I really love my commute on 15x39 gearing. Cruising at 70-75 mph with traffic is nice so I was wondering if anyone has knowledge on an even smaller rear sprocket for my SF. I might like to try a 15x36 to see how it goes on my interstate commute at lower rpm. Where can I get one? Do I just get any 520 sprocket with the right bolt pattern? I can live with 15x39 but want to see. Maybe I will need a 351 kit to pull a gear like that. I don't mind not pulling wheelies for commuting. I would rather give up 1st gear acceleration and gain top speed. My dirt wheel will get the 43T gear I recently sampled.

 

I think Sprocket Specialists will make any size you want. Simple computer in put for the cutting process.

You are right, eventually you will hit the point where there isn't enough power to push the air. From what I felt - seat of the pants with guess work - the 15/39 is close to the max without any clean up of aerodynamics. You could consider a small windshield to help there. It will also make the commute a bit better.

Here's one I did on the KLX650 that could be done on the 250"



Mounted on the crossbar:



Smooths the wind flow, less power required to push the wind.

 

Thanks. It appears I can get any number of teeth I desire from sprocket specialists. The question I wonder is how low can I go. Will have to think a little about it.
Edit...So I lifted these words from Wiki and using the worst numbers they(http://www.wallaceracing.com/Calcula...or%20Speed.php) cite for Cd and frontal area come up with less than 15HP needed for aero and rolling resistance combined. Perhaps for commuting duty I can install lower narrow handlebars in addition to a small windscreen. Of course torque not HP is what results in tire to ground force which will be reduced as I go smaller. I think I will use KLXSTER's dyno reports and see what number crunching finds.


The dimensionless measure of drag coefficient, Cd, varies from .55 to .65 (comparable to a pickup truck), vs Cd .29 for many sports cars and even .20 for high efficiency cars. However a more relevant figure of merit, CdA, factors in the vehicle's frontal area, and thus actual power required to overcome wind resistance.[15] A typical projected frontal area for motorcycles is in the neighborhood of 2.9–3.9 sq ft (0.27–0.36 m2).

 

If you go too low, you could also drop the front a tooth or two if needed.

 

I may not worry about -2T for 75 bucks. Then again, I may.




Hi Dave,
It has come to our attention you are looking for a rear sprocket for your 2009 Kawasaki KLX250F that is our PN# 287 -37 37 teeth is the lowest you can go and we can build in 520 chain pitch $64.99 plus $10.00 shipping if you are in the USA.
Thank you for your interest in our company we look forward in doing business with you.

CHRISTINE
TEAM SPROCKET SPECIALISTS
MADE IN THE U.S.A. SINCE 1950
PH 800-782-8200
FAX 435-896-2944
www.facebook.com/SprocketSpecialists618

 

Just took a look at their site.
They use aluminum. I think steel would be better. The slight weight penalty would be offset by the steel part lasting longer.

 

I did not read the fine print. Guess I am sticking with the 39. No Al for commuting. Bicycles yes, moto no.

 

I don't even care for aluminum gears on the bicycle especially for the small front ring.