Halfast

I bought and have been very happy with a set of XERO XR1s. Have only trued the rear once for a minor wobble. No signs of flex.

Now after riding them 5000 miles I am getting brake rub on both front and rear when I accelerate hard standing. I think part of this is that I am getting stronger after lots of miles and increased speed the last 12 months. I am riding harder, but I did not think it was that much harder.

So the question is, can wear and stress cause wheels to be more flexable with miles and age?

DannoXYZ 

No, there's nothing that wears out on the wheels besides the bearings. The spokes and rim may fatigue over time, but their operating characteristics like stiffness and flexibility doesn't change; even right up to the point right before failure.

What caused your wheels to go out of true are nipples that rattled loose. The spokes loosen at the bottom where you have the load. Hitting big bumps will cause those spokes to loosen completely and have no tension. This allows the nipples to rattle and spin lose. Since each one will loosen a different amount, the wheel will go out of true. That's why double-butted spokes give a "stronger" wheel that's less likely to go out of true over time; the spokes stretch more and allow greater deformation of the wheel before losing all tension. Also use Spoke-Prep or blue Loctite on the nipples to prevent them from turning when the spokes lose tension.

But in your particular case, I suspect it's the frame flexing that's causing your brakes to rub. While standing, I prefer to keep the bike perfectly vertical so that ALL of my weight and leg forces go into spinning the pedals in the plane of the crank. If you rock the bike side to side, a lot of that strength goes into bending the frame.

HillRider

Another possibility is that the spokes weren't tensioned high enough from the very beginning which makes them more likely to loosen with use. Do you have an LBS with a good wheelbuilder? I'd have an experience mechanic re-tension these wheels and I expect the flex will go away.

urbanknight

I've heard it argued that rims get more flexible when they get thinner from the brakes wearing down the sidewalls. This was told to me before machined sidewalls were popular, but couldn't that still make the rims a little weaker side-to-side?

DannoXYZ 

The vertical sidewalls on the rim are stiffest vertically. Imagine bending a metal-ruler; it's stiffest in the direction of the most material in the same plane. It's the lateral box-sections at the base of the rim that determine lateral-stiffness. So yes, if the sidewalls are thinned by braking, the wheel may be more complaint vertically, but lateral-stiffness should remain the same. But even then, the rim is so elastic given the spoke-tension that it's really the spokes that determine wheel's overall rigidity and stiffness.

And according to Damon Rinard's testing, spoke-tension actually has very little to do with a wheel's lateral-stiffness unless the spokes are super-super loose. Once the spokes are even partially tight, the wheel will be at close to maximum-stiffness. That's because in order to deflect a wheel-laterally, you'll have to actually stretch the spokes. Even a single spoke with all your weight hanging on it will not stretch very much. Additionally, the width of the flanges and the angle between the opposing spokes coming together at the rim has a big impact on the stiffness; the wider the flanges the better.

urbanknight

Makes sense, thanks.