2008 NHRA rule book states "A ribbon type or SFI spec 27.1 mesh type window net is mandatory on any full bodied car required by the rules to have a roll cage."
If you go to the SFI web site, the 27.1 expires every 2 years. But the way I read that above, a ribbon net is not required to be SFI certified (ie. it also has no expiration date either). I'm still confused though because I just got a new ribbon net from Stroud about 2 months ago and it has a dated SFI tag on it still. I think it would be cheaper for Stroud to not continue using the SFI tags if they weren't required. I assume they have to buy those tags from SFI or pay some type of 'royalty' to use them??
I just posted about this recently and got mixed opinions. I asked Brian at Skinny Kid about a week ago and he said every 2 years. Unless it's a super new change in the rules that he's not aware of, I trust what he says.
Last month we went to Milan for the track rental the day before the Friday shootout, and the tech guy studied the belts, punched Sept 08, like they were diamond studded and velvet lined, and never even looked at the net, punched Jan 05, so who knows??