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Wheels
Let's continue our attempt to approach automotive technology from a fundamental perspective. After the wheel bearing, it's the wheel's turn. Rim and tire are almost inseparable. Tires made of some kind of springy plastic
without air filling have been making the rounds at various trade fairs for some time now, but such a thing hasn't yet reached series production.
So, here we're describing rims made of steel, alloy, and (as yet untested) carbon, as well as tires made of rubber (carbon black), cord, and steel wire or mesh. Rims and tires have an interface, in most cases a very
specific part of the drop center. It has a very unique shape, with the rim flange on the outside, respectively further inward the tire bead seat, and then a safety feature called a hump. This prevents the deflated tire from
slipping into the drop center and making the ride dangerous.
It's nice to hear a freshly mounted tire being inflated and then finding its final position with a loud plop on each side. Which brings us to the diameter, which, of course, must be the same for both the rim and the tire. The
arrows show the connection point, and in the tire, the wire-reinforced bead. These diameters are getting larger and larger, and at up to 23 inches, have even exceeded those of trucks. Nevertheless, a car wheel - or should
we call it an SUV wheel - is significantly smaller overall than a standard truck wheel.
This is due to the first two parameters. The first refers to the width of the tire. No, not the tread width, but rather a measurement taken from shoulder to shoulder with a giant caliper. For this purpose, the corresponding rim
width 3 and the filling pressure are standardized. Also, the measurements are only given in grid dimensions. For example, the tire above, described as 255 mm wide, can be from 250 to 260 mm wide. And then there's one
more measure where truck tires vary considerably.
It's the so-called height/width ratio in percent. For the tire above, the height of tire 2 is only 40 percent of the width of tire 1. The typical truck tire below would not only be wider, but, in contrast to the SUV, it would also have
twice the width as its height. Both tires side by side would therefore have roughly the same rim size but very different tire sizes.
The inflation pressure is also two to three times higher than that of a car. This all depends on the load capacity, which is usually not taken into account in either the car or truck designation. This is based on the permissible
axle loads with a certain safety margin. There is, of course, a load index that expresses a tire's load capacity, but the above car designation actually assumes approximately 500 kg and a truck 8,000 kg—significantly less
for twin tires, by the way.
The filling pressure is also two to three times higher than that of a car. This all depends on the load capacity, which is usually not taken into account in either the car or truck designation. This is based on the permissible
axle loads with a certain safety margin. There is, of course, a load index that expresses a tire's load capacity, but the above car designation actually assumes approximately 500 kg and a truck 8,000 kg, significantly less for
twin tires, by the way.
If more is required, such as in future electric vehicles with large battery capacities, the load index is usually not printed on the tire, but rather a letter or designation. This is often noted during the vehicle inspection because
these tires are sometimes significantly more expensive. There are two ways to warn about too different air pressure: either through different wheel speeds or through measurements in the tire and radio transmission
(image above).
| Tires on truck steep shoulder rims . . . |
Tires must be balanced and must not be too heavy anywhere along the circumference (static imbalance) or on one side (dynamic imbalance). Tires with reinforced sidewalls (run-flat - video below) can also be fitted to new
vehicles, allowing them to withstand a complete loss of pressure. Adequate tread depth is necessary for road safety, and tires with this symbol are required for ice and snow.
Yes, the maximum permitted speed is important. On test benches, blowouts have occurred after 10 minutes even with slight overshoot. Winter tires, in particular, are often approved for lower speed than the car is capable
of.
| S | 180 km/h |
| T | 190 km/h |
| H | 210 km/h |
| V | 240 km/h |
| W | 270 km/h |
| Y | 300 km/h |
| ZR | >240 km/h |
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