If your rims become too worn, then braking performance is going to be compromised, which is far from ideal, and in extreme cases, you run the risk of the wheel failing because there isn’t enough material in the rim wall to cope with the forces exerted upon the rim by the high-pressure tire.
But how do you know when your rims are worn out? We’re here to help with some top tips for things to look out for.
As we said at the top rims primarily wear out because they are used as a braking surface by caliper rim brakes. If you’ve got disc brakes this is something that is eliminated, as the hub-mounted disc rotor is the braking surface, not the rim.
Many rims have a wear indicator. It’s designed to make it simple to determine when a rim is nearing the end of its life, and remove any risk about when you should replace it.
There are two common types, a groove or a hole. A groove, like the one in the pic above, is added to the entire circumference of the rim at the centre of the braking surface. Over time this groove becomes more shallow and when it disappears, it’s time to replace the rim.
The other common wear indicator is a small hole in the sidewall, like in the photo at the top of the article. When this hole disappears, you know it’s time to replace the rim. Easy.
Why does a rim wear out though?
The constant pressure of the brake block pressing against the aluminium rim slowly wears out the rim. It’s that simple. That black muck that you wash off your bike after a wet and gritty ride? That’s your aluminium rims wearing away.
“In the course of this second function as a braking surface, rims may be subject to wear, especially from intensive or prolonged use. Rims may experience wear for reasons as diverse as the build-up of gravel or mud in the brake pads or the use of worn or poorly adjusted brake pads. These can wear down or damage the rim sidewalls, and may not be noticed by the user,” says Mavic.
How long a rim lasts depends on many factors. Mainly it comes down to how much you ride and how often you brake, and the terrain you ride can be a factor as well. Live in the mountains? You’ll probably use your brakes quite a lot. Commuting in a flat city? Maybe not as much, at least not as heavily as trying to slow down from 80kph!
We asked Shimano’s Ben Hillsdon how long you can expect a rim to last. He says: “It’s difficult to say because it depends on the force of your braking and the cleanliness of your pads and your rim. If you can avoid excessively dragging the brakes and if you clean your bike regularly, paying particular attention to the braking track on the wheels and the face of the brake pad you’ll significantly extend the life of your wheels.”
Hunt’s Ollie Gray says many variables including brake block compound, the frequency of braking, terrain and weather conditions can have an effect on rim lifespan. “The lifespan can fall within a pretty huge window,” he explains. “In the worst cases, riders may experience rim lifespans as short as 1500 miles, and in the best cases perhaps up to 12000 miles! So as you can see, it’s a tricky one to put a figure on.”
Not all rims are the same either. Some are designed to be as light as possible so the sidewalls will be thinner to reduce weight, while rims designed for heavier duty riding like touring and commuting will have more material in the braking surface and will last longer. Ceramic coated rims can improve the longevity of a rim but cost a lot more than a regular rim but can be a good option if you do a lot of miles.
Cleaning your rims
It’s worth paying close attention to the rims. Make it a part of your regular bike wash and maintenance schedule, paying particular attention to the profile and condition of the rims and also the brake blocks. You’ll want to remove any debris or grit that might be caught in the brake block as this can damage the brake surface of the rim.
Regular cleaning of your wheel rims and brake calipers is a good recommendation, as important as cleaning the chain and derailleurs. A regular visual inspection will alert you to a worn rim or brake blocks. For cleaning the rims, you can just use the same cleaner as you use for the rest of the bike, something like Muc-Off or hot soapy water, to clean the rims and brakes. You can use an isopropyl alcohol or a degreaser to clean the rims more thoroughly and use one of those sponges with a scourer on the other side.
Turning your attention to the brake blocks, you’ll want to remove any debris, stones or glass that can sometimes get embedded in the rubber. Left untouched, this debris will damage the rim and also lead to decreased braking performance. It’s easier to remove the wheels to get a closer look at the brake blocks. Use a pair of tweezers and pry out any debris. Also, remove any buildup of crud in the grooves.
The brake blocks will wear out a lot more quickly than the rims, that’s okay they’re designed that way. You can choose different types of brake blocks to suit different conditions. Generally, a softer compound will prolong the rim life compared to a harder compound brake block.
And the compound of the brake blocks you use can make a difference too. Hunt’s Ollie Grey tells us: “Again, there are a few variables to consider but on a basic level: softer compound pads, which offer greater modulation and are best used during the dry/summer months, won’t deteriorate the rim’s braking surface as much as they’ll deteriorate themselves. Harder pads are better for year-round riding, but owing to their firmer compounds will cause more wear on the braking track over time.”
Shimano recommends using a certain quality of pad to match sure brake calipers. “All Shimano rim brake clincher wheels have an aluminum braking surface so with these you don’t need to use a specific type of pad compound, although we recommend a certain quality of pad to match certain brake calipers. Our Tubular wheels require carbon-specific pads. Whether you use Shimano Dura Ace, Ultegra or 105 calipers though they are all compatible with the standard R55C4 or carbon-specific R55C4-A brake shoes,” explains Shimano’s Ben Hillsdon.
A brand new rim has a flat braking surface. A highly worn side is easily identifiable by a concave shape. You can spot it by eye, or you can use something with a straight edge, a small tool, to detect the curved shape of the rim. The reduced thickness of the side causes the concave shape of the rim; the material has been worn away with all the braking.
That reduced thickness will eventually lead to a structural failure with the thin rim insufficient to contain the tire bead under high pressure. It can either happen when you inflate the tire, or worse when you’re riding. The latter is something you want to avoid.
Carbon fiber rims
That’s all for aluminum rims, but what if you’ve got carbon wheels? Generally speaking, you want to follow the same steps as with aluminum rims, regularly checking for debris caught in the brake block and ensuring all surfaces are cleaned daily.
As for checking for rim wear, it’s worth checking with the maker of your carbon rims to find out what they recommend. A curved side, just like with an aluminum rim, is a telltale sign of a worn carbon rim, so that’s one thing to check for.
But it’s harder to tell by eye when a carbon rim is worn out – you won’t find handy wear indicators like on an aluminum rim, so you want to pay particular attention to the carbon surface.
Generally speaking, carbon fiber rims are constructed with an individual layer of material laid over the brake track to give an excellent braking performance, prevent heat buildup and prolong the life of the rim. Underneath this top layer is the raw carbon weave. If you’ve been using your carbon wheels so much that this structural layer has become exposed, then it could be time to replace the rim. Sometimes the edge can become discolored in this area, or you can see through the top layer of carbon.
Hunt’s Ollie Gray backs this up, adding that you need to check for visible carbon weave. “Carbon brake tracks are much more resilient than alloy braking surfaces and generally will not show the same bowing/caving that alloy surfaces do. While they will curve inwards to a degree, this is not normally the tell-tale. What you should look for instead is when the woven fibers of the carbon itself begin to become exposed or frayed,” he says.
“If you run your hand/fingers across the braking surface and it feels rough or fibrous, that’s normally a sign the resin has worn away. As stated above, however, this resin on carbon braking tracks is quite resilient, and of course, carbon brake pads are of a softer compound, again contributing to the increased lifespan. To date, we’ve not known of any of our carbon wheelsets out on the road to have completed a lifespan.”
You might also notice a judder during braking that didn’t occur when the carbon wheels were brand new. This could be a sign of the brake track wearing out and would be an excellent time to get the rims professionally inspected.
Each carbon rim brand recommends a specific brake block and it’s strongly recommended to stick with this recommendation, as using another carbon-specific brake block could harm the rims.
“You must use the proper brake pads for your specific rims. Use of improper or contaminated brake pads will lead to excessively high braking temperatures which can cause premature rim wear and failure which can lead to serious injury and death,” warns Zipp.
So now you know how to spot a worn out rim, what are your choices for replacing the worn side? In most cases, you can return the edge, and all good bike shops will happily do the work for you. Building a wheel isn’t as scary as you might imagine if you fancy doing the job yourself, and there are lots of good wheel building guides available online if you want to go down that route.