Because most full suspension frame designs either have a single pivot that is not concentric with the bottom bracket, or a linkage with a variable center of rotation, the distance between the bottom bracket and rear axle varies throughout the suspension's travel. This requires a sprung chain tensioner, similar to a rear derailleur, to be used. Only relatively uncommon frame designs with single pivots that are concentric with the bottom bracket allow the use of more desirable static chain tensioners. Another consideration is the generally increased amount of pedalling while standing up that is inherent to single speed mountain biking, which increases losses from suspension bob and squat. Therefore, full suspension mountain bikes are fairly rare, with hardtails and rigid bikes being much more common.