Looks like a thing to hold the spare wheel in place. Put the yellow things under a bracket type of thing, put the wheel on top, put the screw through the black thing and the hole in the wheel, turn the black thing (and screw) until it's tight.

What does the writing on the yellow part say?

@frankv:

 

Looks like a thing to hold the spare wheel in place. Put the yellow things under a bracket type of thing, put the wheel on top, put the screw through the black thing and the hole in the wheel, turn the black thing (and screw) until it's tight.

 

What does the writing on the yellow part say?

 

 

 

the post above you has the answer in it

Lucky you found it then - consider using nylock nuts.