great movie - i highly recommend it
ok
replacing the chain on a full sus mtb......
if you have the old chain you can just size off that one and cut to the same length
if not, or you have made some changes to the drive train in terms of chain rings, cassettes or rear mech you need to take into consideration the maximum travel of the rear suspension with respect to chain length.....ie you need to ensure your chain has enough length to work with the respective suspension of your bike
there are some frame suspension designs (single pivots especially) that when they reach full compression result in some chain growth (increased distance between the cassette and chainring)
we thread the chain through the large chain ring up from and largest sprocket in the rear (not through the rear mech) and add 2 links to size the chain
for a full sus bike you need to allow for ALL the travel in the rear shock
you can either completely deflate the shock and bottom out the suspension - a velcro strap or toe clip strap comes in handy here and then size the chain with this consideration
or
remove the lower attachment bolt of the shock, lift it out of the way, move the bikes frame to the maximum allowable movement and then size the chain allowing for that degree of movement
you should be able to cycle the suspension through all its travel without the chain becoming too tight If the chain becomes rigid before the suspension reaches the extreme of its travel you will have to add some links
just as an added note - there is more than one way to skin a cat.....
another popular chain sizing technique is
small up front, small in the back, threaded through the rear mech and then taking the size length to ensure the chain is not touching the pulley body on the rear mech
here is a nice video demonstrating that method
http://www.artscyclery.com/learningcenter/installshimanoroadchain.html