Rolling Resistance Of Bike tires

Earlier we wrote about how the air resistance increases heavily when increasing your speed. Now we will look on the rolling resistance developed between the tires and the road surface. I will try to make you understand how this works. First, we will have to look on how the rolling resistance is found. That can be done by using equation 1 below:

The equation that shows how great the rolling resistance is. The variables are:

• P is the power needed from the rider to keep the velocity. This power is measured in Watt [W].
• C_rr is the rolling resistance. This value varies greatly from tire to tire and from the surface being used. There are some few numbers below here from Wikipedia:
• 
  

Specific tire	Minimum value	Maximum value
Production bike tires at 8,3 bar on rollers	0,0022	0,005
Typical BMX tires	0,0055	0,0055

• 
  
• N is the normal force of the bike and its rider against gravity. This force is measured in Newton [N], and can be found by the following equation, equation 2:
• 
  

 

In this equation, we have the following variables:

1. m is the mass of you and your bike. This is measured in kilograms [kg].
2. g is the gravitational force. This force is 9,8 m/s².
3. Cos(θ) is Cosine to the horizontal angle. If you are riding on a flat road that would be 0 degrees which equals 1. So on a flat road this is nothing to worry about. But of your are climbing on a mountain or hill, or descending, it has to be taken into consideration.

But as we already saw with the drag, the power needed is different than the force you are experiencing. The equation for the power required is shown below in equation 3:

 

There is only one variable that we have not met before. That one is:

1. V is the velocity you are doing. This is easily measured by your bikecomputer http://www.cyclingtipsonline.com/test/bikecomputer-reviews/ . This is measured in meters per second [m/s]. If you want to convert your speed from kilometers per hour to meters per second just divide by 3,6. If you want to change it from meters per second to kilometers per hour, multiply by 3,6.

What this simple equation got, is the rolling resistance multiplied with the velocity. That means that we can put it in equation 1, along with equation 3, and therefore can make it into a single equation, if you want to find the power required at variable speeds. That gives us equation 4:

Calculation of examples

Lets make a few examples at different speeds. We will take 20 km/h, 30 km/h, 40 km/h and 50 km/h. We say that the rider weighs 70 kg, which is a decent average. The road is also 100% flat. The rolling resistance coefficient of the tires will be 0,003.

If we start off at the 20 km/h, that equals 5,556 m/s. So the equation will be:

As we can see, the rolling resistance will account 11,43 Watt. 

At 30 km/h, the equation will be:

Now it has become 17,15 Watt. In a 50 % speed increase, it has increased around 50 % too. At 40 km/h it will be

The value has now increased to 22,87 Watt. As you can see, that is about the double of the value than it was at 20 km/h. So the power required increases linearly with the speed. If you double your speed, you double the power required to overcome the rolling resistance.

However, what if we are having some nice tailwind? How much power will we need to overcome at 50 km/h? That can be found in the equation below:

At 50 km/h the tires are now creating a resistance so great, that you now need to power 28,58 Watt in order to maintain your speed.

Nevertheless, remember, these numbers does not account for much more than just theory. Every tire acts different, and even the smallest difference in production of the same brand of tire can give them two different friction coefficients. And the drag you will experience because of the wind accounts for a much larger value than the tires rolling resistance. Nevertheless, if you want speed, try checking the Continental GP4000S out. They were the fastest clincher tires in the German magazine “Tour” test.

If you want to decrease the rolling resistance as much as possible, ride with a high pressure in your tubes. The higher the pressure the lower the rolling resistance. That is due to the harder the tire will bounce over just the smallest roughness in the surface. But that is different on a mountainbike. On a mountainbike in the forest, a low pressure would actually make you go faster. 

But try to read here why you should focuse on buying good tires before spending much more on a pair of wheelset. You will actually be able to win more speed by changing your tires than your wheels!

Written by René

Read more: http://www.cyclingtipsonline.com/news/rolling-resistance-of-bike-tires/

Continental Ultra Race Test

The Continental Ultra Race are tires made by the German manufacture meant for the people who are not able (or want) to pay enough for a couple of GP4000S-tires. For riders who yet still wants to pay more than the Ultra Sport, along with ensuring that you are getting decent tires, with a low rolling resistance. They were equipped on some Mavic Cosmic Elite that followed with my Scott Foil 40.

On the road

They weigh 220 grams per tire, 15 grams more than GP4000S, and 30 grams less than the Ultra Sport. Weight is a big factor when it comes to wheels, and especially at the tires, because they are the outer part of the wheels. Wheels inertia is very important when it comes to acceleration, and pro riders would rather save 100 grams on the wheels, than 500 grams on the frame and own body. They are just a little bit easier than the Ultra Sport to get in speed (it can just be my imagination of course, but they do feel a bit better at that), but you cannot feel the difference between them and the GP4000S.

I have been using them for about 3-4 months, ridden 2000 km in that time, and they have been doing fine. The rolling resistance seems to be somewhat low. They are easy to maintain speed in, and the grip in corners are just fine. Also in wet conditions. I do not feel insecure in corners with them at least. I have not had any punctures yet, and I do not even think that I have been closing on getting one. They do not seem to pick up stones/gravel just as easy as the GP4000S does. Especially on wet surfaces, the difference is great.

I have tried both the more expensive GP4000S and the cheaper Ultra Sport, and I must say that they are a bit faster than the Ultra Sport, and still slower than the GP4000S. The first day I had the GP4000S, I was riding 1,5 km/h faster than on the Ultra Race, so if you want pure speed, go get the GP4000S, but if you don’t want to put so much money in a couple of tires, you can easily go for the Ultra Race. The Ultra Race was also somewhat 0,5-1 km/h faster than the Ultra Sport. Moreover, if you do not think that the tires really can do so much of a difference, I recommend you to read this. Nevertheless, the price and quality (if quality equals speed) follows each other’s at Continental.

I cannot comment on the wear of them with only 2000 km in them, but they do last longer than the Continental Ultra Sport I am sure of, because at this point, the Ultra Sport already looked quite worn out.

The Verdict

I can highly recommend these tires, if you do not care about the last extra speed you can get by switching to GP4000S instead. Nevertheless, compared to the Ultra Sport, I really think you should spend those extra bucks on these, because they seem to last a lot longer, plus they are better in somewhat everything than the Ultra Sport. They are better at preventing punctures too, than any of the other tires I have tried so far, which is only a big plus.

Written by René

Read more: http://cyclingtipsonline.com/news/continental-ultra-race-test/

Continental Ultra Sport Test

The Continental Ultra Sport tires are made by the German manufacture meant for the people who aims after some cheap budget tires, that aren’t meant for great speed, but more good tires for a small price. They came as standard with my Scott Speedster S20 equipped on AlexRims Race 28 Pro wheels.

On the road

They weigh 250 grams per tire, 45 grams more than GP4000S, and 30 grams more than the Ultra Race. Weight is a big factor when it comes to wheels, and especially at the tires, because they are the outer part of the wheels. Wheels inertia is very important when it comes to acceleration, and pro riders would rather save 100 grams on the wheels, than 500 grams on the frame and own body. They are just a little bit harder than the Ultra Race to get in speed (it can just be my imagination of course, but they do feel a bit better at that), which also counts for the GP4000S that feels a little bit easier to get in speed.

I have been using them for about 1½ years, ridden 4000 km in that time, and they have been doing somewhat decently. The rolling resistance seems to be higher than what I am used to, but that was expected. They are still easy to maintain a good speed in, and the grip in corners are just fine. Also in wet conditions. I do not feel insecure in corners with them at least.

I have had a single puncture with them, which is due to them wearing out quickly. The rear tire was more or less ready to be shifted out after 3500 km, which I did not do, and paid the price of a flat, due to not much rubber was left. That is a big minus in my book that the tires cannot last longer than that. The front tire seems more used too than they should. But that is what you pay the price for.

I have tried both the more expensive GP4000S and Ultra Sport, and I must say that they are the slowest of the three. Compared to the Ultra Race, I was 0,5-1 km/h slower on the Ultra Sport. And the GP4000S are additional 1,5 km/h faster than the Ultra Race. There is really a lot of speed to gain with better and more expensive tires. If you do not think that the tires really can do so much of a difference, I recommend you to read this. Nevertheless, the price and quality (if quality equals speed) follows each other’s at Continental.

The Verdict

I will not recommend these tires. They do not last very long, and they are really slow. For not much more euro’s will you be able to buy the Ultra Race that is a lot better overall. Especially with the endurance of the tires, there is quite a big difference, asw well as in the speed. I really think you should spend those extra bucks on the Ultra Race, or go after the GP4000S if speed is the thing you want.

Written by René

Read more: http://cyclingtipsonline.com/news/continental-ultra-sport-test/