The armrests come with a metal supporting plate. The idea is that if you use the inside holes, if you apply pressure to the outside of the armrest it could (will) snap. So you should use the metal plate under the armrest to limit the flex and therefore stop the armrest breaking.

They are not carbon fibre, they are some sort of plastic.

I find this a really strange design - the metal plates obviously add a lot of weight, so why not just design the armrest to be more robust in the first place?

I used the metal plate under mine, but last weekend whilst riding the left armrest snapped in two when I went over some rough road. Arm slipped out and somehow managed to stay on the bike.

I returned to aerocoach and they have given me a full refund. Now I've ordered some wattshop dirty aero carbon fibre ones, that don't need a metal plate and won't break!