I had similar problem. What helped was talking a vice grip, protected with a towel in grip, around the knurled ****, and tightening it.

 

I have a bud who had these on his 2 wheeler and he and his wife had a slow rolling fall over. Nothing big but she said she could not get off and out of the way of the bike with these installed on it.
Made him take them off. This was after riding for many years and miles all over the U.S. Less of a problem on a trike though.

 

I can understand that on a two wheeler, if you roll a trike I think your have more problems to worry about.

 

They just made her feel trapped on the bike.

 

Sherriff,

Can you mount a drink holder with these?

 

yes, but I think it has to be a Rivco cup holder.
although I don't see why a guy couldn't fab one already in hand to fit the supplied hole you see on the right side rest.