I found these IKF rules online. It's my understanding that WKA rules are similar. I know that the Rotax series require the plastic bumpers, but the Bergfelt bumpers are IKF and WKA legal.
IKF 201.6 Starting January 1, 2011 all karts must be equipped with a rear bumper to protect the driver and kart from rear impact and to keep a following kart from reaching the rear tires. The bumper may be either CIK style plastic "rear wheel protection" or steel tube of a double bar design.
The bumper shall extend to at least the center of the rear tires. The ends of the bumper shall not extend beyond the outside of the rear tires, except in a declared rain race. See 200.2.2
201.6.1 The rear bumper must have at least two steel components. The upper bar will be steel and have a minimum diameter of .625" with the lower steel cross bar having a minimum diameter of .625". The rear bumper will be attached to the frame by at least 2 fixation points on the two main frame rails.
201.6.2 There will be a bar between 5" and 12" from the ground. The maximum width will be no wider that the outer edge of the rear tires (55.125"
. The minimum width will not be less than the lateral width of the two main frame rails. All measurements will be with driver seated in the kart as raced.
201.6.3 The lower cross bar height as measured from the ground to the center of the bar 4" maximum. The minimum height of the lower bar will be no lower that the bottom of the frame rail as measured from the ground.
This rule can be found in section 200 starting on page 42 of the IKF Karter News 2011 Competition Regulations & Technical Manual Volume 50 Number 12 December 2010