Jim’s Tech Talk
By Jim Langley
My neighbor Jessica brought her circa 2006 Trek Pilot 5.2 to me for a tune-up. She and her husband Kent are avid road riders. Her Shimano Ultegra Trek had the signs of a well-ridden machine.
Road grit covered the fork and frame, probably from our fog here; there was a black grimy build-up on the chain, cassette, derailleur pulleys and chainrings; the front brake was about to bind up; the seatpost collar was slightly loose and crooked; her dry derailleur pulleys cried for overhauling when I turned them; the Shimano MTB clipless pedals needed bearing grease and screw tightening; and she had a rear flat.
Her primary complaint was shifting hesitation. My chain checker showed that a new chain would be the likely solution. I asked her about the rear flat and whether she had a spare tube or if I should supply it and charge her for it.
She said that she had plenty of tubes because she had suffered a string of mysterious punctures recently and Kent had bought a supply. That made me take a closer look at her Bontrager low-spoke count wheels. Jessica said she had upgraded to them about three years ago.
Tubeless-ready Not Always Tube-ready
All it took was spotting two words on the rims and I was pretty sure I knew the source of her frequent and mysterious (to her) punctures. Those words were tubeless-ready (TR). I’ve seen the same TR issue with riders who use tubes in them several times now and I expect to see it more.
The problem is that someone, and it’s hard to know who – had installed rim strips that are too narrow to work properly with tubes. The rim strips’ job is to prevent the tubes from getting punctured from the holes inside the rim.
It could be that the maker of the wheels provided “temporary” rim strips that should work for a while. And since they think the owner will be setting the wheels up with tubeless tires soon (removing the rim strips and taping the rims in the process), it won’t be a problem.
Creeping Rim Strips
But, if you have too-narrow rim strips in any rim, the rim strips can move sideways. When that happens the rim holes can be exposed. These have sharp edges that quickly cut into the inflated tube, which expands down and into any part of the hole that the rim strip isn’t covering.
So if you run tubes in these tubeless-ready wheels, the next thing you know you have a flat. And, if you put a new tube in, you’ll just get another flat.
Even if you inspect the rim strip and make certain it’s covering the rim holes, it’s likely that it will creep out of position. This can happen during tire installation. The tire bead rests on the rim strip and even if you’re careful, it’s easy to move the strip while popping the tire onto the rim.
The other thing that causes it to move is the normal cycle of tires losing air and topping them off. As the tube shrinks and expands, it grips and can move the rim strip.
Rim Designs are Part of the Problem
Many tubeless-ready rims present another challenge when it comes to rim strips covering the holes. They have offset spoke holes that are closer to one side of the rim.
This means the rim strip only has to move a slight amount and the holes are exposed. Also, the holes can then be on the uphill side of the rim where the rim strip tries to slide down from. Offset spoke holes are great for wheel strength, but they make it even more important to carefully select rim strips.
In the photos I hope you can see that the rim strip is narrower than the rim. The strip measures and is labeled 21mm wide and the rim is 23mm. That 2mm difference doesn’t seem like much, but because the rim strip is not touching both sides of the rim, the gap encourages the rim strip to move. And when it does it is more likely to get pushed down into the rim holes exposing the sharp edges.
Adhesive rim strips are better than Jessica’s that are only held in place by friction. But, most adhesives don’t last forever so even sticky strips need to be right-sized.
If you have tubeless-ready wheels and you’re running them with tubes, plus you’re suffering frequent flats, I would take the tires off to inspect the rim strips and see if they’re properly sized for your rims.
If the rim strips are too narrow, you’ll probably see how some sharp spoke hole edges are exposed and have been in contact with the tube(s). You should remove those rim strips and install new rim strips or tape that fills the inside of the rim from rim wall to rim wall, no less, no more.
Depending on your inside rim width it might be difficult to find the right-width rim strips. If you can’t, it’s okay to use tape as long as it is strong enough not to get cut from the rim holes. Stan’s No Tubes makes rim tape in various widths designed for tubeless setups that will work with tubes, too. Be sure to read and follow the instructions.
Gorilla Tape (it’s like a heavier, thicker duct tape) will work, too. But, electrical tape usually doesn’t hold up very long (though you can double wrap it for longer life).
For Jessica, because I have it on hand, I used 23mm wide Gorilla Tape (I cut it to size from a wider roll – you can buy it in narrower widths, too). 23mm wide tape provides wall-to-wall coverage and will end her mystery flats hopefully forever.
Ride total: 9,730