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Cycle Tyres and Tubes
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by Sheldon "Tyres" Brown
and John "Tyubes" Allen"
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Parts of a Tyre

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How a Tyre Supports its Load

It is commonly thought that the air pressure in a tyre supports the rim. If you think about it, this can not be true because the air pressure against the rim is equal, top and bottom. How, then, does a tyre support its load?

First of all, the role of air pressure in the tyre is to hold the fabric under tension -- in all abut one place, the contact patch with the road surface.

At the contact patch, the tread of the tyre is flattened against the road. Air pressure can only push directly outward, and so here, it pushes directly downward. The downward force of the air must equal the weight load, and so the area of the contact patch approximately equals the weight load divided by the air pressure. (Edge effects and skewing of the weave of the fabric may result in some difference.) For example, if the air pressure is 50 PSI, the contact patch will be about two square inches.

The threads of the tyre fabric can only transmit loads lengthwise and in tension. How then, do they transfer the load from the contact patch to the rim?

Because the contact patch is flat against the road, the curvature of the sidewalls is increased, decreasing their tension, and the angle at which they approach the contact patch becomes shallower. These effects produce the bulge seen at the bottom of a tire under load and transfer the load from the contact patch to the tyre's sidewalls. The threads of the fabric are pulling downward less and outward more. The load is similarly transferred from the sidewalls to the rim. The sideways forces at the two sides are equal and opposite, and cancel out.

Wth a bias-ply tyre, the load is carried lengthwise in both directions along the tyre by the diagonal threads, so the bulge is longer and less deep than on a radial-ply tyre. In the early days of radial-ply car tyres, people often thought they were underinflated, because the bulge at the bottom was more pronounced.

A tyre, then, supports its load by reduction of downward pull, very much the same way that spoking of the wheel supports its load. The tension-spoked wheel and the pneumatic tyre are two examples of what are called preoaded tensile structures, brilliant, counterintuitive designs working together remarkably to support as much as 100 times their own weight.

Bias plies also help to transmit lateral and torque loads, by triangulating the connection between the contact patch and the rim -- much like the way the spokes of a semi-tangent spoked wheel transmit lateral and torque loads. With tubulars, the diagonal plies also work like a Chinese finger puzzle: the air pressure makes the tyre fatter, and so makes it shorter and helps hold it to the rim. Radial-ply tyres for bicycles have been tried -- Panasonic made them for a short time in the 1980s -- but they proved to have an odd feel due to their reduced lateral stability.

If you would like to get into mathematical details, there is an excellent technical description in an old Britannica encyclopedia article online.

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Traction

"Traction" refers to the tyre's resistance to skidding/slipping. There are three areas where traction is at issue: braking, climbing, and cornering. Different tyre designs, particularly in the tread, may enhance or degrade traction in each of these cases.

The traction of a tyre is determined by three things: inflation pressure, rubber formulation, and tread pattern.

Traction is also influenced by the presence or absence of suspension, and by the rider's posture and technique (see also my article on Braking and Turning.)

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Tread Patterns

Tread for off-road use

Treads can help improve off-road traction in two ways:

Tread for on-road use

Bicycle tyres for on-road use have no need of any sort of tread features; in fact, the best road tyres are perfectly smooth, with no tread at all!

Unfortunately, most people assume that a smooth tyre will be slippery, so this type of tyre is difficult to sell to unsophisticated cyclists. Most tyre makers cater to this by putting a very fine pattern on their tyres, mainly for cosmetic and marketing reasons. If you examine a section of asphalt or concrete, you'll see that the texture of the road itself is much "knobbier" than the tread features of a good-quality road tyre. Since the tyre is flexible, even a slick tyre deforms as it comes into contact with the pavement, acquiring the shape of the pavement texture, only while in contact with the road.

People ask, "But don't slick tyres get slippery on wet roads, or worse yet, wet metal features such as expansion joints, paint stripes, or railroad tracks?" The answer is, yes, they do. So do tyres with tread. All tyres are slippery in these conditions. Tread features make no improvement in this.

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Hydroplaning

Car and lorry tyres need tread, because these vehicles are prone to a very dangerous condition called "hydroplaning." This happens when driving fast in very wet conditions, which can lead to the tyre riding up onto a cushion of liquid water. When this happens, there is a sudden and total lack of traction.

Cars can hydroplane because: Bicycles canNOT hydroplane because:
A car tyre has a square road contact, and the leading edge of the contact is a straight line. This makes it easier for a car tyre to trap water as it rolls. A bicycle tyre has a curved road contact. Since a bicycle leans in corners, it needs a tyre with a rounded contact area, which tends to push the water away to either side.
A car tyre is quite wide, so water from the middle of the contact patch can have trouble escaping as the tyre rolls over it, if there are not grooves to let it escape. A bicycle tyre is narrower, so not as much water is in contact with the leading edge at once.
Car tyres run at much lower pressure than bicycle tires. The high pressure of bicycle tyres is more efficient at squeezing the water out from under.
Cars go much faster than bicycles, again leaving less time for water to escape. At high speeds, hydroplaning is just possible for car tyres, but is absolutely impossible for bicycle tyres.

Even with automobiles, actual hydroplaning is very rare. It is a much more real problem for aircraft landing on wet runways. The aviation industry has studied this problem very carefully, and has come up with a general guideline as to when hydroplaning is a risk. The formula used in the aviation industry is:

Speed (in knots) = 9 X the square root of the tyre pressure (in psi.)

Here's a table calculated from this formula:

Tyre Pressure Hydroplane Speed
Miles per hour
Hydroplane Speed
Kilometers per hour
P.S.I. Bars
120 8.3 113 183
100 6.9 104 167
80 5.5 93 149
60 4.1 80 129
40 2.8 66 105

Squirm

Knobby treads actually give worse traction on hard surfaces! This is because the knobs can bend under side loads, while a smooth tread cannot.

The bending of knobs can cause discontinuities in handling: the tyre grips OK for mild cornering, but as cornering force exceeds some critical value, the knobs start to bend and the traction suddenly goes to Hell in a handbasket.

Combination Treads

Many tyre makers market "combination-tread" tyres, that are purported to work well on both pavement and dirt. Generally, they don't.

The usual design is to have a smooth ridge down the center of the tread, with knobs on the sides. The theory is that the ridge will provide a smooth ride on pavement, with the tyre inflated fairly hard, and the knobs will come into play off-road, with the tyre running at lower pressure (or sinking into a soft surface.) Another aspect of this design is that the knobs are intended to come into play as you lean into a turn.

In practice, combination-tread tyres don't work all that well. They do OK in dirt, but they're pretty lousy on pavement. They're much heavier than street tyres, and if you corner aggressively, the transition from the center strip to the knobs can cause sudden washout. They aren't quite as slow and buzzy as true dirt tyres, but they're much worse in this respect than smoothies.

If you mostly ride on pavement, but also do a fair amount of dirt, a combination tyre on the front may be a good choice for you, with a road tyre on the back. See the section on mixing/matching tyres.

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Sizes

Traditionally, each major bicycle-producing country had its own system for measuring and designating tyre and rim sizes. As the industry became more international, these national sizing systems have become a source of considerable confusion, especially since some tyres with different numbers were actually interchangeable, while others with the same numbers were not interchangeable!

There is a relatively new, international system of tyre sizing which eliminates these confusions. This is explained in considerable detail on this site in the article on Tyre Sizing.

Rolling Resistance

"Rolling resistance" is the mechanical friction generated as the tyre rolls. As a segment of the tyre tread rolls into contact with the road, it deforms from its normal curved shape into a flat shape against the road, then back to the curve as the tyre rolls onward. The deformation of the rubber in this process is what causes the friction. A bias-ply tyre has some additional friction because of the "Chinese finger puzzle" effect of the bias plies. The edges of the contact patch scrub against the road as a segment of the tread becomes shorter and wider where it flattens out, then longer and narrower as it becomes round again.

There are fou rways to reduce this friction, each subject to trade-offs:

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Width and Pressure

Tyre width and pressure are inextricably linked. It is a serious mistake to consider one independently of the other. Generally, wider tyres call for lower pressures, narrower tyres call for higher pressures.

Consider, for example, a tyre one inch across, at a pressure of 100 PSI (pounds per square inch). Air is pushing down against the bottom half of the tyre cross-section with a force of 100 pounds per inch of length. Each sidewall of the tyre bears half that load, and so each inch of length of tyre sidewall will be under a tension of 50 pounds. Now let's consider a tyre twice as wide, two inches across, at the same 100 PSI. Each inch of sidewall will be under a tension of 100 pounds. So, a wider a tyre would ride harder, and need stronger fabric, if inflated to the same pressure,

The part of the tyre that is actually touching the ground at any moment is called the "contact patch." Generally, the area of the contact patch will be directly proportional to the weight load on the tyre, and inversely proportional to the inflation pressure. For instance, if the rear tyre of a bike is supporting a load of 100 lbs, and the tyre is inflated to 100 PSI (pounds per square inch) the contact area of the tyre will be roughly one square inch. If the pressure is reduced to 50 PSI, the tyre will squish out until the contact patch has become 2 square inches (or until the rim bottoms out against the tyre.)

A common debate among cyclists centers on the issue of whether a wider tyre has more or less rolling resistance at the same pressure. The constant pressure is proposed because it appears more scientific to eliminate this as a variable, but this is not realistic in practice. The short answer to this question is that, yes, a wider tyre of similar construction will have lower rolling resistance than a narrower one at the same pressure. This fact is, however, of no practical value. If you are comparing two tyres of similar construction, with the same load, and the same pressure, either the wider tyre is overinflated, or the narrower tyre is underinflated!

A tyre is supposed to deflect a bit under load. This deflection the whole purpose of pneumatic tyres. When you sit on your bike, your tyres should visibly bulge out at least a bit under your weight. If they don't, they're overinflated.

Underinflation Correct Inflation Overinflation
  • An underinflated tyre will have more rolling resistance.
  • An underinflated tyre will be prone to pinch flats.
  • An underinflated tyre will tend to wallow and may even come off the rim during cornering. This is a particular problem with wide tyres on narrow rims.
  • A correctly inflated tyre will have negligible rolling resistance.
  • A correctly inflated tyre will not get pinch flats in normal use.
  • A correctly inflated tyre will absorb minor surface irregularities, improving rider comfort.
  • A correctly inflated tyre will absorb surface irregularities without bouncing and losing traction.
  • An overinflated tyre will have slightly less rolling resistance if the surface is very smooth.
  • An overinflated tyre is more prone to damage from sharp rocks and similar road hazards.
  • An overinflated tyre will give a harsh ride on anything but the smoothest pavement.
  • An overinflated tyre can bounce on surface roughnesses. This can cause dangerous interruptions in traction, particularly if it happens during cornering.

Pressure Recommendations

Most tyres have a "maximum" pressure, or a recommended pressure range marked on the side of the tyre. These pressure ratings are established by the tyre manufacturers after consultation with the legal and marketing departments.

The lawyers want the number kept conservatively low, in case the tyre gets mounted on a defective or otherwise loose fitting rim. They commonly shoot for half of the real blow-off pressure.

The marketing department wants the number high, because many tyre purchasers make the (unreliable) assumption that the higher the pressure rating, the better the quality of the tyre.

Newbies often take these arbitrary ratings as if they had some scientific basis. While you'll rarely get in trouble with this rote approach, you will usually not be getting the best possible performance.

Savvy cyclists experiment with different pressures, and often even vary the pressure for different surface conditions.

Optimal pressure for any given tyre will depend on the load it is being asked to support. Thus, a heavier rider needs a higher pressure than a lighter rider, for identical tyres.

Since most bicycles have substantially more weight on the rear wheel than on the front, the rear tyre should almost always be inflated to a higher pressure than the front, typically by about 10%.

Rough surfaces generally call for a reduction in pressure to improve ride comfort and traction, but there is a risk of pinch flats if you go too far. Even at the lower appropriate pressure, wider tyres, because they also are deeper, are more immune to pinch flats.

Rider skill also enters into this: more experienced cyclists learn to "get light" for a fraction of a second while going over rough patches; newbies tend to sit harder on the saddle, increasing the risk of pinch flats.

The table below is based on my experience and a certain amount of guesswork, and should only be used as a very rough guide to a starting point. Interpolate/extrapolate for your own weight/tyre sizes.

Tyre widths are in millimeters, pressure recommendations in pounds per square inch. (Divide by 15 if your gauge reads in bars/atmospheres.)

Tyre width in mm
Wheel load 50 mm 37 mm 32 mm 28 mm 25 mm 23 mm 20 mm
100 lbs/50 kg 45 60 75 100 110 120 130
70 lbs/35 kg 35 50 65 80 90 100 110

Note that these recommendations are based on the actual tyre width. Many tyres are marked wider than they actually are. See: "Dishonesty in Sizing."

Bicycle Quarterly Magazine has published an article recommending optimum pressures.

Please do not contact us with questions about specific tyre pressure recommendations!

Trailers and Tricycles

Tricycles and two-wheel trailers are very different from bicycles, because they don't lean in corners. Most tyre wear comes from cornering forces. On a bicycle, these forces act on different parts of the tread, according to how far one leans into various corners at various speeds.

With a trailer or trike, all of the wear is concentrated on the middle of the tread. If you overinflate the tyres, you'll be riding on only the very center of the tread, and it will wear rapidly.

In addition, wheel alignment is never going to be perfect. As a result, the paired tyres will always "scrub" a bit. If the tyres are rock-hard, this will cause rapid wear. If the tyres are softer, they can flex slightly sideways to accommodate the scrub, without wearing the tread off.

With trailers, severe overinflation can also lead to flipping the trailer over, due to the tyres bouncing on road irregularities. The load on tariler wheels is often very light. Adjust pressure accordingly.

Dishonesty in Sizing

Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high-performance 700 x 25 tyre; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tyre Company and the Coke Tyre Company had tyres of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tyre would be lighter than the accurately-marked Coke 700-25. This would put Pepsi at a competitive advantage. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25.

This scenario prevailed throughout the '70's and '80's. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements.

Mixing/Matching Tyres

Most bikes come with identical tyres front and rear. This is all right for general use, but if you want to optimize your bike, you should consider using different tyres front and rear. The front and rear tyres have different loadings and different requirements.

Airless Tyres

Of all the inventions that came out of the bicycle industry, probably none is as important and useful as Dr. Dunlop's pneumatic tyre.

Airless tyres have been obsolete for over a century, but crackpot "inventors" keep trying to bring them back. They are heavy, slow and give a harsh ride. They are also likely to cause wheel damage, due to their poor cushioning ability. A pneumatic tyre uses all of the air in the whole tube as a shock absorber, while foam-type "airless" tyres/tubes only use the air in the immediate area of impact.

Pneumatic tyres require pumping up frmm time to time, and can go flat, but their advantages overwhelm these difficulties.

Airless tyre schemes have also been used by con artists to gull unsuspecting investors. My advice is to avoid this long-obsolete system.

Kevlar ®

Kevlar ® is used for two different, almost opposite reasons in bicycle tyres. This results in considerable confusion as people try to buy "Kevlar ®" tyres without understanding the difference.

Folding a Wire Bead Tyre

It is possible to fold a steel bead tyre.

click here for a 2.6 mb Video Demonstration.

Tyre Wear-When should you replace your tyres?

Many cyclists waste money replacing perfectly functional tyres simply because they're old, or may have discolored sidewalls. If you just want new tyres because the old ones look grotty, it's your money, but if you are mainly concerned with safety/function, there are only two reasons for replacing old tyres:

  1. When the tread is worn so thin that you start getting a lot of flats from small pieces of glass and the like, or the fabric shows through the rubber.
  2. When the tyre's fabric has been damaged, so that the tyre has a lumpy, irregular appearance somewhere, or the tube bulges through the tyre.
Cracks in the tread are harmless. Small punctures in the tyre such as are typically caused by nails, tacks, thorns or glass slivers are also harmless to the tyre, since the tyre doesn't need to be air-tight.

Gumwall tyres sometimes get unsightly blistering on the sidewalls from ozone damage. (This is frequently caused by storing the bike near a furnace--the powerful electric motors in typical furnaces can put a fair amount of ozone into the air.) This blistering is ugly, but doesn't actually compromise the safety/reliability of the tyre in the least.

Label placement

Most good bicycle mechanics pay attention to the orientation of labels. The most usual custom for tyres is to locate the label right at the valve, facing to the right. Some justify this on the grounds that having a standard tyre mounting orientation can make it easier to find a thorn or glass sliver in a tyre, once the hole has been located in the (removed) tube. While there's an element of truth to this, placing the label consistently is really more about pride of workmanship and attention to detail.

Tread Directionality

Some tyres have an asymmetrical tread, for instance "V" shaped tread blocks that could be oriented with the point of the "V" facing forward > or backward <. The question then arises, which way should they face?

Road Applications

With tyres for road use, it really doesn't matter, since tyre tread patterns serve no function on hard surfaces.

Tyres with "V" patterns are common for motorcycles, and are generally installed so that the point of the "V" hits the road first. This is to help "squirt" the water out ahead of and to the side of the tyre contact patch, as a protection against hydroplaning . Since hydroplaning is impossible on a bicycle, there's no need to observe this custom.

Off-road Applications

For off-road use in soft surfaces, there may be some merit in paying attention to the tread orientation, though this is far from certain.

Ideally, you would like the front tyre to offer maximum traction in the braking direction, while the rear tyre would normally be oriented to produce maximum traction for drive forces. Thus, if a particular tread pattern is perceived to have better traction in one direction than the other, it should be facing one way if used on the front wheel, and the opposite way if used on the rear wheel.

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Other Tyre-Related Articles

Flat Tyres Everything you need to know to fix a flat tyre on your bicycle
Tyre Rotation Should you rotate your tyres for longer wear? NO!
Tyre Sizing Unraveling the mysterious numbers that tell which tyre fits which rim
Tyres from Harris Cyclery Commercial page with tyres for sale
Bicycle Links This page features links to tyre manufacturers' sites.
My Adventure Cyclist articles One of these articles is about tyres.
rec.bicycles.* FAQ

Note: links to individual articles on this site are often broken by its Webmaster,
and I don't have time to keep up with the changes.

Thanks to Russ Ault for sending the corrected URLs the last time this happened.

If you don't find the article where it is supposed to be, go to the site index.

Many of these articles (the ones written by Jobst Brandt)
now exist in updated versions on this site,
in the Jobst Brandt FAQ Section

I've changed the links for those articles, the ones that don't begin with numbers.

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