Hi,
I recommend you buy
a Rear Elka Stage 5 shock with the longer Hydraulic lines for the rear with also the Pre Load Adjuster.
The rear was custom made with longer lines so as to mount the Hydraulic lines and adjuster on the right side of F3 on the frame above the brake pedal area, (same as Hitman5363 ordered in a post above you).
If you weigh 250# & have the side luggage/topcase you need to add at least another 100#, (order the
Stage 5 rear shock set up for a
350# guy).
http://www.elkasuspension.com/product/stage-5-hyd-rear-shock-for-can-am-spyder-f3-f3-s-2015/
With the
adjustable Hi speed & separate Low speed compression adjustment you can soften up the ride over small bumps, etc to suit you, help your back.
You should try
just the rear Elka stage 5 shock first to see if that helps with your back. (the F3 rear shock has same bottom bushing as the top bushing shown).
The front stock Fox shocks 2015 F3 Spyder are pretty good at absorbing bumps.
(I set up many rear shocks/springs/dampening on my Dirt bikes racing in SoCal Mojave Desert Dist. 37, B to V, in the 70's; wish I had a stage 5 Elka shock back then, ha).
Give Len a call at Pitbull Powersports,417-863-1418, he may have a Elka Stage 5 rear shock, heavier spring in stock from Spyderfest;
(Len is the best around at setting up Elka shocks for Spyders)!!
Hope this info helps your decision.
Jim