Kenda Tire Expert - Why Do We Get Flat Tires

June 01, 2018 / Bicycle News

What does it mean to be an expert?  Is it as simple as knowing all the technical features of a product, or is it something more?  For Kenda Tire, being an expert means more than knowing how a tire is manufactured, it means being an expert in how tires are made and used.  Riding tires in a variety of conditions to know not only how they perform, but to be able to answer expertly the questions of riders. This article is the first in a series of articles that attempt to answer the questions that riders, shop owners and employees have about tires and tubes.

Flats, punctures, mechanicals, whatever we call them, they all mean the same thing, a ride that is potentially cut short.  No matter what type of rider you are, what level, or where you ride, every bike rider is vulnerable to the dreaded flat tire.  This prompts the questions, what causes a flat tire and how can we best prevent flats in the future? 

To answer those questions we must first look at what kind of tires a traditional bicycle uses.  The tires we consider to be conventional tires are made of two different parts, the tires itself and a tube, and are often referred to as Tube Type tires. 

A traditional bicycle tire begins as a sheet of Nylon fabric called the casing.  The casing is the support skeleton for the tire and has the wire or folding beads installed along with the tread rubber is applied to form the tread pattern.  A variety of materials can be added underneath the tread or to the casing of the tire to help supplement the durability of a tire; these are called puncture protection materials.  Puncture protection in a tire at the most basic level is extra material to restrict some object from getting from the outside of the tire and into the tube.  When protection is added under the tread of a tire it can come in the form of a woven material, i.e. Aramid, or it can be an additional layer of rubber, co-extrusion.  In both cases, the added material is to prevent any foreign objects from penetrating the casing and tube.  Woven fabrics are the most commonly used materials because they are soft, flexible, and lightweight.  When rigid materials are used, the tire loses the flexibility and delivers a harsh ride quality.  Elastomers or co-extrusions, an additional layer of rubber, is extruded jointly with the tread rubber to form a single block of material and applied to the casing of the tire.  This layer of flat protection under the tread surface can range up to more than five millimeters thick.

Once all the components are assembled into a ‘green’ tire, as it is called, it is placed inside of a steel mold that has a negative impression of the tread pattern.  Heat and pressure is applied via an inflatable steam airbag that vulcanizes the tire components into a singular structure; which determines the shape of the tire and imprints the tread pattern from the mold onto the tire surface, resulting in a ridable bicycle tire.

The other half of the tube-type tire combination is the tube.  Unlike the tire, tubes do not begin with a fabric structural layer; instead, tubes are straight extrusions of rubber pipe that are cooled in a water bath, cut to length, and then the two ends are heat welded, or spliced, together for form the complete circle.  Prior to welding the seam of the tube, a small hole is punched into the tube where the valve and rubber patch are installed.  Once the raw tube has been assembled, it is placed into a size-specific mold, similar to that of a tire mold, but without a tread pattern, and hot air is used to vulcanize the tube into a complete singular piece.  Because tubes are designed to be the air chamber of a bicycle tire, a final step is added to the production process.   All Kenda tubes, over 800,000 annually, are inflated and checked 24 hours later for defects as part of a 100% Quality Control inspection.

Now that we know what goes into make tires and tubes, let’s talk about the different ways they can be flatted. Technically speaking you do not flat a tube type tire you flat the tube. Without any air in the tube, the tire then becomes flat. First we will talk about punctures. A tire and tube can be punctured by foreign objects coming through the tread surface or the sidewalls. These objects can range from thorns, broken glass, rock, nails or anything that is sharp enough to penetrate the tread rubber, any protective layers added to the tire, the casing, and eventually the tube.

Ways to prevent punctures from going through the tread of a tire start with making sure the tire is in good shape to begin with.  The older and more worn a tire is, the greater the chance of a puncture.  Signs that a tire is worn down can include a flat spot where the tire contacts the ground, or exposed casing threads at the tread surface.  Some tires have built in tread wear indicators that wear with the tire and when they cannot be seen, indicate it is time to replace the tire.    In cases where a tire uses a co-extrusion as a layer of protection, the elastomer is frequently a different color from the tread and can be used to tell tread wear and tire life.  When the black tread rubber on your tire starts to wear away and you begin to see the color of the elastomer, you know it is time to replace that tire.  Ignoring indicators of wear in your tires will eventually lead to a flat.

example of a tread puncture.

example of a tread puncture.

example of worn out tire with hole from skidding.

example of worn out tire with hole from skidding.

Punctures do not only come through the tread on a tire.  Depending on where you ride, sidewall punctures or even worse, slashes can be very common.  Until the advent of mountain bike tires, flat protection was confined to under the tread of tires, but with bikes being ridden off road that mentality needed to be rethought.  As tires started being used on rougher and rougher terrain, riders found themselves faced with flats from branches and rocks cutting into the side of the tires.  To mitigate these issues, tire manufactures started extending flat protection beyond the tread area to enclose the entire casing of the tire, offering bead-to-bead protection.  Bead-to-Bead protection added strength to the sidewalls of the tire but it came at a cost.  The extra material now running across the sidewalls of the increased the weight of a tire slightly and it also changed the ride feel of the tire.  The sidewalls lose a little of their suppleness when there is additional material, causing some riders to comment on a slightly stiffer ride quality.  To mitigate potential adverse changes to ride quality, Kenda developed a three-piece construction that provides the same level of protection without hindering ride quality.  However, as technology for mountain bike tires improved so did the puncture protection.  The introduction of sealants and tubeless tires (we will address in the next Kenda Tire Talk) meant that thinner protections could be used, thus letting the rider jointly rely on a thin protection and sealant. 

However, there is always the one riding buddy who can get a flat no matter where they are.  For those riders, combination puncture protections were designed.  Kenda took the best elements of bead-to-bead protection and then added on an additional layer of protection under the tread to create the Endurance line of tires.  Endurance tires offer twice the normal protection where the tire contacts the ground and a single layer or protection across the sidewall of the tire. 

Up until now, we have focused on punctures coming from the outside of the tire, but that isn’t the only direction flats can come from; meaning a puncture that came from the wheel or at least part of it.  

Almost every bicycle wheel uses spokes in the construction of the wheel and those spokes are connected to the rim of the wheel by specialized nuts called spoke nipples.  Spoke nipples pass through holes in the rim where they thread into the spoke.  Sometime, spokes are longer than they need to be and part of the spoke sticks up far enough to poke at or even cut into a tube.  Other times, the small hole in the rim that the spoke nipple passes though has a sharp edge that can cut into a tube.  The easiest way to prevent damage from your wheel to the tubes is to make sure that a rim strip is properly fit to the inside of the rim.  Rim strips need to be wide enough to cover the entire rim bed from side to side, making sure that the spoke holes are covered and that none of the spokes are stick through the rim strip.

Now that we have addressed flats from outside of the tire and from within, there is a third type of flat that occurs that isn’t a puncture; it is a snakebite.  These kinds of flats are actually compression and pinch flats; occurring most frequently when under inflated tires are ridden, allowing the tire casing to flex and fold over onto itself.  When this happens, the tube is pinched between the edge of the rim and the ground.  With enough pressure applied, such as hitting a rock or a small drop like riding off of a curb, the pinching force will be enough to create two small parallel holes that are only a few millimeters apart from each other, resembling the twin fangs of a snake bite, hence the name.  Keeping tires inflated to the proper pressure is the best way to reduce the chances of getting a pinch flat, but it is not a guarantee.  As many road or urban rider can attest, a tire even at full pressure striking curb or a pot hole at speed can create enough pressure to pinch flat a tube and in worse case situation cause the sidewall of the tire to tear or be cut between the ground and rim surface.  It is instances like these where tires with complete bead-to-bead coverage are ideal, helping to reduce the chances of sidewall tire cuts.  Replacing a pinched tube is a relatively simple and straight forward process, one that every rider should be prepared for and feel comfortable doing; however not every rider carries a spare tube with them.

If you experience a puncture or flat on a ride, there are certain things that you should do to prevent future punctures.  When you experience a flat the first thing you need to do is determine what caused the flat.  If you just hit a pothole or some other object in the road it is likely you suffered a pinch flat.  This can be determined by looking for the two parallel holes in the tube.  If you cannot find any holes in the tube, you can pump some air into the tube and listen for the telltale ‘hiss’ of air escaping.  If there is only one hole in the tube, it is probably from something that penetrated the tire and tube.  Before replacing the tube in the tire, either with a new tube or patched tube, you need to check the tire to make sure that what caused the original puncture is still not in the tire.  On numerous occasions, riders in a hurry to repair a puncture, neglect checking the tire, only to discover a short while later that what caused for first puncture has now caused a second. 

Checking the tire can be done visually by inspecting the casing of the tire from the tread side first and the interior second.  If a visual inspection does not yield results, a second inspection can be carried out tactilely by running your fingers across the interior and exterior surfaces of the tire, feeling for any debris or sharp objects.  When this is done, care needs to be taken as to not poke yourself in the finger with whatever caused the flat.  Once the offending material has been found you can remove it, either by picking or scratching at it to remove it.  In some cases you may not be able to find what caused the puncture, in those situations, it is possible that the air escaping from the tube could have forced out the object that caused the puncture.

If you cannot find anything in the tread of the tire or on the inside of the casing, and it wasn’t a pinch flat, you should check the sidewalls of the tire.  A slash to the sidewall may not be obvious to the eye, so this will likely be a situation where you will have to check the casing by hand.  Cuts to the casings of tires, unlike punctures at the tread area are often unrepairable and necessitate the replacement of the tire overall.

As tire technology improves and lighter and stronger puncture protection materials are introduced into tires, the frequency of punctures will continue to decrease, but there is no such thing as a puncture proof tube type tire.  Until a time comes where the flat proof tube type tire is developed, the tips in this article will serve to help riders know what to look out for in terms of reducing the chances of a flat, either through preparation or through stronger tires.  Moreover, if a flat does occur, what to do mitigate the chances of it happening again.

A quick checklist for ways to prevent flat tires

  • Is my tire in good shape? 
    • Are there signs of wear?
      • Threads showing
      • Flat spot on crown of tire
  • Is my tire inflated to the proper pressure?
    • Tires under inflated are at a greater risk of pinch flats
  • Does my tire have the protection I need for the riding I do?
    • Sub-Tread protection
    • Bead-to-Bead
    • Sidewall only
    • Combination protection (Bead-to-Bead and extra Sub-Tread)
  • Is my wheel going to cause a flat? 
  • Does my rim tape cover the spokes and spoke holes?

I got a flat, what should I check for?

  • Did I hit something hard?
    • Is it a pinch flat (snake bite)?
    • Did I damage my tire by cutting the sidewall?
  • Did something penetrate the tread?
    • Is it still in the tread or casing?
  • Did I cut my sidewall