Updated 2023-06-29
Just 30klm shy of completing our 6000klm ride from Brisbane to Tasmania and back (in March 2019), I snapped my chain on the freeway. I was travelling at 100klm/hr and felt the chain change pitch, I 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 GPz 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).
Sprocket Selection
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 110 link O-ring or X-ring 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 4.2mm narrower than the factory original. The obvious solution is for me to source a washer to sit behind it or get a spacer spun up.
Below are the details on the new sprockets:
But…
One key point I have learned recently is that despite the factory original sprockets being 630 type with 15 teeth, you cant just swap it for a 530 sprocket of the same teeth size. The reason is the distance between the links, a 630 chain has 19.05mm centre to centre while a 530 chain is 15.875mm, this means there are more links on a 530 chain which throws out the link count and ratio. Reading up in the text book “The Racing Motorcycle” tells me that the circumference must be the same as the original sprockets, not the number of teeth.
Recalculating for a 530 setup….
Using the original 630 / 15 tooth front sprocket and the link to link distance of 19.05 mm for the chain, I calculated a sprocket circumference of 285.75mm. If you divide that by 15.875 which is the link to link measurement for a 530 chain, then you get 18 teeth exactly.
Applying that same logic to the rear gives 41 x 19.05 = 781.05 mm then divide the result by 15.875 gives 49 rounded.
The ratio of 18 / 49 sounded familiar, so I searched through some service manuals in my library and flipped through the GPz900R manual. The GPz900R has a 530 chain, with a 17 tooth front and 49 tooth rear with a final drive ration of 2.88. So that’s close for a similar ratio to the 1100 (2.733) but with 530 sprockets. So the math’s appears to be correct or at least close.
Final Drive Ratios
If you check your service manual it gives the final drive ratio for the B2 model as 2.733. It’s calculated by dividing the front sprocket tooth count into the rear tooth count so 41/15 = 2.733 and 49/18 = 2.722. 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 had originally ordered the 15 and 41 tooth sprockets to trial fit and check clearances, that’s before I recalculated the circumferences and what I think is the correct sizes or close to it. So now I’ve reordered an 18 and have some GPz900r rear sprockets to trial fit on the hub.
Fitting The Sprockets
Once the front sprocket arrived I needed to get a spacer made up to space it out from the output shaft bearing and center it with the smaller rear sprocket I received. Below is the factory 630 front sprocket, it has the integrated spacer as shown.
The new sprocket is thinner, in this case there was 4.2 mm difference between the thickness of the original larger sprocket and the new one. Below is the spacer I had made, complete with slight chamfer.
Chain Selection
Over the weekend I fitted the sprockets and wrapped an old 530 chain around to see how many links I really needed and to test how accurate the online calculator was. Well it seams 110 links for my sprocket ratio’s is about right and the online web calculator tool is close but not 100% correct. However since I recalculated the sizes as being larger still, I now need to buy a 120 link chain and cut it down to size.
Armed with the link count I went searching at some Motorcycle Accessory stores on the weekend. After checking out the dealer offerings I went around a few accessory stores. Well bike shops are expensive and accessory shops are cheaper, they also stock a limited range and the reason became obvious once I told the sales guy what I was doing.
Seams most stores here only stock 120 link chains, the expectation is you just rivet the chain to the link count you need… obvious really. I need 110 links and I can use the tool I have to break the chain and peen over the ends to join it back.
I picked up a 120 link VX-530 chain which is the same as I have on my zx9r. Since the horse power is slightly less on my modified GPz, it should work well and being heaps lighter the rolling weight of the chain should be considerably less and then add in the weight reduction of the sprockets!
If you are a GPz1100 owner then follow my updates on the B2 here: https://sidyoung.com/another-classic-is-on-the-road/
I also have a 1981 B1 which I will do this modification too as well (in due course).
-oOo-
My GPz1100B2has been parked since 2002 hasn’t been started since then!
Hold onto it, classics like it are getting hard to find!