The ITU rulebook...the whole section on wheels
5.2. Equipment:
c.) Wheels:
(i) No wheel may contain any mechanisms, which are capable of accelerating it;
(ii) Tubular tyres must be well glued, the levers of the quick releases must be
closed tightly onto the hubs and wheels must be fixed properly onto the
frame;
(iii) There must be a brake on each wheel;
(iv) Wheels can be replaced only at official Wheel Stations, where provided;
(v) Officials at the Wheel Station will give the proper wheel to the athlete. The
athlete is responsible for replacing the wheel in the forks. Wheels
designated specifically for other athletes or teams cannot be used;
(vi) For Elite, U23, Junior and Youth draft-legal competitions, wheels must have
the following characteristics:
A diameter between 70 cm maximum and 55 cm minimum, including
the tyre;
Both wheels must be of equal diameter;
Wheels shall have at least 16 metal spokes;
Spokes can be round, flattened or oval, provided their width does not
exceed 2.4 mm;
The maximum rim dimension will be 25 mm. on each side;
Wheels not complying with the requirements above are allowed to be
used if they are included in the UCI non-standard wheels list.
(vii) For Age Group draft-legal competitions, wheels must have the following
characteristics:
Wheels shall have at least 12spokes;
Disc wheels are not allowed.
(viii) For draft-illegal, competitions, covers are allowed on the rear wheel.
However, this provision may be changed by the Technical Delegate in the
interest of safety, i.e. high-winds
http://www.triathlon.org/...es_december20141.pdf