Changed my rear sprocket out for better RPMs at high speeds

[AmatureWoodsman]

Writing this because there may be others out there who were thinking like me, so here is a quick write up.

I have a 2022 model and I have to travel a highway to get to any gravel. At 50 mph this bike is buzzy, which makes sense, its just a 125. So I changed out the stock 39T sprocket for a 36T (JTR269.34) thinking it wouldn't make too big of a difference on the bottom end but let me cruise at 50 with a little less vibration.

Process:

• The OEM sprocket was not hard to get off and the new one went on easily enough. JT sprocket website said it was a fit for CT125 models but it isn't perfect, I had to smack it with a rubber mallet to get it on, not scraping anything just snug. It fits in line with where the old one did so I didn't have to use spacers or anything.
• I did have to cut a link off of the chain which wasn't too big a deal but I'll have to get a new one if this doesn't work out.
• The space between the mounting holes and the edge of the nuts was not very big so the sprocket got scratched, no chance of returning it.

First Ride:

• I've done one ride so far and it seems like the bike has lost about 3 or 4 mph on the top end. There is almost no buzz, but the 4th gear now just barely pulls from 39 to ~46 on the speedometer but 49 on the GPS. I did not confirm the speed before hand with a GPS but my speedometer would get up to 55 (if anyone can verify that I'd appreciate it).
• There was no notable low-end power loss
• There was a notable top-end power loss
• ABS error light blinked the whole time

I'll probably leave it on there for a few rides, I already spent the money, but I'm not sure I'm going to like it long term.

 

[dmonkey]

I think you may have a typo in there somewhere, JTR269.34 would be 34T and not 36T.

You can use nuts with a smaller head size to fit securely within the ridge of the JT rear sprocket.
https://hondatrail125.com/index.php?threads/sprockets.75/page-5#post-1236
https://hondatrail125.com/index.php?threads/sprockets.75/page-15#post-24352

Thanks for sharing your findings.

 

[AmatureWoodsman]

Well, you just told me why the difference was so much more drastic than I expected... Guess I ordered the wrong one.

 

[SneakyDingo]

Front Sprocket	Rear sprocket	Final Drive	Notes
14​	39​	2.786​	OEM
14​	36​	2.571​	What you thought you were going to buy
14​	34​	2.429​	What you actually bought
15​	39​	2.600​	15T countersprocket option

It's been generalized a little that adding or subtracting 3T from stock on the rear sprocket is approximately the same as going up or down one tooth on the front sprocket. Makes sense with a FD of ~2.8 on the OEM gearing.

If you do find yourself interested in trying the 36T, it would likely be easier and cheaper to take the 15T front sprocket + OEM rear sprocket approach. From memory that's 4 bolts, a locking plate, a cover and the sprocket itself, not to mention you're not wrangling a whole wheel around. Running it through the calculator, I'm not 100% sure the 106 link chain will be able to be used with the 15-39, it'll be close (wheel pushed all the way forward in the adjuster). 108 link chain will work though.

 

[JPMcGraw]

Did we ever figure out how big we can go on the rear sprocket with the stock 14t before the ABS goes off?

 

[AmatureWoodsman]

I'm not sure about big, but my ABS light flashes after I went down 5. Based of other threads I've seen, I think 2 would be the limit.

 

[AmatureWoodsman]

Front Sprocket	Rear sprocket	Final Drive	Notes
14​	39​	2.786​	OEM
14​	36​	2.571​	What you thought you were going to buy
14​	34​	2.429​	What you actually bought
15​	39​	2.600​	15T countersprocket option

It's been generalized a little that adding or subtracting 3T from stock on the rear sprocket is approximately the same as going up or down one tooth on the front sprocket. Makes sense with a FD of ~2.8 on the OEM gearing.

If you do find yourself interested in trying the 36T, it would likely be easier and cheaper to take the 15T front sprocket + OEM rear sprocket approach. From memory that's 4 bolts, a locking plate, a cover and the sprocket itself, not to mention you're not wrangling a whole wheel around. Running it through the calculator, I'm not 100% sure the 106 link chain will be able to be used with the 15-39, it'll be close (wheel pushed all the way forward in the adjuster). 108 link chain will work though.

Good info to have, but honestly I'll probably just go back to stock. I'm not much of a modder this was just an experiment. The bike was already really good stock.

 

[Cpd419]

Were you at in ky? I have a stock chain

 

[SneakyDingo]

Did we ever figure out how big we can go on the rear sprocket with the stock 14t before the ABS goes off?

Given the 13T countersprocket never triggered it on my bike, I'd say you'd probably get away with a 42T, but the 43T would potentially trigger it.

 

[AmatureWoodsman]

Were you at in ky? I have a stock chain

Hey I'm in west ky by LBL. Its fine though, I bought a new chain with it just in case. I really appreciate it though!

 

[Happy Campa]

Writing this because there may be others out there who were thinking like me, so here is a quick write up.

I have a 2022 model and I have to travel a highway to get to any gravel. At 50 mph this bike is buzzy, which makes sense, its just a 125. So I changed out the stock 39T sprocket for a 36T (JTR269.34) thinking it wouldn't make too big of a difference on the bottom end but let me cruise at 50 with a little less vibration.

Process:

• The OEM sprocket was not hard to get off and the new one went on easily enough. JT sprocket website said it was a fit for CT125 models but it isn't perfect, I had to smack it with a rubber mallet to get it on, not scraping anything just snug. It fits in line with where the old one did so I didn't have to use spacers or anything.
• I did have to cut a link off of the chain which wasn't too big a deal but I'll have to get a new one if this doesn't work out.
• The space between the mounting holes and the edge of the nuts was not very big so the sprocket got scratched, no chance of returning it.

First Ride:

• I've done one ride so far and it seems like the bike has lost about 3 or 4 mph on the top end. There is almost no buzz, but the 4th gear now just barely pulls from 39 to ~46 on the speedometer but 49 on the GPS. I did not confirm the speed before hand with a GPS but my speedometer would get up to 55 (if anyone can verify that I'd appreciate it).
• There was no notable low-end power loss
• There was a notable top-end power loss
• ABS error light blinked the whole time

I'll probably leave it on there for a few rides, I already spent the money, but I'm not sure I'm going to like it long term.

I went one up one tooth on my front sprocket since I have a similar use. I lost some top speed as well, but smoothed out the bike vibrations. It maxed out at about 50 MPH. I then compensated by adding power mods. Now it can cruise at 60 MPH. I have not had an issue with the ABS. I recommend a camshaft as a first power mod.

 

[Farmer Mike]

Front Sprocket	Rear sprocket	Final Drive	Notes
14​	39​	2.786​	OEM
14​	36​	2.571​	What you thought you were going to buy
14​	34​	2.429​	What you actually bought
15​	39​	2.600​	15T countersprocket option

It's been generalized a little that adding or subtracting 3T from stock on the rear sprocket is approximately the same as going up or down one tooth on the front sprocket. Makes sense with a FD of ~2.8 on the OEM gearing.

If you do find yourself interested in trying the 36T, it would likely be easier and cheaper to take the 15T front sprocket + OEM rear sprocket approach. From memory that's 4 bolts, a locking plate, a cover and the sprocket itself, not to mention you're not wrangling a whole wheel around. Running it through the calculator, I'm not 100% sure the 106 link chain will be able to be used with the 15-39, it'll be close (wheel pushed all the way forward in the adjuster). 108 link chain will work though.

Not sure how many people venture here to look at this, but the 2023's are shipping with a 38 Tooth Rear Sprocket... Final drive of 2.71

 

[Fishwishin]

Anybody know what, if any difference, putting a 39T rear sprocket on a '23 would do? It would bring the ratio to the same spec as the JA55 models, but would the JA65 design create any performance/riding difference?

 

[Flash]

I am thinking there is info on how the ABS is effected once you change sprockets. Honda must be using the chain ratio to calculate out front wheel rpm miss match. Link to how to do that please. The Canadian model gets 14/38 stock I was thinking about a 16/36 to reduce rev's on highways. Seen reference to a program plug but please redirect.

 

[Farmer Mike]

So yes there is information here about the ABS fix (which also corrects the spedo).

I take delivery on a 2024 Honda CT125 in May

the doc fee for these bikes is 300 dollars so that should be the only "fee" from the dealer end( I paid 4299 plus nevada tax and registration for my 2023, so msrp plus the 300 doc fee and tax) anything else is a mark up, and considering Oregon doesn't have sales tax 5000 is still scummy. not a...

hondatrail125.com

On the sprocket combination you are considering... while in theory that would be great, not sure in practice how that will work out... you are dealing with 9 HP... not sure it has what it takes to push the mass with that setup, and HP. Also before you purchase that 16, you might remove the front sprocket cover, and measure the inside diameter, not sure it will fit inside sprocket housing, that's hidden inside.

If you decide to charge ahead, let us all know how it works out. I would suggest you do one or the other for starters, and just see how performance is affected. My on experience is that with the stock setup, the hill should best described as a "slight rise" rather than a hill, or the bike slows down even at WOT. I personally doubt the sprocket setup you describe will work, unless you have installed a BBK and other mods.

 

[CB Spoolmak]

16 Tooth front sprocket will fit IF you modify/remove material from the metal guard piece that is riveted to the backside of the plastic sprocket cover.
Webike Japan has a pricy/fancy replacement that clears the 16 if you want to order/wait.
I chose to remove/modify since no one can see it hidden away.

I am using 16 on the front w/49 on the rear which is equivalent to 13/39. I tried 14/49 to get better off road gearing but it was still too high to do what I want while removing too much top speed for the street. Worst of both worlds. Honda really should have included the 2 speed gear box of old.

 

[Happy Campa]

I say make them in 200cc from the factory.

 

[vintagegarage]

This forum has long needed a thumbs down button next to the thumbs up and reply buttons.. LOL

 

[Flash]

So yes there is information here about the ABS fix (which also corrects the spedo).

I take delivery on a 2024 Honda CT125 in May

the doc fee for these bikes is 300 dollars so that should be the only "fee" from the dealer end( I paid 4299 plus nevada tax and registration for my 2023, so msrp plus the 300 doc fee and tax) anything else is a mark up, and considering Oregon doesn't have sales tax 5000 is still scummy. not a...

hondatrail125.com

On the sprocket combination you are considering... while in theory that would be great, not sure in practice how that will work out... you are dealing with 9 HP... not sure it has what it takes to push the mass with that setup, and HP. Also before you purchase that 16, you might remove the front sprocket cover, and measure the inside diameter, not sure it will fit inside sprocket housing, that's hidden inside.

If you decide to charge ahead, let us all know how it works out. I would suggest you do one or the other for starters, and just see how performance is affected. My on experience is that with the stock setup, the hill should best described as a "slight rise" rather than a hill, or the bike slows down even at WOT. I personally doubt the sprocket setup you describe will work, unless you have installed a BBK and other mods.

Agreed. Not enough power. Thought maybe try to get ECU napping to richen up the top end. They appear to be very lean hence loss of power to give mileage. Might only go one tooth larger on front to correct speedo. Better 3rd gear around city but compromize highway power in the wind.. lol maybe I'm just to big.and the bike is too small. It is a blast around city thou.