Technical FAQ: Wheel flop and patching inner tubes

Lennard Zinn addresses questions about several topics this week, including a lengthy follow up on patching tubes.

More on wheel flop

Dear Lennard,
Your wheel flop formula in the Dutch bikes answer is confusing:

“The amount of wheel flop is equal to the sine of the head angle (in radians) multiplied by the cosine of the head angle and by the fork trail.”

Why mention radians? Is fork trail the same as trail? Or did you mean fork rake?
— Michael

Dear Michael,
I mentioned radians because of the way I use the formula for wheel flop when I design a bike; I have to make sure the head angle is converted to radians or I will get a screwy answer. That’s because I initially design each bike in Excel. I have embedded the formulae for trail and wheel flop into the Excel spreadsheet, and I look at both results when I’m determining what head angle and fork rake to use on the particular bike, based on what the customer will be using it for and what his or her riding style is. [related title=”More Tech FAQ” align=”right” tag=”Technical-FAQ”]

I have this formula for wheel flop: “=SIN(U34)*COS(U34)*N5” in a cell in my Excel spreadsheet, and if the contents of cell U34 (the head angle) are in degrees, rather than radians, it will produce the wrong answer (the number in cell N5 is the fork trail, or trail).

Trail and fork trail are indeed the same. I certainly did not mean fork rake. Here is an explanation of the difference that also includes the formula for trail — T = (r cosØ – R)/sinØ — where fork rake (a.k.a. “fork offset”), “R,” is the perpendicular distance from the center of the front hub to the steering axis. Fork trail, “T,” is the horizontal distance between the center of the tire contact patch on the ground and the intersection of the steering axis with the ground. Head angle, “Ø,” is the acute angle between the steering axis and the horizontal. The wheel radius is “r.”

From it, you can see that the formula for wheel flop, namely:

Wheel Flop = TcosØsinØ
= (r cosØ – R)cosØsinØ/sinØ
= r (cosØ)2 – RcosØ

This is a much longer, but more complete, answer to Steve’s question. I apologize for taking a shortcut last week.
― Lennard

More on hydraulic levers compatibility

Dear Lennard,
I read your response in your Aug. 14 column to the rider enquiring whether XT/XTR hydraulic levers are compatible with Ultegra flat mount calipers. An alternative solution for a hydraulic disc brakeset that has flat-bar levers with flat-mount calipers is to use the Shimano Metrea brakes. I recently fitted a set of these brakes to one of my wife’s bikes, as she prefers to have her road bikes set up with flat bars.
— Nick

More on patching inner tubes

Dear Lennard,
Right on. I used to sand but for years now I’ve been buffing butyl tubes with lacquer thinner and a rag instead of sanding. The rubber expands a bit with application of the solvent and you’ll see some black on the rag. In my opinion, this results in superior adhesion and is a bit quicker and easier. I feel this works better for punctures on or close to a mold line in the tube despite not removing the mold line.

I do like to disrupt the clear plastic film on the top of the patch by forcibly stretching it after the patch is adhered. I feel the film inhibits stretching of the patch and localizes the strain at the edge the patch, largely negating the benefit of the patch’s feathered edge.
— Lou

Dear Lennard,
Why on earth are you guys patching tubes when you get a flat? When I get a flat 25 miles into a 60-mile ride, there is no way I am patching it on the side of the road with glue and all that nonsense. I simply replace it with a new tube.

I guess I don’t understand the circumstance where you would patch a tube. Doing the water test implies you are at home when you notice or fix your flat, which is the exception, not the rule.

Also, I get about one rear flat per 2,500 miles (one flat per 12,000 miles for a front tire). If you get substantially more than that, you are either riding through debris or you need to replace your tires with newer tires or a different brand. Flats are usually a symptom of bad or worn tires or riding where you shouldn’t. All are forms of operator error.

However, I do routinely patch tires that suffer cuts that go through the casing, so long as the rest of the tire is in decent shape. Basically I use multiple layers of electrical tape (each successive layer is slightly longer than the one it goes over, a technique which has a 100 percent “good to go” fix rate that will outlast the tire). Usually four layers will make it bulletproof and it doesn’t seem to affect balance. These electrical tape strips are pre-cut and stored as functional anchoring tape ends for my handlebar wrap.

My guess is the same people who patch tubes don’t patch tires. They just leave their tires with all these knife-like cuts and slashes in them that expose their tubes to getting more flats, which is probably why they get so many flats and feel the need to patch tubes to begin with! Again, pilot error for patching tubes instead of tires.

I also find it difficult to wrap my head around the supposed cost-saving motive at work here: that people who spend tens of thousands of dollars on state-of-the-art cycling equipment are patching tubes that cost $4 (you buy them online in bulk and avoid the $16/tube bike shop ripoff) — for tubes that are only changed once a year and once every several years for front tires. If your water bottle gets a hole in it, you don’t patch it. If your sock gets a hole in it, you don’t patch it. If your derailleur cable frays, you don’t splice it back together. If your car’s windshield wiper comes apart, you don’t glue it back together. If your toothbrush breaks, you don’t duct tape it back together. So I don’t know why tubes get repaired and yet every other $4 product you own gets replaced. Tubes are a hell of lot more essential to your bike than practically any other piece of equipment on your bike.

So I don’t understand this whole patching-a-$4-tube thing for something that will likely end up staying in the tire for a year (front tires for several years). By using brand new tubes, I go through roughly $8 in new tubes per year ($80/decade). So is saving maybe $45 per decade by patching vs. replacing tubes supposed to be some kind of incredible cost-savings trick I am missing out on? Where’s the math to justify this decision?

And most pinch flats are the result of people who fail to coat their tubes in talcum powder (cover your freewheel and hubs with a plastic bag) or are riding over metal sewer grates, manhole covers, etc.

Plus, the fact that the very same cyclists who are religious about patching tubes routinely ride criteriums and training rides and suffer thousands of dollars in equipment damage to their bike from crashes (I won’t even get into the medical bills) while competing for the possibility to win a $250 purse or that sprint to the next telephone pole is also bizarre behavior to me from a cost:benefit analysis. So the very same people who will gladly pretzel their $1,200 carbon fiber wheels and incur a $2,000 medical bill deductible in a local criterium but will then patch their tubes to “save money.” Yeah OK, Forrest Gump. Good luck with that shrimp boat business.
— Phil

Dear Phil,
I patch tubes because I like to, because I always have, and because I like fixing things that are broken rather than throwing them out. I suppose that is why I write bicycle repair books.

Like you, when I am out on a ride and get a flat, I do not patch the tube; I replace it with a tube from my spare bag (which may be a patched tube). But I do generally bring along a patch kit because there have been times when I ran through all of my spare tubes and kept getting flats descending some crazy steep dirt road I was exploring on a road bike and ended up having to ride home with tubes that I had tied knots in to isolate the punctures. I suppose now at my age those days are over, and I no longer need to prepare for eventualities like that.

And while flats may indeed be indicative of riding a tire too long, they can also indicate that you are taking your bike on some adventures beyond what it is designed for and what other people do. I find it a bit sad that I do so much less of this now, but now that I have an e-bike that lets me get back on sketchy roads in the mountains, I get more flats again.

Yes, I am patching the tubes at home. Once I have developed a pile of punctured inner tubes, I go through and patch them all at once. It doesn’t take much time, once I have them all lined up and marked (I dip them, inflated, into my koi pond to find the holes); the glue is drying on some while I’m already applying patches on others. I also used to pay my daughters when they were little to patch my piles of punctured tubes. I think it was a good experience for them.

I find my life to be more in balance when I am not in such a rush and don’t think my time is so important that I can’t take the time to patch some inner tubes. Of course, some patches don’t work (I usually know immediately or find that out on the first ride), and I don’t even try to patch ones with multiple holes or holes near the valve stems. I never raced on patched inner tubes (or patched tubulars, once I’d learned that lesson once back in the 1970s). I like your idea of patching your tire casings; I don’t do that, nor do I ride tires that have cuts in them.

Eco-Cycle here in Boulder charges $0.50 to recycle each bicycle inner tube, tire or tubular, and I pay it (it adds up, as we also change tires on customers’ bikes and toss them in the recycle pile as well). It is not to provide financial incentive to patch tubes but rather to see tubes I don’t patch get reused in some way as well as to remind myself that I’m not the only one on this planet, and I have a responsibility to live lightly on it wherever I can. That’s the same reason I have a solar array in my yard and an electric car and water heater.

Indeed, I don’t splice broken cables or patch windshield wipers, water bottles, or socks. That said, while I don’t repair broken toothbrushes, I do use toothbrushes that have replaceable heads so that I’m not replacing the entire handle and everything just because the bristles have become mashed out. That’s the way I see a punctured inner tube — as a perfectly good item that just has one minor, fixable, defect.

Each of us has different quirks, and some of us like patching inner tubes.
― Lennard