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Discussion Starter #1
Since my CBR250R will be my commuter and hypermiler (the Ninja250 will be the "fast bike/ fuel hog") I am going to play with longer gearing. Has anyone seen any larger front sprokets available yet? I may eventually go up as much as 17% total with changes to the front and the rear sprokets making it into a 5th gear/ top speed bike with overdrive to put 70mph highway cruising right at the first torque peak of 6000 rpm in 6th.
 

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On bikes I have had it was no problem fitting different sprockets. The same sprockets fit a range of bikes, though probably not all. My guess is a dealer, or an engineer will have something that fits, or that can be made to fit. Just ask around.

Here is a website that will help you choose suitable gearing; the CBR250R is on it already:

Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
 

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Since my CBR250R will be my commuter and hypermiler (the Ninja250 will be the "fast bike/ fuel hog") I am going to play with longer gearing. Has anyone seen any larger front sprokets available yet? I may eventually go up as much as 17% total with changes to the front and the rear sprokets making it into a 5th gear/ top speed bike with overdrive to put 70mph highway cruising right at the first torque peak of 6000 rpm in 6th.
We will look forward to hearing your results with spocket size experimentation...
 

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Discussion Starter #5
16/ 38

If the stock gearing is indeed 14/ 38 there is not much smaller to go in the back. It will all have to be done on the front. I hope there is room to go up 2 teeth in front. I have read of one person using a 17 on his CBR125 which originally comes with a 15. A 16, which is up 2, would be perfect my CBR250R. I played with the Gearing Commander calculator. By comparing my actual values with the charts, my rear tire is running slightly smaller than spec at 77.00 inches circumference according to my verified speedo error which is 1% slow, and, my indicated speed/ rpm which is 66 mph/ 7000 rpm in 6th. Going up 2 teeth in front will give me 68.5 mph at 6020 rpm in 6th and 93.2 mph top speed at 9500 rpm in 5th. 6th will probably also top out near 94 although I have seen youtube videos of people reaching 160Kmh/ 99.4 mph.
 

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When I give it wot in 5th, (up hill, passing, against the wind,) it doesn't do any good to shift into 6th because the bike slows down then. I guess you could get some extra speed on the flats. I don't think it would pull very good at >68.5 mph at 6020 rpm in 6th.
 

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Discussion Starter #7
Roll on gone

When I give it wot in 5th, (up hill, passing, against the wind,) it doesn't do any good to shift into 6th because the bike slows down then. I guess you could get some extra speed on the flats. I don't think it would pull very good at >68.5 mph at 6020 rpm in 6th.
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Roll on in 6th will be gone with the longer gears but I want to try it to see if it will at least maintain 70 mph on the flats. I am an amature bicycle racer so am very comfortable with both awareness and fitness while riding in a tuck. This must make a big difference because I was in and out of the fast lane at 68-73 all the way to work (30 mile highway run) and find that me and my CBR250R can roll on in 6th at 65 to squirt through blind spots no problem. I might have to drop down 2 after the gearing change. If I don't like the 16T I can always just change it back and only be out $30.
 

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Honestly, it's already geared to be at the best fuel economy it can be at 60-70mph. Changing the gears up will probably only make it slower. I personally would rather go the other way with it and go down 2 teeth.
 

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Discussion Starter #9
Best

Honestly, it's already geared to be at the best fuel economy it can be at 60-70mph. Changing the gears up will probably only make it slower. I personally would rather go the other way with it and go down 2 teeth.
We shall see about the fuel economy. Definitely needs at least 1 tooth for efficient cruising at 70 mph. 67 mph is 7000 rpm which is higher than needed. With a slip on, nearly flat torque is available even lower than stock, down below 5800. First is low enough for launches as it is. If you were to drop 2 more teeth you would have to shift so soon, you wouldn't even get your foot on the peg before the engine is wound out.
 

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We shall see about the fuel economy. Definitely needs at least 1 tooth for efficient cruising at 70 mph. 67 mph is 7000 rpm which is higher than needed. With a slip on, nearly flat torque is available even lower than stock, down below 5800. First is low enough for launches as it is. If you were to drop 2 more teeth you would have to shift so soon, you wouldn't even get your foot on the peg before the engine is wound out.
I can't put both feet on the ground so that foot is usually already on the peg lol
 

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When I give it wot in 5th, (up hill, passing, against the wind,) it doesn't do any good to shift into 6th because the bike slows down then. I guess you could get some extra speed on the flats. I don't think it would pull very good at >68.5 mph at 6020 rpm in 6th.
Isn't there a rev limiter or something that makes it bog when slipped into 6th?

Or is it simply the gearing?
 

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Discussion Starter #13
Custom sprocket

The front sprocket on the CBR250R seems to be a rare design. Z-11 splines on a 21.6mm shaft with an oddball twist lock keeper. I can't find any other bike in the JT catalog that has ever used that set up. The parts man at Honda said it reminded him of something that the three wheelers used in the late seventies but they all had even fewer teeth. I can call JT on Tuesday but will probably end up asking Sidewinder to make a custom 16T front sprocket which looks like it will just fit inside the webbing of the block.
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Sprocket | Chain | Motorcycles Sprocket | ATV Sprocket
 

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Discussion Starter #14
In stock

JT Sprockets has 70 of the JTF285.15 sprockets in stock in the US wharehouse that we can use by drilling and tapping for the keeper. Now we just have to come up with a way to buy them. JT doesn't sell direct from the phone or internet (fools) so I have to wait to see if my local authorized retailer (Honda dealer who doesn't want to bother with it) can convince the distributor (middleman who doesn't to deal with it) to get them for us. I will still also try to have Sidewinder make a custom 16T for me but they aren't answering the phone today. On holiday? They might also offer a decent price on a 15T as well if we can't come up with the JT.
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JT Sprockets: Buyers's Guide
 

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Keep us posted as soon as you can make it happen cuz ill hop on it real quick, We have alot of highways here in Texas.
 

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witht he new exhaust i can get up to like 21mph before needing to shift to 2nd
What has an exhaust go to do with 1st gear?,
It wont change the rev limiter lol.

As to gearing it up that would be a big step backwards, go the other way if anything... or.. leave it stock.
 

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Discussion Starter #18
JT

Keep us posted as soon as you can make it happen cuz ill hop on it real quick, We have alot of highways here in Texas.
The 15T sprocket from JT can be ordered through my Honda dealer at any time. But would then have to be drilled and tapped which may be tough depending on how hard the metal is. We may have to settle for that as the design work to have a new one made from scratch is more trouble than most of the custom companies want to put into a new design with unknown demand.
I am holding out for Superior Sprockets to get caught up with current orders to take an interest in making 15 and 16T sprockets. I wish I could tell him that I could sell 20 sprockets to make it worth his while but I can't afford to get stuck with them. There are some guys on the Thai forum that want different sprockets so maybe I can put together a group buy to leverage the quantities up.
 

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What has an exhaust go to do with 1st gear?,
It wont change the rev limiter lol.

As to gearing it up that would be a big step backwards, go the other way if anything... or.. leave it stock.
By putting a decent exhaust on the CBR250r it makes the engine breath better therefor revving faster and higher (you only need to see the Dyno to see that)

as for 1st gear if the engine is bogging down at a certain revs then curing the bogging will make the bike slightly faster even in 1st gear.
 
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