Iron isn’t the only metal in those wheels, some are more poisonous.

Stainless is mostly iron and chromium, sometimes nickel. Some oxidation states of chromium are indeed toxic, but the most stable form is chromium(III), which is harmless enough that you can buy it as a supplement. Nickel also occurs naturally in soil and many foods and is not accumulating in the body. The kidneys can remove it. Only large doses are problematic.