Jim’s Tech Talk
By Jim Langley
This week I’m sharing what I consider an important safety message for anyone riding the road or gravel on wide tires. By “wide,” I mean rubber that’s more than 35mm wide. You can now purchase 700 x 38, 40 and even fatter tires.
Rubber this wide will only fit on bicycles designed with the frame and fork clearance to accept it. There are plenty of “gravel” bikes out there that can, such as the Ibis Hakka as just one example. But don’t go trying to use them on classic road bicycles, which typically top out at 700 x 28c tires (28mm wide).
I’ve been working on some bikes with these wide sneakers lately and last week had an incident that took me completely by surprise and left me dazed and confused for most of a day. I’m telling the story here hoping that you won’t try what I did and possibly get hurt worse. My concern is riders conditioned by regular road tires who might over inflate these wide tires when using them on pavement.
Road tire blowouts
The cause of my tire trouble was the way I’m used to thinking about bicycle tires. I think of them as having the highest likelihood of blowing off the rim because they are run at the highest pressures. It usually happens from improper installation or over inflation.
I’ve blown dozens of road tires off as a mechanic and seen dozens of other mechanics blow them off, too. When high pressure road rubber first appeared in the early 1970’s with the Michelin Elan, some rims couldn’t handle the skinny new tires and would allow the tube to push the bead out of the rim and blow out the side – kaboom!
Sometimes blowouts were related to faulty tire installations, getting the tube trapped beneath the bead is probably the biggest mistake. And over inflation from an overpowered compressor could do it, too. Even with only a floor pump it’s possible to blow tires off – especially when a lot of roadies ran sky-high pressures such as 130psi or more.
As road rubber and rims improved, blowouts became less common because the tires and rims didn’t fail. Today, you should only experience a blowout from bad installation or if you go with a poor quality tire, or seriously over inflate the tire.
Nothing to Worry About?
Because I had this theory that the high pressure of road tires makes them most likely to blow off the rim. I also believed that the 100-plus psi type of blowout on a road tire was the worst you could have.
And, since I’ve been right next to tires blowing off like this – sometimes only inches from my face – and never been hurt in any way, I believed that a bicycle tire blowout can’t hurt you.
So, I wasn’t concerned one bit when I installed a 700 x 40c gravel tire for a customer last week. He asked for help because he had trouble getting the tubeless-ready tire to seal.
He also said that he had put the inflated tires in his garage to see if they would seal overnight and when he came back one had blown off the rim. He had pumped them to 60psi. That’s not very much pressure, so I didn’t think there was anything to worry about.
Sonic BOOM!
I fixed the 700 x 40 tire’s sealing issue with a new rim strip. I next wanted to make 100% sure the tire would stay on the rim. I inflated it to 60 and let it sit and it was fine. No air loss and no signs of the tire creeping off the rim.
The tire had a maximum pressure of 75psi printed right on it. I know riders experiment with tire pressure and that pumps can be wildly inaccurate. So, I decided that I should test the tire at 90psi.
The instant the number “90” appeared on the pump’s digital gauge, the tire blew off the rim. The explosion was unlike any tire blowout I’ve experienced before. It didn’t knock me down, but I was rendered completely deaf and stunned, just standing there trying to regain my senses.
People came running to check on me. I could see they were talking to me, but I couldn’t hear a word. It wasn’t until the next day that my hearing fully bounced back. And I felt a bit woozy and out of it all day after the blow out.
I’ve heard stories of truck mechanics who were badly injured even killed by tires blowing off rims but I never understood how. When that gravel tire exploded, I experienced a whole new type of tire explosion — and it was dangerous and scary. It made me realize something besides pressure is at play. It never occurred to me that 90 psi could make a potentially concussion-inducing sonic boom.
Tire Volume Matters
One of the engineers at work explained the key factor I was missing. He pointed out that it’s the volume of air inside the tire that’s so hazardous. The more volume there is, the more force the tire exerts on the rim.
Since a 40mm wide tire holds so much more volume than a 25mm tire, if it blows off, the explosion is much, much worse. Now I know. It’s too bad I had to learn this lesson the hard way, but at least I wasn’t injured and I won’t make this mistake again.
My advice to you is to always follow and never exceed the tire manufacturer’s recommended pressures (it’s written on the tire somewhere). And remember that the wider the tire, the worse the blowout.
Remember that for most riders and uses, as tires get wider, the recommended pressures are lower because the additional volume of air allows the tires to perform just fine that way. So there should be less chance of a blowout – a very good thing.
Ride total: 8,856
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After your experience, you can just imagine the force involved when a truck tire blows off the rim. THis used to happen much more often when large trucks used split rims. Because the tires were so stiff and hard to mount, one side of the rim was actually removable–made of spring steel. You pried it on and it had to seat in a groove much like a tire bead does when you inflated the tire.
Sometimes these ~5 lb hunks of steel would fail–and blow off–with catastrophic results unless the wheel & tire were first places in a steel cage for protection..
At first order, it’s not the pressure on the rim (which is independent of the tire size – a pound per square inch is an pound per square inch) that’s greater in a larger tire. The effect you observed is due to the volume of air that escaped in the explosion, which creates a larger and longer pressure impulse in a larger tire.
The pressure is what caused the failure, but the force you felt is because there was more air at that pressure escaping. The explosive force dissipates over distance, and with more air creating the pressure pulse it takes longer to become a smaller force (that you experienced). That doesn’t change the facts, and I’m glad you weren’t injured. Thank you for sharing your experience to others can learn from it.
Pressures can be lower in larger tires because the your weight is spread out over a larger surface area (contact area).
Actually the force on the rim does change with tire size. The rim bead hook is resisting the sideways force of the pressure on the tire and this force gets larger as tire width (and thus height) increases. Some carbon rims have maximum pressure/tire size limits for this reason. So you may have to look at more than the max pressure printed on the tire to be safe.
Very interesting article Jim thanks! I now understand the big boom I was getting whenever it happened to me.
What Brian Rees states above is true, the magnitude of the event is related to volume.
Another factor is wall tension in the tire. Smaller radius confers lower tension. Larger radius, higher tension. Recollection is tension increases as the 9th power of radius length!!! Wider tires do not need as much pressure to support rider’s weight, and also CANNOT CONTAIN HIGHER PRESSURES. Here’s a simple reference:
http://hyperphysics.phy-astr.gsu.edu/hbase/ptens.html
Regarding the problem of difficulty getting tubeless tire beads to seat on rim sidewalls, my solution is warm the tire. If beads don’t pop into place on first inflation, I spin the wheel two revolutions while dipped in 4″ hot water in a laundry sink, then apply the inflator. Has worked every time, instantly, with my 700c road rims.
Be very careful, even with semi wide tires. I was running 28mm on HED Belgium Plus Rims and started experimenting with tire pressure. Normally I run 100-110 psi on 25mm and ran the 28mm’s up to 110. While descending Jackson Peak on a hot day with 8-9% downgrades the tire exploded. I called HED and was told I should be around 80psi with those 28mm tires. Learned the lesson the hard way. Wider Tires, lower the pressure.
Great comment but one Q: 80 psi seems low, i run 105-110 on my 28gatorskins. Were these tubeless tires?
I also want to remind folks that the pressure on the tire (max pressure) may not be supported by the rim.
I have seen rims fail when the wider tires were dutifully inflated to “max tire pressure” Unfortunately that exceeded the rim max pressure.
You’ll also want to verify your pump’s gauge accuracy. (My Topak runs 10 – 12 PSI lower than actual, according to several other gauges I checked.)
Im glad you werent seriously injured. A simple analysis: force = pressure x surface area and the internal surface area of a 40mm tire is more than 2.5 times times that of a 25mm tire, i.e., (40 / 25) ^ 2. Inflating a 40mm tire to the same pressure as a 25mm tire means the force on the tire/rim system is 2.5 times larger. Airliner tires are inflated inside metal cages for this reason (very large tires at 200+ psi).
Jan Heine discussed tubeless blowoff in large tires on his blog (https://janheine.wordpress.com/2017/05/29/the-trouble-with-road-tubeless/) and recommends no more than 60 psi.
That’s correct about aircraft tyres being inflated inside a cage. or should be. As well, they generally use nitrogen. Aircraft rim also have fusible plugs that release the pressure if the rims get to hot. I realize that this is not bikes but it certainly show wnat can happen when tyres and rims part company
https://aerossurance.com/helicopters/dangers-of-tire-inflation/
This happened to me nearly a month ago and my hearing has not returned to normal. Be careful and consider wearing ear protection whenever possible. I will be. I can’t afford to lose anymore hearing
Another factor influencing max pressure is rim width, wider rims reduce the maximum allowable pressure because wider rims make the tire inflate wider and deeper, increasing casing tension. Manufacturer recommended pressures for road tires are typically given for old narrow rim widths, modern wide rims need to have lower pressure to be safe.
I had an original tire on a 1985 schwinn bike blow out because the tire was over inflated and it literally tree me over the handle bars and to the concrete.
The spund was like a rifle blast and it took me a couple seconds to figure out what happened and had small rocks stuck in my elbow and me one felt like it was on fire