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#51
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Quill seat posts for MTB?
The Muzi fix is remarkable. We imagine considerable owner handwringing before development.
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#52
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Quill seat posts for MTB?
On 12/7/2016 1:52 AM, Doug Landau wrote:
On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote: On 12/2/2016 9:09 PM, Doug Landau wrote: Why is clamping the seat post with a clamp at the top actually different from clamping the seat post at the bottom? Theoretically at least the seat post is supposed to be sliding fit in the seat tube with minimal play so clamping at one end is very much the same as clamping at the other. - Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it. Google brandt+pretension Certainly the pinch bolt is tightened. But is the pinch bolt, in some manner independent of all else? Just floating there in space absorbing all stresses and strains imposed on it? Or is it simply a clamp that transfers loads imposed on it to the bicycle frame. Is the fact that all seat posts have some minimum insertion limits important? Given that the seat clamp tension bolt "absorbs the tension"? The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post. Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right? I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt. Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube. And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it. A double butted seat tube has a smaller ID at the top, not larger. In traditional format that's 26.8mm post versus 27.2mm -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 How deep does the thicker part at the top? Any dimension you specify to the tube mill, probably. The standard Tange Prestige traditional OD (28.6mm) DB seat tube of 0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm at the (trim this end) top of a 650mm. Champion CrMo touring tubes the same except thicker wall. That said, every brand offers optional length tubes, various materials, diameters, shapes and gauges besides being very welcome to special orders at some healthy manufacturer not hobbyist volume. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
#53
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Quill seat posts for MTB?
On 12/7/2016 7:45 AM, DATAKOLL MARINE RESEARCH wrote:
The Muzi fix is remarkable. We imagine considerable owner handwringing before development. Fix? Women have viewed me as a fixer-upper project all my life. I remain unreconstructed. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
#54
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Quill seat posts for MTB?
On 12/7/2016 10:00 AM, AMuzi wrote:
On 12/7/2016 1:52 AM, Doug Landau wrote: On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote: On 12/2/2016 9:09 PM, Doug Landau wrote: Why is clamping the seat post with a clamp at the top actually different from clamping the seat post at the bottom? Theoretically at least the seat post is supposed to be sliding fit in the seat tube with minimal play so clamping at one end is very much the same as clamping at the other. - Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it. Google brandt+pretension Certainly the pinch bolt is tightened. But is the pinch bolt, in some manner independent of all else? Just floating there in space absorbing all stresses and strains imposed on it? Or is it simply a clamp that transfers loads imposed on it to the bicycle frame. Is the fact that all seat posts have some minimum insertion limits important? Given that the seat clamp tension bolt "absorbs the tension"? The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post. Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right? I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt. Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube. And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it. A double butted seat tube has a smaller ID at the top, not larger. In traditional format that's 26.8mm post versus 27.2mm -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 How deep does the thicker part at the top? Any dimension you specify to the tube mill, probably. The standard Tange Prestige traditional OD (28.6mm) DB seat tube of 0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm at the (trim this end) top of a 650mm. Champion CrMo touring tubes the same except thicker wall. That said, every brand offers optional length tubes, various materials, diameters, shapes and gauges besides being very welcome to special orders at some healthy manufacturer not hobbyist volume. FWIW: Our tandem was custom built (oversized Reynolds 531) back in 1979. Among the things I didn't really like was this: the builder brazed a shim inside the top of each seat tube to reduce the ID to the size of the seatpost. That shim is only an inch or so long (axially within the tube). I was afraid it would give problems with the seatpost rocking, but it never has. He did skimp on the width of the clamping slot at the back of the seat tube, such that the slot closed just before firmly clamping the seatpost. A hard tug on the stoker's handlebars (say, when carrying the bike or loading onto a rack) could slightly pivot the captain's seatpost. I eventually fixed that by widening that slot with a saw. But otherwise, the shim kludge has worked fine. -- - Frank Krygowski |
#55
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Quill seat posts for MTB?
On Wednesday, December 7, 2016 at 10:38:06 AM UTC-5, Frank Krygowski wrote:
On 12/7/2016 10:00 AM, AMuzi wrote: On 12/7/2016 1:52 AM, Doug Landau wrote: On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote: On 12/2/2016 9:09 PM, Doug Landau wrote: Why is clamping the seat post with a clamp at the top actually different from clamping the seat post at the bottom? Theoretically at least the seat post is supposed to be sliding fit in the seat tube with minimal play so clamping at one end is very much the same as clamping at the other. - Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it. Google brandt+pretension Certainly the pinch bolt is tightened. But is the pinch bolt, in some manner independent of all else? Just floating there in space absorbing all stresses and strains imposed on it? Or is it simply a clamp that transfers loads imposed on it to the bicycle frame. Is the fact that all seat posts have some minimum insertion limits important? Given that the seat clamp tension bolt "absorbs the tension"? The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post. Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right? I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt. Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube. And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it. A double butted seat tube has a smaller ID at the top, not larger. In traditional format that's 26.8mm post versus 27.2mm -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 How deep does the thicker part at the top? Any dimension you specify to the tube mill, probably. The standard Tange Prestige traditional OD (28.6mm) DB seat tube of 0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm at the (trim this end) top of a 650mm. Champion CrMo touring tubes the same except thicker wall. That said, every brand offers optional length tubes, various materials, diameters, shapes and gauges besides being very welcome to special orders at some healthy manufacturer not hobbyist volume. FWIW: Our tandem was custom built (oversized Reynolds 531) back in 1979. Among the things I didn't really like was this: the builder brazed a shim inside the top of each seat tube to reduce the ID to the size of the seatpost. That shim is only an inch or so long (axially within the tube). I was afraid it would give problems with the seatpost rocking, but it never has. He did skimp on the width of the clamping slot at the back of the seat tube, such that the slot closed just before firmly clamping the seatpost. A hard tug on the stoker's handlebars (say, when carrying the bike or loading onto a rack) could slightly pivot the captain's seatpost. I eventually fixed that by widening that slot with a saw. But otherwise, the shim kludge has worked fine. -- - Frank Krygowski lot to be said on an adjustment allowance for play when butted hard. |
#56
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Quill seat posts for MTB?
On Wed, 07 Dec 2016 09:00:47 -0600, AMuzi wrote:
On 12/7/2016 1:52 AM, Doug Landau wrote: On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote: On 12/2/2016 9:09 PM, Doug Landau wrote: Why is clamping the seat post with a clamp at the top actually different from clamping the seat post at the bottom? Theoretically at least the seat post is supposed to be sliding fit in the seat tube with minimal play so clamping at one end is very much the same as clamping at the other. - Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it. Google brandt+pretension Certainly the pinch bolt is tightened. But is the pinch bolt, in some manner independent of all else? Just floating there in space absorbing all stresses and strains imposed on it? Or is it simply a clamp that transfers loads imposed on it to the bicycle frame. Is the fact that all seat posts have some minimum insertion limits important? Given that the seat clamp tension bolt "absorbs the tension"? The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post. Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right? I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt. Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube. And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it. A double butted seat tube has a smaller ID at the top, not larger. In traditional format that's 26.8mm post versus 27.2mm -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 How deep does the thicker part at the top? Any dimension you specify to the tube mill, probably. The standard Tange Prestige traditional OD (28.6mm) DB seat tube of 0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm at the (trim this end) top of a 650mm. Champion CrMo touring tubes the same except thicker wall. That said, every brand offers optional length tubes, various materials, diameters, shapes and gauges besides being very welcome to special orders at some healthy manufacturer not hobbyist volume. The Columbus "LIFE" tube set even offers an option of a seat tube that is double butted but the upper end is enlarged on the outside to furnish a straight 27.4mm I.D. nearly the entire length of the tube :-) -- cheers, John B. |
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