Pasqualea..I just did a three day ride with the Grand Tour bag and no backrest. I ended up putting plastic wire ties from the rings under the seat attachment to the bag on both front corners. Then used the straps and crossed them front to back. With the wire ties I was able to crank it tight and it stayed perfect. Prior to that, I just used the straps and at one stop I realized the bag had rotated backwards 90 degrees. It didn't fall off but it was resting on the fender. I just kept a finger nail clipper in the bag and cut off the wire ties at the end of the day to take the bag in the hotel. It took 10 minutes to get it setup but I felt a lot more comfortable that the bag would not fall off.