4 ms·

Undertensioned wheels will start to fail in about 1-2000k from fatigue in the j bend of the spokes. They're a lot easier to build, because spoke windup and nipp

Undertensioned wheels will start to fail in about 1-2000k from fatigue in the j bend of the spokes. They're a lot easier to build, because spoke windup and nipple galling don't start to be an issue until you get to decent tensions. There are three failure modes that I've seen for an over-tensioned wheel: 1) The wheel will buckle in a potato chip shape - This is generally something that will happen at build time, and can be recovered from. 2) The inner wall of the rim will pull away from the side wall. This was particularly bad with the old Mavic MA single eyelet rims, but I've seen it on at least one other rim as well, a lightweight MTB rim laced as a tandem rear. (Completely inappropriate choice of components) 3) Spokes can be improperly tightened without lubrication in the threads, causing windup which later (when unloaded under tire pressure and rolling loads) causes the pinging sound of the wheel going out of true. This is not a structural failure, but it does cause the wheel to require attention. For the buckling limit case, the ultimate load depends on the torsional stiffness of the rim, and a few other minor factors. Box or closed section rims are much better, U shaped rims are bad. Paradoxically, thinner spokes are better, and you need to lubricate the nipple threads and the contact with the rim. I used to really like the Mavic Open4 rims with the double eyelets, but that's gone out of style now.

jacquesm 5y ago
> Undertensioned wheels will start to fail in about 1-2000k from fatigue in the j bend of the spokes. Yes, I've seen a couple of those, typically mail order or equivalent bikes sold without a mechanic involved in the sales process. Guess who gets to fix them :) > They're a lot easier to build, because spoke windup and nipple galling don't start to be an issue until you get to decent tensions. Lubrication during build is the key I think. > The wheel will buckle in a potato chip shape - This is generally something that will happen at build time, and can be recovered from. Interesting, I've never seen this at build time but I've seen people that build their own wheels crash terribly because of this, so far only (fortunately) rear wheels. Just thinking about having that happen to a front wheel while on the move is enough to give me views of dentist offices. > The inner wall of the rim will pull away from the side wall. This was particularly bad with the old Mavic MA single eyelet rims, but I've seen it on at least one other rim as well, a lightweight MTB rim laced as a tandem rear. (Completely inappropriate choice of components) This I've never seen. > Spokes can be improperly tightened without lubrication in the threads, causing windup which later (when unloaded under tire pressure and rolling loads) causes the pinging sound of the wheel going out of true. This is not a structural failure, but it does cause the wheel to require attention. This is the main thing I've come across frequency wise. I suspect some manufacturers don't particularly care about what they ship as long as the customer pays and it makes it out of warranty. > For the buckling limit case, the ultimate load depends on the torsional stiffness of the rim, and a few other minor factors. Box or closed section rims are much better, U shaped rims are bad. Yes, logical too. Box or closed section rims can be ridiculously stiff. > Paradoxically, thinner spokes are better, and you need to lubricate the nipple threads and the contact with the rim. Can you explain the 'thinner spokes are better' bit? I use thicker spokes on the gear side if a cassette is very high so the wheel is almost flat on one side to get symmetrical tension, but otherwise equal on both sides and thicker for heavier loaded bikes (such as tandems). Would love to understand this bit. > You need to lubricate the nipple threads and the contact with the rim Yes, this is key. You won't be able to get the spokes properly tensioned if you don't do that (or you end up eating up the hole around the spoke nipple). That's a great way to ruin a rim or to end up with busted spoke nipples. Oh, and also important: use the proper key! (not those round ones but the ones that go over and on, obviously sized exactly for the nipples you use).
- wiredfool 5y ago
  I don't think I have pics of the MA, but the tandem rim looked like this: http://static.wiredfool.com/wiredfool/bigRimCrackpt.jpg http://static.wiredfool.com/wiredfool/bigRimCrackpt.jpg The MA had 2cm long cracks in a similar location, maybe gapped 1mm or so, at most of the drive side spokes.
  - jacquesm 5y ago
    Wow that looks bad. Another thing I've seen is rims with the spoke nipples pulling so hard that the rim material was flowing upwards, so you got all these little bulges (they definitely weren't designed like that, not all of the spokes had them).
- davidgould 5y ago
  > Can you explain the 'thinner spokes are better' bit? The rim deflects inward as it contacts the ground. This is controlled by the stiffness of the rim. The deflection untensions the spoke. The cycle of tension/untension as the wheel rotates causes fatigue and also if the tension reduces to near zero allows the nipples to unwind. A thinner spoke elongates more for a given tension, so the rim deflection reduces tension proportionally less. For example, given a thin spoke that elongates 5mm under tension, and a thick spoke that elongates 2mm, a 2mm rim deflection reduces tension 40% on the thin spoke and 100% on the thick spoke. So the thick spoke will fatigue and fail faster. Ideally use the thinnest spokes that can deliver the desired wheel tension. This is not commonly done in cheap wheels because thin spokes have more windup and more spokes interact when truing so the build takes more time and care.
  - wiredfool 5y ago
    Yep, that's it. The key is that spokes never fail from tensile overload (barring something like a branch going through the wheel), they fail from fatigue cracks. A spoke will be unbuildable before it has too little tensile strength.
    - jacquesm 5y ago

jacquesm 5y ago

> Undertensioned wheels will start to fail in about 1-2000k from fatigue in the j bend of the spokes. Yes, I've seen a couple of those, typically mail order or equivalent bikes sold without a mechanic involved in the sales process. Guess who gets to fix them :) > They're a lot easier to build, because spoke windup and nipple galling don't start to be an issue until you get to decent tensions. Lubrication during build is the key I think. > The wheel will buckle in a potato chip shape - This is generally something that will happen at build time, and can be recovered from. Interesting, I've never seen this at build time but I've seen people that build their own wheels crash terribly because of this, so far only (fortunately) rear wheels. Just thinking about having that happen to a front wheel while on the move is enough to give me views of dentist offices. > The inner wall of the rim will pull away from the side wall. This was particularly bad with the old Mavic MA single eyelet rims, but I've seen it on at least one other rim as well, a lightweight MTB rim laced as a tandem rear. (Completely inappropriate choice of components) This I've never seen. > Spokes can be improperly tightened without lubrication in the threads, causing windup which later (when unloaded under tire pressure and rolling loads) causes the pinging sound of the wheel going out of true. This is not a structural failure, but it does cause the wheel to require attention. This is the main thing I've come across frequency wise. I suspect some manufacturers don't particularly care about what they ship as long as the customer pays and it makes it out of warranty. > For the buckling limit case, the ultimate load depends on the torsional stiffness of the rim, and a few other minor factors. Box or closed section rims are much better, U shaped rims are bad. Yes, logical too. Box or closed section rims can be ridiculously stiff. > Paradoxically, thinner spokes are better, and you need to lubricate the nipple threads and the contact with the rim. Can you explain the 'thinner spokes are better' bit? I use thicker spokes on the gear side if a cassette is very high so the wheel is almost flat on one side to get symmetrical tension, but otherwise equal on both sides and thicker for heavier loaded bikes (such as tandems). Would love to understand this bit. > You need to lubricate the nipple threads and the contact with the rim Yes, this is key. You won't be able to get the spokes properly tensioned if you don't do that (or you end up eating up the hole around the spoke nipple). That's a great way to ruin a rim or to end up with busted spoke nipples. Oh, and also important: use the proper key! (not those round ones but the ones that go over and on, obviously sized exactly for the nipples you use).

wiredfool 5y ago
I don't think I have pics of the MA, but the tandem rim looked like this: http://static.wiredfool.com/wiredfool/bigRimCrackpt.jpg http://static.wiredfool.com/wiredfool/bigRimCrackpt.jpg The MA had 2cm long cracks in a similar location, maybe gapped 1mm or so, at most of the drive side spokes.
- jacquesm 5y ago
  Wow that looks bad. Another thing I've seen is rims with the spoke nipples pulling so hard that the rim material was flowing upwards, so you got all these little bulges (they definitely weren't designed like that, not all of the spokes had them).
davidgould 5y ago
> Can you explain the 'thinner spokes are better' bit? The rim deflects inward as it contacts the ground. This is controlled by the stiffness of the rim. The deflection untensions the spoke. The cycle of tension/untension as the wheel rotates causes fatigue and also if the tension reduces to near zero allows the nipples to unwind. A thinner spoke elongates more for a given tension, so the rim deflection reduces tension proportionally less. For example, given a thin spoke that elongates 5mm under tension, and a thick spoke that elongates 2mm, a 2mm rim deflection reduces tension 40% on the thin spoke and 100% on the thick spoke. So the thick spoke will fatigue and fail faster. Ideally use the thinnest spokes that can deliver the desired wheel tension. This is not commonly done in cheap wheels because thin spokes have more windup and more spokes interact when truing so the build takes more time and care.
- wiredfool 5y ago
  Yep, that's it. The key is that spokes never fail from tensile overload (barring something like a branch going through the wheel), they fail from fatigue cracks. A spoke will be unbuildable before it has too little tensile strength.
  - jacquesm 5y ago