i have a trek 3700, it has a 7 ring cassette. what would it take to convert it to a nine speed cassette?
Easiest and cheapest option is just to buy a new rear wheel with a 9-speed cassette on it. However, if you want to go the conversion route:
Firstly, identify what you currently have. Remove the rear wheel and take a gander at these pics:
Have a look at the notched/splined hole at the top of your cassette (where the smallest sprocket is) and compare it to the pictures above. Which one does it look like, A, B or C?
If A, then you're out of luck as what you have is a screw-on type freewheel. These don't come in 9-speed versions. Eight was max and even those were like hens teeth.
If B or C, then check your wheel to see if there is a spacer ring underneath the cassette. If there isn't, then the splined freehub body is too short to accept the taller stack 9-speed cassette. If there is, then you're in luck.
Stuff needed to upgrade to 9-speed:
9-speed Shimano or Shimano compatible cassette
Shimano cassette lockring remover
Big adjustable spanner
9-speed Righthand shifter
1 Gear cable
Probably the rear derailleur as well due to reach and the minimum size of the smallest sprocket tooth size that it can handle.