This is also a good opportunity to change your gearing ratio. I originally ordered a 16/39 sprocket combination. Sidewinder mistakenly sent me a 17/39 combo. Instead of changing it out, I tried the 17 CS & it works fine. There is a bit of clutch action required to get my 200# started from a dead stop, but after that, it runs nice!
Good luck
J.
I have a 15t 16t and 17t on hand. Most of my riding on my Ascot is city with some back roads riding but very rarely freeway. I'll break it down how I feel about each sprocket when combined with the stock rear sprocket (42t)
15t: Works great out on some twisty back roads but is a little buzzy when keeping up with traffic. I live on the outskirts of a major metropolitan area so much of my riding is four-lane stoplight to stoplight. The 15t gets up and goes off of the line but first gear is limited to just about walking speed tops. I also can't seem to decide what gear it wants to be in at around 40-45mph, fourth seems too low and fifth seems too high so I usually just decide to leave it in fifth and go 50mph anyways. Also wheelies, you know, if that's your thing.
16t: I may be a little prejudiced and also hopped up on a cocktail of alkaseltzer, Tylenol extra strength cold medicine, mucinex and a few cups of espresso to balance the whole thing out but I absolutely love the 16t sprocket for my style of riding. It gets rid of that annoying, should i be in fourth or fifth issue that I had mentioned while still giving the bike some get up and go acceleration. I've ridden with this combo on a 127mi trip and cruised steadily without feeling like the bike was going to vibrate itself apart at 55-65mph. I also don't have to give it a Fred Flintstone style pedal to lug my 190lb frame away from stoplights with enough revs to frighten pedestrians or people in nearby office buildings. Which leads us to....
17t: I put this on my FT because for some reason I though it would be a good idea to slap on an old spitfire fairing I found at a garage sale, combo it with this sprocket and attempt to ride my over-sized lawnmower on a 22-mile freeway only commute to work. But... we learn from our mistakes. So i ditched the fairing and just stuck with the 17t sprocket out of sheer laziness. Now this sprocket is great if you live somewhere with nothing but straight, flat roads and want to take long trips without stopping, but in that case you probably wouldn't be using your FT for that. All cynical comments aside the 17t sprocket is high strung. It takes a lot for me to scream my way out of stoplights and giving it a little push off with my legs and a hand full of throttle helps me along. This sprocket once again solves the aforementioned "should i be in fourth or fifth" problem by totally eliminating the need for fifth gear. With this sprocket combo i did a close to 200mi loop one night and don't think I used fifth for anything but back road cruising at 60-65mph.
TL;DR: 15t for hooligan fun or sub 50mph road burning, 16t best all around combo in my opinion, 17t better be a flyweight or go for more high speeds.
Hope this helps and I didn't just type an unintelligible mess.
-ThumperDH