sprocket swap for increased acceleration...

[Ninja007]

so I've been trying to research sprocket swaps on our little 250 to get a better acceleration.

tho I have read some of the sprocket threads on our forum, I'm not the smartest guy and I have a hard time understanding all the mechanics/physics of gear ratios and what not.

so I was just wondering if any one had sprocket combos that they have found that give them an increase in acceleration?

any help would be awesome, thanks!

[Alex]

Shorter gearing helps acceleration, taller gearing hurts acceleration. Shorter gearing means that your engine is turning faster at the same road speed. Taller gearing means that your engine is turning slower at the same road speed.

To get shorter gearing, make the front sprocket smaller or the rear sprocket larger. To get taller gearing, make the front sprocket larger or the rear sprocket smaller.

Downsides to shorter gearing is the engine revs higher. As our bikes already rev pretty high at highway speeds, raising it much at all may make things somewhat unpleasant. Fuel mileage may be hurt a small amount. If the gearing is made too short, the bike's top speed may be affected if it is allowed to hit redline in 6th gear. Downsides to taller gearing is acceleration is affected, and available power at the same road speed in the same gear is less. Fuel mileage may be helped a small amount.

[kkim]

yeah...

edit- shouldn't this be added to the wiki/FAQ?

[Ninja007]

thanks Alex! makes sense now. Now the hard part, deciding weather I want to/it's worth it to swap sprockets or not...

[Broom]

the first time i took the 250 to the track we had 13/47 on there and the bike felt hella fast. it was awesome.

only problem is the bike tops out too early. i'm guessing around 70mph or so (i don't have a dash, so i don't know).

[TrueFaith]

Depends if a usable first gear is important to you. I have taller gearing and I found it completely changes the bike's personality. On a stock 250R, first and second gear are just annoyances that you shift through to get to where the power band sweet spots are in 3rd or 4th gear. With taller gearing you actually use 1st and 2nd to accellerate to cruising speed. You can concentrate on where you're going instead of mashing gears for the first 1/4 mile. You do have to keep the revs higher in each gear to get the same throttle response as a stock bike would have at the same gear & speed, but that's the trade-off to be able to actually use the lower gears. I wouldn't recommend it to any speed nuts (even though top speed theoretically should be faster than stock), but it does make riding the 250R a bit more relaxed without all that low-speed shifting from every stop. And it has improved my MPG slightly.

[Alex]

Originally Posted by TrueFaith

On a stock 250R, first and second gear are just annoyances that you shift through to get to where the power band sweet spots are in 3rd or 4th gear. With taller gearing you actually use 1st and 2nd to accellerate to cruising speed.

How much did you change the gearing (what are your new sprocket sizes) to transform 1st and 2nd from what you feel is useless into useful?

[Ninja007]

my bike is mainly a commuter to school/work. I mean, I love riding on the highway at higher speeds, but I don't make it out there as much.

so I was looking for some gearing (i.e. - sprocket sizes for front and rear) that would give me a little bit more acceleration without sacrificing too much gas mileage and not killing my top speed.

correct me if I'm wrong, but is the stock sprotcket sizes 15T for front and 45T for rear?

[Broom]

right. putting a 47 on the rear will probably give you the results you're looking for. its possible that might require a longer chain though... i'm sure someone will chime in. if it does require a longer chain, and you don't want to buy one, then go with the 14 front sproket.

remember that chains and sprockets should always be replaced as a set. if your set up is still pretty new then its not a big deal, but if you have some miles on there then you'll want to replace them all at once.

[Alex]

I believe stock is 14T/45T, for a ratio of 3.214. The higher that ratio, the shorter the gearing; the lower that ratio, the taller the gearing. Many people who do change the sprockets on a ninjette go to 15T/45T for a slightly taller ratio that provides a slightly calmer highway ride at a small cost of some acceleration.

Sounds like you want to go in the other direction. A ninjette hits the rev limiter in top gear at a true speed of about 100 mph. (yes, the speedometer reads higher than that, but it's inaccurate). So going from stock to say 14T/47T, for a new ratio of 3.357, would mean your bike would now hit that same rev limiter at a true 95.7 mph. Say you changed it even more to 13T/47T for a new ratio of 3.615, the new top speed would be 89 mph on the rev limiter.

[Broom]

opps... alex is right. 14/45 is stock

can you even hit the rev limiter in 6th gear? the bike seems to quit pulling before it gets there. maybe a tail wind and going down hill?

[00v_Lucky]

Sooo, did anyone do anything to 6th gear for your cruisers out there to make the gear "taller"? (just learned from this thread)

I do not like how the bike is just staying at 5k rpm at 45. =/ I would like it to spin at around 3 -5k at this speed.

What are the changes I must do to achieve this?

[Broom]

3-5k? your bike isn't going to like that much. not to mention you're going to have to downshift in order to get any acceleration should you need to spurt your way outa trouble.

these bikes are made to run higher in the rpms than that. you might feel like you're being hard on the bike, but you're really not.

[Alex]

Originally Posted by 00v_Lucky

Sooo, did anyone do anything to 6th gear for your cruisers out there to make the gear "taller"? (just learned from this thread)

Hi Howard. I haven't ever seen any posts about people who changed individual gear ratios in the ninjette's transmission. That would require dissassembling the transmission and replacing internal gears. All I've ever seen is sprocket changes to change the overall gearing. By definition, sprocket changes are going to affect all gears an equal amount, as they are after the transmission.

Originally Posted by 00v_Lucky

I do not like how the bike is just staying at 5k rpm at 45. =/ I would like it to spin at around 3 -5k at this speed.

What are the changes I must do to achieve this?

If your bike currently turns 5k RPM in top gear at 45 mph, and you want it to turn 3K rpm instead, the changes you need to achieve this include:

a. posting up your ninjette in craigslist
b. finding a buyer
c. purchasing a replacement motorcycle of significantly greater displacement.



Our machines have such limited torque that our engines have to be spun pretty quickly to make enough power to move the bike along safely and acceptably quickly. There is no way to make the gearing 40%+ taller and still have enough grunt to get off the line. Or start on a hill. Going even 10% taller is very, very noticeable in terms of a performance hit; 20% taller would feel like you're dragging an anchor, and 40% wouldn't be livable at all, if you could even find sprockets of such a size to make that work. (by my math, it would take something like 15T/28T or 16T/30T to have the gearing work out as you are suggesting).

The good news is that our engines are designed to spin at high speeds, and will do so happily for many, many thousands of miles. If it's just that you don't like the noise when it's turning at those speeds, definitely check out that earplugs thread for some suggestions.

[Viper-Byte]

I have not hit the limiter in top gear, but have got to 12krpm in top gear. Speedo was reading 175kph, but we all know how true that is... I guesstimate it to be about 157.5kph based on 10% speedo inaccuracy. But my bike is not stock, so I do not know how a stock bike will do.

There is like no power for accelleration in high gears at even 5krpm, so I really advise against running it that low, you could bog the engine down and this is not healthy for it as it puts far too much strain on it. They are made to run fast, that is just what they do. Basically, Alex is right if you want to run at 3-5krpm without issue!

[kkim]

I have both a 13T and a 15T CS sprockets on my workbench which I have been meaning to swap out with the stocker to see which works better for me for my use of the bike. I agree the 13T will make the lower gears even more useless than they already are, but I'm looking for more pull from corner to corner in the canyons.

On the other hand, I've heard where going to a 15T, although taller, makes the bike a bit more useful for higher speed riding where the engine rpms are a bit more subdued than the stock gearing. I'll swap between them and then decided which of the 3 I like better. once i make that decision, I'll fine tune any further gearing changes by changing out the rear sprocket by adding or subtracting teeth from it.

[00v_Lucky]

lol, kk, im still just not used to an engine spinning so high! so what you guys are saying is, on the freeway just keeping the engine at 8k is still okay?

[Alex]

Perfectly fine. Keep in mind that our piston speeds are actually pretty low, even though the RPM's are high. Thank our incredibly tiny pistons and small stroke for that. The mechanical limit on most piston-driven internal combustion engines is the strength of the connecting rods (and the pistons connected to those connecting rods). Those are the parts that are reciprocating, changing direction many, many times per second. A connecting rod that is moving twice as far as ours (say on a 500cc bike with a similar bore/stroke ratio), will have the same speed at 5000 rpm as ours will at 10000 rpm. There is a de facto limit for street engines, which if I remember correctly is somewhere around 4500 feet per second. If you calculate out what an R6 connecting rod is doing at 17,000 rpm, and what a Ducati 1198 is doing at its redline around 12000 RPM, or even a large vtwin cruiser at 6K RPM, you'd be surprised how close the max connecting rod speed is at that bike's redline, and it always works out to about 4500 feet per second. To go above that requires very expensive materials that don't make it to production engines.

[00v_Lucky]

i just learned something today thanks for the crashcourse on engines, I do not feel bad pushing my baby to 9k now. (It sounds better up there too)

[kkim]

push it to 12k , it sounds great!!!

[Viper-Byte]

I push my bike harder than my car... I have not gone near the red line in my car, I stay about 1000rpm away from it. On my bike, I have hit the red line and even the limiter a few times.

That is just pure fun! usually when I am really pushing it I like to shift around 12500rpm max.

[Angel-be-Good]

Does anyone know at what rpm the rev limiter cuts in on the new bikes? I've hit it once before, but on a couple of occasions I've looked down and--oops!--it looked like the needle had crept past 13k. I know redline on some of the older bikes is 14k, I wonder if that's not when the rev limiter kicks in.

[TrueFaith]

Originally Posted by Alex

How much did you change the gearing (what are your new sprocket sizes) to transform 1st and 2nd from what you feel is useless into useful?

My second 2008 Ninja 250R came already modded with 15/44 sprockets. I had ridden my first 2008 250R completely stock for several months before I totalled it, so the change in gearing was a big difference. It behaves more like a bigger bike in that I don't feel like I'm constantly climbing gears to get to speed. On my stock bike it seemed like I'd have to be in 3rd before I even got out of my driveway. Now I can round the corner onto the main drag and get a little speed up before I'm even thinking of shifting into 3rd. I also find that I use the top gears a lot less around town than I did on the stock bike. The bike just seems to have more gears with the new sprockets, since you get to use the lower ones in a way you can't on the stock set-up. It makes quite a difference in the personality of the bike and having ridden both, I much prefer the 15/44 to what the bike normally comes with.

[kkim]

yeah, but don't forget your motor has been tweaked and is putting out a bit more power than a stock bike. How would it feel geared like that without the power mods? wheezy?

[TrueFaith]

Originally Posted by kkim

yeah, but don't forget your motor has been tweaked and is putting out a bit more power than a stock bike. How would it feel geared like that without the power mods? wheezy?

No doubt. Scotty did all the right mods on this thing to make it behave like it should out of the box. Kawi really messed up this bike's potential with all the emission restrictions.

[Alex]

Originally Posted by TrueFaith

I much prefer the 15/44 to what the bike normally comes with.

All well and good, and if you feel it's better, that's all that matters. But going from 14/45 to 15/44 is still only a 9% change in overall ratio. I find it hard to believe that 1st gear and 2nd gear used to be useless, and now they're great. Either they were great before and they still are great, or they were useless before and they're still useless. I believe you're attributing some of the other improvements that have been done to your bike to the gearing changes, when that gearing change alone will not improve a stock bike nearly as much as you may think.

[TrueFaith]

Originally Posted by Alex

All well and good, and if you feel it's better, that's all that matters. But going from 14/45 to 15/44 is still only a 9% change in overall ratio. I find it hard to believe that 1st gear and 2nd gear used to be useless, and now they're great. Either they were great before and they still are great, or they were useless before and they're still useless. I believe you're attributing some of the other improvements that have been done to your bike to the gearing changes, when that gearing change alone will not improve a stock bike nearly as much as you may think.

Well, for all I know Scotty could have done a few tweaks that he forgot to tell me about. Perhaps a 9% change in gearing ratio is just more noticeable in a smaller displacement bike like the Ninja. All I know is my lower gears have a much more usable power band than they had when they were stock and I don't necessarily have to be in 6th gear when I'm going 40mph like on my stock bike. That's pretty good for a 9% change. The power mods have improved the throttle response tremendously, but like Kelly said without them and just the sprocket change the bike probably wouldn't perform nearly as well. So anyone attempting this mod should probably plan on doing all their carb, airbox and engine mods in addition to the sprocket swap for best results.

[almost40]

Ok I have posted a rather long winded version of this on another 250 site and dont feel like doing it again. Let me just say this. If you have a properly modded bike (full exhaust and Jet Kit and airbox mods) your bike will be faster thru a 1/4 mile if you gear down not up. I have tested this repeatedly and found that a 41T rear / stock front gives the best times. Now there are a few factors to consider, most important being your weight. At 140lbs my bike was slower in a 1/4 mile with the stock 45t rear. Yes my 60 foot times were a little slower but the ability to hold 3rd gear farther down the track more than made up for that loss. (3rd with a 41T is shorter than 4th with a 45T) The lower the gear at the big end of the track the better. Take it for what its worth but when I goto the dragstrip I run my 41T rear. Now if your a canyon carver or a track lover Im guessing (well not really) that the stock gear with mods is the ticket for better pull from corner to corner. The track riders here all gear up for the track and Ill do the same for my soon to begin crooked track sessions.

[Alex]

Perhaps, but pretty much the only reason that would be accurate is if shifting less allowed you to make up the time, and a quarter-mile distance is just the right distance for the shifting gains to make up for the acceleration losses. If you raced to 1450 feet instead of 1320, or even 1100 instead of 1320, I have a hunch that you'd find going to taller gearing slowed you down more than you think.

It's just like 0-60 times in cars. A faster car might have a slower measured 0-60 time because its geared to 58 mph in 2nd and requires one more shift. But if those same two cars went 0-70, the faster car (higher power to weight ratio) would win every time.

[sometimesido]

So, could I have long revving 1st and 2nd gears and tall 6th?
Is this possible? without bike running like crap...

[Alex]

"long-revving" is equivalent to tall. Means the same thing. The taller the gear ratio, the longer you are in the same gear. You can't adjust different gears in different ways without taking apart the transmission and replacing actual gears. Sprocket changes will affect all gears equally. If you're taller in 1st, you're taller in 6th.

[sometimesido]

but if I just changed my sprockets, I will have all tall gears but my acceleration will suffer right? hahaha, I think I will stay stock. I'm happy with what I've got. And like you said earlier, I'd go bigger bike if I wanted more.

[almost40]

I still have to shift to 4th in the 1/4 mile its just a matter of when that shift comes. 45T means you have about 500 feet of track left and a 41T means about 200 feet of track. Trust me I ran alot of passes to figure it out. Theres no way around having to use 4th with a 14T up front. I will try a 15T/43T later this year and let you know how it works out. My guess is without more HP it wont do much good. I know it seems odd but the increased HP that comes with a properly tuned and modded bike plays right into the hands of a taller gear in a 1/4 mile. Ill also stipulate that the 41T would be faster in 1500 or 2000 feet. The only place the 41T will lose ground to the 45T is from the 0-50 time.

[kirana]

me 15 front and back is 43, god in gear 123 and 4,for the five and isjust soso,but i manage to clock 180Kmh 3 time. from160 to 180 it illtake about 3min from160.
if uwant a vaster RPM then just usd trh BRT CDI,they will go 1600RPMin no time. i only have 1 bott withm 1;.

[Snake]

Originally Posted by kirana

me 15 front and back is 43, god in gear 123 and 4,for the five and isjust soso,but i manage to clock 180Kmh 3 time. from160 to 180 it illtake about 3min from160.
if uwant a vaster RPM then just usd trh BRT CDI,they will go 1600RPMin no time. i only have 1 bott withm 1;.

What is the BRT CDI ?