Good question, I also need to know this as I always use to use only the rear bags as I never carried more than 15 kg. As I got fitter more spokes broke as I hit bumps on the road at a higher speed. I am now going to ride Venice to London over the Alps and want to spread the load as I can imagine climbing all day and having all the load on the rear will be hampering my climb. So what is the ratio ?? 40 to 60% on the rear I suspect but would appreciate the stats from some one who has tried and tested this...