Just 30klm shy of completing my 6000klm ride from Brisbane to Tasmania and back in March 2019 I snapped my chain on the freeway. I was travelling at 100klm and hour, felt the chain change pitch, slowed down to 80 but obviously not quick enough, next the clutch lever pulled in and the engine locked up and we halted quickly to the side of the road. Below is a picture of the damage done to the transmission case and on the left, the cleaned up replacement I managed to obtain (it was as dirty as the one on the right when I got it).
Not only was the case smashed, the clutch push rod was also destroyed, the chain smacked it so hard it bent it and snapped it clean off. So as I started to source parts I decided that I would convert the GPz1100 to a modern X-ring style 530 performance chain. They are lighter, rated for more power and longer life. First I would need to source the right sprockets so I looked at JT Sprockets as they have a usable web site and using an online calculator someone sent me a link to, I could calculate ratios and chain lengths. Some links to JT Sprockets (https://www.jtsprockets.com/), and to calculate the new chain length (https://www.rbracing-rsr.com/chainlength.html).
After some calculations, I have come to the conclusion that a JTF517 front sprocket and JTR488 Rear Sprocket should be the closest match. By selecting 15T front and 41T rear, I should end up with the factory ratio of 2.733 and be able to fit a 114 link O-ring or X-ring chain.
Below are the details on the new sprockets, selecting a 15/41 ratio should require a 114 link 530 chain based on 26.5″ shaft to shaft distance . If I went 16/44 (2.75 ratio) then that would mean a longer chain.
The only issue with the front sprocket is the width, The 630 factory sprocket is 12.9mm in width due to a 3 mm integrated spacer, the 530 replacement is 3.9mm narrower than the factory original. The obvious solution is for me to source a washer to sit behind it or get one spun up.
Other ratios that might work are 16T/44T and 17T/46T but the chain link count grow by at least 2 or more links each time and there is the issue of weather a 16 tooth or greater fits with enough clearance?
Final Drive Ratios
If you check your service manual it gives the final drive ration, for the B2 its quoted as 2.733. That’s calculated by dividing the front sprocket tooth count into the rear tooth count so 41/15 = 2.733. As an exercise, I checked my 1977 z650B1 ratio and its 42/16 = 2.625 some what close but it would have a different affect on each bike.
So what’s the relevance of that you might ask? Well, if you increase the rear sprocket you get less top end speed (but more torque), if you increase the front you get a higher top end speed (but less torque). However, the RPM scale also changes when you upset the factory ratio. Larger rear sprocket, means you need to rev higher to get the same top speed as you had with the factory ratio.
It is quite legitimate to change the ratio for touring along the highways and revert it back for city traffic, if you put a 16/44 you get a ratio of 2.75, not much from the factory ratio and only 1klm loss at top gear, however you need to get a longer chain and the torque characteristics change by about 5%. Food for thought.
I’ve ordered the 15Tooth and 41Tooth sprockets to trial fit it and check clearances, then I will go and get the chain for the final fit. I will most likely buy a GB530GXW which are around $190AUD.
If you are a GPz1100 owner then follow my updates on the B2 here: https://sidyoung.com/another-classic-is-on-the-road/