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#21
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Possible to have brake pads too close to rims?
Phil W Lee writes:
Joe Riel considered Thu, 20 Nov 2014 15:03:18 -0800 the perfect time to write: Phil W Lee writes: Frank Krygowski considered Wed, 19 Nov 2014 18:48:44 -0500 the perfect time to write: On 11/19/2014 5:32 PM, Sir Ridesalot wrote: Okay, you've got very nearly perfectly trued wheels, single pivote calipers brakes and decent quality tires. Braking isn't that great. Flip open the brake caliper quick relese and braking becomes much more effective. Is it possible to have brakes so finely adjusted with the pads so close to the rim that braking ability is degraded whereas moving the pads further from the rim actually improves the braking? I suppose it's possible, but that's in the same sense that "anything is possible." I'm trying to visualize the geometry of different brakes, and wondering if some resulting change in geometry might increase mechanical advantage. So - What model of brakes? What model of brake lever? What type of quick release? That is certainly possible - braking designs have been trying for ages to give geometries which give maximum pad movement (with low mechanical advantage) at the start of lever travel, to move the pads quickly into contact with the braking surface, and lower movement with high mechanical advantage toward the end of lever travel, to maximise force applied between pad and braking surface. In fact, understanding this is the key to getting maximum performance out of cantilever brakes. Similar "rising rate" geometries have been at the heart of vehicle suspension design for many years as well. So it's certainly above the level of "anything is possible". But the Shimano 600 brakes don't have any mechanism to do this. The only clear mechanical difference is that opening the QR slightly increases the leverage, probably not enough to be significant. As has been speculated, the more likely cause is that Sir is more comfortable braking with his hands slightly clenched; that isn't unusual. I find that with the brakes freshly adjusted, to allow for the most wear, braking seems effective. An analysis of the changing mechanical advantage of the hand, as it closes, might be interesting. Check the changing mechanical advantage of the lever first. All of them rotate around a pivot, so the mechanical advantage cannot be linear. The question is, which part of the rotation is used at what point in the application? If the cable is at 90deg to the line from pivot to cable attachment point with the brake fully off, and the lever moves 30deg during application, you just increased the mechanical advantage over it's travel (1deg of movement at that point would move the cable less than it would have done at the beginning). Too late at night to do the trigonometry, but depending on the distance between the pivot and the cable attachment point, it can be a very significant effect on longer lever travels. With short travel, there will be almost no increase in MA over the travel, since there's almost no change in the angle of the relative force vectors. Yes, it changes. 30 degrees would be a large movement. On my bike, the brake lever movement required to initiate braking is about 0.75", with a lever arm of about 3.5", which is 12 degrees. The cos(12 degrees) = 0.89, so the change in leverage could be around 10%, though they actual value depends on details which I didn't have time to measure. Probably the more significant fact is that the change of leverage here is presumably greater than what is achived by the increased lever arm of the open quick release. -- Joe Riel |
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#22
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Possible to have brake pads too close to rims?
On 11/21/2014 9:03 PM, Phil W Lee wrote:
...- a better design for brake levers would have the pivot nearest the little finger, giving maximum effect to the far stronger first two fingers). Seems it would also give more precise control. Fiddlers and guitarists know their ring & little fingers are not only weaker, but clumsier. Reverse-pivot brake levers have existed, both for bicycles and motorcycles. A few are still available, but they're pretty uncommon. Is there some reason they haven't caught on? -- - Frank Krygowski |
#23
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Possible to have brake pads too close to rims?
Phil W Lee writes:
Joe Riel considered Fri, 21 Nov 2014 08:12:53 -0800 the perfect time to write: Phil W Lee writes: Joe Riel considered Thu, 20 Nov 2014 15:03:18 -0800 the perfect time to write: Phil W Lee writes: Frank Krygowski considered Wed, 19 Nov 2014 18:48:44 -0500 the perfect time to write: On 11/19/2014 5:32 PM, Sir Ridesalot wrote: Okay, you've got very nearly perfectly trued wheels, single pivote calipers brakes and decent quality tires. Braking isn't that great. Flip open the brake caliper quick relese and braking becomes much more effective. Is it possible to have brakes so finely adjusted with the pads so close to the rim that braking ability is degraded whereas moving the pads further from the rim actually improves the braking? I suppose it's possible, but that's in the same sense that "anything is possible." I'm trying to visualize the geometry of different brakes, and wondering if some resulting change in geometry might increase mechanical advantage. So - What model of brakes? What model of brake lever? What type of quick release? That is certainly possible - braking designs have been trying for ages to give geometries which give maximum pad movement (with low mechanical advantage) at the start of lever travel, to move the pads quickly into contact with the braking surface, and lower movement with high mechanical advantage toward the end of lever travel, to maximise force applied between pad and braking surface. In fact, understanding this is the key to getting maximum performance out of cantilever brakes. Similar "rising rate" geometries have been at the heart of vehicle suspension design for many years as well. So it's certainly above the level of "anything is possible". But the Shimano 600 brakes don't have any mechanism to do this. The only clear mechanical difference is that opening the QR slightly increases the leverage, probably not enough to be significant. As has been speculated, the more likely cause is that Sir is more comfortable braking with his hands slightly clenched; that isn't unusual. I find that with the brakes freshly adjusted, to allow for the most wear, braking seems effective. An analysis of the changing mechanical advantage of the hand, as it closes, might be interesting. Check the changing mechanical advantage of the lever first. All of them rotate around a pivot, so the mechanical advantage cannot be linear. The question is, which part of the rotation is used at what point in the application? If the cable is at 90deg to the line from pivot to cable attachment point with the brake fully off, and the lever moves 30deg during application, you just increased the mechanical advantage over it's travel (1deg of movement at that point would move the cable less than it would have done at the beginning). Too late at night to do the trigonometry, but depending on the distance between the pivot and the cable attachment point, it can be a very significant effect on longer lever travels. With short travel, there will be almost no increase in MA over the travel, since there's almost no change in the angle of the relative force vectors. Yes, it changes. 30 degrees would be a large movement. On my bike, the brake lever movement required to initiate braking is about 0.75", with a lever arm of about 3.5", which is 12 degrees. 3.5" between cable attachment point and pivot? I'd have thought that something between 1 and 1.5" would be more typical. 3.5" would be more appropriate for the lever arm between the cable attachment point and the mid-point of hand force (which as has been said many times, is the wrong way around, giving highest leverage to the weakest fingers - a better design for brake levers would have the pivot nearest the little finger, giving maximum effect to the far stronger first two fingers). By brake lever I meant the hand lever. 3.5" is that length, 0.75 is how far it travels until the brake pads touch the rim. The reversed brake lever makes sense, ergonomically, but doesn't fit as well, routing-wise, with the standard brake lever position. The cos(12 degrees) = 0.89, so the change in leverage could be around 10%, though they actual value depends on details which I didn't have time to measure. Probably the more significant fact is that the change of leverage here is presumably greater than what is achived by the increased lever arm of the open quick release. -- Joe Riel |
#24
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Possible to have brake pads too close to rims?
On Sat, 22 Nov 2014 02:03:01 +0000, Phil W Lee
wrote: Joe Riel considered Fri, 21 Nov 2014 08:12:53 -0800 the perfect time to write: Phil W Lee writes: Joe Riel considered Thu, 20 Nov 2014 15:03:18 -0800 the perfect time to write: Phil W Lee writes: Frank Krygowski considered Wed, 19 Nov 2014 18:48:44 -0500 the perfect time to write: On 11/19/2014 5:32 PM, Sir Ridesalot wrote: Okay, you've got very nearly perfectly trued wheels, single pivote calipers brakes and decent quality tires. Braking isn't that great. Flip open the brake caliper quick relese and braking becomes much more effective. Is it possible to have brakes so finely adjusted with the pads so close to the rim that braking ability is degraded whereas moving the pads further from the rim actually improves the braking? I suppose it's possible, but that's in the same sense that "anything is possible." I'm trying to visualize the geometry of different brakes, and wondering if some resulting change in geometry might increase mechanical advantage. So - What model of brakes? What model of brake lever? What type of quick release? That is certainly possible - braking designs have been trying for ages to give geometries which give maximum pad movement (with low mechanical advantage) at the start of lever travel, to move the pads quickly into contact with the braking surface, and lower movement with high mechanical advantage toward the end of lever travel, to maximise force applied between pad and braking surface. In fact, understanding this is the key to getting maximum performance out of cantilever brakes. Similar "rising rate" geometries have been at the heart of vehicle suspension design for many years as well. So it's certainly above the level of "anything is possible". But the Shimano 600 brakes don't have any mechanism to do this. The only clear mechanical difference is that opening the QR slightly increases the leverage, probably not enough to be significant. As has been speculated, the more likely cause is that Sir is more comfortable braking with his hands slightly clenched; that isn't unusual. I find that with the brakes freshly adjusted, to allow for the most wear, braking seems effective. An analysis of the changing mechanical advantage of the hand, as it closes, might be interesting. Check the changing mechanical advantage of the lever first. All of them rotate around a pivot, so the mechanical advantage cannot be linear. The question is, which part of the rotation is used at what point in the application? If the cable is at 90deg to the line from pivot to cable attachment point with the brake fully off, and the lever moves 30deg during application, you just increased the mechanical advantage over it's travel (1deg of movement at that point would move the cable less than it would have done at the beginning). Too late at night to do the trigonometry, but depending on the distance between the pivot and the cable attachment point, it can be a very significant effect on longer lever travels. With short travel, there will be almost no increase in MA over the travel, since there's almost no change in the angle of the relative force vectors. Yes, it changes. 30 degrees would be a large movement. On my bike, the brake lever movement required to initiate braking is about 0.75", with a lever arm of about 3.5", which is 12 degrees. 3.5" between cable attachment point and pivot? I'd have thought that something between 1 and 1.5" would be more typical. 3.5" would be more appropriate for the lever arm between the cable attachment point and the mid-point of hand force (which as has been said many times, is the wrong way around, giving highest leverage to the weakest fingers - a better design for brake levers would have the pivot nearest the little finger, giving maximum effect to the far stronger first two fingers). The cos(12 degrees) = 0.89, so the change in leverage could be around 10%, though they actual value depends on details which I didn't have time to measure. Probably the more significant fact is that the change of leverage here is presumably greater than what is achived by the increased lever arm of the open quick release. I'm sitting here with a Dia Comp BRS 202 brake and the distance from the cable attaching clamp to the pivot is 3-1/4". However with a double pivot brake there is also a relationship between the two brake arms that would seem to decrease the effective mechanical advantage so the 3-1/4 distance is likely not the entire picture. As far as I can measure with a 6" steel scale there is no change in brake arm leverage between the normal and released position. simply a difference in effective extension of the cable. However as others have mentioned opening the quick release does move the brake lever toward the handle bar and make it, perhaps, easy to squeeze. -- cheers, John D.Slocomb |
#25
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Possible to have brake pads too close to rims?
Joe Riel wrote:
Phil W Lee writes: Joe Riel considered Fri, 21 Nov 2014 08:12:53 -0800 the perfect time to write: Phil W Lee writes: Joe Riel considered Thu, 20 Nov 2014 15:03:18 -0800 the perfect time to write: Phil W Lee writes: Frank Krygowski considered Wed, 19 Nov 2014 18:48:44 -0500 the perfect time to write: On 11/19/2014 5:32 PM, Sir Ridesalot wrote: Okay, you've got very nearly perfectly trued wheels, single pivote calipers brakes and decent quality tires. Braking isn't that great. Flip open the brake caliper quick relese and braking becomes much more effective. Is it possible to have brakes so finely adjusted with the pads so close to the rim that braking ability is degraded whereas moving the pads further from the rim actually improves the braking? I suppose it's possible, but that's in the same sense that "anything is possible." I'm trying to visualize the geometry of different brakes, and wondering if some resulting change in geometry might increase mechanical advantage. So - What model of brakes? What model of brake lever? What type of quick release? That is certainly possible - braking designs have been trying for ages to give geometries which give maximum pad movement (with low mechanical advantage) at the start of lever travel, to move the pads quickly into contact with the braking surface, and lower movement with high mechanical advantage toward the end of lever travel, to maximise force applied between pad and braking surface. In fact, understanding this is the key to getting maximum performance out of cantilever brakes. Similar "rising rate" geometries have been at the heart of vehicle suspension design for many years as well. So it's certainly above the level of "anything is possible". But the Shimano 600 brakes don't have any mechanism to do this. The only clear mechanical difference is that opening the QR slightly increases the leverage, probably not enough to be significant. As has been speculated, the more likely cause is that Sir is more comfortable braking with his hands slightly clenched; that isn't unusual. I find that with the brakes freshly adjusted, to allow for the most wear, braking seems effective. An analysis of the changing mechanical advantage of the hand, as it closes, might be interesting. Check the changing mechanical advantage of the lever first. All of them rotate around a pivot, so the mechanical advantage cannot be linear. The question is, which part of the rotation is used at what point in the application? If the cable is at 90deg to the line from pivot to cable attachment point with the brake fully off, and the lever moves 30deg during application, you just increased the mechanical advantage over it's travel (1deg of movement at that point would move the cable less than it would have done at the beginning). Too late at night to do the trigonometry, but depending on the distance between the pivot and the cable attachment point, it can be a very significant effect on longer lever travels. With short travel, there will be almost no increase in MA over the travel, since there's almost no change in the angle of the relative force vectors. Yes, it changes. 30 degrees would be a large movement. On my bike, the brake lever movement required to initiate braking is about 0.75", with a lever arm of about 3.5", which is 12 degrees. 3.5" between cable attachment point and pivot? I'd have thought that something between 1 and 1.5" would be more typical. 3.5" would be more appropriate for the lever arm between the cable attachment point and the mid-point of hand force (which as has been said many times, is the wrong way around, giving highest leverage to the weakest fingers - a better design for brake levers would have the pivot nearest the little finger, giving maximum effect to the far stronger first two fingers). By brake lever I meant the hand lever. 3.5" is that length, 0.75 is how far it travels until the brake pads touch the rim. The reversed brake lever makes sense, ergonomically, but doesn't fit as well, routing-wise, with the standard brake lever position. A reversed lever position might actually work well with butterfly handlebars. The cos(12 degrees) = 0.89, so the change in leverage could be around 10%, though they actual value depends on details which I didn't have time to measure. Probably the more significant fact is that the change of leverage here is presumably greater than what is achived by the increased lever arm of the open quick release. |
#26
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Possible to have brake pads too close to rims?
I think it's kind of silly from a practical standpoint to have the brakes so close to the rims that you can hardly get a piece of paper in there. One pot hole and you'll likely be rubbing. Ever ride in the rain? Debris clings to the rims and the brake pads and with tolerances so close, you'll be hearing a lot of debris grinding.
I always adjust my brakes so they have a lot of play. This comes from years of riding hard tail mtn bikes and wanting as much of my hands around the bars while still being able to modulate the brakes. I don't do much mtn biking but still tune my brakes like this. If you're really going to have an OH S*** moment, having a real good grip on the bars and brakes is important. |
#27
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Possible to have brake pads too close to rims?
This is such a problem that on some classes of racing motorcycle, bar end fitments must be used to shield the ends of the brake and clutch levers. I've never heard of a class of motorcycle racing that requires covering the clutch and brake to the bar and I've raced a good bit but not everything. We use "bark busters" where a metal guard goes in to the end of the handle bars and wraps around to the front, often tying in to the triple clamp. These guards usually have some plastic on the front to deflect "roost." The metal bar, know as the bark buster is there to do just that, bust bark and save your fingers while flying down some tight single track. |
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