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#11
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spoke length problem
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#12
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spoke length problem
On 11/30/2017 3:23 PM, Frank Krygowski wrote:
On 11/29/2017 11:22 PM, jbeattie wrote: On Wednesday, November 29, 2017 at 7:58:57 PM UTC-8, Frank Krygowski wrote: Warning: Actual bike tech content below. I'm building up an antique bike for a kid. After a false start with the original rims (very light, but in a very obscure size) I ended up replacing the wheels with 20" Sun rims on hubs that I had lying around: a Campy front hub, and a Shimano 3CC 3 speed rear hub with coaster brake. I measured both hubs and used Sun's online ERD value of 396mm for the rim. I plugged dimensions into three different online spoke calculators. It looked like I could use the same spoke length front and rear if I went 2x front and 3x rear. Here were the calculator results: Sapim said front 189mm, rear 188mm. DT said 186 front, 186 rear. United Bicycle Institute said 185.6 front, 185.1 rear. So I ordered 14 gauge Sapim spokes, 186 front and rear. The front wheel built up perfectly, as usual. But the rear wheel won't tension up. The spoke threads are bottoming. I just double checked my hub dimensions. They seem to be spot on. And again, everything was perfect in front. The spokes measure 186 with 10mm of thread. The nipples are 16mm long with an unthreaded 5mm deep hole at the end that slides over the spoke. I can think of several possible solutions: 1) buy shorter spokes. 2) sacrifice a few nipple threads by drilling into the end of the nipple to counterbore another 2mm or so. It's for a little kid, and I doubt he'll be stressing the wheel very much. 3) acquire a die and extend the spoke threads. The cut threads won't have a full thread form, but there should be plenty of strength in the existing threads. 4) put washers under the spoke heads. It's a double wall rim so there's room in there. 5) and next time, maybe go 2mm shorter than the calculators say? But why? These spokes aren't stretching. I can't get any tension at all. I'm curious if anyone's had a similar problem. If so, did you determine what caused the error? I'm prepared to smack my forehead if necessary. They're too long, obviously. Is lacing 4X a possibility? Otherwise, you buy new spokes. You could also find a shop with a Phil or other spoke cutter. 4x won't work. That would require roughly 195mm spokes. I happen to have a set of 170mm spokes (intended for the original odd rims) and the calculators tell me x2 would need 177mm. So I'm assuming 170 is too short. I'll see if some shorter nipples might work. If not, I'll probably go with washers under the spoke nipple heads. I'm more interested in knowing what might have gone wrong. I've never had this problem occur before. I just tried measuring the ERD myself, using a tape measure to get circumference of the rim tape's surface (1260mm) and a metric vernier caliper to get the "drop" from that surface to the face of the hollow rivets on which the spoke head rests (3.8mm). Circumference divided by pi minus (drop x 2) gives me 393.5mm. From https://www.sheldonbrown.com/spoke-length.html I get the advice to add a couple millimeter for the thickness of each nipple head, which gives me ERD = 397.5mm. That compares well with the online catalog value of 396, so I'm still baffled. Our calculator gives 185 left & right for that rear wheel. Sorry but I didn't see any obvious error. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
#13
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spoke length problem
On 11/30/2017 5:17 PM, jbeattie wrote:
On Thursday, November 30, 2017 at 11:37:28 AM UTC-8, wrote: All my experience is with buying too short spokes. I use the calculators to figure the right length. Then always buy a spoke 2-3-4 mm shorter figuring the calculators are over generous and the spokes will stretch when tightened. Went a bit overboard on this on one set of wheel and ended up with lots of threads showing below the nipple. In your situation the other suggestions of new spokes or cutting longer threads on the existing spokes is the best method. But just using washers under the nipples inside the rim would be the easiest fix and still be fine and dandy. Washers are so trailer. It is odd that the length was fine for the front but not the rear. The obvious explanation would be a mistaken flange height/spoke hole measurement. The ERD was correct for the front so it should be correct for the rear, assuming the as-manufactured rim is not out of spec. I assume Frank put the right number of spoke holes into the calculator. The only variable in the equation is the hub. No? Yes, everything was double checked, then double checked again when things didn't fit. I'm using both dial calipers and vernier calipers on the hub dimensions. It occurs to me that the front wheel barely tensioned up. I don't think I could have gotten another millimeter worth of turns out of those nipples. I suspect that the rim ERD is a bit undersized. That's the most difficult measurement to make, and I know from stories of super-tight tires that rim diameters are not super precise. (The front tire was certainly easy to mount, no tire levers required.) The most exact way of measuring the ERD directly would require stripping all the spokes out again, then precisely measuring the diameter using a couple spokes and a caliper. But I don't plan on going through that trouble. It's also interesting to me that the various spoke length calculators don't precisely agree. Four different calculators gave 188, 186, 185.1 and 185.3. At its heart, the calculation is just three dimensional Pythagorean theorem. The variations indicate some assumptions that are hidden and different. For kicks, I used Sketchup (a three dimensional drafting program) to draw a simple model of the hub and rim. (Details on request. It's overkill for standard spoking patterns, but could be handy for spoking oddball wheels.) I drew in a couple spokes with the proper number of crosses and measured a spoke length of 186.1mm. That's straight line point-to-point geometry, and it certainly confirms the 186mm spokes I bought. Too bad they don't fit. This afternoon I dug in my junk piles and found some shorter spoke nipples that thread a millimeter further onto the 186 spokes. I think those plus washers should fix things. At least washers in a rim are less visible than hose clamps on a stem! ;-) -- - Frank Krygowski |
#14
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spoke length problem
On 11/30/2017 6:07 PM, AMuzi wrote:
On 11/30/2017 3:23 PM, Frank Krygowski wrote: On 11/29/2017 11:22 PM, jbeattie wrote: On Wednesday, November 29, 2017 at 7:58:57 PM UTC-8, Frank Krygowski wrote: Warning: Actual bike tech content below. I'm building up an antique bike for a kid. After a false start with the original rims (very light, but in a very obscure size) I ended up replacing the wheels with 20" Sun rims on hubs that I had lying around: a Campy front hub, and a Shimano 3CC 3 speed rear hub with coaster brake. I measured both hubs and used Sun's online ERD value of 396mm for the rim. I plugged dimensions into three different online spoke calculators. It looked like I could use the same spoke length front and rear if I went 2x front and 3x rear. Here were the calculator results: Sapim said front 189mm, rear 188mm. DT said 186 front, 186 rear. United Bicycle Institute said 185.6 front, 185.1 rear. So I ordered 14 gauge Sapim spokes, 186 front and rear. The front wheel built up perfectly, as usual. But the rear wheel won't tension up. The spoke threads are bottoming. I just double checked my hub dimensions. They seem to be spot on. And again, everything was perfect in front. The spokes measure 186 with 10mm of thread. The nipples are 16mm long with an unthreaded 5mm deep hole at the end that slides over the spoke. I can think of several possible solutions: 1) buy shorter spokes. 2) sacrifice a few nipple threads by drilling into the end of the nipple to counterbore another 2mm or so. It's for a little kid, and I doubt he'll be stressing the wheel very much. 3) acquire a die and extend the spoke threads. The cut threads won't have a full thread form, but there should be plenty of strength in the existing threads. 4) put washers under the spoke heads. It's a double wall rim so there's room in there. 5) and next time, maybe go 2mm shorter than the calculators say? But why? These spokes aren't stretching. I can't get any tension at all. I'm curious if anyone's had a similar problem. If so, did you determine what caused the error? I'm prepared to smack my forehead if necessary. They're too long, obviously. Is lacing 4X a possibility? Otherwise, you buy new spokes.Â* You could also find a shop with a Phil or other spoke cutter. 4x won't work. That would require roughly 195mm spokes. I happen to have a set of 170mm spokes (intended for the original odd rims) and the calculators tell me x2 would need 177mm. So I'm assuming 170 is too short. I'll see if some shorter nipples might work. If not, I'll probably go with washers under the spoke nipple heads. I'm more interested in knowing what might have gone wrong. I've never had this problem occur before. I just tried measuring the ERD myself, using a tape measure to get circumference of the rim tape's surface (1260mm) and a metric vernier caliper to get the "drop" from that surface to the face of the hollow rivets on which the spoke head rests (3.8mm). Circumference divided by pi minus (drop x 2) gives me 393.5mm. From https://www.sheldonbrown.com/spoke-length.html I get the advice to add a couple millimeter for the thickness of each nipple head, which gives me ERD = 397.5mm. That compares well with the online catalog value of 396, so I'm still baffled. Our calculator gives 185 left & right for that rear wheel. Sorry but I didn't see any obvious error. Thanks for checking. At least I don't have to slap my forehead just yet. -- - Frank Krygowski |
#15
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spoke length problem
On 11/30/2017 5:15 PM, Frank Krygowski wrote:
On 11/30/2017 6:07 PM, AMuzi wrote: On 11/30/2017 3:23 PM, Frank Krygowski wrote: On 11/29/2017 11:22 PM, jbeattie wrote: On Wednesday, November 29, 2017 at 7:58:57 PM UTC-8, Frank Krygowski wrote: Warning: Actual bike tech content below. I'm building up an antique bike for a kid. After a false start with the original rims (very light, but in a very obscure size) I ended up replacing the wheels with 20" Sun rims on hubs that I had lying around: a Campy front hub, and a Shimano 3CC 3 speed rear hub with coaster brake. I measured both hubs and used Sun's online ERD value of 396mm for the rim. I plugged dimensions into three different online spoke calculators. It looked like I could use the same spoke length front and rear if I went 2x front and 3x rear. Here were the calculator results: Sapim said front 189mm, rear 188mm. DT said 186 front, 186 rear. United Bicycle Institute said 185.6 front, 185.1 rear. So I ordered 14 gauge Sapim spokes, 186 front and rear. The front wheel built up perfectly, as usual. But the rear wheel won't tension up. The spoke threads are bottoming. I just double checked my hub dimensions. They seem to be spot on. And again, everything was perfect in front. The spokes measure 186 with 10mm of thread. The nipples are 16mm long with an unthreaded 5mm deep hole at the end that slides over the spoke. I can think of several possible solutions: 1) buy shorter spokes. 2) sacrifice a few nipple threads by drilling into the end of the nipple to counterbore another 2mm or so. It's for a little kid, and I doubt he'll be stressing the wheel very much. 3) acquire a die and extend the spoke threads. The cut threads won't have a full thread form, but there should be plenty of strength in the existing threads. 4) put washers under the spoke heads. It's a double wall rim so there's room in there. 5) and next time, maybe go 2mm shorter than the calculators say? But why? These spokes aren't stretching. I can't get any tension at all. I'm curious if anyone's had a similar problem. If so, did you determine what caused the error? I'm prepared to smack my forehead if necessary. They're too long, obviously. Is lacing 4X a possibility? Otherwise, you buy new spokes. You could also find a shop with a Phil or other spoke cutter. 4x won't work. That would require roughly 195mm spokes. I happen to have a set of 170mm spokes (intended for the original odd rims) and the calculators tell me x2 would need 177mm. So I'm assuming 170 is too short. I'll see if some shorter nipples might work. If not, I'll probably go with washers under the spoke nipple heads. I'm more interested in knowing what might have gone wrong. I've never had this problem occur before. I just tried measuring the ERD myself, using a tape measure to get circumference of the rim tape's surface (1260mm) and a metric vernier caliper to get the "drop" from that surface to the face of the hollow rivets on which the spoke head rests (3.8mm). Circumference divided by pi minus (drop x 2) gives me 393.5mm. From https://www.sheldonbrown.com/spoke-length.html I get the advice to add a couple millimeter for the thickness of each nipple head, which gives me ERD = 397.5mm. That compares well with the online catalog value of 396, so I'm still baffled. Our calculator gives 185 left & right for that rear wheel. Sorry but I didn't see any obvious error. Thanks for checking. At least I don't have to slap my forehead just yet. Because Shimano tricoaster and the trig are well known, I suspect the actual ERD is not what we assume. There are several rims whose dies were changed over the years with various retoolings. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
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