Installed a 13T sprocket and the rpms are still the same? - Honda CBR250R Forum : Honda CBR 250 Forums
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post #1 of 9 Old 12-02-2012, 12:18 PM Thread Starter
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Installed a 13T sprocket and the rpms are still the same?

Hey guys its my first post. Been reading on this site for a while decided to join.

Anyways I just installed a 13T sprocket from JT and my revs appear to be the same as with the stock 14T. I still have 5400 rpm on the tach at 50mph indicated (6th gear). This seems strange to me. Is the rpm and speed locked until you get a SpeedDRD to fix it?

Anyone have any insight?

Steve

Last edited by stevo585; 12-02-2012 at 12:24 PM.
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post #2 of 9 Old 12-02-2012, 01:32 PM
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Hi Stevo and welcome to the forum!
I'm certainly no expert on this, but it is my understanding that the speedometer and tach take their readings from the crankshaft, (or possibly camshaft?), meaning inside the engine - before any external gearing changes. Therefore, the speedo and tach have no idea that you might be running a different countershaft or rear sprocket.

So, when you're engine is turning at 5400 rpm, it just assumes you are doing 50, though in your case you are actually going slower now than before the gearing change.
I'm sure others can add more particulars, such as percent changed etc but yes, you will need to some sort of unit to correct the readings if you want them to be accurate. Gearing comander can give you an idea of the percentage change you've made for making quick calculations in your head ...
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post #3 of 9 Old 12-02-2012, 02:06 PM Thread Starter
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Thanks DD!

Now that I think about it your probably right. Its just now my speedo reads high. So I'm now, for example, actually at ~47 mph at 5400rpm.

The bike does seem quicker especially in 3rd-6th gear. Over all good mod for $10 even though there is more chain noise. Only downside.
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post #4 of 9 Old 12-02-2012, 07:39 PM
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Your tachometer reads crankshaft revolutions, whereas the speedometer gets its info from the speed sensor, located in the transmission, which reads revolutions of the transmission output shaft (aka countershaft). So the speedo is factory calibrated for a 14T drive sprocket, and a 38T driven sprocket. Any changes in sprocket sizes that result in a final drive ratio which differs from the stock ratio will change your true road speed relative to what the speedo indicates. The Speedo DRD from 12 o'clock labs allows you to correct the speedometer. Basically it takes the signal from the speed sensor in the transmission and factors it before the signal is received by the speedometer. If you go to the 12 o'clock labs website, they have a calculator in which you plug in your sprocket sizes, rear tire size, and factory speedo error (typically 4 - 5%). When you change to a smaller front sprocket, you will get a negative correction factor (changing to a larger front sprocket, or taller ratio, would give you a positive correction factor), which you then program into the Speedo DRD.

Once you have corrected the speedo, your tach will display higher rpm's relative to your road speed in 6th gear. On my CBR (with a 13T front sprocket & stock rear sprocket, and SpeedoDRD) running in 6th gear at a steady 60 mph, the tach displays 7000 rpm.

Last edited by MotoMike; 12-02-2012 at 07:55 PM. Reason: Typos
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post #5 of 9 Old 12-08-2012, 11:06 PM
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What's with the speedometer it is approx 10% out !
It reads 110kmh when it is only 100kmh 90 when it's 80kmh etc !
I have checked mine with my cars and a GPS . It took me a week to realise why everyone was passing me !
Now I add 10% to all my speeds and I keep up with the traffic and I no longer have people flashing their lights at me .
Has anyone else checked their speedo for accuracy ? Can the speedo be calibrated ? Apart from the speedo I love the bike !!


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post #6 of 9 Old 12-11-2012, 05:47 PM Thread Starter
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Wow so its a 500-600 rpm increase....didn't think it was that much. Either way worth 10 bucks.

I also noticed a bit more chain noise. Have you experienced this as well? I figured its just the chain dragging across the follower more.
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post #7 of 9 Old 12-14-2012, 04:25 PM
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Just took my bike for a test ride with the 13T front sprocket installed and the difference is pretty noticeable. Every gear feels a lot more responsive now and even in 6th the bike moves a lot more when I give the throttle a twist. From a stop sign I'm 3-4 mph faster now over the same distance.

IMO this is how the bike should have been set up from the factory.
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post #8 of 9 Old 12-14-2012, 09:32 PM
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Quote:
Originally Posted by Asako View Post
Just took my bike for a test ride with the 13T front sprocket installed and the difference is pretty noticeable. Every gear feels a lot more responsive now and even in 6th the bike moves a lot more when I give the throttle a twist. From a stop sign I'm 3-4 mph faster now over the same distance.

IMO this is how the bike should have been set up from the factory.
Nice, huh? ... did you go with the SpeedoDRD unit as well?

What amazes me is the guys who swear up and down that a 15T or even a 17T front sprocket is the way to go for this 23 HP bike... makes no sense to me.

Last edited by MotoMike; 12-14-2012 at 09:36 PM.
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post #9 of 9 Old 12-15-2012, 07:02 PM
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Quote:
Originally Posted by MotoMike View Post
Nice, huh? ... did you go with the SpeedoDRD unit as well?
Not yet. For now I just have to remember that the speedometer reads faster than my actual speed. It is kind of funny to look down and see 84 mph when I'm cruising down the highway though.
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