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Discussion Starter #1
OK, I am a bit perplexed with my new project bike, 1987 Big Wheel 350. Lots of much needed parts are discontinued and near impossible to find. As some here will know the BW350 uses 4 sprockets and has a jack shaft to accommodate the giant rear tire. Using the sprocket calculator for a standard bike with just 2 sprockets is simple but not so simple where 4 sprockets are concerned.

Here is my issue and I need some help. Stock gearing for this bike is 14 tooth on the engine that goes to a 17 tooth on the inner jack shaft. Then from the outside jack shaft with a 13 tooth back to the rear wheel with 37 teeth. The engine and inner jack sprockets use a 530 size chain and the outside and rear wheel use a 520 chain. The 17 tooth 530 inner jack sprocket is simply not available but I can find a 16 tooth. The rear wheel 37 tooth is no longer available either, it is a dished off set sprocket but there are adapter plates to allow regular 37 tooth flat sprockets to be mounted.

I want to keep the final gear ratio as close to stock as possible and can't calculate what going down one tooth on the inside jack shaft sprocket will do to the final drive ratio.

To give a bit of background, with the stock set up on this bike 14-17 and 13-37 first gear is amazingly low and similar to a creeper gear in a 4 wheel drive in low range. I can walk faster and have the clutch out at idle with no problem. I really don't want to change that. There is also the possible issue of chain length being too long by going down a tooth on the 530 sprockets from engine to the jack shaft and I don't know just how much adjustment there is on this portion. Wish I could find a 17 tooth 530 sprocket that fit the splines of the inside jack shaft but no luck with that after a month of searching. If anyone cares and has a local dealer that might have old stock the Yamaha part number is 93852-17126. My bike has all original sprockets that are in OK shape but over this winter I plan to replace the chains and hate the thought of new chains on partially worn sprockets.

Who here can calculate what going down one tooth will mean in the end?

Thanks

GaryL
 

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Gary, if you go with the 16 on the shaft, increase the rear wheel sprocket to 40 and it should be a wash. Think of it this way; down 1 in the front equals up 3 in the rear.
 

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Discussion Starter #3
Gary, if you go with the 16 on the shaft, increase the rear wheel sprocket to 40 and it should be a wash. Think of it this way; down 1 in the front equals up 3 in the rear.
That is exactly what is messing with my head. This 16 tooth sprocket on the jack shaft is actually down one in the rear between the engine output and the jack. I am pretty sure there is some way to mix and match the other three sprocket tooth counts to achieve the same or close final drive but I just don't know how to figure it where 4 sprockets are in the mix.

GaryL
 

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I'm thinking of the two sets separately; the drive sprocket on the shaft is remaining constant, the driven is changing so think of it as the front and the wheel is the rear
 

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Simple math, dropping from a 17 to a 16 makes a 16/17ths overall change, so you will want 17/16ths change in the rear to compensate. This works out to a theoretical 39.31 tooth rear sprocket. If you buy a 40 tooth rear then the change from stock will be a 40/ 39.31ths change, or stated otherwise an approximately 1.75% greater numerical increase in final drive ratio ( a 1.75% lower granny gear)
Installing the 16 tooth with a 40 tooth rear should not dramatically change your riding experience, rear axle will spin about 1.75% slower for a given engine rpm from the stock unmodified ratios . You would need a very sensitive and well calibrated fanny to detect any difference.
 

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Discussion Starter #6
Simple math, dropping from a 17 to a 16 makes a 16/17ths overall change, so you will want 17/16ths change in the rear to compensate. This works out to a theoretical 39.31 tooth rear sprocket. If you buy a 40 tooth rear then the change from stock will be a 40/ 39.31ths change, or stated otherwise an approximately 1.75% greater numerical increase in final drive ratio ( a 1.75% lower granny gear)
Installing the 16 tooth with a 40 tooth rear should not dramatically change your riding experience, rear axle will spin about 1.75% slower for a given engine rpm from the stock unmodified ratios . You would need a very sensitive and well calibrated fanny to detect any difference.
Nice Fred, Thank you for the great explanation. See if you can calculate this. Say I drop a tooth on the inside jack shaft from 17 down to 16 but on the outside jack shaft I increase a tooth from 13 to 14, Those are readily available. Does that do the same?

GaryL
 

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16/17 x 14/13 = 1.01357. Thus a 1.36% change in overall gearing with the proposed 14 tooth jack shaft sprockets. Theoretically a little closer to stock than previous proposal. Ignores very minor changes in the parasitical friction losses from chains flexing around different diameters in your overall riding feel.
I'ld say change the affordable jack shaft sprocket pair as per your last post.
Sounds like a nice project, wish I was closer to experience and appreciate a test ride once you are done.
 

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Discussion Starter #8

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Gary here is a picture of the spread sheet I created to show you the changes. The higher the final final drive value the slower you go.
image.png
 

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Correct Gary!
Looks counterintuitive but the changed 16 tooth driven sprocket will spin the jack shaft faster so the larger 14 tooth driving sprocket is needed to slow down the effect on the rear wheel's 37 tooth sprocket.
Plus I can see from the drawing that with the changed sprockets you've also managed to tighten the rear chain up nicely.
Difference should be about a mild 1.3% if I did my Gazintas right.
.
 

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Discussion Starter #11
I am kind of pissed you relate me to Jethro but where Gazintas are concerned I guess I has to just takes it. Nice try Fred but Ellie May never had any issues with my Gazintas!:D

Thanks to you both.

GaryL
 

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Discussion Starter #12
Gary here is a picture of the spread sheet I created to show you the changes. The higher the final final drive value the slower you go.
View attachment 187938
Very interesting! I do have a nice new 43 tooth rear wheel sprocket and if I am computing this right I should be able to crawl at a babies speed even lower than the stock 14/17 and 13/37 combination when I switch to the 14/16 and 14/43. I don't really plan on hitting any trees with this BW but I should be able to climb them.

GaryL
 

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Discussion Starter #13
BTW, I don't much care about how fast the top speed will be with any of these combinations. I never plan to go blasting out on any roads or trails that allow for much over 30 MPH anyway. Scottie158 up in middle NY thinks he can be the wheelie king so with 150 more CCs than his TW200 he will beg me for a shot on this beast. So far I have only blasted up and down my short road in 25 degrees but the seat of my pants got real hot real quick and actually a bit scary for these old bones. I like the rubber side down these days but this beast has no problem lifting the front wheel way farther than I care to go.

GaryL
 
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