Fixing Repeated Tubeless Flats on the Zinn Fondo

Also, photos from our staycation

The second Zinn Fondo at 12,000 feet on Trail Ridge Road 36 years ago. Mark Adams photo

The 2025 Zinn Fondo on Wall Street. Ben Delaney photo

Ben and Lennard through the Switzerland Trail rock cut. Chris Case photo

The author near the top of Switzerland Trail. Chris Case photo

Dear Readers,

Today is my birthday (my 67^th—my how time flies!), and I’d like to offer some learning takeaways from a rash of flat tubeless tires on the annual Zinn Fondo, which this year was a gravel event in the mountains. It took place on Saturday (the summer solstice) and culminated a week’s staycation with my wife creating a functional metal sculpture to close off the end of our driveway. I’ve included photos at the bottom.

The Zinn Fondo informal ride started from our shop in Louisville, CO with 26 people. We would pick up 20 more people at a park at the base of the Boulder Canyon before riding up into the mountains.

We left late because I was sealing up tubeless tires of a couple of the riders whose rear tires were leaking. But then I and another rider flatted our rear tires before we had reached that second meet-up point in Eben G. Fine Park. I had already deployed a canister of Vittoria Pit Stop into one of those tires I was working on before the start, and I had none with me. I normally carry multiple Pit Stops with me on a ride like this, and that would have been my first line of defense in this case. Instead, we added more sealant to both of our tires and fortunately had a friend with a Silca mini electric pump, so we were neither using up CO2 cartridges or our time and energy on a very hot day flailing away with a hand pump while so many riders were waiting for us up ahead at the park.

The other flat victim’s tire held air, but I had to pump mine a second time before reaching the park. At the park (after meeting up with those who had patiently waited), it became clear why it wasn’t sealing—sealant was shooting out of a small hole in the sidewall when I leaned the bike over with that spot down so sealant flowed there.

Despite minutes of sealant shooting out, it didn’t seal or even slow the rate, so I “Darted” it. The (Stan’s) dart initially seemed to make it much worse; sealant was flying around all over the place. But pushing the tails of the Dart around while keeping the sidewall tipped down into the grass stopped the flow.

Everybody’s tires were now holding air, and I joyously rode on with a great group of friends high up in the mountains until my rear tire hit hard against an unseen sharp rock in a shady section of the Switzerland Trail. Oops. This time, there would be no Dart or other tire plug that would fix it, because sealant was now squirting out along the bead at the edge of the rim.

Resorting to an inner tube. Chris Case and Ben Delaney photos

I put in my spare tube and hit it for the fourth time with the Silca Elettrico Mini Inflator. However, it didn’t hold air. Some of you may already know the issue—the Dart has a sharp plastic point on the end, and I hadn’t removed it before putting in the inner tube! Another oops.

The Stan’s Dart after pulling it out of my tire. It has a sharp tip that can puncture an inner tube. LZ photo

BTW, this same issue could occur with a DynaPlug; its pointy brass tip also stays inside the tire, ready to puncture an unsuspecting inner tube. A sticky-worm tire plug, however, does not leave anything sharp inside the tire to cause such problems.

I was lucky that it was my birthday ride, because my buddies not only waited (again), but a second inner tube was also promptly offered. It wasn’t easy to remove the Dart, but eventually I pulled it through, so this second tube did not get punctured. However, my buddy Matt’s Silca Elettrico Mini Inflator finally ran out of charge. It had pumped my tire four times plus another guy’s tire once, so it had been worth its weight in gold. And then Ben Delaney whipped out another Silca Elettrico Mini, and I was back on the (rocky) road again!

Now with a tube inside, the site on my Challenge of my first flat, where I inserted and then removed the Stan’s Dart. LZ photo

The tire had a wobble in it, since due to fears of bulging the tube through the Dart hole or using up precious electric charge in Ben’s Silca mini inflator, I didn’t want to risk going to high enough pressure to completely seat the bead. Thankfully, the tire held the entire rest of the climb, the long descent back to Boulder, and the hot (~100F) roll back to the waiting barbequed brats and burgers and cold drinks at Zinn Cycles in Louisville.

Lessons (hopefully) learned:

1. Be prepared

The long course of the Zinn Fondo this year included a section of the Switzerland Trail, a jeep road following the former tracks of the narrow gauge railway line that connected the mining camps west of Boulder. It is quite rocky, including sections of very jagged loose rock that has slid onto it from the scree slopes above. It can be hard on tires, and the 40mm Challenge Getaway tires I was using that day had already done this same ride four other times, and the sidewalls were looking dry and delicate.

Challenge Getaway too old and worn to be up to the challenge. LZ photo

I had new Challenge Gravines laid out to install for this ride, because I didn’t want to be “that guy” on my own birthday ride having tire problems. Decent plan, but my wheels didn’t get back into my hands until the last minute, hence my using the beat-up tires anyway.

(I had loaned my wheels to a customer with a new Zinn gravel bike whose wheels were being returned to the manufacturer for warranty service. But due to all sorts of delays, I didn’t get my wheels back until a few days before the Zinn Fondo—during my week of staycation with my wife where we were working dawn to dusk every day building a metal sculpture gate closure for the end of our driveway. I didn’t want to take the time away from that task, nor did I like the idea of riding the event on tubeless tires that I hadn’t had a chance to ride before and make sure they were all sealed up properly.)

I also only had my little spare tire bag with a few tools and a tiny hand pump; I forgot to bring the array of tools and supplies I would normally take on a group gravel ride on this sort of terrain. The other four times I mentioned that I have done that ride since last fall, I brought three Vittoria Pit Stops, a sticky-worm type tire plugging tool and a DynaPlug Air tool (which refills the tire through the hole you are plugging rather than bothering with the valve at all). And of course I didn’t need them on those rides! I had been preoccupied with welding and cutting metal late into the night all week leading up to it as well as making sure friends were invited and logistics were in place for the ride that take along tool details fell by the wayside. Second oops.

2. Avoid blowing tires by slamming into sharp objects

‘Nuff said on that—another oops on my ride that day.

3. Remove a sharp tire plug before installing an inner tube

We’ve already discussed Stan’s Darts and Dynaplugs; there may be other tire plugs with sharp tips. That was my fourth oops.

4. Get a mini electric pump

Thankfully, there was a lull in the tariffs that had stopped Silca from taking possession of the Elettrico Minis that had landed at the port, so you can get them for now. These pumps are much easier, faster, and less likely to damage a valve than a mini hand pump. They also are less wasteful than a CO2 inflator and will inflate more tires while being less prone to hardening up the sealant inside the tire than is CO2.

Staycation!

I didn’t write a column last week because I was on vacation. It was my wife and my first staycation! For decades, my wife has wanted a better gate at the end of our driveway that would look better and be dependable for keeping our dog contained in the yard. I did too, but not enough to actually do anything about it. Both of us also wanted it to be an artistic endeavor. Clearly, it wasn’t ever going to happen unless we set aside the time to do it and committed to not allowing ourselves to be interrupted. We were optimistic to think that we could complete it in a week.

We wanted to move the gate away from the garage so trucks going into our backyard would no longer clip the gutter and eaves. So, to offset the gate latch from the garage, we made this heavy steel panel, which illustrates our love of the mountains, my wife’s passion for horses (she trains horses in dressage), and our mutual love of bike riding, whitewater rafting, cats, bunnies, and other animals. The gate is strong (and heavy), made out of angle iron, steel rebar, and corrugated galvanized steel. We still have to dig two more post holes, mount the gate and the hinge, weld the swinging part of the latch to the gate, and make the fence panels, which will be wood-framed corrugated galvanized steel. Now, to find the time to do those things…

― Lennard

Subscribers can send brief technical questions to Lennard at: veloqna@comcast.net.

As a frame builder, Lennard Zinn has been designing and building custom bicycles for over 42 years; he founded Zinn Cycles in 1982 and co-founded Clydesdale Bicycles in 2017. His Tech Q&A column on Substack follows his 35-year stint as a technical writer for VeloNews (from 1987 through 2022). He is a former U.S. National Cycling Team member and author of many bicycle books including Zinn and the Art of Mountain Bike Maintenance, Zinn and the Art of Road Bike Maintenance, and The Haywire Heart. He holds a bachelor’s degree in physics from Colorado College.

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