The International CBX Owners Association

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!!

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.

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

AH! For the difference, that 25% might decide.

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.

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

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

Dave

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....

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!