Broken spokes

Discussion in 'Cycling Equipment' started by wsommariva, Jul 27, 2010.

  1. wsommariva

    wsommariva New Member

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    Ok, I'm no lightweigt at 255 lbs. I've broken two spokes on my Trek Navagator 2.0 in the last two months. (bike is 13 months old). I just ride on the streets. My LBS says my size should not break spokes. I tend to disagree. Any thoughts?
     
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  2. Froze

    Froze Well-Known Member

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    I more then tend to disagree with your LBS!!

    What wheels are on it now and how many spokes are on the rear? I assume the rear spokes are the ones breaking?

    Are they Bontrager 550's with 36 spokes? They normally would be strong enough but obviously your having problems. The cheapest fix would be to have your current spokes and nipples replaced with DT Alpine III spokes installed with brass nipples and laced 3 cross which is probably the lacing used originally anyways.
     
  3. Peter@vecchios

    [email protected] New Member

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    Breaking spokes means the wheel is sick. The rim is now deformed and the tension erratic. The wheels that came on that bike are not suited for you. They need to at least build you a rear wheel more specific to you and your riding. 36 hole, appropriate rim, double butted spokes, built well.
     
  4. wsommariva

    wsommariva New Member

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    Seems my first response and a post or two have disappeared.

    Yes the rear spokes broke. My bike is stock. My LBS said if it happened again they would replace the wheel under warranty. Good to know that there are other stronger spokes available. The wheels are wide not those thin ones, other than that I don't know what they are. So, hopefully I'll have no more issues; if I do I hope my LBS comes through. Thank you for the advise.
     
  5. Froze

    Froze Well-Known Member

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    Fear not, your LBS will save you. But your wheel is not deformed! If it was a simple thing as a mild deformity then re-tensioning the wheel would fix the problem, rim deformed like a pretzel then no to fixing. Quite simply the spokes were not strong enough to handle the load, and you shouldn't use standard double butted spokes, look into replacing the current spokes with the DT Alpine III's their actually triple butted and their designed specifically for heavy loads that tourist and clydesdales provide.

    More then likely the LBS will get a wheel under warranty from Bontrager (if that's the brand of wheel you have), and that wheel will be the same as you have now and more then likely in a year a rear spoke will break due to the stress placed upon the wheel.

    If they won't or can't build the current rim with the new spokes ask your LBS if they will take the warranty replacement wheel from Bontrager as a trade for a custom built wheel using Sun Rhyno rims, DT Alpine III spokes, brass nipples, and laced 3 cross. And ask them what they think of the wheel I proposed and if they would accept the trade idea. With the trade idea you may have to pay a little bit of out of pocket expense, usually just the labor to build the wheel, but your LBS will determine appropriate charges.
     
  6. Peter@vecchios

    [email protected] New Member

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    No, I don't mean it's a pretzel but 'mildly' deformed resulting in erratic(low) tension in spots that results in broken spokes. A heavier rim is what is needed, not necessarily beefier spokes, IMO, but I think we agree, not the wheel that comes standard on that bicycle.
     
  7. wsommariva

    wsommariva New Member

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    Froze, thank you. I will recommend this to them.
     
  8. dhk2

    dhk2 Active Member

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    Peter, agree it's a better rim and better build that will do the trick, not just heavier straight-gauge spokes. Several people I know on Bontrager wheels have had problems with them. My opinion, perhaps unjustified, is that Trek is putting some pretty cheap and poorly-built wheels on bikes, even expensive Madones.
     
  9. alienator

    alienator Well-Known Member

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    I won't disagree with that sentiment, but then I may be biased. After all, I once had a set of Bontrager Race-X-Lite wheels with the self-destructing rear drive side hub flange. Oh, Keith Bontrager, whither did your paired spokes and all their marketing benefits go?
     
  10. wsommariva

    wsommariva New Member

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    I'll ask for a stronger wheel and stronger spokes.
     
  11. Froze

    Froze Well-Known Member

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    Ok, now I understand what you meant. Supposedly though, according to Bontrager, that the rim should have been strong enough, that's why I suspect the spokes. Regardless if the LBS will replace the rim then going with the Sun Rhyno which a inexpensive but very durable rim, combine that with the DT Alpine III and you got a heck of a strong rim for lower cost then trying to go with Mavic XC 717 for example. Fortunately the OP can get the wheel adjusted under warranty, we'll just have to wait to see if he can get a better, stronger rim with a trade in.
     
  12. dhk2

    dhk2 Active Member

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    Yeah, exploding hub....that's what happened to my buddy the first time he tried to climb a 10% grade with his new Madone. When he brought the bike into the dealer, the LBS was quick to loan him a wheel, said that Trek knew they had a batch of bad wheels and it would be replaced no problem.

    I just "checked" his replacement pair-spoke wheel today on a ride and found it pretty soft laterally. He recently had it rebuilt with new spokes (after two seperate spoke-break incidents. The LBS Trek guy (a solid mechanic) told him he set up the tension on the lower end of spec to avoid the spokes failing. Not impressive for a 160 lb rider after two years (10K miles) IMO. At tleast the wheel does look pretty cool now, with red bladed spokes.

    Not sure that makes me feel very good though.....a big part of what you pay for with Trek and other major brands should be quality control, which is a lot more than "yeah, they all do that, we'll replace it for you". Figure I want to be a QA tester, I can buy bikesdirect frames and wheels a lot cheaper.
     
  13. alienator

    alienator Well-Known Member

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    The hub shell, particularly the flanges, are/were done badly on Bontrager paired spoke wheels. I think that massive slot, into which two spoke heads were placed, was too long for such an unsupported structure. Methinks there should have been individual holes for each spoke head.

    Paired spokes don't really have any engineering upside, except for aerodynamics.
     
  14. tafi

    tafi Member

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    The problem is not the spokes, it is a poor wheel build which causes this problem.

    255lbs = 116kg

    This weight is equivalent to 1137N. Less than the yield point of a single spoke (which is somewhere in the region of 1500N). So you could easily hang all your mass from a single spoke.
    If the spokes could not take the load they would break as soon as you got on the bike, not after 13 months.

    The problem here is fatigue. When weight is applied to the axle of the wheel the tension in the spokes above the hub remain the same whilst the spokes below the hub loose tension. As the wheel rotates the spokes go through successive cycles of loading and unloading and this is what results in breakage.

    The place most likely to see spoke breakage is in the rear non-drive side where spoke tension is lower to begin with and where the spokes may actually slacken completely as they unload.

    This is combined with the fact that the wheels on bikes like the navigator are almost always machine made and not hand made in the factory. From experience of building hybrids (of all brands) from the box, these wheels always have too little and rather uneven tension, which is a recipe for problems of this sort. A lighter person than yourself would still suffer the same problem, it would just take longer to happen. Unfortunately there is often not the time when building bikes in the shop to go through and retension the wheels completely (often this is what is really needed).

    In short there is no such thing as a strong or weak spoke in respect to the loads applied to them, if the wheels are built properly any spoke should be fine.
     
  15. Peter@vecchios

    [email protected] New Member

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    Rim too light, too few spokes, built poorly...three strikes and yer out, the Bontrager.
     
  16. wsommariva

    wsommariva New Member

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    They mentioned tension to me; said it was ok. Said stock spokes should handle my weight. Said if it happened again they will replace the rear wheel. Seems they think the rim is bad.

    I hope I don't break another spoke, but I'm thinkling it will inevitable.

    I'll see what happens when I do my 20 miles this weekend.

    Thank you for all the thoughts and advice.
     
  17. alinewton

    alinewton New Member

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    Spokes will only break due to metal fatigue which makes them brittle. This occurs if the spokes aren't tight enough to stay in tension at all times, and as a result they are allowed to oscillate between tension and compression as the wheel turns.

    If it is an expensive wheel then have it completely respoked and then retensioned after a short while of use and then periodically thereafter. If it is a cheap wheel then buy a new one, but again have the spokes retensioned after a couple of weeks.
     
  18. dhk2

    dhk2 Active Member

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    tafi, 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.

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

    wsommariva New Member

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    Trek makes a tandem. How about if I ask that they replace my wheel with a tandem wheel. I ride for exercise only, don't care about bike weight.

    And I do love my bike.
     
  20. alienator

    alienator Well-Known Member

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    You're right. The majority of spokes fail because of cyclic unloading and loading of the spokes. When a wheel hits a bump, the spokes around where the bump impact is located unload some--or completely in a poorly built wheel or massive bump--and then load again as the rim rebounds to its original shape. The properties of concern are stress and strain in the spoke. Cycling of stress leads to failure and stress is linearly related to the cross-sectional area of a spoke. Double butted spokes work by moving the majority of the stress away from places where stress risers are, namely the spoke elbows.
     
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