A detailed analysis of tubeless tires for cyclocross

A topic that causes much contention on internet message boards, and is generally pretty interesting to a lot of people is that of tubeless tire setups for cyclocross. I would like to give a detailed look at where we are at with the technology, and where things might be headed.

Historically, tubulars have ruled the discipline, and for good reason. You can safely run them at pressures suitable for the amount of traction required, and even ride them flat to the pit if needed. Most importantly, good tubulars have extremely supple casings (on the order of 300+ tpi), so they conform to the ground extremely well aiding traction and comfort.

Clinchers on the other hand, offer anywhere from 60 tpi to 320 tpi casings, though somewhere around 120 is probably average. The ride of the nicer clinchers is better than the cheap ones, but still does not come close to the performance of tubulars. Also, running tubes (even latex) at the pressures required for cross almost invariably results in a pinch flat costing you the race. When this does happen, you can't really ride it to the pit because the tire is likely to peel off the rim.

The mtb world has been familiar with tubeless tires and rims for some time, and the technology is now quite mature. There are 2 camps, generally speaking:

1. UST which is a certified system encompassing a spec for both tires and rims. The rim and tire have matching bead profiles, which are much more square than conventional folding beads to give a secure lock. The tire includes a butyl layer - essentially a tube - so that it will hold air. The system may be used with or without sealant as a preventative measure from punctures.
2. Converted setups, which involve a rimstrip (originally Stan's Notubes, but now often simply a split innertube) and the use of sealant to seal both the tire/rim interface and the tire casing itself (if a non-UST tire is used).

Since sealant affords such good protection against punctures, almost everyone uses it in their tubeless tires and suddenly UST becomes a lot less attractive due to the extra weight of the UST-spec tires. This has led to the development of "tubeless-ready" tires, or those which have stronger/tighter beads but not an airtight butyl layer so they can be used tubeless with sealant at more competitive weights. Parallel to this, non-UST spec tubeless rims have been developed, most prominently by Stan's Notubes where the bead hook is designed specifically to hold on to non-UST beads in a much more secure way than option 2 above. In my mind, this is really the gold standard of tubeless setups, and thus 3 new distinctions are formed, each with loyal followers:

1. Those who prefer tubeless ready tires and rims (eg. Bontrager TLR setup, Hutchinson tubeless ready tires) with sealant, sometimes on UST rims.
2. Those who use Stan's Notubes rims, any clincher tire of their choice and sealant - with the rim being designed to fit standard tires tightly, a secure and reliable setup can be had without the weight penalties associated with UST and tubeless ready tires.
3. Those still using converted rims and tires - definitely becoming a minority, and IMO this setup does not have the same level of reliability at the bead interface, since it depends on a rimstrip to make a tight fit, rather than a special bead hook or tighter beads on tubeless ready tires.

So getting back to cyclocross from all of this: there are very few tubeless ready tires available, and no UST spec exists. As such, the primary systems in use to date are either converted rims or Stan's Notubes rims, both with standard tires. Performance, including bead security at typical cross pressures, has been an issue especially with converted setups. Most people using Stan's rims and some of the tighter tires do not have this problem.

This takes me to my inspiration for this post - having installed cross tires on my 29er wheels before, I notice that they have a great profile: wide and high volume, which is sure to increase performance.

Indeed this is what many tubeless users have been enjoying, since the only Stan's Notubes rim appropriate for cross has been the ZTR 355 29er rim, which is 24.4 mm wide (19 mm internal). The problem with this is having a wider rim means setting your brakes up for said rim, so when a standard road wheel is used they are not in the right place (far beyond what can be accounted for with a barrel adjuster). Also, at 410g the 355 is not really as light as it could be for cyclocross use. In response to this demand, Stan's has released a road/cross rim called Alpha 340, which is 22.35 mm wide (17 mm internal) and approximately 340 g. It also has machined sidewalls, which will improve braking greatly over the non-machined 355. This is still wider than the average road rim, but narrow enough that with careful setup one might be able to get their brakes to work with both using only a barrel adjuster. Unfortunately, much of the advantage of the wider rim has been given up, and I think if you were to really commit to tubeless use (i.e. switching all your wheelsets) that having a ~24 mm rim but with machined sidewalls and low weight would be ideal.

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Lets consider however, the Alpha 340 as the best available tubeless rim for cross at the moment. Building these up fairly light with American Classic hubs and DT Revolution spokes 28h/32h front/rear you get a wheelset that is 1274 g ready to mount tires (incl. rimtape and valves). If we consider a typical high end clincher is 380 g and use 75 g of sealant per wheel, that amounts to 2184 g for the whole setup.

Now for a "typical" tubular setup of about the same calibre we'll take some Velocity Escape rims laced to the same AC hubs with DT Revolutions 28/32h. This comes out at 1327 g. Add some FMB tubulars (370 g each) + Mastik One glue (75 g for the pair), for a total of 2142 g. This is 82 g less than the top of the line tubeless setup above, and probably costs about the same when all is said and done (cheaper rims balance out more expensive tires).

So how does this wash out?

• for similar money to a mid-range tubular wheelset (+ good tires), you can have the best tubeless setup currently available
• the Alpha 340 rim is marginally wider than your average road rim - this is good for users who want to be able to use their road wheels or perhaps a set of tubulars they already own on the same bike, but this is not ideal if the goal is the best tubeless performance possible.
• the tubeless setup is still dependent on low tpi tires, so the grip and ride will not be as good as tubulars until this situation changes. Low pressure performance is unknown at this point, with the rims not yet on the market for significant testing.
• tubulars still come out lighter (though not by much, if you put the money into light tubeless wheels) and have inherent advantages (being able to ride them flat, etc.)

This shows promise for the tubeless camp, but it does not yet appear to be a viable idea for serious racing, considering that it costs as much as a set of reasonable tubular wheels and the best tubs you can buy and the performance is still less.

If we can get wide enough tubeless rims (dedicated cross rims, i.e. machined brake track, lightweight) and good supple clincher tires, ideally with beads optimized for tubeless use (i.e. tighter) there may be a case for making the switch, but at that point you have to commit all of your wheels to ensure brake compatibility.

In other words, we're not there yet. Keep buying those tubular wheels when the road racers dump them at the end of the season, and keep gluing up your Dugast/FMB/Challenge/Tufo tires with care, because it's the best we've got for the foreseeable future.