Technical FAQ: Pinch flats, non-tubeless road tires set up tubeless, wires in tires

Flats and tubeless woes are in the spotlight this week.

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Dear Lennard,

Pinch flats are ruining my ride aurora. What is causing them? (Besides the pothole I just rode thru.) Seems I’m getting more occurrences since I went to a wider rim (24mm) over the standard 17mm. I would greatly appreciate it if you would weigh in on this “Snake” Bitten ordeal. Could the inner tube fit/size have an effect on the situation? IE: Is a tube sized 17mm-25mm more or less prone to pinching opposed to say a tube sized 23mm-30mm fitted in a 25mm tire?? As it would seem that the smaller size would be more stretched out thus rendering the lining thinner and possibly more prone. Then again, I wonder about the larger size perhaps being more bunched up by not fully forming inside the tire casing rendering it to be more prone to getting that “snake bite”!! Second thought, do pinch flats only/ mostly occur over the spoke hole recess vs the rim bed?? I await your most revered wisdom response on this HORRIBLE ordeal. Thanks, signed waiting for my wife to bring me a 3rd tube…


Dear Doug,

First, yes, the wider rim with the same tire will be more prone to pinch flats, because the tire is more flattened out. The top of the tire has less far to go to hit the rim edge on the wider rim than on the narrower rim. Furthermore, the rim wall is sitting under less of the sidewall on the wider rim, so less of the tire and tube comes down to the rim, requiring less force yet to cause the pinch.

As for the inner tube, I believe that the larger tube (the one sized 23-30mm) will be less likely to pinch-flat than the skinnier tube (the one sized 17-25mm) inside of a 25mm tire, because the wall thickness of the tube is higher when inflated inside of the tire. This is assuming the tubes are otherwise identical (same brand and model, same wall thickness, same material, same manufacturing process). The larger tube will still be stretched a bit and won’t be “bunched up” inside the tire.

Pinch flats happen where the tire pinches down to the rim wall; it is irrelevant whether it is adjacent a spoke hole or not. The rim wall is essentially the same all of the way around the rim, whether adjacent spoke holes or not. Obviously, having a good rim strip is critical to not getting flats, but a puncture due to the end of the spoke puncturing the tube would not be a pinch flat.

― Lennard

Dear Lennard,

Thanks so much for writing your column, the tech Q&As entertain me every week! My girlfriend recently bought a Trek Checkpoint ALR 5 (2021 version, not the new updated geo). As part of the purchase, we asked for the tires to be set up tubeless. The bike shop did this, however over the next couple of months the front then back wheel deflated, and I could not get the tires to reseal. I thought the deflation was a bit strange, so I looked into it, and I realized the tires are not tubeless ready. The bike shop never indicated that the tires were not tubeless. Is there a safety issue here? I am concerned my girlfriend was riding around on an unsafe set up for 2 months.

– Josh

Dear Josh,

While you might be able to get away with a non-tubeless-ready tire run tubeless on a mountain bike, it’s an absolute no-no on a road bike or gravel bike. Many XC riders got away with running non-tubeless tires tubeless early on in the history of bike tubeless tires, as the UST tubeless tires of the day were super heavy—too heavy for them to consider racing on them.

It’s not just that the sidewalls of non-tubeless-ready tires leak like sieves (which in itself is a safety issue). The primary issue is that the bead is not designed to be run tubeless and can burp more easily when she is cornering, which would take the wheels out from under her and send your girlfriend sliding across the road. Not something you want to risk happening.

― Lennard

Dear Lennard,

Re: your post about wires in tires follow up for us Luddites with tubes in our tires. A few days later, tire went soft on a ride, and I found a very small wire protruding less than 1 mm in the inside of the tire. Took me 20 minutes to remove it. Now I’m carrying some tweezers in my tool bag for next time.

– Jeff

Dear Lennard,

Catching up on the recent chain discussions in your technical FAQ and wondering if you have an opinion as to, when installing a Shimano 11-speed chain, if the connector pin or the master link is superior?

I always felt the master link had more points of possible failure and so continued using connector pins even when Shimano stopped supplying them with chains. My reasoning is partly based on never having had a chain failure using this method. I went so far as to buy a 100 pack of connector pins so I have them on-hand for years to come.

Am I being overly cautious? Should I get with the current times and start using the master links?

– Kevin

Dear Kevin,

That’s an interesting take on it. My opinion had been the opposite. Since the master link is a complete unit, manufactured to spec and tested extensively, I figured that was the connection you could bet on more.

I can get your point about the connector pin, and I am certainly not going to argue with the success you’ve had with connector pins. However, the fact that the user damages the outer link plates when pushing out the old pin leaves me questioning the longevity of a junction using those a lot more than I question a master link.

I’m not saying that’s what you should use; I’m just telling you my opinion, which is not based on data.

― Lennard

Dear Lennard,

I have always enjoyed your articles and letters, always looking forward to sharing with my fellow coworkers and customers. I really enjoyed your articles on Hopf bifurcation! Comparing the gains or lack of losses between aerodynamics and weight savings has been really helpful and interesting lately as well!

As far as brake pad wear goes, I love answering these types of questions with a semi assumptive generalization, before “braking,” down the actual answer; it depends.

“How long do the brakes last?”

“About 5,000 stops.” (Based on an assumed mileage and number of times you actually stop compared with just using the brakes on a given ride; maybe I need to double or even triple this number?!)

Customer looks up and out in dismay and wonderment.

As they return to the conversation, I let them know, “It really depends on riding frequency, riding intensity and riding conditions, things like load, grade, speed, the weather…”

But that quizzical look of bewilderment is priceless…

“How fast does it go?”

“About 1.21 Jigawatts.”


“Speed is money, how fast do you want to spend?!”

– Leif

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom huge bikes , a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn

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