sprockets, drag bike

[greasemonkey]

Does anyone have any info on a good sprocket setup to run on my drag bike? The bike is a slighly modifieid fj1100. I'm extending the swingarm and I have strapped/ lowered the front. I will also be swapping to a 17" gsxr rear wheel. I'm using a 5 degree timing advancer along with stage 3 jet kit full yoshi pipe and pod filters. I only weigh 140 so I need a good sprocket setup to get me off the line quick but also not stand up on me at the same time. Any info will be appreciated.

[andyb]

Assuming you're using a wheel around the stock size and want to use all the gears, you'll be in the 127mph range at the big end.  17/39 or 17/40 will put you over the line at the top of fourth.  If you want to use 5th, drop to 16/41, give or take.  Best times will be likely reached when you're indicating just under 9k accross the line.

I'd use 4 gears personally.  That'll let you launch around 5-6k and go WOT right off the line without wheelie worries, but I was doing it at 58" WB.  If you can set it up for using all the gears and still use WOT in first without wheelie trouble, that could be worthwhile.

The FJ motor with mild mods doesn't really want to be spun all that hard.  With the stock tach, I found that shifting about 500rpm short going into third and fourth gave the best times, carrying first and fourth as long as you dare, but experimentation will show you what works best.

[fj1289]

 Andy beat me too it. Found some of my old notes and mid 120's for trap speeds. With a 180/55-17 rear, I was geared 16/42 with a stock engine with exhaust, Dyna ignition, and flat slide carbs. The launch should be your only concerns for the front end coming up -- a stock engine FJ won't have enough power to come up between the the 60 and 330 like a Busa, ZX10, etc.

How much experience on a drag strip with a bike?

[greasemonkey]

I used to run my daily riden zx7r.

[fj1289]

Cool!  I'll spare the newly advice then. Let us know how it goes. 

[andyb]

By comparison, the FJ should be a lot more forgiving than the 750 at the launch.  Anything within a relatively wide range will work for a launch RPM, because of the pretty broad swath of torque.

When I ran the FJ SWB, I was 17/42 and going through the traps at the top of 4th.  Going to 17/40 didn't really change the 60 time at all, as it was chassis limited rather than gearing limited, but it made the bike pull more strongly to the finish, and I gained pretty significantly both in ET and MPH.  The shorter gearing was really past the power peak, so giving it long gearing meant it was pulling strongly through the finish line.

I was about 740lbs in race trim, and usually 123-124mph if the air was decent (1-2kft DA).  Saw 126mph one night in insane air (negative DA!).  I was a more stock motor than fj1289, just pipe/jets/pods, geared, lowered, and strapped, using a 170/60R17.

Honestly, you'll want a 40 or 41 rear sprocket, and then just buy a 16 and a 17 for the front.  $30 worth of extra parts, yes, but it'll give you some adjustability to play with.  Going through in 4th leaves you one less shift and imo makes tuning relatively easy, because you'll run pretty much all of the backhalf in one gear, so you can use that to tune your main jets appropriately.


How much swingarm are you going to put on it?  At your weight, you should be just getting into the 1.6xx 60' range at the stock 58".  You don't really have enough power to get decent weight transfer if you go much past 64" or so.  The current obscession with going straight to 70" is for guys with 75hp more and another gear in the trans... and even then, they'll piss and moan about a lack of grip on anything but an ideal track.  The shorter you keep things, the more forgiving it'll be if the track isn't ideal; short bikes are a good way to make money bracket racing on a track that varies significantly.

[greasemonkey]

I'm looking at around a 6" extension. I believe I will try a 16 and 17 on the front to see what works better for me. Luckily we have plenty of extra front and rear sprockets at work. I appreciate everyones input it is very helpful. Next test and tune is September 28th and 29th so if you check back then I will post my times.