Why did my wheel go all slack?

[Amanda P]

Hoping for some expert advice here.

I built a wheel for the back end of my yellow fixed/singlespeed winter going-to-work bike, as the old hub bearings were knackered, and, unable to unscrew either the freewheel or the sprocket, I couldn't get in to replace them. Anyway...

It has a brand new hub, but I built it using the spokes and rim from another (front) wheel. It's a stiff, double-walled rim and 36 stainless steel spokes, built 3x both sides. It went together easily enough; I dipped each spoke's thread in a bit of grease as advised by Sheldon and others when I laced it up, and got it nice and tight and true.

I'd ridden it maybe twenty miles before today, and checked it for truth and spoke tension to see if it had settle down OK. It was fine. Today, a damp, drizzly, day, I rode it to work.

After about five miles, I became aware that it had a wobble. Stopping to see how bad it was, I found that almost all the spokes had gone really slack - the nipples weren't even finger tight. Erk!

So, at work first thing this morning, I've retensioned re-trued it. Fine, but my question is - why did it go so slack, so suddenly? I've built one or two wheels previously, in exactly the same way, but never had anything this dramatic happen. What's going on?

 

[fossyant]

Dunno - I don't bother with grease.

Actually did you de-stress the spokes during the build/true process ?

I put the wheel on its side, hub to ground, and press very hard on each side of the wheel, rotate, then press again, flip over repeat. Then true again, then de-stress, repeat until when I de-stress the spokes the rim doesn't go out of true - this is an important process, and can take a while.

If you've built it up true without taking the stress out (i.e. twists in the spoke) I suppose the spokes could come loose.

 

[accountantpete]

I would guess that the rear hub might need a small amount of dish in it.

 

[Ian H]

Slackening spokes are most often caused by insufficient tension. A decent minimum tension can be hard to achieve both sides on a wheel with a large amount of dish. Also as above - de-stressing - or squeeze pairs of spokes in turn until none of them ping.

 

[Amanda P]

Hmm. The wheel needs no dish as it's a flip-flop hub, so the wheel's symmetrical.

I did de-stress, but maybe not enough.

Anyway, since re-tensioning it this morning, it's been down to town and back (20 miles or so) and it hasn't gone slack again.

 

[ushills]

I would guess at tension, how did you know the spokes were tight enough. Reusing spokes you would need a rim with the same or almost the same ERD, the spokes may be too long.

 

[Amanda P]

The hub was the same type as the one I was replacing, and so was the rim, so I knew everything would fit. (I've got spoke length wrong before).

I assessed the spoke tension by comparing the feel of them, and the sound they made when plucked, with a professionally built wheel from another bike, and kept tensioning until they were similar.

Nonetheless, I suspect the tension wasn't enough. My theory is that a little bit of rainwater, making an emulsion with the grease on the spoke threads, coupled with not quite enough tension, allowed the wheel to detension itself. It's a bit tighter now, so we'll see what happens next.

It's the first rear wheel I've built, and the first to be used on a fixed-wheel bike, where I would guess it's quite stressed. One learns from experience!

 

[actonblue]

You might just have had one or two of the spokes slightly under tensioned and that could have led to the loss in true.
The Roger Musson book has lots of info on getting the build correct before you ride.