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08-04-18, 06:15 AM
#26
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So I take it that the SunTour pulleys from the same era don’t have this problem? I’m not buying the shrinkage theory.
08-04-18, 10:40 AM
#27
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But the fact remains that dark gray Campagnolo pulley wheels of this type are often found in this cracked condition, even NOS ones. I have two or three in the bin. I have seen other brand pulleys go bad in a variety of ways, but this radial crack is, in my experience, unique to this version of dark gray Campagnolo pulley wheels.
Last edited by rhm; 08-04-18 at 12:07 PM.
08-04-18, 10:53 AM
#28
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FWIW 10t Tacx pulleys are good , their nylon, chain passes over them quieter than a harder plastic.
and certainly quieter than metal ..
They're stock on R'off chain tensioners,
and certainly quieter than metal ..
They're stock on R'off chain tensioners,
08-04-18, 06:13 PM
#29
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We should remember how long it has been since the tooling and processes for making Capagnolo's pulleys was created.
I've designed plastic parts that were injection-molded, and it seems that, when choosing from the plastics that had the highest strength and/or temperature resistance, that the process of injection molding parts was a challenge because of the higher viscosity of the resin.
When injecting higher-viscosity resins, there is some tendency for a sort of skin to form on the advancing flow, so where two surfaces come together the surfaces don't exactly meld together as a liquid, but instead act like a bonding surface, along which strength is lower. Here is where a failure may later occur due to shrinkage from aging and/or from the initial cooling process leaving tension stress in the part around any metal bushing (polymers contract radically more than metals during the cooling process, leaving residual tension stresses).
Newer polymers arrive periodically which improve the compromise between molding ease and the properties of the finished product. But back in the day it was harder to make good parts our of plastic. Campy's pulleys were still world's stronger than those made by Simplex, but Huret perhaps achieved the best of two worlds with their adjustable ball bearing pulleys having extremely tough plastic teeth molded on to a metal core.
I've designed plastic parts that were injection-molded, and it seems that, when choosing from the plastics that had the highest strength and/or temperature resistance, that the process of injection molding parts was a challenge because of the higher viscosity of the resin.
When injecting higher-viscosity resins, there is some tendency for a sort of skin to form on the advancing flow, so where two surfaces come together the surfaces don't exactly meld together as a liquid, but instead act like a bonding surface, along which strength is lower. Here is where a failure may later occur due to shrinkage from aging and/or from the initial cooling process leaving tension stress in the part around any metal bushing (polymers contract radically more than metals during the cooling process, leaving residual tension stresses).
Newer polymers arrive periodically which improve the compromise between molding ease and the properties of the finished product. But back in the day it was harder to make good parts our of plastic. Campy's pulleys were still world's stronger than those made by Simplex, but Huret perhaps achieved the best of two worlds with their adjustable ball bearing pulleys having extremely tough plastic teeth molded on to a metal core.
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