Both sides need to be ribbed.
I had to replace mine last year and I ordered part #11287841529 and it came ribbed on both sides. I was facing the same dilemma and had to order 3 times before i got the right one I needed.
I had to wait for about 2 weeks for the double sided belt to come in and my old one was shredded so I ran the single sided for a couple weeks while waiting and it worked fine but I was paranoid that the belt would slip off.
EDIT- I changed the part number to the correct part number for the double sided ribbed
11' E92 M3 Space Grey | DCT | ESS VT2- 625 | Gintani Race (valved) + Quad Res Catless X-Pipe | KW Sleeve's | GC Camber Plates|Arkym Diffuser | Vorsteiner VRS Lip | P3 Boost Guage | Stud Conversion
: Click Here ESS VT2 625 PHOTOS: Click Here
Last edited by j2m; 03-06-2013 at 12:48 PM.