### November 28, 2014

# FLO Cycling - Why Do You Use Less Tire Pressure for a Bigger Tire or Wider Wheel?

During the last several years, bicycle rim width at the brake track has been increasing. Manufacturers who are producing the new wave of high-tech wheels are ignoring the old 19mm width standard, and commonly producing rims that are 23-25mm in width. This is done to improve both performance and aerodynamics.

This increased rim width has manufacturers asking their customers to inflate their tires to a lower psi, and the obvious question is why. Why does a wider rim and/or tire require a lower psi? We get this question so frequently that we decided to write a blog article to explain the science behind it.

This increased rim width has manufacturers asking their customers to inflate their tires to a lower psi, and the obvious question is why. Why does a wider rim and/or tire require a lower psi? We get this question so frequently that we decided to write a blog article to explain the science behind it.

**Tire Background**

Before starting, let's look at the basic parts of a clincher tire.

**Casing:**The fabric that is used to shape the tire and support the air pressure in the tire.

**Tread:**The rubber coating put on the casing for grip on the road and to make it airtight.

**Bead:**This is the metal hoop that hooks into the clincher hook on the wheel.

Parts of a Tire |

The casing is the important part for today's discussion. The casing is made from a thread that is stitched together in a diagonal orientation to form a bias ply tire. This bias ply is what holds the tires structure when inflated with air. Without it, the tire would stretch like a ballon.

Bias Ply Weave |

**Casing Tension and Hoop Stress**

In order to understand why we use a lower pressure for a bigger tire or wider wheel, it's important to understand hoop stress. To understand hoop stress, it helps to have a few visual examples. The picture below shows a large propane tank. It has a cylindrical body and cupped ends.

If we were to cut a section of the tank (Section A), we would get a circular shape like the one below.

There are three dimensions for the section. There is the outer diameter, the inner diameter, and the mean (average) diameter. For our calculations we are concerned with the mean diameter.

When the propane tank is full of propane, the pressure of the gas exerts a force on the wall of the tank as shown below.

This force from the internal pressure creates a stress on the wall of the propane tank. Wikipedia describes stress as follows:

"Stress is the force per unit area on a body that tends to cause it to change shape."

In the case of a thin-walled vessel, (like the propane tank) the stress that the wall of the tank experiences is called hoop stress.

So how does this relate to a clincher tire? If we were to section a clincher tire when it is mounted on the rim, you will see a shape that is very similar to the section from the propane tank. Yes, the rim and tire is not perfectly round but the principle is the same. As we inflate the inner tube, (increase the internal pressure by increasing the air pressure) the casing of the tire is stressed. When stressed, the fibers of the bias ply weave are placed under tension. This tension is referred to as casing tension.

**Calculating Casing Tension**

To determine the casing tension, we need to define the equation for hoop stress.

In the above equations we define the following as:

Wall thickness = the thickness of the tire casing

Mean diameter = the tire size

Pressure = the pressure the wheel is inflated to

We eliminate thickness since we can assume all of the tires in the calculations below have the same wall thickness.

**Setting the Base**

As a starting point, we will use a 20mm tire at 120psi to calculate the casing tension.

From the calculation above, you can see that the casing tension on the tire is 8,273.709Pa. When calculating the appropriate psi for a new tire diameter, we will want to keep the casing tension at a constant 8,273.709Pa. To get the psi for each new tire size, we will want to rearrange the hoop stress equation used above to solve for pressure.

Now that we have solved for pressure, let's use a 23mm tire as an example and calculate the tire pressure needed to create the same casing tension we saw in the 20mm tire.

As we can see, the tire pressure that creates the same casing tension for the larger 23mm tire is less then 120psi. Since bigger tires do not have thicker tire casings, we want to make sure we are not creating too much casing tension. This is why we keep the casing tension the same when we calculate the new tire pressure. It is also why you will see tire manufacturers lower their max tire pressure recommendations for wider tires. Here is a list of tire pressures that relate to 120psi for a 20mm tire all the way up to a 30mm tire. Remember, the tire casing tension is the same in each example.

**Lower Tire Pressures for Wider Wheels**

As we widen the rim of a wheel, we by default widen the tire. It is not by a huge amount, but it does get wider. The picture below shows what I mean.

The tire on the wider rim (left) is wider at the mid point then the tire on the narrow rim (right) |

Since the tire gets wider with a wider rim, we increase the dimeter in our pressure equation above. This results in a lower tire pressure for a wider rim with the same size tire. The proper way to calculate it is to determine the percentage that it increases. Each wheel will be different and it will depend on what wheel you are comparing it to.

I hope this blog article was easy to follow and that you enjoyed it. I wanted to thank our good friend Tom Anhalt over at http://bikeblather.blogspot.com/ for helping me sort out my thoughts and teaching me a thing or two about casing tension. Please let me know if you have any questions or comments below.

Take care,

Jon

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## 12 comments :

So for a 23mm tyre on a 19mm rim vs 23mm rim, how much pressure should I drop? The table shows different tyre width on a 19mm rim or?

Lee Wen Yang,

That all depends on your weight. I would use the tables in the following blog article and add about 4% to the PSI value if you are using a 19mm rim.

http://flocycling.blogspot.com/2014/09/flo-cycling-tire-pressure.html

Chris

Interesting. Good to see some maths, but the whole argument rests on this claim: " When calculating the appropriate psi for a new tire diameter, we will want to keep the casing tension at a constant". I don't think that has been established. Failure of a bicycle tyre due to casing stress is extremely rare, so reducing pressure for that reason isn't important. Most riders' selection is more about comfort, resistance to pinch flats and 'performance'. I'm not clear how size and pressure relate to those factors.

Paul,

I am not recommending that you use the psi values in the tables as a standard. Selecting a tire pressure is extremely complex when you take everything into consideration. However you should lower a psi with a wider tire or wider wheel. Tire companies recommend the same thing.

I am using the casing tension to show the reason why. I am not trying to say that you use a casing tension to select the proper psi.

Reducing pressure is important for safety reasons and also will relate to your comfort, resistance to pinch flats and performance. Again, the article was to show why not to set a standard.

Take care,

Jon

How does rolling resistance figure in to this. What pressure is ideal at the varying width to optimize rolling resistance? I get more confused the more I read about this.

In a previous post (http://flocycling.blogspot.com/2014/09/flo-cycling-tire-pressure.html) I found that 23mm and 25mm have a constant difference through all weights of 1 bar (15psi). Same diference between 25mm and 28mm.

What I understand is that to have a 25mm tyre at the same pressure of a 23mm one, I have to deflate it of 1bar/15psi.

BUT using the formula of this post, in order to have the same pressure, the difference between 23&25mm and between 25&28mm is 0,6bar/9psi (at a reference pressure of 7bars/100psi)

Did I "mix apples with pears"? there is something wrong with my comparison?

Paolo

P.S. sorry for my terrible english

Rich,

There are number of factors that play into rolling resistance. It's a tough answer the question without getting into more details. Look at this from Schwalbe's website.

"Tire pressure, tire diameter, tire construction, tire tread and other factors all have an effect on rolling resistance. The higher the tire pressure, the less is tire deformation and thus the rolling resistance.

Small diameter tires have a higher rolling resistance at the same tire pressure, because tire deformation is proportionally more important, in other words the tire is "less round". Wider tires roll better than narrow ones. This assertion generally generates skepticism, nevertheless at the same tire pressure a narrow tire deflects more and so deforms more.

Obviously, tire construction also has an effect on rolling resistance. The less material is used, the less material there is to deform. And the more flexible the material is, such as the rubber compound, the less energy is lost through deformation.

Generally, smooth treads roll better than coarse treads. Tall lugs and wide gaps usually have a detrimental effect on rolling resistance."

Ultimately if you keep the tire pressure the same a larger tire will have less rolling resistance. That's assuming it's the same model of tire.

If tire pressure get's too low the rolling resistance will increase. Velo News performed a test a while back which is a great read. You can find it in the link below.

For a base line you should take a look at our blog post that helps you pick the right PSI. http://flocycling.blogspot.com/2014/09/flo-cycling-tire-pressure.html

http://velonews.competitor.com/2014/12/bikes-and-tech/resistance-futile-tire-pressure-width-affect-rolling-resistance_355085

I hope this helps.

Take care,

Jon

Jon,

I'm interested in calculating the effects on case tension in mountain bike tyres when using different width rims. Eg what's the difference between a 19mm ID rim and a 30mm ID

I can calculate an approximate internal circumference of the case by measuring across the back of my the tyre (145mm on a 2.4 schwalbe racing Ralph) and then adding the ID of the rim (19mm on a mavic 719 and 30mm on a velocity blunt 35)

Could I just calculate the diameter for the equation from this?

Would the difference in air volume between wheel sizes (26, 650b or 29er) be a factor in case tension?

Ian,

This should give you a decent estimate. In reality, the inside of the rim is not a true 19mm or 30mm since the shape isn't flat. Don't worry about the size of the wheels. I hope this helps.

Take care,

Jon

Cool, thanks for this

I've been looking at the difference that larger ID rims make to mountain bike wheels. I've just replaced my very old Mavic 719's (ID 19mm) with Velocity Blunt 35 (30mm ID). I always ran my 2.4 tyres at 30 psi. The cross section of the 2.4 tyre I measured is 145mm

Therefore by calculating the approximate circumference of the tyre (145mm + 19mm) and then calculating the diameter (divide by 3.14) I can calculate the casing tension in this set up to 5401 Pa

To see what an equivalent casing tension would be with the 30mm ID rim I did the reverse calculation and got 28psi which is about 6% difference. It will be interesting what I find is a suitable pressure for running them at.

Ian,

That's cool. Let me know what you find.

Take care,

Jon

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