't stick out to the side like sidepulls. It seems to me that that problem can be cured by using v-brakes, though that introduces problems of its own (e.g., mating them to road levers). How far??? What is the issue here anyway. Is it perhaps streamlining or just someone who suffers from the "loose ends" syndrome (things that protrude, as in toilet paper rolls that pay off the front instead of the back [hidden ends])? He says that even the standard-reach (what would be called long-reach by most) dual-pivot brakes offered by Shimano don't leave much room for fenders. Is that true? NO. Just how big a tire would you have to run before you would crowd out the dual pivots? (Elsewhere in the Reader, Grant says that centerpulls will clear a 40mm tire or a 38mm tire with fenders; what's the limit with the current long-reach dual pivots?) Furthermore, why not long-reach single-pivots, which are still available? Probably his most controversial claim is that single-pivot sidepulls "offered inferior performance" and became widespread mostly because of Campagnolo's advertising and gruppo packaging. Contrast that with Jobst's claim in the FAQ that, "The centerpull brake of the 1950's, was popular for nearly a decade, in spite of being entirely without merit, being worse in all respects than the side pull brake with which it competed." A conspiracy! The sidepull brake is the obvious mechanism for road bicycles to all who understand mechanical design. All this other stuff is amateur thinking guided by misunderstanding of the concept. Even places like Campagnolo are not immune. After Tullio died the place was run by incompetents who made the Delta brake, a non linear response brake with huge cosine error. It was an Ide? fixe of someone with "loose end" syndrome, externally clean but a mess inside. http://draco.acs.uci.edu/rbfaq/FAQ/8f.15.html In the past, I've been pretty fed up with Grant's talk of reviving centerpulls, because it so often appeared side-by-side with complaints about Rivendell's financial straits and lack of popularity. I do notice a more refreshing tone in this issue: he mentions a bike planned for next year that will have braze-on pivots for centerpulls and adds, "But we don't expect anybody to follow that lead., and we fully expect to sell fewer bikes because of it. Centerpulls, for as much sense as they make, will still scare off most customers." .... and they should. Now let's hear it again concisely. What are the advantages? Jobst Brandt |
#6
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Centerpull brakes
Tim McNamara writes:
Every decent brake- whether sidepull, centerpull, cantilever, V-brake, disc brake, drum brake, what have you- provides enough stopping power to make the wheel skid. In that regard, none of these brakes provide more stopping power than the others. The question of choice really rests on other factors (lever feel, cable routing, clearance, etc). That may be true today but for the older brakes, before dual pivot, it was not. 4:1 leverage was more than a non athletic rider could handle, especially with two fingers. The different ratio hand levers caused problems for which people found solutions. One of these was the Travel Agent, a clever and appropriate name that we don't hear much about now that the dust has settled. http://tinyurl.com/r6la Jobst Brandt |
#7
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Centerpull brakes
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#8
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Centerpull brakes
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#9
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Centerpull brakes
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#10
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Centerpull brakes
Tim McNamara wrote:
From a design perspective, it seems to me that (1) centerpulls are much more complicated than either single- or dual-pivot sidepulls; More so than single-pivot, less so than dual-pivot as the pivots are symmetrical and there is no proportioning mechanism. and (2) centerpulls are more prone to flex under the force of the rim acting on the brake pads, mainly due to flex in the hanger with the center bolt and pivots. If you have developed such an impression, it was probably due to the reach length of any centerpull brakes you have tried being well in excess of whichever sidepull you were using for comparison. Centerpulls locate the mechanical pivots, and their accompanying play, further down the lever arm, for less effect on overall pad deflection at the rim. The "bridge" element containing the pivot studs is typically stouter than the analogous portion of sidepull arm. I've never tried centerpulls mounted on braze-ons, which might reduce or eliminate the flex problem. A number of people seem to claim this is the case, including Tony Oliver, Jan Heine and Grant Petersen. They've used 'em and I haven't, so at this point I have to take their word for it. Brazed-on pivots work. That is the definitive difference between the road bike centerpull as used during the bike boom, and the U-brake as used during the 80s MTB craze. The stopping power of a U-brake dwarfs that of any road bike brake ever made, and U-brakes were no more difficult to set up than a road caliper. Brazing centerpull pivots to the frame usually reduces the overhung length by more than half, and it usually anchors the brake to a stiffer piece of the frame than a centerbolt does. What flex remains can be attenuated by a booster plate, unlike flex that occurs at a centerbolt. Every decent brake- whether sidepull, centerpull, cantilever, V-brake, disc brake, drum brake, what have you- provides enough stopping power to make the wheel skid. --in the rear, if you are a single lightweight rider on a short-wheelbase upright bike without a heavy load. The incapacity of the road caliper brake as furnished to provide more stopping power when appropriate is its most serious shortcoming. The ability of cantilever brakes, U-brakes (and other stud-mounted centerpulls), v-brakes, and hydraulic rim brakes to be set up to deliver more stopping power than most riders require is what makes them versatile and valuable, even if it is of no benefit to most sport riders. Chalo Colina |
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