sprockets *yes another one, but this is different*

[WRT_GPX]

ok, this is yet another sprocket question, but the opposite, i was looking at putting my front sprocket up a tooth or 2 if i can, but i need to ask a few questions... how much approx with fuel economy increase with that 1 tooth up front and about how much will it rev at 100km/h in 6th *for highway work*

im doing this just because soon i will be doing some fairly large highway runs and im not sure if it would be that fun to be sitting on 7000-7500 rpm for those hours

and im not worried about acceleration THAT much as im not racing around, im a commuter... lol but will 1 or 2 teeth up front absolutely kill the ninja?

oh, and for the record its my 87 GPX250.

[mike-s]

One thing to remember is that as the rpm reduces via sprockets the torque available to sustain a given speed is reduced, and you may have to use more throttle to keep that same speed up.

Also the idle and low speed handling will suffer. i.e. my bike does 8-9km/h with the current sprockets, but it might jump up to 12-13km/h with a taller final gear from the sprockets.

[WRT_GPX]

good point, although at 100km/h it shouldnt matter that much, even if it dropped to 5000 rpm theres plenty of power still to keep it rolling, i plod around town in 6th at 50km/h and about 3000rpm with miniscule throttle to keep it moving, but then again at highspeed the wind resistance would suck, itd probably be like how it feels at 80km/h in top gear, if theres a head on it doesnt help, but just plodding along its fine
if 100km/h could feel like 80 currently does id be very happy. and as for the low speed stuff... thats ok, my main commutes and area is pretty open.

[krusty]

this may be of help
http://ninja250.kingston.net/speed.html

I've read that you need to remove some metal around the front sprocket if you want to put a 16 tooth on it.

If your engine is in good condition and you change the oil every few'000 km then it will have no problem being run at 7000rpm or 8000rpm on the highway.

Going 1up on the front and 2 down on the back would drop the revs ~1000 for a given speed in 6th gear (and make it a little less willing to overtake in my experience).

[mike-s]

i take it stock is about 14 teeth? i'd suggest if shaving metal is the answer to fitting a 16toother front in, i'd just jump down 2-4 teeth at the back. You just shouldnt have to shave metal to make things like that fit.

[Neka79]

for what its gunna save ya in fuel, its gunna be a pain in the ass to do..id just leave it as is..its only a 250..u dont have that much power/torque too bugger around with,....

[photomike666]

I went down two teeth on the back of my 600 (couldn't go up at the front without shaving casings), and my fuel consumption went up! Basic theory is in order to get more speed from the same revs = more fuel. Also kills getting off the lights.

This wasn't me just cruising around town, but 40K freeway and 10k town twice a day. On standard gearing, 300k = 15ltrs, 2 teeth down at rear 300k=16ltrs. 1 tooth down at front back to standard at rear 300k=15.2ltrs

The difference may be more noticable on a 250 due to the lower power output. Put simply, a car runs top gear almost 1:1. As a bike gearbox is much smaller, so the ratio is cut down, and the difference made up through the sprocket gearing. A slight change to the sprockets makes a fair difference to the overall gearing. Expext the bike to struggle off the lights, have slow acceleration once at speed and fail to attain usual max speed.

[GlennB]

I was commuting for a while and did the same thing, I put a 15 tooth sprocket on the bike and found I was cruising at about 7500rpm and got about 20km extra per tank. I wouldn't go a 16 tooth (if you can get one that size) I also found the bike struggled a little more to overtake cars at 90-100ks with a 15 tooth.
Hope this helps.
Glenn