From a comment on another forum, I was inspired to define the main ride handling benefit of front bags. I have been witness to quite a good few falls where people were using rear panniers only.
not a big fan of front panniers who ever said that 4 panniers balances the bike needs there head examined
If you plan on riding on gravel roads or other unsealed surfaces (rocky tracks, loose pea-gravel), through country with any steep descents, or carrying heavy loads including multi-days' food supplies; then low-mounted front panniers are
highly recommended. They stabilise your steering, making it less twitchy - more predictable; and they act to keep your tyre on the road, rather then bouncing or sliding.
I've been witness to a number of nasty falls (with broken bones) when people have been riding on rough track descents, and all of them were not using front panniers. Others riding the same roads with four panniers were fine.
Come to think of it, my fall in early Feb that has left me with a broken collarbone was also a front tyre slide when I had just the rear bags
Why it works?
1. A heavy load of gear on the rear rack acts like pendulum with your steering as the axis. Every pedal stroke pushes the pendulum and it swings to and fro. Your rear load acts via this to "steer" your bike. Not a huge problem in normal riding, where you just correct the steering to cope. Pedalling at higher speeds - not good. It can trigger a speed wobble. I have witnessed a very bad fall that occurred just this way.
2. Front wheel weight on the road is very important on loose surfaces. Heavy load on the rear with no weight on the front is not a huge issue most of the time. However on a gravel road descent (or even on wet tarmac) some speed, combined with a bit of a bump or rise, can leave your rear tyre sticking to the surface (good) while your front tyre lifts off or is un-weghted (not good). Combine this with some loose gravel, a bit of a rut-edge, or even a front wheel lift and steering correction, and very rapidly everything goes pear-shaped. I have also witnessed a number of falls that occurred in this sort of manner.
I have actually done overnight tours with
solely front panniers and a rack-top bag. The bike rode very well and I was able to stand up on the pedals for climbs, something that is often quite destabilising with just rear bags. On my fully-loaded bike with 25-30kgs of gear I will frequently (and happily) do standing climbing. This would be impossible with all that load on the rear rack
So basically the weight in your front pannier will keep the tyre firmly planted on the road, will help keep less weight on the rear (especially behind the rear axle or high-mounted), and as long as the front panniers are low-mounted, balanced and fairly central to the steering axis (for & aft and side to side) they will not mess with your steering too much.