I've got an old patio door that is very hard to slide backwards and
forwards. It runs on 2 pairs of 30mm plastic wheels, running on a steel
rail. At least one of the wheels is disintegrating, and it looks like I
would have to partially dismantle the door in order to remove the wheel
bogie.

Rather than hunting around for a compatible replacement bogie and then
taking the door apart to fit it, I was thinking of buying a nice big block
of PTFE from RS, and fixing a bit of it to the bottom of the door, with a
channel cut in it to keep it on the rail.

Has anybody tried this? Would this have low-enough friction? Would bits of
grit get stuck in the PTFE and then abrade the steel rail?Presumably there's
a reason that patio doors use wheels rather than blocks of PTFE?

Thanks.