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I have caliper brakes on my 700C road bike wheels. I notice that the surface of the rims, both front and back, where the brake pads make contact is starting to look a little worn (after eleven years).

Is there a recommended minimum thickness for the rim before needing to replace the rims/wheels?

I've measured mine to be 1.5mm remaining. I don't know what they started out at.

Thanks.

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  • Hi, welcome to bicycles. Do you happen to know the make/model of your rims?
    – DavidW
    Commented Mar 19 at 2:42
  • Yes. Thanks for asking. They are Shimano R500 rims. On the other bike I have some white DT Swiss wheels but the sticky labels on the rims have all come off so I can't identify the model.
    – Lummo
    Commented Mar 19 at 3:54
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    I replace my rims simply when they start to break, which has happened probably less than a dozen times in 40 years of frequent biking. I have never had a catastrophic failure; they usually start to show a longitudinal split somewhere, and the ride starts to feel "bumpy", or the brake pad rubs. I am, however, using the bike mostly for commuting on level streets; the question may be more virulent if you, say, use it for downhill mountain biking. (And, of course, this is not advice to be followed but just a report of what I'm doing.)
    – Peter - Reinstate Monica
    Commented Mar 20 at 7:26
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    @Peter-ReinstateMonica or indeed the sort of long road rides or tours where the bike is the only way to get home, hundreds of km away (hopefully there's a bike shop on the way, but not many shops keep much stock of major parts, relying on next-day delivery)
    – Chris H
    Commented Mar 20 at 13:44

3 Answers 3

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Many wheels have wear indicators

From documentation resources of Shimano's R5000 wheels:

The hollow on the opposite side to the valve hole is an indicator for the amount of rim wear. If this hollow can no longer be seen, stop using the rim. If you continue using the rim, it may break, and the bicycle may fall over and an accident may result.

So, there seems to be some wear indicator opposite of the valve hole on the Shimano rims and many wheels have such in form of grooves or little indents.

Just for illustration, here's how that looks like on one of my bike's rims:

enter image description here

Update

Out of interest, I checked my other wheels, some fairly modern Fulcrum Racing Zero and older Ambrosini (both alloy rims). Both don't have any grooves or other wear indicators.

Fulcrum's manual states that this should be part of inspection and that wheels should be checked every 10000 km by a professional for stress marks, deformation, cracks etc. - but no specified thickness, at least not in public documentation.

Just looking at my wheels, even without dedicated markings, there are often good indicators you can use as a reference over time, he's what I've seen one set of my wheels

At least with my rim/tire fit, I can see the full wall thickness without taking the tire off. I see at least an estimated 2mm. Criggie's answer reported failure at around 1 mm thickness, so my example still has some life in it (but these wheels are just 3 years old and did ~15000 km)

enter image description here

I don't think this is meant as an indicator but the brake tracks have a little step over the side of the rim, knowing your wheel and looking on that going away over time might also help to judge.

enter image description here

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    I don’t have my R500 wheelset in front of me right now, but just to point out: The wear indicator dimples are surprisingly small and shallow and hard to see. Very similar to what a center punch would produce. Photo from the internet: i.ytimg.com/vi/TMwyBwQa-fM/maxresdefault.jpg
    – Michael
    Commented Mar 19 at 9:39
  • @Michael Yeah but if you know your wheel should have one and you can't find it even when dedicatedly looking for it, it is a good sign that something's wrong. I'm pretty sure the hollows/dimples/indents are usually place in a defined place such as opposite the valve hole so that they can be easily found and not confused with anything else. I added a pic from my OEM wheels for demonstration.
    – DoNuT
    Commented Mar 19 at 10:34
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    Thank you for the info about the small wear indicators. I was able to find just such tiny dimples in the rim more or less opposite the valve hole on my Shimano R500 wheel rims. They aren't exactly opposite the valve hole but slightly offset to the right, more or less over the first spoke nipple. I took a look at my DT Swiss wheels and they have dimples all the way round the rim at roughly every two spoke intervals. Given that I found them I guess that this means that there is life left in the rims and that 1.5mm is thicker than the limit. Thanks all of you for your help.
    – Lummo
    Commented Mar 20 at 3:17
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    @DoNut I've taken another look at your photo on a "real" computer. My mistake. What I'm seeing is the leading edge of the slot for the mounting stud for the pad holder. Sorry to have confused things.
    – Lummo
    Commented Mar 20 at 21:29
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    @Lummo No worries, it made me actually aware that these must have been swapped at some point. Definitely correctly mounted now because matching the forward markings. ;-)
    – DoNuT
    Commented Mar 20 at 21:54
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Most newer rims have wear indicators. Either a groove around the rim, or a hollow at a specific point, like the example DoNuT mentioned. If those wear indicators can no longer be seen, the rim is at the end of its life.

For older rims that don't have wear indicators, the recommended minimum wall thickness depends on the manufacturer, and the material of the rims.

One way to estimate if the rim is still okay(-ish) is the shape of the side walls of the rims. Since the tire exerts a force that pushes the side walls out, a worn rim will start to bend. you can use calipers to see if the side walls are still parallel. When they start to get wider on the outside, and the side walls are no longer straight but start forming a curve, they are used up and should be replaced immediately.

Failing to replace them will eventually lead to the rim breaking: the side wall will be pushed out by the tire pressure, resulting at minimum in a loss of the brakes on that wheel, and in ugly cases can cause injury when the tire spontaneously parts with the rim. Obviously, this will always happen in situations where the stress on the wheel in question is greatest, so don't take that lightly.

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  • Luckily, that curvature has more to do with uneven abrasion than with actual bending of aluminum. Because if it were the aluminum deforming, sudden catastrophic failure would be imminent.
    – cmaster - reinstate monica
    Commented Mar 19 at 19:43
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    @cmaster-reinstatemonica: Actually the difference in width between zero pressure and full pressure can be significant and can be a good indicator on when to replace. When it’s more than ~0.5mm or so it’s probably a good time to replace.
    – Michael
    Commented Mar 19 at 20:46
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    @Michael Ah, you mean the elastic deformation. I wasn't aware that that can be measured and consequently jumped to the abrasion effect which can be very visible. But, of course, an increase in width of 0.5mm means that each wall flexes only by 0.25mm, and is easily measured with calipers. I'll need to remember that trick.
    – cmaster - reinstate monica
    Commented Mar 19 at 22:26
  • I assume that, if the side walls are bulging outwards in a curve, then the brake pads are going to develop a corresponding concave curve. Could this be another sign of approaching problems?
    – Lummo
    Commented Mar 20 at 7:59
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    @Lummo Nope. The brake pad material is too flexible and too dull optically to show a rounding any sooner than the corresponding side wall. And it's surface is not even visible unless you take it off entirely. The abrasion curve in the side wall is much, much more visible.
    – cmaster - reinstate monica
    Commented Mar 20 at 9:15
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Minimum: 1.5mm

Here's a dead rim that failed in this way.

Surprisingly, the tyre was fine and I still ride it. The inner tube has a massive ragged burst with lots of tentacles so was unpatchable.

This happened while doing a normal inflation to around 80 PSI. There was a brief "tink" noise like what one might get from spokes when stress-relieving a wheel, and perhaps a second later BAM I had the neighbours checking what the noise was, and my hearing was affected for hours afterwards.

I had been standing and using a track pump; I hate to think what would have happened if I had a minipump and had my head right by the wheel.

enter image description here

enter image description here

Update:

enter image description here

At the middle of the failure, the exposed metal is 0.7mm thick. enter image description here

Closer to the ends, the exposed metal is 1.2 mm thick. enter image description here

As an approximation, your rim should be no thinner than 1.5 mm wall thickness at any point.

This assumes a rim brake, with wear on the brake track. And that the rim is made of Aluminium with no additional brake track added.

To measure these distances you can't use a caliper or a micrometer because the bead hook will be in the way.

To do this measurement you need a tool called a Double Ended Caliper or sometimes a "Figure 8 Caliper" that looks like this:

enter image description here

Source: https://federatedtool.com/robert-sorby-977-double-ended-calipers-7/

These calipers simply transfer the gap from one place to another, and you read off the measurement at the other end using measuring calipers.

A fancy metal set would be nice, but you could make something adequate from cardboard, or I have designed a printed one at https://www.tinkercad.com/things/7GkwHr7pZJb-double-ended-calipers It is functional but could be better.

enter image description here

My "good" rim turns out to be about 2.0 mm thick. However the tolerance of this tool is suspect - it would be better to use gauge blocks or feeler gauges to measure small distances.

enter image description hereenter image description here

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    I just use a standard Vernier calliper gauge and a 15mm long piece of an old spoke. I hold the piece of spoke up against the rim wall below the bead on the edge and then measure that. Then subtract the spoke thickness. Could always take a spare gauge and cut away some of the jaws so that they will reach over the rim bead.
    – Lummo
    Commented Mar 26 at 3:29

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