Why Are Road Bike Tires Thin?
The previous theory that skinny, high-pressure tires were more efficient than wider tires pumped to lower pressures was incorrect. The latest research has proven this incorrect, and so while modern road bikes use thin tires, it has little to do with tire efficiency or rolling resistance.
Road bike tires are thin because they offer less wind resistance when traveling faster. Wind resistance rises by the square of the bike’s speed, which means that a bike traveling at 20 miles per hour will face wind resistance 4 times higher than a bike traveling at 10 miles per hour.
The common misconception that the amount of friction produced by the bike tire contacting the road is why road bikes have thin tires is incorrect. In this article, we explain why road bikes have thin tires.
Road Bikes Have Such Thin Tires For Two Main Reasons
Newton’s second law of motion (don’t worry, we are not going to get all technical) can be used to describe the bike wheels’ effect on increasing the speed.
From this, it can be worked out that two “tire” factors influence a bike’s speed.
- The aerodynamic benefit
- The weight saving.
The Aerodynamic Benefit
The degree of wind resistance is controlled by three factors.
- The speed at which the bike is ridden.
- The area (size) of the tire moving through the air.
- The shape of the tire moving through the air.
The Speed At Which The Bike Is Ridden
Air is made up of molecules. As a bike is ridden, it will collide with the air molecules.
Two factors change as the speed increases.
- While the number of air molecules remains constant, as the bike increases speed, it collides with more air molecules.
- The speed of the bike and air molecules increase.
The effect of this is that the force needed to pedal through air increases exponentially (V2) as the bike speeds up.
The Area (Size) Of The Tire Moving Through The Air
The wider the tire, the greater the area which has to “push back” the air molecules.
| Speed At Which The Cyclist is Riding The Bike (Mph) | Increase In Wind Resistance Factor Over 10 Mph |
|---|---|
| 100 | 100 times |
| 50 | 25 times |
| 40 | 16 times |
| 30 | 9 times |
| 20 | 4 times |
| 10 | 0 times |
Theoretically, the air pressure will deform the front of the tire at very high speeds, increasing wind resistance.
The Shape Of The Tire Moving Through The Air
Just as the area of the tire affects wind resistance, so does shape.
An example of drag being increased by the shape of a tire is the knobby tread found on mountain bike tires. Each of these becomes a barrier that the cyclist has to power through as the speed increases.
Result Of The Bike Wheel Aerodynamics
The wheel’s aerodynamics are the most important factor when trying to add speed to a bike.
The Weight Saving
The thinner the bike tire is, the lighter it becomes.
As the tire weight reduces so, less effort is required from the cyclist to ride at the same speed as on a heavier bike.
The weight saved on the wheel (rotational weight of a bike) has a bigger impact than weight savings on non-rotational parts such as the frame (Link.)
In reality, outside of a laboratory, this is an insignificant factor.
Do Road Bikes Have Thin Tires To Reduce Rolling Resistance?
It is a common misconception that road bike tires are thin because this means that the contact area with the road is reduced.
While it may seem counterintuitive, the opposite is true, and the narrower the tire, the greater the size of the tire’s contact area on the road.
The confusion is because most people automatically assume that the tire’s width (lateral area) increases in area and causes greater friction with the surface.
The real issue is that the narrower the tire, the greater the longitudinal area of the tire which contacts the road.
This is excellently shown in the illustration on the following page (link.)
The result of the greater longitudinal area on the road is that each part of the tire remains in contact with the friction, causing surface for longer periods.
While a greater tire width may contact the road surface compared to a thin tire, each contact point remains on the surface for a shorter time, reducing the rolling resistance.
How Does This Work In Real Life?
It is great to understand how these factors work together and help manufacturers and riders to get the optimal performance out of a bike.
Despite this, there are real-world conditions that must also be taken into account.
The Road Surfaces On Which The Bike Is Ridden
The road surface the bike is ridden over will determine the rider’s tire choice.
Perfectly smooth bike tracks in a velodrome will allow fro the thinnest tires to be selected.
On the other hand, if the road network in which the bike is ridden is in poor condition, the biker will need thicker and sturdier tires to survive the trip!
In addition, if water is involved (rain) and the roads are damp, thin tires with no effective tread will be useless and dangerous.
For general riding conditions, 26-30mm will work at racing speeds.
If you are not riding to achieve the fastest speed possible, we recommend installing wider tires.
One of the benefits of using wider tires on a road bike is that they can be ridden at lower pressures.
This helps them absorb small imperfections in the road, making them more comfortable.
Thicker Tires Are More Comfortable
Wider tires pumped to lower pressures offer more comfort than thin tires pumped very hard.
They absorb small imperfections in the road and transfer less vibration to the frame and handlebars, making for a less tiring ride.
Most research shows that tires pumped to a softer pressure (even too soft) are better for normal use than tires pumped up to very high pressures.
The latest trend is for manufacturers to fit wider tires on road bikes which can be pumped up to lower pressures.
In the distant past, 25mm tires were used for “off-road” bikes, and 23mm was used for standard (non-racing) road bikes. Racing road bikes were fitted with 19mm tires.
It is not uncommon to see 28mm tires fitted to road bikes used on rough courses like the “Cobbled Classics” in Europe.
Conclusion
Road bikes are fitted with thin tires to reduce wind resistance and, to a lesser extent, weight. There is a growing trend to replace these with wider tires that provide more comfort and better grip.
