If you could watch your chain, you’d see waves of energy literally bounce the chain as it moves up and down the cassette or when riding on rough roads. With the shorter chainstays of folding bikes, the waves just get amplified in size. In order to help prevent the chain from derailling the standard guide is fitted to many models of Dahon and Tern folding bikes. (For a more robust solution on suitable models you can use the Tern Mainstay Guide, this however does not fit all models of Dahon) One certain older steel frame models it is necessary to use a piece of rubber or plastic packing not supplied in order to tighten the guide on the seat tube.