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Technical FAQ: Avoiding that frightening blow-out

  • By Lennard Zinn
  • Published Jan. 26, 2010
  • Updated Oct. 11, 2010 at 2:30 PM EDT

Dear Lennard,
First, some background: Last season I experienced two sudden (and very frightening) front wheel flats on long, two-mile, twisty and steep descents on hot summer days.

Neither of these flats were puncture-related but instead were caused by the inner tube/tire blowing off the rim because of heat. These are DT Swiss RR1450 wheels with Michelin latex tubes, Veloflex Pave tires and yellow SwissStop brake pads. By the way this is my only wheelset on which I have to use levers to install tires, the tire-to-rim fit is really tight. The inner tubes both times showed a long (6-inch) rip, not a cut. Needless to say these episodes have taken away some of my descending mojo since I do not want to repeat the event. I was able to stay upright both times but consider myself very lucky to have made it safely without crashing. I am an experienced descender with a week in the Dolomites with Andy Hampsten as part of my knowledge base.

I have been thinking about a switch to tubeless road wheels anyway, probably Campy Shamal 2-way fit.

My assumption is that tubeless tires would eliminate the possibility of the tire blowing of the rim. Is it valid to assume that tubeless wheels are the answer? Or is there a change I could/should make in my current tire/tube/brake pad combination that would reduce the chance of heat-caused failure without buying a new wheelset?

Please help me get back some of my descending mojo.
-Terry

Dear Terry,
Provided you use a rim recommended by the tire manufacturer for use with a road tubeless tire, and the Shamal 2-Way Fit would qualify, then, yes, this problem should not occur with a tubeless tire.

That said, I am willing to bet you that both of those flats were caused by the inner tube being caught under the rim bead. And catching the tube under the bead is far more likely to happen with a tight tire that you have to mount with levers. There is no way to get a long rip in a tube like that without blowing the tire off of the rim, and there is no way to get a tight tire to blow off of the rim (at least one that is sized appropriately to the rim – see below) without getting some inner tube underneath it to lift it off. Yes, the heat of braking precipitated the occurrence, but it was the trapped inner tube edge that, once the pressure came up enough due to the heat, blew the tire off.

When mounting any tire, but especially a tight tire, you need to make sure to finish at the valve stem, and not start at the valve like so many people do. This will significantly reduce the chance of that happening.

Not only is tire installation best by finishing at the valve stem, but removal of the tire is also most easily accomplished by starting near the valve stem. That way, the beads of the deflated tire can fall into the dropped center (“valley”) of the rim on the opposite side of the wheel, making it effectively a smaller-circumference rim onto (or off of) which you are pushing the tire bead. If you instead push the tire bead onto (or off of) the rim on the side opposite the valve stem as a high percentage of people do, the circumference on which the bead is resting is larger, because the valve stem is forcing the beads to stay up on their seating ledges opposite where you are working.

I would not be at all surprised to find out that you had such a struggle mounting these tires that you had to resort to tire levers because you had started mounting the tire’s second bead at the valve stem and worked around from there, finishing opposite the valve.

And if you’re considering tubeless tires – and it is no secret that I’m a big advocate, and agree with you that they could eliminate this risk — you should know that adhering to this mounting method is particularly critical with a tubeless road tire, because the bead is very tight, it is carbon fiber and does not stretch at all, and you must install the tire without tire levers to avoid damaging the tire’s edges that seal the air in.

With a standard, tubed clincher tire, there is another reason to finish at the valve stem besides ease of mounting, namely exactly what I think happened to you: ending up with a little bit of inner tube trapped under the tire bead where you finished pushing the tire onto the rim. You may even have taken the (appropriate and highly recommended) precaution of starting with some air in the tube to keep it from twisting and getting under the bead as you push the tire on, finally deflating it as is normally required only when it prevents you from getting the final few inches of the tire bead on. However, the edge of the flat tube can slip under the bead edge as it pops into the rim at that point, and this is even more likely to occur with a thin, latex tube.

When you pump it up to pressure with some tube caught under the bead, it may immediately blow the tire off of the rim (temporarily deafening you, the explosion is so loud), or it may explode at some point soon while riding. In either case, you will have an un-patchable tube, just like the two you’re now the proud owner of, with a long rip down its length. And, as you have now seen firsthand twice, you may experience risking your life if it is your front tire blowing at high speed, especially on a turn.

Minimize explosion likelihood by finishing installing the tire bead at the valve stem from either side. The edge of the deflated tube may still slip under the tire bead as you push the last bit on, but it tends to do it less because the tire is less tight when mounted this way. But those caught tube edges, if they’re there at all, will be under just the final sections of bead extending in either direction from the valve stem. If you then push up on the valve stem after the tire is on, you can suck the adjacent sections of inner tube completely into the tire chamber and ensure that none is caught under the edge, waiting to blow the tire off of the rim. In any case, before pumping, always inspect around the edges of the tire bead by pushing inward on them so that you can see down into the rim alongside the tire.

So here’s the mounting protocol for the second bead, after the first bead is on and the slightly-inflated tube is in place inside the tire:

  1. Starting at the side opposite the valve stem, push the tire bead onto the rim with your thumbs. Be sure that the tube doesn’t get pinched between the tire bead and the rim.
  2. Work around the rim in both directions with your thumbs, pushing the tire onto the rim. Finish from both sides at the valve, deflating the tube when it gets hard to push more of the tire onto the rim. Using this method, you can often install a tire without tools. If you cannot, use tire levers, but make sure you don’t catch any of the tube under the edge of the bead. Finish the same way, at the valve.
  3. Reseat the valve stem by pushing up on the valve after you have pushed the last bit of bead onto the rim. You may have to manipulate the tire so that all the tube is tucked under the tire bead; check that it’s all under before pumping.

As for a tire appropriate to the rim, I assume your tire is 23mm or perhaps 25mm wide or you wouldn’t care enough about weight to be using a latex tube. According to ISO, a 23mm tire is recommended only on hook-bead rims whose inner width is 13-15mm, and a 25mm tire is recommended only on hook-bead rims whose inner width is 13-17mm. That DT Swiss rim, which I think is about 14mm wide interior to the hook beads, would be ideal for 23mm and 25mm tires.

This letter originally was sent to my colleague Zack Vestal, and here is his response as well:

I’ve run latex tubes quite often and never had a problem. I don’t think you can fault the wheels — any aluminum wheel is going to absorb heat at about the same rate. I wonder if it could have been the yellow Swiss Stop pads. There are all-around pads but designed for carbon and therefore might be softer, and possibly hotter, than standard, when used on aluminum rims.

The fact that both your flats were on the front wheel suggests a problem with the rim tape, or perhaps a burr at the welded seam on the rim. Inspect that wheel carefully for any possible culprits.

The other thing that I wonder about is the latex tube being twisted, folded, or weakened during install. With the difficulty you describe in mounting, it could well be a case of the latex having a fold due to tight install, then weakening under pressure along the fold (which might be acting like a stress riser).

I just wrote a Q&A for the next VeloNews magazine issue describing the importance of partially inflating a latex tube, and using lots of tire talc on install to ensure that it goes in straight.

It’s always a bad idea to use a tire lever to mount a tire. Pinching the tire to the center of the rim, where the tire bed is deepest, often helps create enough slack in the bead to mount it by hand. Tire talc also helps the bead slide up and over the rim wall. Finally, you could try a thinner rim tape. Sometimes the thickness of cloth Velox, while being the best rim tape money can buy, causes the tire bed to be too tight for easy tire mounting.

However, after all that, to answer your question, I think you will find a tubeless road setup to answer your issues. For one, you can start with lower tire pressure, like 85 or 90 PSI, without fear of rolling resistance or puncture problems. So when it heats up, the pressure increase won’t be over 100 or 110, which is within a safe range.

Two, without an inner tube containing the air, the much sturdier tire itself plus some sealant provide much better air retention overall, with fewer failure modes.

If you are thinking about road tubeless anyway, I would absolutely give it a shot. See our Buyers Guide for a few options reviewed and tested. We’re huge fans of road tubeless due to the minimal incidence of flats, plus reduced rolling resistance and improved ride quality.

If you would rather not go that route, try installing your latex tubes as I have described.
-Zack

There you have it; everything you could want to know about running clincher tires with latex tubes!
-Lennard
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Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

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Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, 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 degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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