38 t rear sprocket chain length?

38 t rear sprocket chain length?

Postby rekit » Fri May 21, 2010 1:07 pm

81 cbx. Going to 18/38 530. CBX Perf thinks I need 4 less links. (104 vs. 108) Can someone that's actually done this mod tell me how many links they needed. Thanks
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Re: 38 t rear sprocket chain length?

Postby EMS » Fri May 21, 2010 7:04 pm

Without going into a detailed calculation with lengths, radii and diameters, a quick estimate:

From 40 to 38 teeth is a 5% reduction in circumference, ergo in diameter. 4 links out of 108 is close to 4%, which means it is probably right. 4 or 5 links shorter chain will suffice. You can pick up any variation by adjusting the chain.
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Re: 38 t rear sprocket chain length?

Postby Will » Sat May 22, 2010 7:53 am

EMS wrote:Without going into a detailed calculation with lengths, radii and diameters, a quick estimate:

From 40 to 38 teeth is a 5% reduction in circumference, ergo in diameter. 4 links out of 108 is close to 4%, which means it is probably right. 4 or 5 links shorter chain will suffice. You can pick up any variation by adjusting the chain.



You are comparing chain length to sprocket circumference. That doesn't work. Going from 40 teeth to 38 teeth is a 2 tooth reduction in circumference. The sprocket doesn't know how long the chain is.
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Re: 38 t rear sprocket chain length?

Postby rekit » Sat May 22, 2010 9:56 am

Yea, my thinking is that circumference won't get you there since you're only routing the chain around half of it at a time. So I was thinking 4 teeth less from a 42 would need 2 less links. (I'll count what's on there now, but I read stock is 42.) I went ahead and bought a chain-breaker and standard length 108 chain, so I'll post the results when it gets here. BTW, I took off the old chain and didn't see any o-rings. 13,000 miles on the bike. Did the original chain have o-rings?
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Re: 38 t rear sprocket chain length?

Postby EMS » Sat May 22, 2010 12:05 pm

Will wrote:
You are comparing chain length to sprocket circumference. That doesn't work. Going from 40 teeth to 38 teeth is a 2 tooth reduction in circumference. The sprocket doesn't know how long the chain is.



Following my original statement, Will:

Without going into a detailed calculation with lengths, radii and diameters


what do you think will work?
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Re: 38 t rear sprocket chain length?

Postby daves79x » Mon May 24, 2010 6:51 am

Since you are going with a new chain (yes, the original one had o-rings) anyway, just install your new sprocket(s) and wrap the new chain around and see where you need to cut it to make the adjustment range normal. Keep in mind that you will be too short if you don't like the new gearing. I personally like no smaller than 40 tooth rear sprockets (yes, 42t is stock). It still is a little doggy rolling on with a 40 compared to stock, but not too bad for cruising. A 38 may make the bike feel like a 900.

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Re: 38 t rear sprocket chain length?

Postby rekit » Mon May 24, 2010 12:20 pm

Thanks. For me downshifting is no problem and I'm mostly in the flatlands. On my other bikes 4000 is 70-75mph. I went up 1 tooth on the front pulley on a Vulcan 900 Custom and it was like a new bike, and still accelerates hard enough. I just hate low gearing when you're droning along.
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Re: 38 t rear sprocket chain length?

Postby daves79x » Mon May 24, 2010 8:45 pm

I understand your perspective - it's a common comment with new CBX owners (I'm making that assumption - sorry if that's not the case). I can just tell you that the CBX likes to rev and Honda geared it's bikes, for the most part in that era, so that they would pull redline in top gear. Gearing up will not allow that, and Honda wanted the quickest bike back in the day. That said, the CBX will still pull taller gearing acceptably, but overall performance will suffer.

I can assure you that the CBX will run all day long at 7500 rpms and love it. They just like to rev. But lots of guys are happy with taller gearing for their needs and that's great too. To each his own. Let us know how you like the changes.

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Re: 38 t rear sprocket chain length?

Postby Will » Mon May 24, 2010 10:04 pm

EMS wrote:
Will wrote:
You are comparing chain length to sprocket circumference. That doesn't work. Going from 40 teeth to 38 teeth is a 2 tooth reduction in circumference. The sprocket doesn't know how long the chain is.



Following my original statement, Will:

Without going into a detailed calculation with lengths, radii and diameters


what do you think will work?



I thought I had replied to this. But, maybe I didn't push the right buttons.

Basically, nothing changed except the rear sprocket. The chain length between centers did not change, the front sprocket did not change, only the circumference of the rear sprocket changed. The chain only touches half of that. So, for that half, you would lose one tooth, for which you should lose one link. Now they used to make what they called a "half link", which would take up one link, and would have done the job. But, I haven't seen one in decades. So, take two, the rear axle will move forward that extra link distance, or about 1/2".
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Re: 38 t rear sprocket chain length?

Postby rekit » Thu Jun 03, 2010 5:08 pm

OK, got the chain on and it required removing a section that is 4 pins (links) long. So, 108 is stock, 104 for 18/38 combo. Thanks for the replys.
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Re: 38 t rear sprocket chain length?

Postby 17Mkensith » Tue Jun 14, 2011 3:34 pm

I too am looking to change my ratio a little I do a lot of highway travel if I go to a 40 t what will i have to get for chain lenght and is 18 t for front
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Re: 38 t rear sprocket chain length?

Postby EMS » Tue Jun 14, 2011 3:58 pm

17Mkensith wrote:I too am looking to change my ratio a little I do a lot of highway travel if I go to a 40 t what will i have to get for chain lenght and is 18 t for front


You need to determine what chain you have. 630s and 530s used different sprocket sizes. Do you have a 79 with an original 630 chain and 18/42 sprockets?
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Re: 38 t rear sprocket chain length?

Postby Don » Tue Jun 14, 2011 4:31 pm

EMS wrote:Do you have a 79 with an original 630 chain and 18/42 sprockets?
That would be the only one in the world then, right Mike? - An 18 tooth 630 counter sprocket would be a bit of a tight squeeze

I think the 630 equipped bikes came geared 15/35

If going from an 18/42 equipped bike to an 18/38 required 4 less links (104 vs 108) it should be a pretty safe bet that an 18/40 setup would require 106 links . . . . I think

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Re: 38 t rear sprocket chain length?

Postby EMS » Thu Jun 16, 2011 8:19 am

I am still totally messed up with my sprocket sizes :crazy:
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Re: 38 t rear sprocket chain length?

Postby ericfreeman » Thu Jun 16, 2011 10:57 pm

Yes, 1979 with 630 chain had 15/35 sprockets as stock. When I bought my '79 from friend it had a 15/38! Accelerated like gangbusters but was in 5th gear by the other side of the intersection!!

I tried the OEM 35 and it was better but I was still bothered about highway revs. Bought an aluminum 32 for the rear and loved it's highway performance, but it did lose some snap. Recently, discovered the aluminum sprocket has deformed quite a bit so I fitted an OEM, steel 34 rear. Really like this combination. Just a few less revs on the highway but plenty of snap everywhere else. Too bad Honda didn't fit one of their Hi/Lo gearboxes to the CBX! We could have been kings of the drag strip and still enjoyed a leisurely cruise on the highway.

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