The load capacity is an equally important value, even if it is not so relevant for a car. The ordinary tire is designed in terms of its load-bearing capacity in such a way that it includes the common passenger car. It can perhaps be critical with the SUV, even more critical with the transporter, and do you know where it is most critical, with the trailer.

These six wheels have to fit under the low chassis and then also carry up to 3.5 tons permissible total weight, with four wheels it is still 2.7 tons. Well, a transporter has to do that too, but it has much larger and maybe wider wheels.

In general, trailer wheels are among the most neglected of all. The category shown above should not be addressed here, but rather caravans. They stand around most of the time, maybe even robbed of their softeners by the sun and then they are supposed to trot faithfully and well-behaved behind the family vehicle, possibly even loaded beyond the limit.

Although that's a bit of an understatement, because some countries even allow more than 100 km/h. That would all be fine if they didn't also reach an almost biblical age. Sure, a lot of tread is not worn in the comparatively few kilometers and that's how you usually judge whether you need new tires.

Sometimes it takes over 10 years. Have you ever seen how such a partially huge vehicle behaves when a tire bursts? Owners of double-axle vehicles have a bit of luck (picture above), but if there is only one tire installed on each side, then 'good night'. When a train starts to swing, it's hard enough as it is, but when a tire blows out . . .

You might have first encountered the load capacity of a tire as the proud owner of a VW 'Bulli'. A lot of work put into it, donated good used tires and then to the TÜV. They were specialized in the tire designation. It was thought that the cheaper '185 R 14' would do, but TÜV required the 'C' behind it. Without that there was no getting through.

So hopefully being able to return the set of tires and look out for the more expensive '185 R 14 C' for 'Commercial' as that letter guaranteed the appropriate load capacity. Alternatively, the then already outdated designation '8 PR' for 'Ply Rating' could have been written on it, which would have corresponded to the load-bearing capacity of the eight precisely defined cord layers in the carcass (inner tire structure).

If you look at the table above, then the 'C' would need to be replaced with a Load Iindex of around 100: '185 R 14 100 S', where the speed index 'S' would of course not be used. There just weren't any slower tires in this category. Just like the tire width, the load index summarizes different masses. It is important that these are maximum axle loads.

So if a car in the compact class has a permissible total weight of, say, 1,700 kg, one would choose '78' based on a quarter of the value as a first approximation. But that is not possible, because the car has probably a front- wheel drive and is either heavier at the front or at the rear because it is allowed to carry a lot of luggage.

It is therefore always important to pay attention to the axle loads. But even these do not add up to the exact permissible total weight. It is usually a little more to allow greater freedom in distributing the loads. The maximum load capacity of the tire must be based on the axle loads and not on the total weight.

And then there is a certain safety margin. Because no air pressure can be maintained exactly. In order to be recognized by the system, it has to fall off a bit. Presumably, test drives under extreme conditions at the prototype stage clarify which tires can be fitted by the manufacturer with a clear conscience for the respective vehicle.

In the case of dual tires, about 5 percent must be deducted from the addition of both tires. If a tire on the given vehicle does not use its maximum permissible speed, the load capacity may be increased between 10 and 42 percent, the latter e.g. max. 25 km/h.

Where can you find newer whitewall tires such as those shown on the vehicle above? Yes, that looks chic on such a classic car. You have to know that tires were originally white like latex milk (picture below from 1918). Only later did they became black, presumably due to the addition of soot. That was expensive at first, even just for the tire walls. Later, the premium feeling reversed.

You can still buy whitewall tires today. In the USA you have fewer problems with their mediocre qualities, e.g. the grip in the wet. Therefore, either the expensive post-vulcanization of your favorite tires or rings that are clamped into the rims and have a white plastic ring are recommended.