hhk25

I have an old mountain bike with v-brakes. The rims are original and I'm definitely feeling wear on the back wheel from the brake pads. I don't see any wear markers at all. Is there any way I could measure and know if the rim needs replacing?

DiabloScott

1. If you think it's worn out it probably is.
2. Worn out brake track will be sort of concave, or you might see alternating shiny and dark patches between spokes.
3. Most wear markers are visible when the rim is still good - and disappear when the rim is worn out.
4. Pretty hard to get most measuring tools in there, but they do make some.

LKA 

I don't know if verniers would work to measure in this case. I'm thinking that a tire removal & the best line-of-sight eyeballing of the wall thickness difference that you can get might be your only recourse. Or you could take the bike to your LBS. If they are good citizens, you should get an honest evaluation. I'm curious what a more knowledgeable cyclist will have to say.
Good luck!

edit: Verniers above, FTW. My verniers wouldn't fit inside the rim like that due to their design.

dsbrantjr

Something like these would work: https://www.amazon.com/Stainless-Iwa.../dp/B0087HKWCO

79pmooney

Or just lay a straightedge across the rim sidewall. If the mid-point of the sidewall is a long ways from the ruler, you have a lot of wear. (You can see that clearly on Scott's photo.) This check isn't accurate or definitive, but it is easy to do with the tire on; a real plus.

Also consider - replacing a rim too soon is spending unnecessary money but waiting too long and having the tire blow, tearing the rim sidewall out could lead to scraped seat or chainstay and maybe even a cut calf. I've seen some bikes (and calves) at the coop that weren't pretty pictures. If you bike is carbon fiber, more reason to replace early than late.

Edit: typos

Ben

ThermionicScott 

Something I've been thinking about lately: we know what the failure mode is if the sidewall wears through between the bead hook and the bead seat of the rim, and it's not pretty. What happens if the rim wears through the hollow space that we can't easily measure first? Seems like it would be akin to wearing through the sidewall of a tubular rim.

Ghrumpy

Crap. Now I want a new tool. Thanks @DiabloScott

Crankycrank

Some mfrs will list the acceptable wall thickness of their rims, I would do a search there. If you have some measuring device that only measures flat surfaces you can use some round wire about the gauge of a spoke (or just use a spoke), measure the dia of the wire then place in the valley of the brake groove in your rim, measure and then you can find the thickness of the wall by just subtracting the wire dia.

79pmooney

I would expect not nearly so big a deal. Sidewall would collapse on say a pavement break from lack of vertical structural strength, the tire might blow but I would expect the outer portion of the rim to not peel off as a long, jagged shard. There would be the danger of the tire coming off the rim, always a risk with clinchers and that outcome can be bad.

I've collapsed the sidewall hopping a pothole on a very worn tubular rim. Had to ride the 10 miles home on a 1" indent. Thump, thump. Other than that, not a big deal. The day may come when I go back to those pain-in-the-a** sewups for the peace of mind. Good glue - the tire stays on. Blow outs, rim failures - speed doesn't matter.

To your point of that inner portion of sidewall being hard to measure: another reason for the straightedge check.

Ben

fietsbob

if course if running tubulars you need a width, base measurement, to start with from new rim.

then you may note the wear over time.. with future measurements..

sweeks

That's a "crown thickness gauge". If it's decently made, it's accurate to 0.1 mm. Not as accurate as the micrometer, but a heck of a lot cheaper!

This is an easy test, but like measurement of the rim wall thickness it's just a data point that indicates wear, but doesn't really tell *when* to pull the plug on the rim.

In my experience, when the rim wall wears thin enough, the tire pressure will begin to push part of the rim wall out. This will be readily apparent as a new "thump" that's only present when the brake is applied. This is the time to replace the rim! Once it begins to crack and bulge out, the brake pads will flex the rim wall every time the bulging part passes through the brakes. This flexing will propagate the crack faster, and could eventually lead to a sudden and (probably) catastrophic rim wall failure.

I've attached images of one of my folding commuter bike's rims in "failure" mode. When I saw the bulge, I reduced the tire pressure significantly and rode home and took the wheel out of service. This is a 20" rear wheel... the only kind of wheel I've seen severe wear on. However, if you Google "Bicycle Rim Wear" you will see full-sized rims with the same problem.

So... short answer: Replace the rim when it begins to thump when the brakes are applied.
Steve

hhk25

This is what mine looks like right now. There is noticeable wear but no bulging. I'll leave it for now until I can get to a bike shop.

The problem with replacing the wheel is that selection of 26" wheels is pretty slim these days.

ThermionicScott 

There's still a pretty good selection of 26" replacement rims, though. If you don't currently have the ability to relace/build wheels, I highly recommend picking it up.

JohnDThompson 

I have to admit, I am still riding wheels I built almost 40 years ago, and see no compelling reason to retire them. But the modern fashion of machined sidewalls may well reduce the life expectancy of a rim, so caveat emptor.

Jeff Wills

Says you. I had a piece of rim 10" long break off when a well-worn rim blew. This was on my recumbent so it was a long way from my leg, but if it happened on an upright at speed I can see a chunk of rim skewering a calf.

ThermionicScott 

That was from the section of the rim holding the bead, though, right? I was inquiring about what might happen if the other part wore through first, marked crudely here as "A":

Retro Grouch 

That's what I think too. My test is just to feel the brake track with my fingers. If I feel a noticeable concave feel I assume that rim is nearing it's end-of-life.

It's mid-March. This is the time of year that many of us are prepping our bikes for the nice riding season. If you are going to have to take a bike out of service for a wheel or rim replacement, now is a good time to do it.

gsa103

There's a decent selection of older model stuff. The real problem is that brake tracks vanished from MTB rims before 26" became obsolete.

High-end stuff is still available on closeout sale, and lower end stuff is still stocked as replacement parts.

Darth_Firebolt

you can also check it with the tire deflated, then inflate the tire and check again to see how much the rim flexes. if it's too much for your taste, you can feel better about replacing it.

Jeff Wills

OK- that was not clear from your post.

However, the portion of the rim you've indicated gets much less stress than the portion above the inner "wall" of the rim. The outer portion is only supported on one side plus it must resist the outward pressure of the inflated tire. On mine, the rim tore apart just above the inner wall of a double-wall rim (Sun CR-18).

woodcraft

A mnt bike wheel at ~35 psi is less likely to fail dramatically

than a road wheel at ~100 psi.

The only rim failure that I've seem on the road was NBD but did result in hitch-hiking.

I say just ride it.

ThermionicScott 

That's fair -- I had trouble verbalizing what I had in mind and should have just posted a picture from the get-go.

Jeff Wills

Pictures? You show me yours and I'll show you mine... ahem...

NJgreyhead

Somewhere I read a recommendation to over-inflate the tube/tire (how much? dunno) and look for a rim bulge. If none, then air down to your preferred pressure and ride.
I've done this inadvertently, and was glad I saw the rim bulge on the workstand in my basement.

I like the straight-edge test too.