Singlespeed & Fixed Gear - spokes breaking at the hub... grrr...

Soo... I just snapped my second spoke on my front wheel in 4 months. Pulled into the alley heading home at very low speed and *ping*. I know the wheels (http://somafab.com/somax.html) ain't the greatest, and I ain't the smallest guy, but it's pretty annoying.

Are the stresses greater at the hub flange with high flanges or it is just a case if a bigger guy and sub-par wheels? Do deeper profile rims contribute to the problem somehow?

It sounds to me like you have a machine-built set of wheels which have insufficient tension.

If the wheel hasn't been tensioned and stress-relieved correctly, it might be rideable†, but on each revolution, the spokes at the bottom or the wheel will be detentioned to the point that they will be loose, and will move, and then as the wheel rotates, the spokes will be under tension again, and so on and so on. This puts enormous stress on the spokes and they will break sooner rather than later, usually at the elbow as that is where most of the stress occurs.

A properly tensioned wheel will have the spokes under sufficient tension so that when the wheel turns, those spokes at the bottom will be detensioned, but not enough so that they move.

Wheels can be built by machines, but for a good wheel, the final stages of truing, tensioning and stress-relieving have to be done by hand. I recently bought a "hand-built" set of wheels for my MTB - I noticed when I checked, that the tension on each spoke was about a half of what I would have expected. I'm sure they would have ridden OK, but I'll bet the spokes wouldn't have lasted very long - so much for "hand-built" wheels from eBay… ;)

- Wil

† I've no idea as to your weight - if you're a Clydesdale, the spokes would tend to break quicker than if you were a skinny 120 lb lightweight riding the same set of wheels…

What's the best way of tensioning a wheel or making sure it's tensioned properly?

-a Clydesdale :)

take em into the LBS and have them throw the wheels on the stand and check them with a really expensive tool.

it might cost you $5, you might get it for free if you bring beer for the wrench.

tool's not that expensive. The park one works well and is worth its weight in gold...especially if you are buildingtruing more than one set of wheels in the next 20 years.

Nice asterisk/cross-looking thing. Those don't get used enough these days.

For the sake of curiousity I started 'twanging' the spokes on the one new rim. Some were very noticeably loose. Glad I read this before putting them on - when the second one comes in I think I'm going to let my lbs true & tension it and then record the pitch. Hopefully they'll let me watch (they do have an open work area). I also have an extra rim that I could practice on without worrying about destroying a fully functional one.

you want somewhere around 70nm if you got a tensionmeter.
take the readings off 10 spokes and figure out the average of em'.

and just a note. machine built wheels are always only about 70% tensioned.
I would suggest getting your LBS to always bring your machine built wheel to the correct tension when you buy it.

Spokes almost always break at the flange, and flange size doesn't have much to do with it. Spokes start out as just wire, so they're going to break where they've been bent and then put over a stress riser (the flange). Breaking fronts is weird, though, I would retension as others have suggested.

Great advice, thanks!

you want somewhere around 70nm if you got a tensionmeter.
take the readings off 10 spokes and figure out the average of em'.

and just a note. machine built wheels are always only about 70% tensioned.
I would suggest getting your LBS to always bring your machine built wheel to the correct tension when you buy it.

70 is a bit low for many applications (and riders). When I'm building I generally aim for about 80-90 for a box front, 90-100 for a V front. 100-110 for a box rear, 110-120 for a V rear. When spokes fail, it is generally due to too little tension, rather than too much. The limiting factors for tenion are spoke pull-through on the rim (ask rim manufacturers for recommended tensions) and how compliant you want the wheel.