How do I determine shock compatibility? I have a Trek Fuel EX 9 2013 with shock DRCV FOX FLOAT CTD 7.25" x 1.875" and I want to know if Rock Shox Monarch RT3 184mm x 48mm (same that FOX in inches) for Trek bikes is compatible.

There are the compatibility list:

Fuel EX 29 2014
Remedy 26/27.5 2010-2014
Remedy 29 2014

Both shock are designed for Trek bikes, but is strange that Fuel EX 9 2013 is not in list if Remedy is compatible.

Thanks.
The 2 dimensions listed for each shock are;


The eye to eye length (sometimes listed as i2i, this is the distance between the 2 mounting points)
The shock stroke, i.e. how far the the shock shaft will move


If the two shocks have the same eye to eye length and the same stroke then the Monarch will physically fit into the frame and allow full travel without fouling the frame (See note below).

The other thing to consider is the damping tune of the shock. Shock manufactures produce a variety of shock tunes to suit different suspension designs (and a large company like Trek might even commission custom tunes). While this will not stop a shock from working in a frame it may result in undesirable handling. Typically the damping tune of a shock can be altered during a service (full service not air can service).

In your case I would expect that the Monarch will be just fine. Be careful to check the clearances before riding.

Note: some shocks are wider or have a piggy back reservoirs. Care should be taken to make sure the new shock does not foul the frame at any point during its travel. Typically you would do this by removing all air from the shock and cycling the suspension by hand and checking the clearances.