Broken spokes

[tafi]

Just discussing fine points here: Peter summed up the OPs problem well.

While we are on the subject of finer points, we may as well do it properly.

appreciate that you included some figures on load and yield strength of a spoke in your discussion. But to consider only static loads doesn't seem sufficient, since real-world bumps can put several g's of load into the wheels. Thus the wheel in your example that's so slack as to put all the load on one spoke, would likely have a spoke failure in very short order after just a few bumps or minor potholes in the road.

Agree that the issue isn't straight-gauge vs DB spokes causing the weak wheel. But question your conclusion that all spokes are the same in load-carrying and thus wouldn't affect the durability of a well-built wheel. Seems to me that a 14 ga spoke (or 14/15/14 DB) will be stronger than a 15/16/15 in terms of ultimate yield strength, and thus have a longer fatigue life, assuming the quality of the spoke materials and processing equal.

Anyone who has read any literature on bicycle wheels as pre-stressed structures is aware of the fact that bike wheels accept loads (static or dynamic) by decreasing the tension of the lower spokes and not by increasing the tension in the higher spokes. Several engineering studies (using actual wheels and also computerised finite element modelling) have been conducted on this and all have shown that the spokes at the top of the wheel do not change in tension when the wheel is loaded. The spokes at the bottom UNLOAD. To say that load results in increases in tension is incorrect.

A useful reference for this is Burgoyne and Dilmaghanian, Journal of Engineering Mechanics, Vol 119, No 3. 1993 and Jobst Brandt’s The Bicycle Wheel also gives a good description based on sound Engineering principles of structures.

Given that spokes will never increase in tension beyond the pretension applied when building, it is entirely accurate to say that a spoke is strong enough if it can support the rider’s weight on its own. If a bump were to increase the tension to the point of failure, it would occur at the first such bump and not 10 months down the track.

The main cause of spoke failure, as I said above, is invariably fatigue. This is due to the persistent cyclic unloading and reloading of spokes under the rider’s weight whilst riding. It is not due to occasional loads from bumps. Spoke failure after 10 months of use is a classic indication of a premature fatigue failure. Premature fatigue failure is usually due to insufficient tension in building and/or stress raising features on the spokes. The double butted spoke actually performs better in fatigue than plain gauge as the narrowed shaft helps concentrate stresses there, and away from the stress raising features of the thread and elbow, where fatigue failures are common. At any rate I was never arguing for or against a particular spoke. Built properly any type of spoke would be better than the OP's current situation.

If you want to know more about fatigue, I would recommend reading a book on engineering materials or solid mechanics.

I raised the issue of insufficient tension because it is a persistent problem with prebuilt (machine made) wheels of the type which is at issue here (who manufactured them doesn’t tend to matter), and time isn’t always available at dealer shops to tension the wheels properly as they come out of the box. Human wheel-builders all know, or are taught, that sufficient tension is a good thing.

 

[Froze]

That is why I recommended the DT Alpine III's Tafi because of all the science and reasons you gave. I also think that a tandem wheel would be a great idea as Wsommariva said, it may be an overkill but it would probably never fail. The only "problem" with the tandem wheel is the cost, while their not outrageously priced, it's something the OP needs to check into if he considers going that route.

By the way, the warranty on the old wheel was a year?. Warranties are funny things, if the wheel is replaced under warranty with the exact same wheel as before, then the warranty time period only covers from the original purchase date of the first wheel...You don't get a new 1 year warranty with the new wheel. Thus if this new wheel doesn't work out and fatigues out in 8 to 12 months you will be paying the full cost of a new wheel. I think you need to check to make sure the warranty will extend an additional 12 months, OR, do the swap I was telling you to do now so you don't have a problem down the road and have no warranty.

 

[wsommariva]

I just broke my third spoke. I heard it snap. I just finished a fast downhill and started to climb the next hill. Put alot of pressure on the gears and it popped. Off to the LBS. I'd rather have better/stronger spokes properly installed than a new wheel. I'll let you know what they suggest.

 

[wsommariva]

Just back from the LBS. They replaced the rear wheel with a Weinmann Zap 19. Original was a matrix 550. Spokes are the same size at 2 mm.

No charge even though the warranty ran out in June.

The tech mentioned that tension could have been a problem, so I assume he checked tension on the new wheel.

Hopefully no more problems. Thanks for the input.

 

[Froze]

Just back from the LBS. They replaced the rear wheel with a Weinmann Zap 19. Original was a matrix 550. Spokes are the same size at 2 mm.

No charge even though the warranty ran out in June.

The tech mentioned that tension could have been a problem, so I assume he checked tension on the new wheel.

Hopefully no more problems. Thanks for the input.

Why did they replace with that wheel? That's a very inexpensive rim, they sell for as much as $75 and some times on e-bay they get no more then $32 with Alivio MC12 hubs and 14 gauge spokes. Their not particularly stout, thus perhaps not a whole lot better then what you had! I have those on my Giant, while I haven't had any issues but I only weigh 165; other's I've heard that weigh over 200 have had problems.

Again their just straight gauge spokes and not double butted or triple butted like the Alpine III's, thus straight gauge spokes don't handle the load as well as others so scientifically written about.

Did the LBS give you a new warranty on the Weinmann's? If so, then use them and see what happens. But do yourself a favor after about 30 days of riding take the bike back in and have the BOTH rims re-tensioned and trued, then about 3 months later do it again.

Hopefully these will hold up for you, but I'm not optimistic.

 

[wsommariva]

They thought that it was a bad bunch of spokes installed on original wheel as they said the tension was good. I guess it was easier to replace the wheel than the spokes. As long as the wheel operates as it should I'm ok with what they did.

I'm 54 and remember my Schwinn that my dad bought me 40 years ago. My sister still rides it with all originl equipment except tires, tubes, etc. Today things are not made as well IMHO.

Regardless, I love the bike and need it to perform well for many years. If I break another spoke I will have to go with a tandam wheel or a stock wheel with the better spokes.

Question about re-tensioning and truing. How many miles between this service should I wait. I ride only 20-30 miles per week.

 

[dhk2]

You really shouldn't need any retruing or retensioning of a well-built wheel, at least not for a long time. My current wheels have 26K miles on them, and have never been touched. Once wheels start coming out of true, my experience has been that they are just about at the end of their useful life.

Suggest you start thinking about wheels as a "system". Sure, most are built from seperate hubs, rims and spokes, but if the wheel system is properly designed and built for your weight, you shouldn't have to worry about it for many years (barring major potholes or crashes). Once the wheel comes out of true, or spokes start breaking, the best thing to do is generally to replace the wheel, or have a new wheel built on your old hub.

Actually, I'd say most bike stuff is built to higher quality standards today than it was decades ago. The quality of components on my 1973 Raleigh GS, or the French bikes from that era, left a lot to be desired. The problem with today's equipment is that everything is designed to be as light as possible, because manufacturer's have learned that lightweight sells, and that they can afford the warranty cost because most buyers don't ride their new bikes enough to break stuff anyway.

 

[Froze]

Normally a well built wheel shouldn't have to be retensioned, this is true. HOWEVER, due to the load placed on the wheel and not knowing for sure if it was built right from the factory, it's best to follow a path of caution as I stated earlier. Look, what will it hurt for you do that? A little time? but in return you gain a lot of piece of mind. The LBS should do it for free knowing the situation.

I had a very good wheel builder build my Torelli Master Series rims on the Trek and he wanted to see the wheels again after 2 weeks to make sure everything was ok because I was placing demands on the wheels like racing and mountains. I still have those wheels, they have over 45,000 miles on them and rarely do I have to true them and then it's just very slightly.

 

[wsommariva]

Yes I guess it's the balance of lightweight and strength. Tandem wheel seems like a good alternative to me. But maybe my new wheel will last many years. And IF I lose some weight, well, we can only hope. Anyway case closed for now. Thanks to everyone

 

[wsommariva]

Two tuff rides 10 miles each over the weekend and no problems with the new wheel.

Question. My max tire pressure I think is 50psi. What psi should I ride with to ensure good wheel life? Remember, I'm no speed guy, just like good tuff workouts up to 20 miles.

 

[dhk2]

While we are on the subject of finer points, we may as well do it properly.



Anyone who has read any literature on bicycle wheels as pre-stressed structures is aware of the fact that bike wheels accept loads (static or dynamic) by decreasing the tension of the lower spokes and not by increasing the tension in the higher spokes. Several engineering studies (using actual wheels and also computerised finite element modelling) have been conducted on this and all have shown that the spokes at the top of the wheel do not change in tension when the wheel is loaded. The spokes at the bottom UNLOAD. To say that load results in increases in tension is incorrect.

A useful reference for this is Burgoyne and Dilmaghanian, Journal of Engineering Mechanics, Vol 119, No 3. 1993 and Jobst Brandt’s The Bicycle Wheel also gives a good description based on sound Engineering principles of structures.

Given that spokes will never increase in tension beyond the pretension applied when building, it is entirely accurate to say that a spoke is strong enough if it can support the rider’s weight on its own. If a bump were to increase the tension to the point of failure, it would occur at the first such bump and not 10 months down the track.

The main cause of spoke failure, as I said above, is invariably fatigue. This is due to the persistent cyclic unloading and reloading of spokes under the rider’s weight whilst riding. It is not due to occasional loads from bumps. Spoke failure after 10 months of use is a classic indication of a premature fatigue failure. Premature fatigue failure is usually due to insufficient tension in building and/or stress raising features on the spokes. The double butted spoke actually performs better in fatigue than plain gauge as the narrowed shaft helps concentrate stresses there, and away from the stress raising features of the thread and elbow, where fatigue failures are common. At any rate I was never arguing for or against a particular spoke. Built properly any type of spoke would be better than the OP's current situation.

If you want to know more about fatigue, I would recommend reading a book on engineering materials or solid mechanics.

I raised the issue of insufficient tension because it is a persistent problem with prebuilt (machine made) wheels of the type which is at issue here (who manufactured them doesn’t tend to matter), and time isn’t always available at dealer shops to tension the wheels properly as they come out of the box. Human wheel-builders all know, or are taught, that sufficient tension is a good thing.

Sorry I'm late in replying; for some reason missed your detailed response. Appreciate your discussion of the primary mechanism for spoke fatigue; certainly I was wrong in thinking the "opposite" upper spoke carried a significant addition load. The correct explanation then must be that as the wheel rim deforms slightly when it contacts the road under weight of the bike/rider, the unloading of single spokes would be a much larger cyclic variation than the resulting additional loads shared by the remaining spokes.

I don't have any textbooks on fatigue as I never studied it formally (as you've already correctly concluded). However, I do own the book Bicycling Science by David G. Wilson, which does contain a brief discussion of spoke fatigue, even referencing the Jobst Brant book and 1993 paper by Burgoyne and Dilmaghanian you mentioned. However, Prof Wilson suggests that LCF resulting in severe unloading from bumps and other events that occur "only" hundreds or thousands of times in the life of a wheel is likely is the key contributor, not smooth road HCF cycles. "In our case, the factor of 3x suggests that the typical smooth-road load is essentially irrelevant to spoke life". He does footnote the sentence to say that a specific fatigue curve would be needed to "be more certain of this conclusion".

I'm just parroting what I've read and certainly not equipped to argue the issue of LCF vs HCF with you. However, based on the text I've quoted, it appears that it's possible both types could contribute to spokes failures.

 

[Froze]

Tire pressure isn't going to effect wheel life, but it will effect tire life. I would say at your weight level you should run 55psi in the rear and 50 on the front. I know you said 50 was the max, but that is a safe margin, for heavier loads then the stated max was intended to handle, which is usually 220 pounds, you can easily go another 10% above stated max without any issues with tire.

 

[wsommariva]

Thank you. Max in the front and max plus 10% in rear.