Iowa Code § 321.442: Projections on wheels.

No tire on a vehicle moved on a highway shall have on its periphery any block, stud, flange, cleat, or spike or any other protuberances of any material other than rubber which projects beyond the tread of the traction surface of the tire except that it shall be permissible to use:

1. Farm machinery with tires having protuberances which will not injure the highway.

2. Tire chains of reasonable proportions upon any vehicle when required for safety because of snow, ice, or other conditions tending to cause a vehicle to skid.

3. Pneumatic tires with inserted ice grips or tire studs projecting not more than one-sixteenth inch beyond the tread of the traction surface of the tire upon any vehicle from November 1 of each year to April 1 of the following year, except that a school bus and fire department emergency apparatus may use such tires at any time.