I have the read a bunch of gearing threads and I still have a question.

I have the 2010 klx250sf, which the stock gearing is 14/39
I've searched and cant find rear sprockets in the 40t, 41t... the next size is 42t?!? Now It is my understanding that -1 on front is the same as +3 on the rear....soooo, 13/39 is the same as 14/42. correct???? That being said, I have not heard of the 16t front being used. Anyone! if u have personal experience, post up please! Anyway...If it fits....the stock equivalent would be 16/45. correct??? Then i could run a 16/43 and that would be equal to 15/40.

Also, Is there any benefit to running a 14/39 over a 15/42?? like chain and sprocket wear or the engine turning the smaller front easier you know any effects either good or bad.

Thanks in advance
Ride On!!

 

Note sure what way you are wanting to go, Higher gearing or Lower?

Yes, 13/39 is the same as 14/42. Divide 39 by 13 equals 3. Divide 42 by 14 equals 3.

“Anyway...If it fits....the stock equivalent would be 16/45. Correct??” Yes close. Divide 39 by 14 equals 2.78. Divide 45 by 16 equals 2.81. Note sure if the 16 would fit between the case and chain.
If you want to run the 16/43 that is going to be 2.68 for a slightly higher gearing, is that what you want? If so just get a 15 front that would be 15/39 or 2.60 to one.

 

I don't recall a successful 16 tooth CS install on our KLX.

 

Im gearing for less rpm for highway. I will be going BB351 in the next few months so the 15t cs is here already. I switched to the 13t end of 2013... and while i like it on back roads and trail, it sucks when the speed becomes very spirited on open straights. Also less range per tank. I will be putting back on the 14t for now until the BB is done. Problem is if u have a stock 42t in the rear like all you S owners you can go up 1 or two teeth because they manufacture them in one tooth incraments. For me it just jumps from 39t to 42t soooo.... NO FINE TUNING!!! Perfect gearing is a matter of riding areas, style and ability. What works for me may not work for others. Looking to make it perfect for me!

 

15/39 is the lowest you can go. 15/40 is taller, meaning more rpms. 39-40 won't net naught but maybe 200rpm difference

 

I have a gear calculator to see what happens when you swap sprockets or go larger diameter tires. It is an excel spreadsheet. If you email me I can send it to you. I have it with the gearbox ratios for my 09. You need to check with the Kawasaki parts diagram to learn the gear tooth counts if you have different models.

Now for the question, the bigger the sprocket the less the chain has to wrap, the more teeth it is in contact with creating less loading per tooth. That said, going to a 15 tooth with the appropriate rear sprocket is the best trick. Less wrap, stress and wear on the front sprocket, which is the fastest wearing. Front sprockets wear and hook much quicker than the rear.

My plan is to use as big a front sprocket as can be had without any cutting or grinding, with a fairly small back sprocket. I can always drop to a 13 if I really want to drop the gearing. I am doing it because I need to buy the set of sprockets, so the timing is perfect. I plan to plug the numbers into the calculator to see what works out to what.

Custom sprockets can be had from a few places like Sprocket Specialists, getting the size you want as long as it will work with the hub. A bit more cost than the standard items listed in a catalog.

 

Gearingcommander .com

So after reading on front sprockets a month ago, it seems the only 13 or 14T sprocket available (JTsprockets) is narrower than OEM and it has to be shimmed?

I do have a 13 and 14 JT that are narrower and I have the shims to stack and use behind, but I'm wondering if there is a choice for something that fits correctly?

Did I catch something about using an XR600 sprocket..can't recall

 

I have a 15T JT on my bike, no shims needed. I got it on ebay, might be able to find the part # around here somewhere.

 

I put the one JT on my bike and it was a little hard to get it off-then I read how they are something like 1mm thinner..maybe on the engine side.

 

Just saw this in your signature...

If I remember right you're the rider who wants to put a "sewer pipe" 40mm carb on your KLX figuring bigger was better? If you read the article you linked, you should now realize why the smaller carb is better for the smaller engine - intake velocity. It ain't just at the valves. It's all about keeping velocity high enough through the necessary range of performance preferred, along with cams/timing/etc. The reason trials bikes have such small carbs. Ever wonder why Bill Blue uses a 34 instead of a 36? I doubt it's because he got a good deal on a bunch of 33s to bore out.

One "fly in the ointment" for Motoman's comments has to do with surface conditions. Rough is better only to a degree, which is part of the "polished" port problem, imperfections are huge and unnoticeable but cause huge turbulence. It is simple fluid dynamics. Gordon Jennings touched on this and the D port in his work and in an article for the SR500 in 1978.

Not much new under the sun, just have to read between the lines.

It would be interesting to see more on how Motoman arrived at his reduction limits. Obviously there is a limit, otherwise the intake would be a pin hole. How big/small should an intake or exhaust be. There is the perfect size for the general applications, but how to arrive there. I haven't read everything, but I've not seen where he talks about how to arrive at the ideal port size.

I will say his comments on D ports misses one vital fluid flow dynamic - the flow is slowest at the inside of a bend in the port. Raising the port and going to a D shape proves to keep the flow more uniform through the port eliminating the slower flow through the port shape.

What I learned when it comes to porting is to simply deflash the ports if necessary and eliminate any sharp steps between manifolds and such. Hogging ports out doesn't always work except at max flow, as Motoman mentions, like a drag racer that stays near max rpm the whole time. Too big can kill low and mid-range.

Now back to the topic... gearing.