18 ft. router

I am in the process of building a large router out of oak as I already have
the material. Some on this list felt that expansion and contraction would be
a problem and I agree. I am thinking that I could put a rotary encoder on
each axis with a piano wire running the length of the axis with a couple of
turns around the encoder. This would measure the expansion and contraction
of the machine and I could enter this factor into my cam program to
compensate.
I plane to use rack and pinion drivers with the rack attached at one end
only to the machine. I am expecting that in this way the rack will not
expand with the machine.
The Y axis will only be 6ft so expansion will be much less than the
18ft. axis. Watcha all think.
Doug Pollard