These are optional Gray Kyosho Inferno Big Bore shock springs, this is the medium length and are used on the rear only of the MP9s.These are optional front springs for the Inferno MP9 TKi2 and 3.
Here is a spring rate chart of the available springs for the Kyosho Inferno Big Bores:
These are some guidelines for choosing different spring rates for your setup on your Kyosho Inferno. Remember that every change you make to your setup will effect something else. It is a good idea to experiment by changing settings to get a feel for how changes effect your car with your driving style. Further reading on the subject of chassis set will also help. Try and hang with experienced drivers at the tracks you race at will also help immensely!
Text Version of the above Chart:
Explanation for the spring rates in the charts above:
The part number of the Kyosho spring has the wire thickness and number of turns included in it.
IF350-716 equates to:
7 turns of 1.6 thick wire.
This can be extrapolated to all the springs and you will see the pattern evolve for the spring rates.
Thicker wire = stiffer spring
More turns in wire = softer spring
Just think of this like a spring is a piece of wire. If you straightened out the wire and held it on one end, the longer it is, the easier it would bend by pushing on it from the opposite end. The same would hold true for a thinner piece of wire, it would be easier to bend as well. This is how springs are actually working. The longer the wire the easier it is to bend and therefore the softer spring rate it has. An advantage of lower turns and thinner wire is you can create a spring with lighter weight vs. a spring with thicker wire and more turns. Keep this in mind when choosing springs to run as well.
I hope this explanation helps rid some confusion about how the above charts work.