Chinook landing gear question

[P6 Driver]

With the CH-47 series, the aircraft has a twin wheel arrangement at the front, but single wheels at the back.

I know the rear wheels are steerable, and that each end of the helicopter has a rotor blade and gearbox assembly and the fuel tanks are broadly between those areas, with the rear of the aircraft also containing the engines.

What puzzles me is why the wheel arrangement is twin at the front only, rather than twins all round? Visually, and in very simplistic terms, the helicopter looks heavier towards the rear than the front.

My advance apologies if this proves to be a stupid question with a blindingly obvious answer...

[ORAC]

Bit of background here (http://cs.finescale.com/fsm/modeling_subjects/f/16/t/58220.aspx).

[minigundiplomat]

I recall coming back from a night resupply to Nowzad in early Herrick days and finding the aircraft listed heavily to the right when we tried to land on one of the old spots.

I went off the back to locate the problem - which was a 2 and a half foot boulder jammed between the two front left wheels (jammed enough to stay in place for a 35 min vibrating recovery to BSN).

We ended up holding it on the back wheels whilst the engineers unscrewed the outer wheel.

Happy days.....

[P6 Driver]

Every day's a school day!
Thank you for your informative replies.
:)