Constant Slow Leak Bike Tube Punctures, Why?

Jim's Tech Talk

By Jim Langley

While I was helping reader "Fixieguy" with this week's topic, which we’ll get to in a minute, I heard back from Stephen Weeks. You’ll remember that Steve had switched to trekking handlebars and he was having shifting issues with his new lever and triple chainring shifting. (If you don't recall Steve, here's a link: https://www.roadbikerider.com/malfunctioning-front-shifting/.)

Steve sent photos and this great ride report:

"It's like I have a new bike! The front shifting is very smooth; in fact it's not much different than it was with the STI road shifter. There's no grinding, and the chain shifts quickly and quietly. No jumping off to the inside or outside… not surprising, since the stops are the same as they were before. The rear derailleur is just as smooth as before.

I got in 35 miles on a beautiful day. Mid-way through the ride I moved the new trekking bars to a more "flat" position because I felt some strain on my wrists. I think the new position is better. I see no reason to rush into a replacement shifter either. Even though the SunRace unit has a steel bracket, it doesn't weigh much and it's sort of neat having a cheap component doing its thing among all the other high-end stuff. Thanks again to you and RoadBikeRider's readers for all your help!"

To explain a little more, the solution for Steve's balky front shifting was going to an old-school friction thumb shifter, remember those? I found a pair by SunRace that cost less than $20 https://amzn.to/3xX1CAA and he used only the left one. That's the "cheap" component he's talking about.

Friction shift levers don't have set positions for each shift. You just push the lever until the chain is on the chainring you want it on. They’re simple and effective and usually compatible with new and old components.

What follows is Fixieguy's and my back-and-forth email exchange, which covers a lot of possible problems and solutions. Please have a read (it's long) and comment with any ideas that come to mind. "Fixieguy" is a regular contributor to the comments who shares his great tips and he’ll be reading along.

"A plague of flat tires"

"I’m taking the liberty to ask this question of you, Jim out of nothing less than desperation, and I am hoping you might have some ideas.

Over the last couple of months I have been experiencing a plague of flat tires. Uniformly, these are neither tire punctures nor pinch flats. They are tiny pinholes in the tube on the rim side of the tube. Sometimes they will occur while riding as very slow leaks; other times I’ll go into my basement and find a bike with a flat tire.

If relevant, I generally use Michelin Air Stop A1 butyl tubes for 18-25mm width tires. I do use them also on 28mm wide tires, but I inflate these tires only to 67 – 70 psi, which ought not stretch the tubes terribly.

Most of my wheels are Weinmann DP18 wheels that have deep side walls, and the spokes’ ends are well inside the rims. Some of my wheels are, e.g., Mavic Open Pro or Mavic CXP series, and there are a couple of sets of seemingly indestructible Araya rims from the 1980's. I have never observed any spoke ends protruding through or even into the rim tape or rim strips on any.

When it is obviously older tape, I do observe a darkened ring around the spoke holes and a slight depression in the tape. When I have removed the rim tape or strip, I have not observed any spokes poking up. Of course, it is possible that I am inspecting the spoke holes improperly, which might change my whole analysis.

At one point, I thought the flats were caused by degraded rim strips. I had used Ritchey plastic rim strips on several bikes. I noticed (by matching up the location of the pinhole relative to the valve to the rim strip) that often it appeared that there was a spot on the rim that matched the pinhole where the rim strip had slid to one side leaving the part of the rim where the base meets its side uncovered. On removing this rim strip, it felt quite dried out and stiff. I replaced it with new Jandd Velox tape. In most cases, that has seemed to resolve the problem with no recurrence on that wheel.

A couple of times I have found there to be old Velox tape, and I have replaced that with new Velox tape. Again, that has seemed to resolve the problem.

However, yesterday I had another such flat. It was a bit larger than a pinhole and the air escaped fairly quickly, so this may be an aberration and unlike the other tiny pinhole flats. Nevertheless, it was again on the inside of the tube. It was about 3 cm from the valve and matched up with the first spoke hole to one side or the other of the valve. While it looked like a spoke hole puncture, this was one of those Weinmann DP18 wheels, and there was no evidence of a spoke protruding up.

To further complicate my analysis, the rim tape appeared to be fairly new Velox tape. My maintenance log stated that I had installed that rim tape exactly one year ago, and my training showed I had only ridden that bike about 550 miles since. The rim tape appeared in good shape except that it had slid down in a couple of places, but neither were near the location of the hole in the tube. I elected not to replace the rim tape. Instead, I put about a 6" piece of older Velox tape over the existing tape over the valve hole, just to see whether that would work. After about 24 hours, it seems not to be leaking air.

It's one thing to get a puncture while on a ride. You replace the tube, make sure you remove whatever sharp object punctured the tire, perhaps use a boot between the tire and the new tube, inflate, and ride on. With a pinch flat, you curse yourself for riding through a pothole, replace the tube, and providing the rim is not bent, ride on. However, these mystery flats of mine are disconcerting because another flat could shortly occur (and has on more than one occasion). I carry two spare tubes. After that I’m at the mercy of Providence, a passing Good Samaritan, or my wife's availability.

After this long presentation, which may contain much more detail than needed, for which I apologize, I wonder whether you have thoughts on what else I might examine or what might be the cause of these flats. It seems consistent that replacing the rim tape has solved each of the problems. Ideally, I’d like to engage in some preventative maintenance to catch these problems before they occur. I could replace the rim tape on all the wheels, but with more than a dozen bikes and having replaced rim tape on only 4-5 rims thus far, that seems like overkill without knowing the cause of the problems. I’ve seen discussion of these types of flats on the internet, but no one seems to have a definitive answer. I’m hoping you’re the guy who does."

I appreciate all the details and I have an idea what might be the problem. What could be causing those flats is not protruding spokes but your other thought, the rim holes. Their sharp edge can cut a small and also a large hole in the belly of the tube.

This is only a problem on rims without ferrules covering the rim holes – the brass "grommets" that cover the rim holes on some rims – not as common anymore.

Rim strips are supposed to cover the sharp rim hole edges but if the rim strips move or they’re cut by the rim, the sharp edge can wear through the rim strip and eventually puncture the tube. I’ve had this issue with plastic rim strips. The rim holes damaged the rim strip and ended up popping the tube. This was on a HED wheel with a HED rim strip and I refused to believe the HED rim strip was the problem, but replacing it fixed my flat problem.

Another way I’ve seen it happen is that when the tire is inflated the tube pushes the rim strip down into the rim hole. If the rim strip gets pushed down far enough the edge of the rim hole becomes uncovered and can cut into the tube. This problem is worse if the rim strip is not wide enough for the rim and/or the rim strip is installed a little offset to one side or the other. Since only one edge and maybe only a few mm of the rim hole edge is rubbing it can create a tiny hole.

Rim strips are really fussy in other ways. Depending on the design of the rim you might need a narrow or a wide rim strip to prevent it moving. And, sometimes it's not logical. The wide rim may need the narrow rim strip and vice versa. You have to test fit the rim strip to make sure it's held by the rim in such a way that it can't move on its own. It's easy to get fooled and choose the wrong one.

Also, if the rim strip is adhesive and applied like tape, it tends to stick and stay put better you’d think. But that's not always the case. Sometimes they don't stay stuck and if so they can move, squirm out of position, which you’ll see when you go to fix the flat. I’ve had that happen with the Velox rim strips even though they usually stick and work great.

There are different ways to fix the problem. The right rim strip should do the trick but you’ve been using standard rim strips and they’re not. So you could try the belt and suspenders approach and put a wrap of electrical tape down and then the rim strip on top of that. The problem with this is that if it's a modern undersize tire, the extra thickness of the tape can make the tire more difficult to install.

Another approach is to switch to tubeless rim tape. Some mechanics like Gorilla tape which can be bought in 1″ wide rolls https://amzn.to/3bA5pwo and is pretty easy to cut down to whatever width you need. You’d experiment first to get the width just right. Then you can rest the blade of a utility knife on a wood block the right width and carefully turn the roll of tape against the blade to size it.

Or Stan's No Tubes makes tubeless tapes in different widths that works on different width rims:

https://www.notubes.com/shop/tubeless/valves-tape. Stan's tape is adhesive and made to not move so that it can seal tubeless tires. Both it and Gorilla tape are stickier and should be less likely to move than regular rim strips. That also means they’ll be harder to remove when that time comes.

Those are a few ideas for you, Fixieguy. It's a great question so I will use it and my answer in my column this week. That way we’ll hear from other readers on what their theories are and hopefully hear about their favorite rim strips, too.

"Thanks for your quick and very detailed response. Apparently, these pinhole flats are a common and complicated problem. The cause and solution are as I suspected the rim tape. I use 16mm wide tape on all my wheels as they are not the newer, wider rims in vogue. I do have some old Mavic MA2 and MA3 rims that are a bit wider, but these have not been a problem yet. I know the Weinmann rims do not have grommets. I think the Mavic Open Pro do and am not sure about the Mavic CXP series.

In reading your response, I realized one more point that may be relevant. I have recently switched over to 28mm tires on about 2/3 of my bikes. Three have Conti Grand Prix 5000 tires, and I’ve had no problem with these, which I ride at 70 psi per Silca's Pro Calculator. They’re expensive and at my septuagenarian pace, I decided that I could live with lesser expensive training tires. I started buying Vittoria Zaffiro Pro V tires. They have folding beads and are quite light and comfortable, which I ride at 67 psi; they are on four bikes. Another bike has an old pair of Kenda 28mm tires that have given me no trouble.. Most, but not all, of these pinhole flats have occurred on wheels with the Zaffiro tires, a couple have been with Michelin Pro4 25mm tires. The Zaffiros are a bit hard to mount initially. I wonder whether the bead is moving the rim tape when it is being mounted, but I doubt it.

In addition, I used to ride much higher psi until I started using Silca's calculator, which you brought to my attention about a year ago. I’m wondering whether the lower pressure, which allows more flex in the tire has some bearing in causing the rim tape to slide. I can't quite figure out how that might work. I assume that once the tire bead is set, it is not going to move. Perhaps the tube itself is moving around, e.g., when cornering, which moves the tape, which exposes a rim hole edge, etc.

Nevertheless, the solution seems to be to replace the rim tape. Your suggestion of using electrical tape (I’m assuming you mean the cloth friction tape, not the plastic kind) seems the simplest idea. I just checked my friction tape and found it to be 18mm wide. I‘ve a Mavic Open Pro lying around that I’m not using that has 16mm Velox tape on it. I laid some friction tape over it, and it seems to fit. I suspect that were I to lay friction tape directly on the rim and then put Velox over it, the tire would still mount properly. I’ve doubled up on Velox in the past and had no trouble mounting or seating tires, so the additional thickness ought not be a problem. It's the additional 2mm width that might be the problem.

I still would like to know what causes the rim tape to move, because that seems most likely to be the problem.

I’ll be interested to learn whether other readers have other ideas before I embark on inspecting and/or replacing rim tape on two dozen wheels, which seems a dreadful project."

On the electrical tape, I should have explained that it's regular plastic not cloth/friction tape that I use. That's thinner and stickier. On its own it won't protect the tube but underneath a rim strip it can unless the holes are really sharp. It just adds another layer and it's less likely to move.

The main thing that prevents rim strips from moving is having the right width rim strip. Both edges should be in contact with the rim. If not, then the tape can move. With the tube and tire pressure holding it down and the edges keeping it in place it shouldn't be able to move.

You’re right about too tight tires. With them if the beads push the rim strip as the tire is put on they can wrinkle and scrunch the rim strip and push it to one side exposing the rim holes. The key thing when installing tires like this is to do a final check squeezing the tire all around and looking inside on both sides of the tire to be certain the rim strip is still where it needs to be. A lot of people don't know about this check or forget to check.

Actually, it's even better to just work very carefully with too tight tires to make sure the beads do not push on the rim strip.

Regarding different tires, tubes and psi, if the rim strip is in there correctly, different tubes, tires and different tire pressures shouldn't cause it to move. But, if you ever had latex tubes that lost air every night, you might have noticed that the loss of pressure can cause the valve to go crooked. Same thing with kids bikes that don't get aired up and get ridden a lot with skidding tires, etc. Those tires and tubes will try to rotate on the rim. I don't think that's your problem.

I just had another idea. You could probably find a round/ball-shape grinding/sanding bit the right size to fit inside the rim holes and use a Dremel or a drill/driver to smooth the sharp edges of the rim holes. I haven't tried this but if the sharp edges aren't sharp they shouldn't be so likely to cut the rim strip and tube – I think, anyway.

Fingers crossed we’re getting close to a solution.

"Thanks, Jim. For now, if it recurs, I’ll try the plastic electrical tape underlayer before installing a new layer of Velox. What's your recommendation on making a hole in the plastic tape to allow for the valve hole? I’m thinking of using an ice pick and then widening it with a tube valve and trying to cover the valve holes's edges before adding the Velox."

The ice pick is good for making the first small hole so you have it centered, Fixieguy. Then if you use a utility knife and make a little X centered on the hole, when you push the valve through the X, the X should open and the V flaps should fold down against the edges. That's the way I do it with tubed and tubeless tapes and it works well.

If the valve is loose in the hole or it's at risk of getting cut you could also try putting a wrap of electrical tape right where the valve meets the metal. But there isn't always room for tape if the valves fit tightly. And it depends on the type of valve in the tube, too.

Whew, we covered a lot of ground there, but repeat flat tires can be maddening like that. Thanks for reading and if you thought of anything to help Fixieguy, please share it in a comment. Thank you and here's to flat free rides for everyone!

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 cycling streak ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim's full bio.