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Technical FAQ: Follow-up on blow-outs

  • By Lennard Zinn
  • Published Feb. 2, 2010
  • Updated Feb. 2, 2010 at 9:49 AM EST

Dear Readers,
I am at the Park Tool Tech Summit in San Jose, learning more about working on bikes. I’ll tell you about it in a future column.

I received interesting feedback on my column last week on blowouts. Here is a representative cross-section:

Dear Lennard,
I read your recommendation for avoiding a tire blowout and got some good tips. Thanks. I just thought I’d mention that from my perspective it was a little confusing trying to understand when to put the tube valve into the valve hole in the rim.

You write, “…you need to make sure to finish at the valve stem, and not start at the valve…” This was a little confusing, because here I think you’re already talking about the point in time defined by, “…after the first bead is on and the slightly-inflated tube is in place inside the tire ….” In fact, to follow your method, most people are thinking of the tube in the tire being completely separate from the rim, and then it seems like you do need to start at the valve, putting the valve stem into the valve hole in the rim, then putting the first bead on. I think it would be helpful to others if you specified when the valve stem is inserted into the hole.

Great tips though. Amazing how you can improve on things that you thought you’ve known all there is to know about for years.
Aaron

Dear Aaron,
Thanks for pointing that out. In my maintenance books, I actually do go into the step by step of putting the first bead on, then putting some air in the tube, then sticking the valve stem in, then shoving the tube into the tire all of the way around, and then continuing pretty much as I described last week. I should not have cut corners to try and keep the column a bit shorter, since I’m sure others were wondering that, too.
Lennard

Dear Lennard,
I’ve been riding narrow clinchers since the first Michelin high performance 700c clinchers came out in the mid 70’s. Over the years I’ve had my share of flats but only one blowout on a Weinmann rim. As you pointed out the trick is to make sure there is no tube trapped under the tire.

I do a full pass around the tire to make sure the tube is inside the tire (a bright coloured rim tape helps here). I think it’s important to emphasize the need to use lots of tube talc with the latex tubes. With normal tubes you can usually get away without using talc, but with the latex tubes it’s an absolute necessity especially if you’re reusing the tubes for new tires. Out of the box the Michelin tubes are well lubricated but that only works for the first use.
Bob

Dear Lennard,
Regarding your most recent columns on tire mounting, I’ve had a couple of incidents with pinched tubes and your advice is right on. I blew a tire off the bead once, fortunately on a straight road with no braking and it was a real head-turner for me.

I would only do one additional thing, something that’s allowed me to discover a couple of potential problems. I read this somewhere long ago.

After fully mounting the tire and tube, I inflate the tire slightly, then push the bead back from the rim all the way around the wheel on both sides.

This allows me to see if the tube is pinched in the bead bed and may also allow the tube to un-twist if that has happened. A couple of times I’ve found the tube under the bead. Pushing back the bead allowed the tube to escape and move into the casing of the tire. If necessary I massage the area until the tube retreats. Then I seat the valve stem and inflate, inspecting once more all around as I do so.

By the way, I usually leave the discharged CO2 cartridge in my inflator after a flat. It never fully discharges on the original fill and it retains enough gas to do the initial tube fill, leaving the full new cartridge for final inflation.

I don’t race. I ride the lightest butyl tubes I can find (usually Specialized Turbo) and I have about 2 punctures a year. With this mounting regime, I’ve had no problems. I am 6’5″, 190 lbs so I’m not so easy on tires and I ride fast (I pass all my friends on descents), but haven’t had a flat on a descent in years and never from a bead pinch, even on the lousy roads and in the summer heat of the rural Tucson area. I ride light 25c tires (Conti GP 2000S), aluminum rims with standard Campy pads.
Larry

Dear Lennard,
Back in the day (late `80s) when I raced I used latex tubes … briefly. I was racing at the Spokane stage race and during the criterium had an explosive, catastrophic blowout on my front wheel. Now mind you these were on my racing wheels which were carefully prepped (I used to be a bike shop mechanic) so I can assure you that they were properly mounted.

The other issue is that the terrain was rough road surface and a fairly twisty, up down course. Luckily I did not go down as I was going in a straight line downhill and was able to clip out and coast into a hay bale and since I was at the front I could get out of traffic safely. On the post-mortem I looked at the tube and tire and the tube (a Michelin latex) was annihilated and the tire was somewhat damaged as well due to the speed and the circumstances of the blowout. I thought about this for a long time and as I never had this sort of thing happen (catastrophic, lose all air instantly front blow out) thing happen from a butyl tube, I played around a bit with both the latex tubes and a butyl tubes and came up with the hypothesis that the problem is derived from the inherent flexibility/slipperiness of latex versus butyl.

I have no way to prove this but I’m pretty convinced that latex tubes are inherently “slippy” when inflated inside a rim/tire and because of this they can move around more inside and under conditions of extreme activity (e.g. a criterium) I actually think that if you are at the limits of the tire/tube/rim interface they can actually work their way into the bead area and you get an induced snake bite….but due to the nature of the latex this tends to be catastrophic rather than progressive like with a butyl tube.

Anyway, that’s my theory and I stopped using latex tubes because of this concern and have never had something like this ever happen again.
Eric

Dear Lennard,
This is actually for Zack Vestal’s piece-in-the-piece regarding the yellow Swiss Stop pads.

The amount of heat generated is a function of the amount of energy that’s dumped. Thus, if you can propel yourself up a hill with 200W at 10 mph, and you’re descending at a constant speed of 25 mph (ignoring air friction!), you’ll be dumping 2.5 times as much energy/heat into the brakes, or 500W. That part of the equation isn’t a function of brake pad material at all.

So where can that heat go? Three places that I can think of:

  1. The wheels – a nice, big aluminum heat sink with lots of air rushing past.
  2. The brake arms – not so big, and probably bolted to a carbon fiber frame. Much less potential to dump heat there, and
  3. Direct cooling to the environment.

The only effect that the yellow pads could have is to reduce No.2 by possibly being a poorer heat conductor. But that probably isn’t a large proportion of the loss anyway, especially if it’s a carbon frame. So I’ll go with Lennard’s suggestions.

I greatly appreciate the columns that both of you write – I just thought I’d throw my two cents in this particular case.
Peter

Got some mountain bike tech questions? Check out Lennard’s FAQ on Singletrack.com


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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.

FILED UNDER: Bikes and Tech / No Spoil / Technical FAQ TAGS: / / / /

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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