Gotta agree about the Heli-bars! But when it comes to suspension, you want to improve your suspension's 'compliance' (reciprocal of 'stiffness') so 'adding air to the air bag
IS NOT necessarily what you want to do!! That could be the worst possible thing to do for your back.... :shocked:
Altho the air bag is mainly used to maintain ride height & to reduce the likelihood of bottoming your suspension out, it does have some degree of involvement in how 'hard' the suspension is; so once the ride height is set correctly, adding any
more air than the minimum necessary
will be making your ride harder and more jarring!! Is that really what you want to do?? :dontknow:
I'd think your best course of action (ignoring budget for now) would be to upgrade the rear suspension to minimise the air bag's involvement (check out
h0gr1der's suspension thread! :thumbup: ) or remove the need for the air bag completely (talk to a suspension/shock tuner like
M2Shocks 
hyea: ) OR, go the other way completely, and toss the coil spring entirely, upgrading the air bag to take the coil's place in carrying the load of you & the Spyder gently while still maintaining the ride height (you'd still need good shock absorbers tho!

) Sorry, but atm I can't recall the mob who've already done this for Spyders.... :sour:
The other thing to bear in mind is that your tires are an important and integral part of the suspension & critical to providing you/your Spyder with a smooth ryde! Low profile tires and high pressures are going to be your greatest enemy!! The OE Spec Kendas are lightly constructed with relatively low profiles and so they
NEED more air pressure in them simply to carry the load than would a stronger sidewalled higher profile tire!! So even without considering their 'lesser performance' & potential for inherent problems, you should be thinking about tossing the Kendas up front ASAP and getting something better suited to your needs - ideally something that your suspension tuner can work with to improve your suspension's compliance even more! And while the Altimax is generally a great choice for the rear, what size is it?? You might want to consider going a bit narrower in tread width with a corresponding increase in profile so that you can run a lower pressure back there too; but that does beg the question... what air pressures are you currently running now? (And it warrants a warning too -
DO NOT vary the pressure in your front Kendas from the Spyder's tire placard recommendation - those OE Spec Kendas
NEED all the air they can get/safely run to maintain their structural integrity! :lecturef_smilie: )
I would suggest you might want to consider running quality auto tires all round, maybe a 'sport/touring' 175/60R15 tire at saaay, 16 psi up front; with a similar 'sport/touring' or possibly a 'luxury/performance' tire in a 205/65R15 size, also at about 16 psi on the rear - but you'd really need to confirm those pressures are safe for the load they are carrying before committing to that!! You can do a rough calculation to work out the optimal pressure for your tires if you know the max load & max pressure info printed on the sidewall and the weight of your loaded Spyder at each tire, or at least at the front and the back. If you want more info on that rough calc, you'd need to get me/tell us all that info. :thumbup: That said, as a very rough indication and something that may not necessarily apply to anyone else/their application, I've tested a number of auto tires on Spyders (RT's & F3's) - various brands & tread patterns, and as BK911 has also stated, I haven't yet met an auto tire that actually fits on these machines that
needs more than a max of 18 psi in it to do the job, front or rear! Some well constructed auto tires even work best a touch lower, altho while there's been tires that 'worked' as low as 8psi, I wouldn't recommend anyone running any less than say, 14 psi in them for 'normal road use', and at least a couple of pounds more if you plan on working them hard on the highway/thru the twisties!
But as a final comment Bert, did you take an RT for an extended ryde before buying an F3?!? I don't have spondylolisthesis, but I
do have the lingering & permanently damaging effects of a number of 'traumatic injuries' to my back et al; and I simply
can not ride for long on
ANY machine like the F3 that puts my feet forward in the 'birthing position' - it's just WAAAYYY too painful because
ALL of the impacts & shock loads from uneven or poorly maintained road surfaces have nowhere to go but straight into and up/down my spine!! :yikes: At least on an RT I can sometimes use my legs to support & cushion some of the larger shock loads; and according to ALL of my surgeons & physical therapists etc, the more upright ryding position is the least damaging and more natural 'load carrying' & 'shock load withstanding' position for the human spine! So while it might be a ryding position you've become used to, maybe an F3 isn't really the ideal choice of ryde for your spine?? :dontknow:
Sorry 'bout the epic tome.....

pps: but you
did ask! :thumbup: