The suction cups by Roth are a quality product made for short-term installation while camping, or around the house and garden. The metal parts are made of stainless steel, the rubber disc is made of a material that is resistant to salt water. So this suction cup is ideal for beach lovers.
How much weight does the suction cup really hold?
That depends on the state of the rubber disc - it should be dry, clean, and free of damage. In this condition placed on a glass pane, the suction cup carries 25 kg.
It also depends on the surface: the cleaner and smoother the area, the easier it is to create a vacuum and keep it. You can test the surface yourself: pull your fingernails over a glass pane. You shouldn't be able to hear any sounds or feel any unevenness.
Now do the same with the area you intend to place the suction cup: The more unevennesses and the more sounds you hear, the more the load is impacted.
Car paint in good condition usually is no problem at all. However, it can be a different story with foils, stickers, etc. You also need to be careful that the suction cup does not peel off the stickers and foil when you remove it.
A few tips to keep your suction cup in the best condition:
After use remove it and store it in a relaxed position, ideally in a clean, dark place.
Extreme temperatures, e.g. due to long-term direct solar radiation, can make the rubber age faster.
The rubber disc should always be clean, and free of oils and lubricants. Otherwise, it may slide off...