Technical FAQ: Road Tubeless Tires

Published February 11, 2020 07:32AM

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Dear Lennard,
I have a lot of experience with tubeless tires on cars. While going to high school and university in the 1960s, I worked in my dad’s service station on weekends and all summer. I changed tires, installed new tires, and plugged more holes than I’d care to count.

Now, I’m just an old, retired guy who rides his bikes more miles every year, than he drives his car. Most of my use is road, with a bit of gravel thrown in for variety. No MTB, though I have a second home on the edge of a mountain, at a national park.

I’ve been reading about tubeless for several years now, and am wondering when I’ll convert. I’m prepared to deal with tight fitting and getting beads seeded, but I am really struggling with the whole sealant concept. I’ve been watching the plug solutions that seem to be hitting the market weekly these days. I remember how much easier those things made our lives when introduced for cars in the mid-60s, and some of them look pretty interesting for bikes. I get about a dozen flats per year, so don’t spend a lot of time or money dealing with them. I also have several sets of wheels so am intimidated by the idea of sealing several sets of wheels a couple of times per year. Is there anything wrong with the idea of using tubeless on the road (I understand the value of sealant for tires with very low inflation) without sealant, and just using plugs to fix flats? It just seems a lot simpler than fiddling about with sealant.
— Jim

Dear Jim,
The problem is that tubeless tires generally no longer guarantee air impermeability. The first generation of tubeless tires were heavier, because they had a thick layer of rubber inside to seal them; they weighed as much as a tire and tube. They were also very expensive to make, because getting the “UST” (Universal System Tubeless) certification required that they be individually tested and shown to be airtight. Literally, every tire was mounted by hand on a wheel, inflated, and dunked in a big tank of water to ensure that it held air. Besides all of the labor involved, high tire rejection rates drove the costs up as well.

Now, tubeless tires generally come with a “TLR” (Tubeless Ready) designation, which does not guarantee that they are airtight. In order to ensure that they hold air, sealant must be used.
― Lennard

Dear Lennard,
I’m writing about your post in response a guy whose 28mm Specialized Turbo 2Bliss tires blew off his Hunt rims, which had a 21.3mm internal diameter. You gave us a response from Specialized. They said those tires were approved for use as tubeless tires only on rims with less than or equal to 17mm internal diameter (ID). They also said that those tires were not approved for use at all on wheels with over 21mm ID. I have a number of issues with Specialized’s response.

If Specialized is really correct that these tires can’t be mounted as tubeless tires to rims over 17mm ID, then this apparently means that a bunch of current-generation performance road wheels can’t accept these tires if mounted tubeless. I think the S-Works Turbo 2Bliss tire is a late 2017 model, but plenty of tubeless rims had a 21mm ID in that year.

I have a pair of wheels built to Pacenti SL 23s with about a 17mm ID, but those are 2011/2012-era rims. I think that the current generation Shimano and Fulcrum road wheels are around 17mm ID, also. In fact, there are going to be a number of wheels that apparently can’t accept these tires at all, like the Hed Eroica or the Bontrager Paradigm Elite. Hed explicitly said that tires 28mm and up are OK [for tubeless] on [their web] page. Trek said similar in comments by a customer service rep on their page.

Second, if the tires are that sensitive to rim specifications, why isn’t this warning made a lot more prominent on the website and in other channels? It does say that the tire should not be used on hookless rims. However, it gives a PSI range of 85 to 95, with no mention of acceptable rim widths. Roval’s page for the CL50s gives pressure recommendations for tires ranging from 24mm to 32mm, which implies that tires of those widths should generally be compatible with the rims. After all, a lot of people would think that those tires are generally compatible with current tubeless rims by default.

Third, I question the number of significant digits in Specialized’s warning. They say >21mm. However, do they mean > 21.0mm, or >21.49mm? I recall (but can’t find on the page) that the Boyd Altamont rims have an ID of 19.86mm, which gets stated as 20mm on their page. Similarly, if the Boyd rims had an ID of 21.3mm, that might get rounded to 21mm in literature. Roval’s page rounds their ID of 20.7mm to 21mm.

Your article linked to the Hunt 30 Carbon Aero Disc wheel. You said that, “Your Hunt Aero 30 wheels have crochet-type rims with a 21.3mm internal rim width, which is outside of the size range, even for use with an inner tube.” However, Hunt’s page has a diagram showing the internal width as “20mm”. I can’t find an exact width on Hunt’s site. Two reviews that Hunt linked to state the ID as “21mm”. I realize that if Specialized is correct, the tires can’t safely be used on those rims as tubeless tires, which is what your correspondent was doing. However, this raises the tangential question if that rim is totally out of spec for that tire or not.
— Weiwen

Dear Weiwen,
In answer to your first point, fit of the tire to rim is obviously critical. The European Tire and Rim Technical Organization (ETRTO) sets standards for sizing of rims and tires for the European Union, creating the ISO 5775 international standard for labeling the size of bicycle tires and rims. For road tubeless, this is not helpful for the current discussion, because the ETRTO specifies a 17mm internal-width, hook-bead rim for tubeless road tires. As you pointed out, modern aero rims tend to have internal widths of 19-21mm or greater, and some of those are hookless rims to boot. It’s clear that new standards need to be developed to ensure that all tire and rim manufacturers are on the same page.

Your second point I can’t answer, since I am not privy to Specialized’s thinking or motivation on that.

As to your third point, I wrote that 21.3mm for the Hunt rim’s internal width in my article based on an illustration I found online. I thought that I had found it on the Hunt website, but now I am only finding 20mm internal width labeled in the drawings there, as you indicated.
― Lennard