Tektro Chain Aligner (Anti-Chain Suck Device): RPB-102A
There are many reasons for chain suck, some of which can be eliminated with some persistence. However if you have a drive train you like and can't find the cause, then this device will prevent chain suck from happening. Also know as an anti-chani jam plate, this little device bolts to the chain stays and aligns with the three chain rings.
See also RPB-100A