Calculations to be made...

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CBX1000FAN
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Calculations to be made...

Post by CBX1000FAN »

I bought my 1979 CBX as a "modified" bike. I am now nearly done with correcting a lot of the stuff done to the bike. However, it will remain modified: It has a 180/55/17 rear wheel compared to the OEM 120/90/18. That has a reduction of speed, measured at 60 mph of 6.4%, meaning the rpm will be higher at true 60 mph.

The second factor is the gearing: It has a 18T front sprocket and a 44T rear compared to the OEM 18T / 42T - this means a reduction of speed by 4.5% or in other words, the rpm will be 4.5% higher...

Combine the two issues and the problem is clear - the rpm at 60 mph is much higher then stock!

I have one simple option to rectify this - change the rear sprocket. The bike has a 530 chain with a master link, so no issue to shorten the chain. However I have to figure out the rear sprocket teeth.... Keeping the 18T up front it seems I have to go with either a 41T or a 40T. The 40T seems to be nearest to the stock gearing.... Can someone help me check my calculations please??

Regards!!

Larry Zimmer
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Re: Calculations to be made...

Post by Larry Zimmer »

Quick SWAG on my part says that the 41 tooth would be closest -- just slightly higher than stock rpm at 60mph with the 180 tire. 40 tooth just a tad lower rpm than stock. Let's see what others say.
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CBX1000FAN
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Re: Calculations to be made...

Post by CBX1000FAN »

Thanks Larry - I also have it so. The 41T is 25% more expensive than the 40T!! :-) Regards!

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Re: Calculations to be made...

Post by Larry Zimmer »

AH! For the difference, that 25% might decide.
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bobcat
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Re: Calculations to be made...

Post by bobcat »

You say it's a modified bike. If you have, say, a big bore kit or other engine performance enhancements,
go with the 40T. If the motor is stock and acceleration is more important than cruising speed, 41T.
My 2 cents.
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82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

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Re: Calculations to be made...

Post by CBX1000FAN »

Yep, I think the 41T will be the best option. Thanks all.

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Re: Calculations to be made...

Post by daves79x »

Why not just use the stock 42T and call it a day?

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Re: Calculations to be made...

Post by CBX1000FAN »

I do not have a 42T... I have to buy a sprocket and it will be wise to go 41T as the 180 rear tire needs 6.4% correction to be OEM. A 41T will bring it to about 2.2% over OEM RPM @ 60 mph....

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Re: Calculations to be made...

Post by CBX1000FAN »

I drove the 220 km today to buy the sprocket... At first the salesman said they do not have a sprocket that will match, but I saw he was looking at Honda sprockets, so I said try Kawasaki. And for sure he had one - a 41T!! It is now fitted and the chain shortened. Will be testing when the weather clears up. Regards!
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