Front sprocket rubber & 630 to 530 chain

millsmobile

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I did a bunch of research to find the right size sprockets to convert from 630 to 530 chain on this bike. got a JTF339-17RB with the RB being a rubber cushion, opted for the rubber version at the last minute. The sprocket itself is the exact width of what I took off the bike, but the rubber makes it wider. Stock is 14.5mm at the hub, the new one is 14.5mm at the hub, but the rubber pushes it out to 18mm. Has anyone used on of these sprockets successfully, or should I order the one without the rubber? My "bike garage" isn't here otherwise I'd go slap it on and see if it clears, mainly concerned about behind the sprocket, there's plenty of clearance in front.
 

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Well if you're here and looking, both the JTF339-17 and the JTF339-17RB with the rubber cushion fit. There's way less clearance behind the rubber of course, but it does fit : ) I went with 17 up front and 48 in the rear and ended up with 109 links of 530 chain.
 
I did a bunch of research to find the right size sprockets to convert from 630 to 530 chain on this bike. got a JTF339-17RB with the RB being a rubber cushion, opted for the rubber version at the last minute. The sprocket itself is the exact width of what I took off the bike, but the rubber makes it wider. Stock is 14.5mm at the hub, the new one is 14.5mm at the hub, but the rubber pushes it out to 18mm. Has anyone used on of these sprockets successfully, or should I order the one without the rubber? My "bike garage" isn't here otherwise I'd go slap it on and see if it clears, mainly concerned about behind the sprocket, there's plenty of clearance in front.
hello millsmobile

so what did you end up doing … did it fit after all ? did you replace with a different one ?

I’m trying to do same — go from 15T to 17T on front, and would love to also convert to 530

thanks for any help you can send this way !

— Luis
 
hello millsmobile

so what did you end up doing … did it fit after all ? did you replace with a different one ?

I’m trying to do same — go from 15T to 17T on front, and would love to also convert to 530

thanks for any help you can send this way !

— Luis

Hey Luis,

I've kept the rubber one on and have about 1,200 miles on it with no issues. That's the JT339.17RB in front, and JTR284.48 in back with a 530 JT chain. I got a 110 link chain and had to take out one link for it to fit. I never rode it with the 630 chain but the math says it's under geared about 10% from the stock 15t & 38t setup and I gotta say it's an absolute hoot lol.
 
Hey Luis,

I've kept the rubber one on and have about 1,200 miles on it with no issues. That's the JT339.17RB in front, and JTR284.48 in back with a 530 JT chain. I got a 110 link chain and had to take out one link for it to fit. I never rode it with the 630 chain but the math says it's under geared about 10% from the stock 15t & 38t setup and I gotta say it's an absolute hoot lol.
so the rubber did fit ok, I see

so I have a question I hope u can answer … with your current 530 setup ( 17T / 48T / 109 link ), what are your RPM in 5th gear at any given speed, do you have any numbers ?

my current 630 setup ( 15T / 38T / 90 link ) is insanely short-geared, and at 50 MPH I’m doing 7,200 RPM … !

I’m so grateful for your help !

— Luis
 
Whoa, there's no way that's correct IMO. That's even shorter than what I'm running and I went shorter on the numbers vs stock. At 60mph I'm at about 5k in 5th. Have you changed tire size, tach drive, anything else like that? TBH that's so far off I'd almost guess you're not really in 5th at that speed/rpm if the tach is accurate.
 
7200rpm at 50mph in 5th would suggest a top speed of 69mph at 10k rpms. Have you tested top speed?
… my bad — been doing too many numbers and my brain is numb !

at 50 MPH I’m doing almost 4,000 RPM, and I would like to bring the RPM’s way lower

this chart pretty much matches the numbers I have recorded
 

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Ah yah those look like the right curves. Using an 18 up front is closer to the factory spec, but having the extra pull with the shorter gearing is a blast. I don't remember what all was available for the back, but I think there's a 45 available from JT also. You can go taller on the gears to drop cruising RPM, but you pay for it off the line and accelerating. Running through the gears is worth a little higher RPM on the highway for me haha.
 
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