Rear pannier placement should be far enough aft to eliminate any interference with your feet. This is commonly known as "heel strike". What determines whether you'll suffer heel strike or not depends, largely, on chainstay length and size and shape of your panniers. I have some Overland Equipment Panniers that are really really nice, but they are rectangular in shape, and so they hit my feet even though the chainstays on my 85 Peugeot Canyon Express MTB are 18-1/2" long. My Lone Peak panniers, OTOH, are shaped with a diagonal section to make clearance for my heels, and therefore heel strike is not a problem. These are smaller than my Overlands, however, so a greater percentage of the load ends up in my Overland panniers on my front wheel. This works OK for me though, as most MTBs are designed with somewhere in the range of 55/45 rear/front weight distribution.
With the added weight of the (loaded) larger panniers over the front, my weight distribution ends up closer to 50/50 F/R.
I wouldn't think rear pannier position would change much if you're using them in conjunction with fronts. Remember, they can only go so far forward due to heel strike, and they can only go so far aft due to rack/pannier/chainstay design limitations.
Are you using lowriders on the front or is it a platform type rack over the top of the front wheel? I have a platform type (Blackburn, actually) on the front and the weight distribution doesn't seem to slow down steering too much or otherwise adversely affect my handling....
And said handling will be more affected by rake/trail and load distribution side to side than anything else.....
Just my .02 and I'm sure someone with more knowledge than me will chime in.
Last edited by Smokinapankake; 01-03-11 at 05:36 PM.