Bicycle Tires Explained
Since the choice of tire is the single most effective and reasonably low-cost way a rider can affect and improve (or degrade) the performance of his bicycle, it's time to take a long, serious look at this component. Too often the choice of tires is decided by the answers to two questions:
- 1. How long will this tire last?
- 2. How much pressure will it hold?
I hope that after reading this essay the reader will take a more nuanced, careful approach to his tire selection. You went to a lot of trouble to evaluate the bike you bought, carefully balancing its handling, weight, feel, materials, workmanship and appearance. You can really improve your cycling pleasure by taking the same trouble when choosing your tires.
Let's start with a few definitions. In any area where sales drive a lot of the discussion, clarity is, to be charitable, often lost. The way I have defined things below is not necessarily the way everyone does. Your bike shop may not use the terms exactly as I have defined them. But this is the way the words are generally used in the trade.
Tubular Tire. Often referred to as a "sew-up". A cloth casing is wrapped around an inner tube. The edges are sewn along the inside circumference. The rim for such a tire has a mild concavity. The tire is glued to the rim.
Clincher Tire. For the purposes of this discussion, it is a tire with a woven fabric casing with either metal or folding fabric beads. This is the kind of tire usually used by road cycling enthusiasts.
Open Tubular. It looks like a clincher but uses the unique construction method of high-end tubulars.
TPI, or Threads Per Inch. Tire fabric casings coming with different thread counts. On a woven fabric casing it may be as low as 20. On a high-end tubular, it may be as high as 320. Only the threads on one axis are counted. At least one clincher tire company wanting to have its tires favorably compared to tubulars has started counting both the warp and weft (sometimes called "woof"). This is deceptive. Higher thread-count casings are stronger, lighter, more flexible and more expensive.
Hand Made. The technique of inflating the casing then hand gluing the tread onto the casing. This is sometimes called "cold treated construction".
Vulcanization. Uncured natural rubber has little industrial use because it is basically unstable. In a short period of time it will rot. It is sticky and smells as it breaks down. It is generally accepted that in the 19th century Charles Goodyear discovered that by cooking natural rubber with sulfur he could create a long-lasting rubber that had an almost infinite variety of uses. This irreversible process of causing the rubber molecules to link together with a curing agent such as sulphur is called vulcanization. Today other compounds can serve the same purpose as sulphur. Notably carbon and silicon are used in modern tire rubber vulcanization.
A short history
Pneumatic tires were invented in 1888 by a Scotsman named John Dunlop. The first tires were an immediate hit with racers because they were so much more efficient than solid rubber tires. But they were too expensive and Dunlop's design was very hard to repair. Edouard Michelin advanced the state of the art with his detachable tires held to the rim with a series of clamps. This eventually led to the modern clincher tire. Racers adopted tubular tires which were glued onto wooden rims. They could hold more pressure and handled better.
Mavic produced the first aluminum rims in 1926. Fearing that aluminum tubular rims would not dissipate heat properly, Tour Boss Henri Desgrange prohibited their use in the Tour de France until 1931. Things remained relatively unchanged for racers and high performance cyclists until the 1980's. A wide array of tubular equipment, both in rims and tires, was available and avidly used by performance riders.
Tubular tires weren't and aren't without problems. Repairing a flat is difficult because the casing must be unstitched, the tube patched and then the casing re-sewn. Tubulars are held to the rim with glue. This must be done with care so that under hard cornering the tire doesn't roll off the rim. Long descents are a problem with tubulars, especially for bigger riders. Heat build-up in the rim with long, heavy braking can melt the glue, causing the tire to shift. In the 2004 Giro d'Italia many spectators were baffled as winner Damiano Cunego would switch bikes for seemingly no apparant reason. He was getting new wheels because of the melting tubular glue.
Miguel Indurain, a big man by stage racing standards, actually switched to clinchers for some descents. Since he was sponsored by tubular maker Wolber (owned by Michelin), he would quietly ride a bike mounted with Michelin clinchers.
In the mid 1980's, tire makers, understanding that there was a deep yearning for a high performance clincher, began a series of product innovations. Until then clincher tires and their rims were heavy. Michelin came out with their Hi-Lite clincher tires. While not up to tubular performance, they were good enough for most riders who had long dreamed of being emancipated from the difficulties of tubular tires. Rim manufacturers developed lightweight rims that would sustain the high tire pressures that heretofore had not been needed for clinchers. By the early 1990's Michelin was paying top-end racers such as Claudio Chiappucci to ride their clincher tires. The regular victories on clinchers in the pro scene (Chiappucci achieved UCI World No. 1 status for 2 weeks) proved that the performance gap had been narrowed.
Still, to this day, the choice of most professionals is tubulars. Yet the sales volume, hence the money, is in clinchers. So a large number of pros are paid to use clinchers.
The different kinds of tires
Tubular tires. As noted above, for years the only choice for a high performance cyclist was tubular tires.
There are two basic kinds of tubular tires, Vulcanized and Hand made.
Vulcanized Tubulars. The vulcanized tubular has the tread vulcanized to the casing before the casing is sewn around the tube. This is far less expensive than cold treating. You can expect that cheap tubulars will be vulcanized, have a low thread count and when inflated on the rim are rarely straight. Cheap tubulars usually have butyl inner tubes rather than the light, more efficient latex tubes found in better tubulars. Cheap sew-ups fail easily because their low-thread-count casings are very fragile. The ride quality is still pretty good, but I think this partly attributable to the flexible, supple design of a box tubular rim. There are good vulcanized tubulars. They make an intermediate step between cheap tubulars and often staggeringly expensive hand-made sew-ups. I know very good riders whose judgement I value who say that a cheap sew-up has a better ride quality than the best clincher. I'm not so sure. The easiest way to spot a vulcanized tubular is by looking at one without air, uninflated. It will be flat, looking almost like a thick strip of cloth.
A vulcanized tubular is more prone to flats. The tread was vulcanized to the flat fabric of the casing. With 100+ psi in the inflated tubular, the tread is stretched several centimeters longer around the outer circumference of the tire. Once the tread receives a road cut, the tread's tension will cause the cut to open up. The strain of this tension also causes the vulcanized tire to be less flexible and very slightly increases rolling resistance.
Hand-Made Tubulars. Hand-made tubulars are works of art. The casing fabric of a hand-made tubular is not woven. Very fine threads, up to 320 per inch, are laid next to each other, compressed and then bonded with a latex coating. A good tubular will have over two kilometers of thread. These threads were traditionally cotton or silk. Now polyester is usually used. The threads are not interwoven as in a normal fabric. The first ply is laid on a round form. Then, another ply is laid at a 45 degree angle on top of the first one. They are bonded with heat (but not vulcanized) and the edges are folded over. The result is a casing of incredible strength and flexibility.
The edges of the fabric are sewn together as an inner tube, usually of natural latex rubber, is inserted. A base tape is glued to the inner circumference to protect the stitches.
The tire is then inflated and the tread is hand-glued to the tire casing.
This method of construction has several excellent consequences for the rider.
First of all, the tension between the casing and the tread that was found in the vulcanized tire is eliminated, making the tire more flexible. The tire doesn't flat as easily because cuts don't open up as quickly. The tire, with it superior suppleness and flexibility has a much nicer feel.
The unwoven casing also has benefits for the rider. The casing is so supple that when the tire rolls over irregularities in the road, a smaller area of the casing deforms compared to woven casings. This decreases rolling resistance. The tire is therefore faster. The high-thread-count casing resists the penetration of sharp objects. Flats occur less often. The tires is less prone to other road damage because the casing is so strong.
In professional racing, if the rider is not paid to ride clinchers, he will, if at all possible, ride tubular tires. The flexibility of the casing not only makes the tire faster, it handles better. The supple casing allows the tread to grip the road. The reduced harshness of the ride means more miles with less fatigue. Hand made tubulars are very expensive, but they remain the finest tires a rider can put on his bike.
Open Tubulars. An open tubular is a tubular tire whose edges have not been sewn together. Instead, a flexible bead of kevlar is sewn along the edges turning it into, if I may be allowed to mix up my definitions here for a second, a clincher that performs almost as well as a tubular. All the techniques of making the tubular; the high-thread-count unwoven casing and the cold-treated tread application are used in the manufacture of Open Tubulars. For this reason, a rider who is not willing to suffer the trouble of dealing with sew-up glue and the tricky job of repairing a flatted sew-up, the open tubular is a wonderful choice.
Almost everything that was written above about hand-made tubulars applies to open tubulars. Their high-thread-count casing makes them strong, while able to roll fast and resist punctures. The cold-treated tread application along with the fine casing give the tires a wonderful feel that must be experienced to understand.
Clincher Tires. As we defined the term above, a clincher tire has a woven fabric casing. This casing is almost always nylon. Clinchers, being vulcanized, are generally cheaper than open tubulars. A clincher can have a metal and therefore rigid bead, or one of flexible kelvar or glass fiber. These tires can be folded and are far lighter than their metal beaded equivalents. The metal beaded tire, while far cheaper, gives a harsher ride.
Generally, a tire factory will employ one or another of the two technologies, tubular or clincher, but not both. As I write this, the present limit for woven nylon casing is 120 threads per inch. This yields an entirely satisfactory tire with a very good road feel. The nylon rides harsher than polyester, silk or cotton, but it's not bad. The woven casing is stiffer and transmits more road shock. But even though the tubular technology remains the apex, a good, high-end clincher on a hand-built cross three spoke wheel gives a wonderful ride. The tread vulcanized onto clinchers is generally thicker. For this reason, a clincher tire will usually last longer.
So what tire to get?
If you are going to ride an important professional race, unless you are paid to ride otherwise, you will most likely use tubular tires. If you truly value the sensual aspect of your cycling, that is the basic quality and feel of the ride, or you want the highest possible performance and the high cost doesn't trouble you, again tubulars are the way to go. There are a couple of other problems with tubulars that the racing professional does not have to worry about. If you flat, putting on a new tire on the rim is very fast, far faster than replacing a tube as in a clincher flat. But the tire does not adhere well to the dry glue on the rim. Cornering must be done with care. On a rainy day a rider must assume that there is no adhesion between the rim and the tire. At least one complete spare tire, usually strapped under the seat, must be brought along. Twenty years ago ago on a club ride that one spare was enough. If a rider suffered more than one flat his buddies, each carrying a spare, could help out. Now, since almost everyone uses clinchers, the tubular user must bring along enough tires to satisfy the misfortune of a multiple flat day.
If the higher level of care required by tubulars is off-putting (and it is for almost everyone), but you still want to get the most performance possible, then open tubulars should be considered. Torelli has open tubulars with 240 TPI casings (Torelli Torino Open Tubular) that start in the mid $30s, so this technology is not beyond reach. For many, even riders with $7,000 bikes, getting a tire that lasts a long time is of primary importance. For them, because open tubulars have thinner, faster treads, while well-suited to their magnificent bikes, they will turn out to be a bit of a disappointment because of their shorter life.
So, for those wanting to get durability at some small sacrifice to performance, good clinchers with 120 tpi casings are the way to go.
Don't dynamite your tires. In the last decade it has become an article of faith that the more pressure one puts in a tire, the better (that is, faster) the bike will ride. One brand of tires actually requires 140 psi. This kind of tire pressure makes the bike ride like junk. With super high tire pressures the bike bounces, transmits road shock, fatigues the rider, causes the premature failure of bike frames and components and makes the tires more prone to flats. Running the tires at 105 - 115 psi will yield a bike that corners better and gives a far nicer ride. Try it. It will still go fast.
Tread hardness. How long the tread on a bicycle tire lasts is no accident. The tire designer has a certain total tire in his mind when he decides upon the casing, tire size and tread hardness. Does he want the tire to be primarily one that is durable? If so he will use a lot of the curing agent (see the definition of "Vulcanization" above). For instance, in general a hard rubber may have as much as 50% of the curing agent while a soft rubber may have as little as 5%. The durable, long-lasting tread comes at a price. The hard rubber does not grip the pavement as well as a softer rubber. This becomes especially apparant in the rain when long-lasting, durable tires break loose from the pavement with the slightest provocation. A softer tread will give better handling and performance at the cost of a shorter life. Torelli tires tend towards better gripping, softer tread compounds to give the bike superior handling. Riding a bike should be fun.
Colored treads are vulcanized with silicon and are generally softer than black treads vulcanized with carbon. Manufacturers of dual-compound tires use soft silicon compounds for their side treads. However, carbon vulcanized rubber can and is made to be very "grippy" by controlling the amount used in the vulcanization.
Tire Width. There is a view that a 20 mm width tire is faster than a 23 by virtue of its smaller cross section and lighter weight. Interestingly enough, this is not true. The people making the Torelli tires had noticed that the pro teams that they sponsored asked for 23s because they felt they were faster. When they investigated and did the testing, they found that the riders were correct. Let me explain.
Let's assume a 200 pound rider and bike unit. Let's also assume that the weight is distributed half over each wheel. That means that each wheel is supporting 100 pounds. Now, with a pressure of 100 pounds per square inch, the contact patch is one square inch. This is true no matter how fat the tire.
What changes when the tire gets fatter is the shape of the contact patch. With a 20, the contact patch is a long oval. With the fatter tire, the contact patch gets shorter and wider.
When a rider is using a skinnier tire, the long contact patch means he is flexing a wider arc of the tire casing, flexing more of the tire, causing more wasted energy from the internal friction of the tire and tube. The rider with the fatter tire is flexing fewer cords at a time.
There is clearly an optimum size, and the fact that racing tubulars are around 22 should keep us from getting super wide tires looking for yet more speed. Other losses probably kick in as the tire gets still fatter. For me, the bike feels like it doesn't have any snap or jump when we stray from the optimum which I believe to be in that 22-23 mm range.
Some have suggested that the skinnier tires make up for their losses because of their lower aerodynamic drag. This could be true for the solo time-trialist, I'm not sure. For the pack rider, it clearly is not an important consideration.
I am grateful to the generous people listed below for the help I recieved in writing this essay:
- Larry Theobald of CycleItalia Cycling Tours
- The kind folks at http://www.tdu.com.au/
- David Herlihy, author of Bicycle"
- Sam Aichouba, formerly of Hutchinson Tires
- Alex Brauns of Challenge Tires