The roller is subject to impact load as it mates with the sprocket
teeth during engagement of the chain with the sprocket. After
engagement, the roller changes its point of contact and balance.
It is held between the sprocket teeth and bush, and moves on the
tooth face while receiving a compression load. Therefore, it must
be resistant to wear and still have strength against shock, fatigue
and compression. (RS25 and RS35 are bush chains and do not
have rollers)