Ask a Mechanic: Non-tubeless tires on UST wheels?

This is the fourth edition of a weekly column we’re running here at Art’s Cyclery where our expert mechanic Daniel Slusser answers your bike maintenance questions.

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Question: Can I use regular tires (non-tubeless rated tires) setup tubeless on my Mavic Crossmax UST wheels?     From: Brian

Answer: Tubeless tires are arguably the best innovation for mountain bikes since suspension came along in the early 90’s. Flat tires with a tubeless system are far less common than with inner tubes. Add to this advantage the potential for greater traction and lower weight and you have a truly winning combination. But this system has its limitations. I purposely chose to use the word “system” because these setups have to be viewed as a combination of tire, rim, and sealant. All of the parts have to work together in order to have a safe and reliable setup.

The consequences of having a tire come off your rim and deflate instantaneously can be disastrous. Imagine going though a rock garden and you suddenly get sprayed in the face with sealant at the same time that you loose all traction and much of your suspension. You will get hurt, and your rim will be destroyed. This is a best-case outcome for this scenario. Now imagine you are hauling the mail around a rocky off-camber turn or a high-g burm when your front tire suddenly comes off your rim causing a loss of traction, dumping you violently onto the ground. Not pretty. Your life could depend on getting your tubeless setup right. At the very least, the integrity of your skin, bones, and teeth depend on it. Don’t take the risk of an unknown setup. It is not worth it.

Mavic XM 819 UST Disc Rim

The Mavic 819 UST Rim is free of holes and integrates two lips into the rim bed to lock the tire bead in place.

With that said, let’s first take a look at rims. Rims that are made to the UST standard have a rim bed that is free of holes and integrates two lips into the rim bed to lock the tire bead in place against the bead hook. Other rims approximate this standard by using a plastic rim strip with similar lips built in that seal a traditionally drilled rim. Some examples of this are the Bontrager and DT Swiss systems. Then there are rims that use a heavy-duty (yet light) tape to seal traditional rim beds for tubeless use. Some examples include select rims from Velocity and Sun, as well as all Stan’s Notubes and Roval rims.Lastly, you have traditional rims that are converted to tubeless using rubber rimstrips or tape. Each of these can be good systems, but they all have their advantages and disadvantages. Because this question involves compatibility issues, I will not delve into these beyond discussing tire and rim interfaces.

On the tire end of the equation there are a few options as well. There are traditional tires that are not designed to work as tubeless but can be converted to tubeless in some cases. Then there are tires that are often marketed as “tubeless compatible” meaning that they use fairly traditional casing construction but have been fitted with a tubeless bead. These beads are square, rather than triangular like traditional tires, and have a small lip molded in to act as a seal against the rim bead hook. Lastly, there are tires that are rated as UST. These tires must meet an international standard that limits their porosity. This requires a heavier casing be used to achieve this in addition to tubeless beads. A true UST tire is designed to hold air in a UST rim without sealant.

Now that the main options are spelled out, which ones are compatible? Any rim that uses an inner lip to help lock the tire in place (i.e. Mavic UST, Bontrager, DT Swiss) should ONLY be used with tires that have a tubeless bead. The shape of the bead hook on these rims is only designed to seal with tubeless compatible or UST tires. As such, traditional tires tend not to seal and are more likely to come off the rim when riding. You may get traditional tires to seal on such a rim, but don’t confuse this with compatibility.

Stan's ZTR Arch 29er Wheelset

On Stan’s NoTubes rims, traditional tires can be used. However, we don’t recommend using them on a front wheel because the consequences for failure up front are too great.

For rims that are designed to be tubeless compatible such as Roval or Notubes, traditional tires can be used and will seal well. However, I will not use them on a front wheel because the consequences for failure up front are too great. The same holds true for traditional rims sealed with rubber rim strips or tape. There are huge forces being applied to tires in a high-speed corner and I need the assurance that if I crash it will be because of pilot error rather than a mechanical failure. It is my opinion that tubeless compatible or UST tires are safe to use front and rear with such a rim although they are not as secure as a rim that uses lips in the rim bed to lock the tire in place.

As for sealant, check with the tire manufacturer and the sealant manufacturer for compatibility. Any more on this particular topic is worthy of another lengthy post.

Just keep in mind that every tubeless system has its limits and that failure can even occur with proper setup in extreme conditions. Pro downhillers generally do not use tubeless tires even though they drastically reduce the potential for pinch flats. This is because they generate so many g’s in the corners that they can rip the tire bead off of the rim even when used with pressures north of 30psi in a heavy-duty tire. This is because the mechanical friction provided by a tube against the tire bead will help to prevent it from coming loose from the rim.

A more common tubeless failure comes from using air pressure that is set too low. Giving exact numbers for what this should be is impossible as there are too many variables including tire casing, rubber compound, tire and rim width, suspension setup and rider weight. To give you an idea though, I will tell you how I set up my tires. I am currently using Stan’s Flow 29er rims with a Specialized Purgatory Control 2.4 up front and a Specialized Captain Control 2.2 out back. Both are tubeless compatible and setup tubeless. I am 175lbs with gear and I am an aggressive rider that doesn’t hesitate to hit jumps out on the trail. I use 24psi up front and 28 out back. Any less and the tires become too squirmy for me in high g turns. All things being equal, if you have 26” wheels you should go slightly higher with your pressure, as the lower the tire volume of 26” wheels require more pressure to get the same tire feel (just imagine a road tire at 80psi compared to a mountain tire at 40, and you will get the idea). If you are using excessively low tire pressure, this is a sign that your suspension is likely incorrectly adjusted or your riding technique may need work because the extra low pressure is likely enabling and masking this bad technique (i.e. poor weighting of the bike to maintain traction when climbing, not counter weighting the bike when going through slightly rough corners, etc).

Just remember: Setup your tubeless right and your next ride is sure to be more fun, and faster. Set them up wrong and it could be a disaster!

Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly University. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.