While it is pretty likely that your vibration is the belt, if your Spyder still has the OE tires then there is the possibility that it's being caused by dodgy tires!! And I'm not trying to implicate poor wheel alignment or balancing here, cos poor &/or rapid tread wear due to poor wheel alignment IS usually correctable if caught & identified soon enough, & tire balance issues are rarely only felt at certain speeds.... and they too are readily correctable! What I'm talking about is the irregular & patchy tread wear issues caused by poor tire construction or failures/defects in the tire &/or tread compound itself! :shocked:
So just to try & eliminate any of the tires, start out by checking the front tires & making sure they have the same pressure! Once you know the pressures up front are very close to each other, check ALL your tires carefully for tread or sidewall issues. Look for lumps or bulges, even 'blisters' in the sidewall or tread, and only then start checking for wear issues. Start out by lightly running your hand (bare if you can, but do be gentle & careful & yes, you will get dirty! ) along the tire & feel the way the tread blocks pass under your hand... if it feels like one side of any of the tread blocks may have a 'sharp' edge, run your hand the other way to confirm, and if there IS a sharp edge on any of the tread blocks, see if you can identify WHERE the sharp edge actually is on those blocks, ie, front edge of the tread blocks as the tire rotates; back edge as the tire rotates; inside or outside on which tire; or left edge or right edge of one or more tires.... cos if you DO have sharp edges on any tread blocks (& any corresponding smooth edges on the other sides of those tread blocks) or uneven tread wear around or across the tread, your vibration MIGHT be caused by dodgy tires.....
The OE tires are renowned for poor construction issues causing this sort of tread wear issue & possibly also vibrations, making it extremely difficult if not impossible to align correctly &/or balance the tires &/or stop the vibrations, as well as creating rapid tire wear & in some cases creating quite dangerous tire &/or traction or steering problems!! In a few cases the tires have delaminated & that could cause a very dangerous & impossible to control steering &/or drive issues if it occurs at speed or in traffic, the twisties, etc!! So while the most likely cause of your vibrations probably IS the belt, with a fairly easy & relatively inexpensive long-term solution (I am ignoring the cheap & nasty BRP 'solution' when I say that!) you shouldn't overlook the potential for shoddy tires being involved!! Check them carefully, & if any of your tires do show dodgy or uneven wear in a way that isn't clearly the result of poor wheel alignment or balance issues, then I'd suggest you get rid of the OE Spec tires as soon as you can & replace them with something better - & btw, anything with the same or better load & speed rating & a matching bead profile is likely to be a fair bit better!! :thumbup: