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.
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
Next – Shoe Wedges or Q-Factor?
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s streak of consecutive cycling days has reached more than 8,000. Click to read Jim’s full bio.