Launched by Mavic, Michelin, and Hutchinson in 1999, the tubeless UST rim and tire system has been used by World Cup racers and weekend riders alike for many years, but the topic is still one that can cause some head scratching from even the most technically savvy of mountain bikers. This is compounded by the large number of "tubeless compatible" tires and rims out there that, while not being UST certified, can also work quite well. With a list of questions in hand, we reached out to Mavic's Zack Vestal for an explanation of how the UST design works, and to clarify some rim and tire compatibility reservations that we had. What does UST stand for, and what exactly is it?
UST stands for Universal Standard Tubeless, and it refers to a very specific, two-part
system consisting of the rim and the tire. The UST system was developed and introduced
in 1999 as a partnership between Mavic, Michelin, and Hutchinson. On the rim side,
Mavic contributed technical expertise as to the rim shape and keeping the tire bed
airtight. On the tire side, Michelin and Hutchinson developed tires with airtight casings
and beads that would lock into the rim, not unlike a tubeless car or motorcycle tire.
There is a very specific shape and dimension to a UST rim's bead hook (the lipped
inner edge or hook of the rim sidewall that the tire bead locks into), with it having
a square shape rather than rounded profile of a standard rim. Also, there is a “hump” on
the edge of the rim bed that helps lock the tire bead into place. The dimensions of the
bead hook, including its height, diameter, plus the width and shape of the “hump,” are
very closely controlled. The last attribute of a UST rim is that it must be airtight, which
means that any spoke holes must be fully sealed with tape or plastic, or the rim bed is
left intact and not drilled at all. The UST rim shape (particularly the shape of the
bead hook and hump) is patented by Mavic, and Mavic licenses the shape to other
companies, for a fee. An independent lab grants technical approval for any products
from other companies to wear the UST logo.
And what makes a tire UST-specific?
On the tire side, a UST tire must have a bead that is shaped correspondingly to the
UST bead hook - it’s more square than round, and it has a small flap on the inner edge
to help create an airtight seal. This flap also helps the tire gain an airtight seal against
the rim when it’s first being inflated. The dimension and mechanical characteristics of
a proper UST tire bead are critical. Any variance in diameter or profile, or the bead stretching
under inflation pressure, can cause the tire to not fit correctly and therefore not remain
airtight. Finally, a proper UST tire casing is airtight and holds air over time. Again, an
independent lab certifies the proper bead dimensions, allowable bead stretch, sealing
capability, security of the bead lock even when deflated, and ability to retain air over
time. However, a company that wants to make a UST tire doesn’t have to pay a license
fee. It just has to make the tire such that it passes the certification test.
Note that on the tire side, “UST-ready” systems are ubiquitous. A UST-ready tire has a proper
UST certified bead, but it requires sealant in order to make the casing airtight. Several brands
of tires have sought and achieved UST certification with the use of sealant. Of course the new
Mavic Charge, Roam, and Roam XL tires fall into this category, as do the TCS tires from WTB.
How does a UST system create a seal? Do non-UST tires work on UST rims and vice versa?
The number one aspect of the UST system is the bead hook interface. The precise tolerance between the rim and tire bead, plus its specific square shape, help the tire bead lock securely into place on the rim. Furthermore, the tire bead is controlled for how much it can stretch when inflated, which is very little to not at all. These characteristics create a very safe and secure bead lock. The tire tends to stay in place even when deflated. And, the tire can be run at different pressures with no fear of the tire exploding or rolling off the rim. Of course the valve is a critical component of the system as well - it must be properly mounted in the rim to maintain an airtight seal.
Non-UST tires will work on UST rims, but they don’t have the security of the bead
locking securely into the rim channel (bead hooks). And the significant discrepancy
in the shape of the bead will cause trouble with air retention. The tire bead just won’t seal
very well against the rim. Finally, non-UST tires are not as tightly controlled for the tire bead
diameter and stretch, and this means that a loose fitting tire could be more prone to rolling
off the rim or exploding off of it under high inflation pressure.
A person would have more success putting a UST tire on a non-UST rim, but again, the
lack of precision between the tire bead and the rim bead hook would cause sealing problems.
And, a non-UST rim doesn’t have an airtight tire bed. Also, you’d have to find a way to seal
the spoke holes on the rim bed before you could set it up tubeless.
The systems from Stans and other tubeless conversions do work, in that they allow you to
create a mostly airtight tire and rim interface. But these systems don’t have nearly as secure
of a bead lock between the tire and rim. This can lead to inconsistent or at worst, unsafe tire
security on the rim. Of course, this all looks like promotion of Mavic’s patented system, but
other rim and tire manufacturers have clearly tested and seen the benefits of going with proper
UST certification. I’ve spoken to a tire product manager at WTB who was very candid about
the unsafe conditions they found in some “tubeless” systems, and it’s why they created their
rims and tires with proper UST certification. The same is true for other brands like Easton