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January 2018

January: Tire Safety: Tubular, Wired-on, Tubeless

by John S. Allen

The earliest bicycles had steel-shod wooden wheels like the wagons of their time; then in the time of high-wheelers, solid rubber tires gave a somewhat softer ride and better traction. Dr. Dunlop's pneumatic tire followed in the 1890s, with a much smoother ride and lower rolling resistance. But, with pneumatic tires came the possibility of punctures and blowouts.

Pneumatic tires come in different flavors. Wired-on tires have been the norm for many decades, except that tubulars (with the casing sewn around the inner tube) were for a time nearly universal on road-racing bicycles. And now tubeless tires are gaining in popularity.

Any of these can blow out due to a large puncture, possibly resulting in a loss of control of the bicycle. A slow loss of pressure, on the other hand, can often be detected before it becomes hazardous. With the front tire, you can look down and see how wide the bulge at the bottom is, but not with the rear tire. I've had flat tires that didn't reveal themselves until I felt little road bumps as the rim bottomed out.

A soft tire is always hazardous when leaning in cornering, as it can "wallow" -- deform so the bicycle weaves from side to side, even falls over. Certainly, as part of a pre-ride inspection, you always should at least pinch your tires between thumb and finger before starting a ride. That will at least catch slow leaks -- though unfortunately, most flat tires occur during rides. Sorry about that! If the bicycle begins to feel unsteady in cornering, bring it carefully to a stop and check the tire pressure.

Now let's look at the failures which are unique to each type of tire.

Tubulars

Tubulars and their rims are the lightest; a tubular can be replaced quickly on the road, but the glue that holds a tubular to the rim can fail and the tire can roll off the rim even if fully inflated. Sheldon Brown explains, here https://www.sheldonbrown.com/tires.html ,

This almost always causes a serious crash. If you replace a tubular on the road, you cannot corner safely at high speeds until you go home and re-glue the tire. For safe high-speed cornering, the glue needs to dry for at least several hours.

Is there any good reason to put up with this? Not really. Despite being lighter, tubulars are slower than the best wired-on tires. Tubulars "squirm" on the rim, adding rolling resistance, as Jobst Brandt has described; https://www.sheldonbrown.com/brandt/rolling-resistance-tubular.html. Patching a tubular requires work with a needle and thread (don't puncture the tube again!) and is time-consuming, so a patch kit is impractical as backup to a spare tubular, and you're stuck if you have a second flat during a ride. I gave up on tubulars -- when was it? Sometime around 1980.

Wired-on

So, then, are wired-on tires immune to sudden failure? Not, and not only due to large punctures. A wired-on tire can blow off the rim if seriously overinflated (more likely with a folding tire) or poorly installed.

One part of a checkover is to spin the wheels and see that the tires are evenly seated, without bulges. These commonly result from a rim strip's being too wide, or off-center. (The rim strip is the fabric, rubber or plastic band which covers the spoke heads or spoke holes in the rim). A tire which is not evenly seated bulges out at one place, and sinks farther into the rim at another.

Too-wide, off-center rim strip. The ride will be lumpy and this risks a blowout.

Also look for structural damage when checking a tire. We may think of tires as made of rubber, but the tire's structural strength is from fabric under the rubber covering. Two layers (plies) of threads run at opposing 45-degree angles between the wires at each side of the tire (called "bead wires" for some reason I don't know). Punctures from small shards of glass or nails pose no structural risk to the tire, but damage which cuts more than a few adjacent threads results in a weak spot even if it doesn't puncture the inner tube. Abrading away the outer ply of fabric may still leave the inner layer intact. The tire will have a bulge, but it will still hold the inner tube in place. But, over time, more threads can break, and then eventually the inner tube will balloon out through the hole and blow out. This kind of damage is often from poor braking technique which skids the rear tire, or to the tire sidewall from a stone, or from repeated contact with a misaligned brake shoe. In an emergency, a moderate-sized gap can be bridged with a tire boot -- a piece of strong but flexible material. Dollar bills are commonly used as tire boots.

If a wired-on tire suddenly deflates, it may or may not stay on the rim. If it comes off the rim, it will flop around, and the bicycle becomes uncontrollable. A tire which is a tight fit on the rim, and harder to install, is more likely to stay on the rim. As anyone who has worked as a bike mechanic could tell you, the tightness of the fit varies greatly: dimensions of rims are not closely controlled. In an article in the journal Human Power, http://ihpva.org/HParchive/PDF/hp51-2001.pdf, Dr. David Gordon Wilson (MIT professor, now retired, author of the book Bicycling Science and stalwart of the Boston bicycling community) describes experiments which he and his students conducted. The safest fit is one in which the tire rests tightly on shelves ("bead seats") inside each edge of the rim. Not all rims have these shelves: some hold the tire in place only by hooked flanges which capture the wires. The difficulty of installing the tire does not depend, though, on whether the rim has bead seats: it depends on the depth of the rim between the bead seats. Deeper is better, but then the rim must be heavier to withstand the air pressure's pushing the flanges outwards.

[CAPTION: Illustration showing a rim with bead seats and hooked flanges.]

Some rims have only one or the other.

Tubeless?

Tubeless tires have been standard on cars for decades now. Off-road bicyclists have been using them for a few years and they are just beginning to see use on road bicycles. Only certain tires and rims are suitable to run tubeless. Special rim tape seals off spoke-hole recesses in the rim. Usually, a sticky sealant is injected into the tire after it has been installed on the rim. The sealant achieves an air-tight seal with the rim, and also seals small punctures. Sealant dries out and has to be replenished from time to time. While tubeless tire users praise them for the freedom from flats during rides, they are definitely high-maintenance when it comes to fix a flat or replace a tire.

Tubeless tires are not immune to large cuts and scrapes. You need to carry an inner tube in case of a flat: patching over sealant isn't practical; also, the inner tube is necessary to hold a tire boot in place.

Tubeless tires add another mode of sudden failure. A puncture isn't necessary for a tubeless tire suddenly to lose all its air. This is due to "burping" -- colorful term!

A tire with an inner tube can move around on the rim without losing air, because the inner tube holds the air. A tubeless tire must maintain a perfect seal all the way around both sides of the rim. "Burping" is well-known among off-road cyclists who run their tires at low pressure. It occurs when a side load on the wheel pulls one side of the tire away from the rim. It may also occur due to running over an obstacle which compresses the tire to the rim, or a heavy landing. I have a friend who is recovering from a broken hip as a result of a heavy landing in a cyclocross race which burped all the air out of a tubeless front tire.

Tubeless tires on road bikes can also blow off due to high air pressure, when a tire with an inner tube would stay in place. The tube, pressing against the tire, actually helps hold it in place. Jan Heine, of the Off the Beaten Track blog and Bicycle Quarterly magazine, doesn't recommend running tubeless tires above 60 PSI. On the other hand, he has conducted extensive testing to show that tires with enough width to run at this pressure are no slower than narrow, high pressure tires, and more comfortable. Still, a 60 PSI limit makes tubeless tires unsuitable for heavy loads, as on tandems. Jan Heine's comments about running tubeless tires on the road are here: https://janheine.wordpress.com/2017/05/29/the-trouble-with-road-tubeless/ . Of interest, he reports that tubeless bicycle tires do not have significantly lower rolling resistance than ones with thin tubes -- probably because the sealant liquid inside the tire increases drag. He also points out that sealant may be used inside an inner tube, so you can have either the best or the worst of both worlds. Sealant should be installed when the tube is new. If you have decided to use sealant in a tube, then you have decided to replace the tube if it does puncture: sealant makes patching a tube difficult too.

Summary

To summarize: as with many innovations in bicycling which increase performance, there are tradeoffs with tires, both in safety and in the difficulty of repair and maintenance. This was the case with tubulars (though the performance improvement was mostly an illusion with them) and it is the case with tubeless tires (though the same may also be true). My own choice is to stay with tubes, though that may change as tubeless-tire technology becomes more mature. You pays your money and you takes your choice, as the saying goes. Be safe out there.

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