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Cracked road bike rim

Old 04-21-24, 11:25 PM
  #26  
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Originally Posted by Duragrouch
It's a confluence of bad: Aluminum has a high coefficient of thermal expansion, small diameter rim (20"), so less heat sink, and short spokes, so less elasticity. In my case, also add only using rear brake on that descent, because front rim sidewalls were more worn and I was trying to baby that. A particular small wheel bike company hypes their 20" wheels as "stronger"; Sort of. Stronger rim "hoop strength", so less likely to dent from impact. Short spokes are more rigid, but you don't want that, you want longer and more elastic, that is the driver for double-butted spokes, thinner diameter in the middle, away from bending loads on ends.
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Old 04-24-24, 01:25 PM
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ADAM31: If you are going to stay with your current LBS, I strongly suggest that you educate your self about bike repair, expected component lifespans, sings of trouble, etc. In my estimation, you should not have had to ask us about that rim - it needed to be replaced, period. (Yes, it was safe enough to get you home, but not for going out on another ride. I'm glad your LBS has arranged the appropriate warranty replacement.) Expanding your bike-mechanic knowledge increases you ability to be your own "reality check" when you get advice that sounds sketchy. BF is a great place to learn such things.
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Old 05-11-24, 08:42 AM
  #28  
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Update I got a new rim and I am going to try riding with lower tire pressure as the lbs belies that this may be the cause of cracking my rims as this is the second time in last year I have cracked my rim
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Old 05-12-24, 07:10 PM
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Originally Posted by ADAM31
Update I got a new rim and I am going to try riding with lower tire pressure as the lbs belies that this may be the cause of cracking my rims as this is the second time in last year I have cracked my rim
My usual disclaimer applies, "I could be wrong". And always good to try different things. However, in my mind, I'm not seeing air pressure as the stressor; That is a double-wall rim, so the inner "pointed" wall where the spoke nipple seats, and an outer radiused wall where the tube rests on, and that has somewhat large access holes to be able to access the spoke nipples. The air pressure in the tube exerts force in all directions; Circumferentially, radially outward to support the tire, radially inward against the outer rim wall, and laterally (sideways) outward on each rim sidewall. That last force is the critical one, pushing both sidewalls apart, and it stresses the outer radiused wall in lateral tension, and fatigue cracks happen mostly in tension, sometimes in torsion or shear or bending, almost never in compression. And yes, no cracks in the outer radiused wall. That same lateral outward force that pushes the sidewalls out and tension in the outer radiused wall, puts the inner pointed wall in lateral compression, which normally would not cause a fatigue failure. Only if the lateral compression force were so great as to try to increase the tightness of that pointed bend in the rim (bending is both compression on one side, in this case the inside of that bend, and tension, the outside of that bend where you see the crack), would there likely be fatigue. It's a possibility. But I think that tension from the spokes is a greater factor, although those forces can and do combine. I'll be interested if lower pressure results in longer rim life. I can't recall if the rim is radially spoked, that can require higher spoke tension and thus fatiguing rims quicker. Oh, you said disc wheel, so probably not radially spoked, though I have seen some that are crossed on the drive side and radial on the non drive side, though usually not with discs I think.
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Old 05-13-24, 07:52 AM
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What pressures do you run? I suspect your LBS is not your friend.
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Old 05-13-24, 08:19 AM
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Originally Posted by ADAM31
Update I got a new rim and I am going to try riding with lower tire pressure as the lbs belies that this may be the cause of cracking my rims as this is the second time in last year I have cracked my rim
How come my rims haven't cracked? I suspect I run my tires at a significantly higher pressure than you do.
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Old 05-13-24, 10:52 AM
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Do you know if those rims spec a maximum rated pressure?
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Old 05-13-24, 12:02 PM
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I still say the spokes were most likely improperly tensioned. I find it difficult to believe it was a result of too high pressure. I do understand the theory though, I just don't buy it. You would have to be bashing into curbs and that type of thing for pressure to make a difference, and it would have to be awfully high for it to crack at a spoke. Near impossible due to pressure. The crack would have been on the tire side of the rim, not at the spoke.
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Old 05-13-24, 12:57 PM
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I don't think that's where a rim cracks due to high pressure. There is a max pressure for many rims though.

When the LBS owner said it was safe, he was thinking, "for me." Granted, it's probably not going to cause injury, but it might cause a long walk.
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Old 05-13-24, 10:26 PM
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Originally Posted by ADAM31
Update I got a new rim and I am going to try riding with lower tire pressure as the lbs belies that this may be the cause of cracking my rims as this is the second time in last year I have cracked my rim
Alu rims come in a variety of quality, and wheels in a variety of build care. This and a poorly balanced wheel build, add in some hard roads hits, and a spoke carrying much more load than the ones surrounding it, will pull thru.
If your rim is of decent quality, and you replace instead of buying a new wheel, then make sure you have a wheel builder who really knows how to build a wheel.
Putting the wheel together in some fashion is quite easy, but building that wheel to it's optimum strength and service is a very real skill, a craft... ANd a good builder will know if the rim and hub are worth the effort, expense and service quality.
Wheels and tires are THE contact points with the riding surface, the only contact points... good wheels mean everything, and often they need not be the most expensive...
Ride On
Yuri
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