Sprocket Conversion?

[M-Oorb]

So I was reading on another forum about people doing a sprocket conversion on their 08 250's. Im pretty new at this modding bike thing and I was just wondering what is gained by doing this? I think the thread stated that the guy was getting a sprocket with two extra teeth in the back...what do you gain/lose by doing this and why? Just tryin to get educated...

[kkim]

Divide the rear sprocket number by the front sprocket number and that will give you the final gear drive ratio. The higher the number, the more the engine needs to turn at a given speed... the lower the number, the engine will turn less at a given speed.

There are pros and cons to going each way from the stock gearing.

[M-Oorb]

So having a larger rear sprocket while keeping the front the same will basically give you more acceleration and less top speed and vice versa...does that pretty much sum it up in a nut shell? And has anyone actually done this...id like to hear some feedback

[k-os]

Originally Posted by M-Oorb

So having a larger rear sprocket while keeping the front the same will basically give you more acceleration and less top speed and vice versa...does that pretty much sum it up in a nut shell? And has anyone actually done this...id like to hear some feedback

That would be correct.

[M-Oorb]

Ok so just wondering again...what would happen if you increased the size of the front and the rear to keep the ratio between the two the same compared to the stock set up? Im guessing you couldnt increase both top speed and acceleration because that would be too easy...and no one is saying physics is to easy...would things just remain the same and you would just have sprockets with more teeth?

[Alex]

Originally Posted by M-Oorb

would things just remain the same and you would just have sprockets with more teeth?

Yup. It's all about the ratio. If you make both larger yet keep the same ratio, the only difference is there may be slightly (very slightly) less chain wear as it doesn't have to wrap as tightly around the front sprocket. But certainly not worth the cost and effort to change sprockets for that extremely minor effect.

[M-Oorb]

Yea thats what I thought...so has anyone actually swapped out the stock sprockets out for either smaller ones (Im guessing you would do this if you rode the bike on a highway a lot) or larger ones? If so....whatd ya do and how do you like it?

[Alex]

On our 2004 ninjette, we swapped out the stock 14-tooth front sprocket for a 15-tooth, to make the gearing slightly taller. This lowered revs on the highway noticeably (by 14/15ths), which works out to 600-700 rpm depending on the speed you normally travel. On the low end, this made the ninjette slightly harder to roll away from a stop; it required a little more concentration with the clutch.

I didn't think it was a problem, and preferred this setup for the more calmer highway revs; my wife had the opposite opinion and didn't like it. She thought it was too hard to pull away, especially if she was starting on an uphill grade.

So we've left the stock gearing on the 2008 ninjette, and aren't likely to change it.

[M-Oorb]

If someone was to gear their bike to be taller would this increase the fuel economy of the bike since less rpms are needed to achieve the same speeds?
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[kkim]

Theoretically it should. Of those that have done this mod, some will say it helped their mpg, while others will tell you it stayed the same.

[Alex]

Originally Posted by M-Oorb

If someone was to gear their bike to be taller would this increase the fuel economy of the bike since less rpms are needed to achieve the same speeds?

Less RPMs are necessary, but the motor still needs to provide the same horsepower (or more specifically, thrust to the rear wheel). So the question is really, at 70 mph, does the ninjette's engine produce the required 17 hp or so more efficiently at 8500 or 9000 rpm. It will likely require more throttle at 8500 rpm to hold the same speed. Theoretically, the engine should be less efficient due to higher friction at the higher engine speeds, but there are other factors at work (airflow both in and out of engine, for two) that together provide much more variability than just the small difference in friction.

Practically, any gains or losses will be very minor in the scheme of things.

[Mo1981]

I did the 15/41 swap on my '08 250 and it transformed into a different bike. I can get an indicated 40 mph in 1st. It has settled down so much on the highway now, running about 10 mph per 1k rpm. I would definitely recommend this swap. Loss of acceleration is almost negligible. You will just spend a little more time in 3rd...Posted via Mobile Device

[M-Oorb]

How much did the new sprockets cost? Also did you have to get a new chain and if so how much did you pay for that too
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