Reifen Rollwiderstand - uiuiui . . . | Posts 16+

Ernst S. wrote:

Let's assume someone is changing tires at an outside temperature of 10°C (50°F). The amount of deformation in the tire's contact patch will be the same, but it takes much more force to mount a summer tire because it's already very hard at 10°C. A winter tire, on the other hand, is still very soft at 10°C, which is why it requires so little effort to mount. And if that's not true, then I'll blame it on the TDI winter tires.

Hi Ernst,

So, I probably don't have a TDI winter weakness or summer weakness myself, if I sometimes have to push my tractor back and forth in the garage (because it's not worth starting the engine).

Even on a mostly quiet, downhill stretch of road, where I often let the car roll in neutral, there are noticeable differences. From 50 km/h, it reaches approximately 70 km/h after 1.2 km with the winter tires and even with the old, worn summer tires, while with the new summer tires, under practically the same conditions (temperature, no wind, dry road), it only reaches about 63 km/h.
At first, I thought it might be worn brake pads, but the manual check of the wheel and brake disc temperatures consistently indicates "no."

The kinetic energy "destroyed" by the higher rolling resistance on the aforementioned inclined section alone amounts to approximately 43,000 J (or Nm). The braking force required to achieve this over the distance is calculated as F = W : s = approximately 36 N.

In addition, there's a "replacement function" for rolling resistance that accounts for air resistance, which is approximately 30 N higher at 70 km/h than at 63 km/h. Assuming the same initial speed, I'm simplifying things by estimating an average of 15 N of "rolling resistance instead of air resistance."
The total difference in rolling resistance between new summer tires and winter tires (or old summer tires) amounts to approximately 36 + 15 = around 50 N.

It also fits the picture that my fuel consumption has increased by about 0.4 liters since I got the new summer tires.
From 0.4 liters of fuel, the TDI engine releases (an additional) mechanical energy of approximately 5 MJ. Distributed over a distance of 100 km, this represents a force of about 50 N, which, with the same distance and driving style, can apparently be attributed to the higher rolling resistance of the summer tires.

The magnitude is also roughly consistent with the subjective impression one gets when pushing it.

To not completely lose touch with the forum :
Given that rolling resistance of tires appears to be a significant but rather uncertain factor in a vehicle's energy balance, anyone who wants to determine the performance of their engine using a dynamometer should also conduct a rolling resistance test.
Even the 50 N difference between my tires (which does not necessarily mean the maximum tolerance) already results in an error of approximately 2% with an acceleration force of 2500 N, and this error fully affects the calculated transit time and "backwards" also the assumed motor power .

I also noticed that the winter tires (195/65/15) roll much more easily. Also, I'm using about 0.3 liters less fuel. Tire pressure is normal (2.3 + 2.5).
My tires (size 225/50 R16) are performing worse when inflated to 2.8 bar.
I believe it's primarily due to air resistance, and then to the contact area of the tires (more rolling resistance).

To Ulf: What was the reason for installing the 15mm spacer discs?

kenny wrote:

ulf, what did you install the 15mm spacer plates for?

Hi Kenny,

"Due to the somewhat narrow entrance to our garage, there are planters that are about axle-height. I'm worried that I might get the tires stuck on them if I don't align the car properly, before I end up denting the metal."

"It definitely looks a little better. But for certain reasons, I don't like using aluminum, and spacers are the cheapest solution in that case."