Tubeless tires are much, much better than the alternative, but that’s not to say they’re perfect. Sometimes, just getting a tubeless tire to hold air can be a challenge, and diagnosing the problem — let alone fixing it — can be frustrating. If you find yourself staring at a flat tire in the garage and muttering “why” over and over again, here are some tips for getting rolling again.
This article won’t cover mounting tubeless tires; we’ll assume the tire has been mounted successfully, but that it won’t hold air for more than a day or so.
Break out the bubbly
No, we’re not celebrating yet, but this will get us closer to our goal by allowing us to zero in on the part that’s leaking.
Fill a spray bottle with water and some dish soap, or really any soap that forms bubbles. Next, inflate the tire to 30psi or so. Spray or just pour the soapy water all around the tire and rim, one section at a time. Note any places where air bubbles appear.
On the tire
If there is a puncture in the tire itself that is leaking, this is generally an easy fix. Make sure you have plenty of sealant in the tire and move it around until the sealant pools at the puncture. Larger punctures may benefit from a tire plug.
With a sidewall leak, you’re usually better off replacing the tire. It might be possible to patch it, or if you’re really lucky, plug it, but in my experience sidewall repairs rarely last very long.
While rare, some tires have been known to absorb or even weep sealant. Tiny pores in the tire compound fill with sealant when it’s initially added, so you may need to add a bit more liquid to replace what’s been lost. If a tire still leaks through the tread or sidewall in multiple locations despite having good sealant coverage and no punctures, you may want to check with your local bike shop or tire manufacturer to see if the tire should be replaced.
Between the sidewall and the rim
Check to make sure the rim wall isn’t dented. If it is, your tire won’t be able to seal. If you find your rim is slightly bent or dented, it might be possible to get things straightened out enough to hold air. According to Gerow, “a couple of small planks of wood, a vice, and a hammer will get you started.”
Even if the rim wall isn’t visibly dented or misshapen, there could be a small gap between the tire bead and the rim that’s leaking air. Make sure you have plenty of sealant in the tire, and hold it horizontally and tilted to allow the liquid to pool around the part of the rim where you saw the bubbles forming. Shake the wheel gently for a minute or so to let the sealant do its work.
In some cases, the tire-rim connection may not be tight due to buildup of old sealant. “Older tires can have a buildup of dry and hardened sealant at their bead that will create space between the rim and the rubber, causing air leaks,” says Gerow. “When mounting a tire that has already been mounted previously, be sure to remove as much dried sealant from the bead as possible.”
Sometimes a tire bead may not be fully seated in the rim. Try pumping the tire up to maximum pressure. The loud popping sound you’ll hear is the bead popping into place. If you didn’t hear that noise the first time you mounted the tire, chances are that is the issue.
After running through the checks above, try soaping the tire again and checking the same spot to see if the fix was a success.
Around the valve
In my experience a leaky valve is often the cause of air pressure losses over time. If the soapy water reveals bubbles at the valve, it’s time to investigate further.
First, check the easy stuff: Is the core screwed in tightly? Is the inlet screw loose or bent? A dedicated valve core tool is helpful for proper tightening, and if your fingers aren’t getting the inlet plunger tight enough, needle nose pliers can do the job. Just make sure you don’t over tighten and break the valve, or get it so tight you can’t add air later. If any part of the valve is bent or broken, don’t try to fix it; it’s time to replace it.
If soapy bubbles are forming around the base of the valve, it may not be attached to the rim properly. Most valves have a nut at the base that tightens the valve to the rim. Get this as tight as possible with your fingers, and if necessary give it a slight turn with a wrench. Just be sure to avoid tightening too much because you could damage your rim, especially if it’s carbon fiber, and you may need to be able to remove the nut on the trail in the case of a puncture.
Next, investigate the valve from the other end, which means taking the tire off the rim. Most valves have a soft rubber gasket that forms a seal around the valve hole in the rim, so check to make sure the valve is seated properly in the rim channel. You can also add a bit of teflon tape around the base of the valve to seal things up.
Sometimes sealant can fill in small gaps around the valve. If you notice air leaking while on the trail, try rotating and shaking the tire so the liquid sealant can get to the valve.
At the spokes
If bubbles form around the spoke nipples, the good news is you’ve found your leak! The bad news is, there isn’t a quick fix. This generally means the rim needs to be re-taped, or at the very least the tape needs to be patched over. If the tape appears wrinkled, ripped, or punctured that’s probably what’s causing the leak. Tire levers can often puncture the tape while installing the bead, causing the tape to leak air through the rim.
There are many tutorials online for taping a tubeless rim, but in general, the idea is to get the rim as clean and as dry as possible before taping all the way around in one go. Be mindful of any gaps where air might be able to leak, keeping the tape flat and taut to avoid blisters or pockets.
No bubbles. Now what?
Sometimes, tires can be sneaky leakers. Pump them up and they’ll stay rock solid in the garage for weeks, but as soon as you roll in the parking lot, or take a turn, they go soft. You soap ’em up, and there’s not a bubble in sight. In fact, this has happened to me a couple times in just the past few months.
This is usually due to a small cut that only opens up when there’s weight on the tire, or if the tire is pumped up to a higher pressure. In your garage, you can try to simulate the effect of riding by adding pressure above what you normally ride, or by deforming the tire with your hands and looking for air bubbles as the tire squirms.
Gerow notes, “Some tires need to be ridden immediately after setup in order to hold air properly. A new tire that won’t hold air in the garage might be good to go after a short romp on the trail.”
Once you find the sneaky leak, getting sealant to the right spot can do the trick, though a plug may work even better.
Be the air
Ultimately, tubeless mountain bike tire systems are pretty straightforward and there are only so many spots where air can escape. Imagine that you’re the air inside the tire, and you’re looking for a way out. How would you do it? That’s exactly the mindset you need to solve this mystery.