为什么是be going bike riding而不是be going riding bike

只能说be going to ride bike,不能说be going riding bike,这样累赘,如果一定要说be going riding bike,此时外国人会省略going,直接说 be riding bike,riding bikes中的riding 是动词.bikes是名词.通常用于进行时,比如We are riding bikes.【我们正在骑车】
. 而 be going bike riding其实也可以写成be going bike-riding,后面可以看成一个词,bike-riding 是复合名词,就是两个名词连接在一起构成的复合词,字面意思,就是名词.如We are doing bike-riding.【我们正在骑车】.
因为骑马会说成horse riding,骑自行车也就会说成bike riding
口语中经常会这样说,这是习惯用法.
如：I went bike riding in the park yesterday.我昨天去公园骑车了.
口语中很多不一定按照严格的语法来,bike riding是习惯口语说法.