Rim Damage - concave brake surface?

[chuckred]

I rode my Velocity Aerohead /C-Record wheels in last July's Monsoon Triple By Pass.... posted about it back then, but am trying to figure out how much rim damage necessitates repair.

The brake surface of the rims has a very slight concavity to it now - from coming down Squaw Pass in inch deep rivulets filled with grit and trying to keep speed down to safe visibility and control, not to mention not freezing...

Anyway, with a straight edge against the surface, I can see a "bit" of daylight - would estimate half a millimeter max.

I know concave rim = bad... but at what point is replacement needed? ANY concavity, or is there some point that it becomes critical?

[Xyzzy]

The worry is if you wear too much rim surface away the rim may not be strong enough to hold the bead of the tire. Most rim manufacturers publish a minimum safe value for the rim wall thickness, or, on cheaper rims, engrave rim wear indicators.

KS pads will greatly reduce the wear your rims take in wet/gritty situations. I severely gouged a set of Ksyrium Elite rims on my very first rain ride because the stock Ultegra brake pads picked up a piece of radial tire wire.

Edit: Found this Usenet article in my saved items folder and this picture in one of my Campy PDFs. The thinking with the Campy picture is as the rim wears it loses strength and spreads out due to the force of the inflated tire bead pushing out. In the Usenet article they talk about measuring the concave section with a feeler gauge. I checked Velocity's website and they do not publish a minimum wall thickness or even a nominal wall thickness.

http://groups.google.com/group/rec.bicycles.tech/msg/414cd2f179e8feb9?hl=en&

Half a millimeter sounds like a lot of wear.

[chuckred]

Thanks!

Got this from another forum (can't vouch for who the other experts are...)

the Aerohead rim is about 1.5 mm thick at the braking track. Most rim experts say that this type of rim is still serviceable to a thickness of about .8 mm at 110-psi tire air pressure YMVV.

I'm prett certain that I've got a ways to go before I get too concerned. Next major monsoon I'll ride my "old" wheels!

[Too Tall]

I resisted giving "internet" advice but think you should rebuild these. Back in a day when I actually wore out clincher wheesets the rim brake surface would deform as it became thin and at that point was only a few rides from being ready to toss out...cracks etc. You would be well served to junk them now.

[znfdl]

chuck:

I am very conservative whith wheels, if I start to question a wheel's reliability, it gets replaced. In my humble opinion my well being depends upon the wheels that I ride. Just my .02

[chuckred]

They've only got probably 1000 miles on them, and all of the wear was from one ride. I think I'll take them in to the wheel builder and have him look at them... I agree - better safe than sorry, a rebuild is better than worrying!

[DfCas]

With mtb rims,the easiest test is to pump it up to `80 or 90 psi.if it survives,I reduce the pressure down to my usual 35-40.I quit rim brakes after the pictured rim failed on a ride.trip to the ER,broken fingers,etc.Yes,it was on the front.

Now I ride disc brakes. If your rim is a front,I would probably replace it.

dan

[pdxmech13]

I have yet to find a measurement that really is fool proof on all makes and models. I have tried measuring the expansion when inflated to max pressure. most modern rims are coming with wear indicators that are no deeper than
.5-.8 of a mm. better safe than sorry.

Oh and I see about 5 worn out rims a week !!!!!!!!!!!!!

[chuckred]

[QUOTE=pdxmech13] most modern rims are coming with wear indicators that are no deeper than
.5-.8 of a mm. QUOTE]

Anyone know if the Velocity Aerohead has wear indicators? I can't say that I've ever seen what they look like on any of my rims - what do they look like?