"Rothaireachd" means "riding a bike": Tha mi a' rothaireachd. In some contexts it might be better to use some verb of motion + "air (muin) rothair" ("on a bike"): Chaidh mi air rothair ("I went on a bike"); thàinig mi air muin rothair ("I came on a bike") "Baidhsagail" is also used as an alternative to "rothair".