How wheel size affects driving performance and vehicle performance
Content
Clothes make the man, wheels make the car. For many years, it is clear that a large number of motorists drive. But some have gone even further, following the motto: "The bigger and wider, the better." Is it really true? Let's look at the problem in more detail and describe the advantages/disadvantages of standard narrower tires and optional wider tires.
Discs are available today in a variety of shapes, sizes, colors, so a potential interested member feels they can pick just about anything that will suit their father. Thus, the data in the data sheet and the space under the wings remain the only limitations. In reality, however, there are several limitations that, if ignored, can significantly affect driving performance, driving comfort or safety. It should also be borne in mind that the wheels are the only point of contact of the vehicle with the road.
Wheel weight
Few people interested in a beautiful and large bike will ask themselves this question. At the same time, the weight of the unsprung masses has a relatively significant effect on the driving performance and handling of the vehicle. Also, a decrease in the inertia force of a rotating wheel increases the dynamics of acceleration and deceleration. In the case of a change in size of 1 inch (inch), the weight gain is relatively small, in the case of an increase of 2 inches or more, the weight gain is more pronounced and reaches several kilograms. Of course, the material from which the disc is made must also be considered.
Simple physics is enough to explain the important role of wheel weights. The kinetic energy of the spinning wheel increases in proportion to the speed of rotation.
Ek = 1/2 * I * ω2
The fact that this is a considerable quantity can be shown by the example of rotating bicycle wheels. They are lightweight, but if they spin at a certain minimum speed, they can hold the bike with an adult in a straight line without gripping or driving. The reason is the so-called gyroscopic effect, due to which changing the direction of movement is more difficult, the higher the speed of rotation of the wheel.
It's the same with the wheels of cars. The heavier they are, the more difficult it is to change direction, and we perceive this as so-called power steering. Heavier wheels also make it more difficult to soften their motion when passing bumps. It also takes more energy to rotate or rotate them. braking.
Vehicle dynamics
The width of the tire also has little effect on the vehicle's dynamic performance. A larger contact area means more rolling resistance when using the same type of tread. This is more pronounced with weaker engines, where acceleration from 0 to 100 km / h can be reduced by a few tenths of a second. In the case of more powerful engines, this difference is negligible.
In some cases (with powerful engines) this effect is even the opposite, since the wider wheel has a larger contact area with the road, which is reflected in less slip during fast acceleration and therefore in a better resulting acceleration.
full speed
The tire width also affects the top speed. However, in this case, the effect of higher rolling resistance is less pronounced than in the case of acceleration. This is because other resistances to motion come into play, and the most significant resistance occurs between the air of the body, but also between the wheels themselves, which rise by the square of the speed.
Braking distances
On dry surfaces, the wider the tire, the shorter the braking distance. The difference is in meters. The same can be said for wet braking, as there are many more small areas (edges) of the tread pattern rubbing against the road.
The opposite situation occurs when the car is driving/braking on a wet surface with a continuous layer of water. Increasing the width of the tire reduces the specific pressure of the tire on the road and removes water from the contact surface worse. The larger area of a wider tire needs to carry a fairly large amount of water, which becomes more and more of a problem as speed increases. For this reason, wider tires start much earlier, the so-called Swim - hydroplaning when driving in a large pool, like narrower tires, especially if the tread of a wide tire is heavily worn.
Maneuverability
On dry and wet surfaces, wider tires with a smaller profile number (smaller dimensions and stiff sidewall) provide better traction. This means better (faster and sharper) handling with a sharper change of direction, as there is significantly less deformation than with a narrower or narrower body. standard tire. Better traction also results in a shift in the shear limit during fast cornering – a higher g-value.
As with braking, the opposite situation occurs on a wet surface or on a wet road. when driving in the snow. On such roads, wider tires will start to slip and slip much earlier. Narrower tires perform much better in this regard, as significantly less water or snow gets stuck under the tread. It goes without saying that comparing tires with the same type and tread thickness.
Consumption
The width of the tire also has a significant effect on the fuel consumption of the vehicle. It is more pronounced in weaker engines, where more accelerator pedal pressure is required for the expected dynamics. In this case, changing the tire from 15 "to 18" can also mean an increase in fuel consumption by more than 10%. Typically, an increase in tire diameter of 1 inch and a corresponding increase in tire width means an increase in fuel consumption of about 2-3%.
Comfortable driving
Narrower tires with higher profile numbers (standard) are more suitable for driving on poorer roads. Their high height deforms and absorbs road irregularities better.
In terms of noise, the wider tire is only slightly noisier than the narrower standard tire. For most tires with the same tread pattern, this difference is negligible.
Speed change at the same engine speed
In addition to the above factors, tire size changes can also affect the speed of the vehicle at the same engine speed. In other words, at the same tachometer speed, the car will move faster or slower. Speed deviations after tire change acc. disks differ in percentage. Let's simulate an example on the Škoda Octavia. We want to change wheels 195/65 R15 to 205/55 R16. The resulting change in speed is easy to calculate:
Tires 195/65 R15
The size is indicated, for example: 195/65 R15, where 195 mm is the tire width (in mm), and 65 is the tire height as a percentage (from the inner diameter to the outer) in relation to the tire width. R15 is the disc diameter in inches (one inch equals 25,4 mm).
Tire height v we believe v = width * profile "v = 195 * 0,65 = 126,75 mm.
We calculate the radius of the disk in millimeters r = disc diameter * 25,4 / 2 "r = (15 * 25,4) / 2 = 190,5 mm.
The radius of the whole wheel is R = r + v »126,75 + 190,5 = 317,25.
Wheel circumference O = 2 * π * R "2 * 3,1415 * 317,25 = 1993,28 mm.
Tires 205/55 R16
v = 205 * 0,55 = 112,75 mm.
r = (16 * 25,4) / 2 = 203,2 mm.
R = 112,75 + 203,2 = 315,95 mm.
O = 2 * 3,1415 * 315,95 = 1985,11 mm.
From the above calculations, it can be seen that a seemingly large 16-inch wheel is actually a few mm smaller. Thus, the ground clearance of the car is reduced by 1,3 mm. The effect on the resulting speed is calculated by the formula Δ = (R2 / R1 – 1) * 100 [%], where R1 is the original wheel radius and R2 is the new wheel radius.
Δ = (315,95 / 317,25 – 1) * 100 = -0,41%
After changing tires from 15 "to 16", the speed will be reduced by 0,41% and the tachometer will show a speed of 0,41% higher at the same speed than in the case of 15 "tires.
In this case, the change in speed is negligible. But if we change, for example, when using wheels from 185/60 R14 to 195/55 R15 on a Škoda Fabia or Seat Ibiza, the speed will increase by about 3%, and the tachometer will show 3% less speed at the same speed than in the case of tires 14 ″.
This calculation is only a simplified example of the effect of tire dimensions. In real use, in addition to the size of the rims and tires, the change in speed is also influenced by the tread depth, inflation of the tires and, of course, the speed of movement, since the rolling tire deforms during movement depending on the speed. and structural rigidity.
Finally, a summary of the advantages and disadvantages of large and wide tires over standard sizes.
| better grip on dry and wet roads | Poor driving performance (handling, braking, grip) on snow-covered or water-covered surfaces |
| better vehicle handling on dry and wet roads | the appearance of aquaplaning at lower speeds |
| better braking properties on dry and wet roads | increase in consumption |
| mainly improving the design of the car | deterioration of driving comfort |
| mostly higher price and weight |
