Figuring out gear ratios- 1983 z50r with ct70 10" wheels

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RadRacer203

Active Member
So I've been continually upgrading my '83 z50r that started as a bin of parts a few years ago. This time I've got a set of 10" ct70 wheels, I believe from 1979 with a 35 tooth sprocket. I'm just having a little trouble wrapping my head around the ratios and was wondering if anyone had any suggestions. I've got the stock front sprocket (12 tooth) so I have some room to make that smaller and I have room to put a larger one on the rear too. Ideally I want it to be about the stock ratio so a top speed of 25 or 30 with enough grunt to haul me around.
 
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69ST

Well-Known Member
Unless the engine has been tuned to make a lot more power, with an emphasis on torque...i.e. increased displacement...the gearing needs to be the same, in revs per mile, regardless of tire/wheel size. Measure the circumference of both tires, divide the larger number by the smaller number, to get the % change. Next, multiply the existing sprocket combo by that number, which will be 1.xx, to get the new final drive ratio. From there, you either multiply the CS sprocket tooth count, or divide the wheel sprocket tooth count by the target ratio.
 
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RadRacer203

Active Member
Awesome, thanks! It's a stock engine so I'll definitely be going for a stock ratio. Hopefully in the future I can get my hands on a cheap ct70 engine and throw that in there but that's a long ways off
 
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69ST

Well-Known Member
You only need a CT70 top end: cylinder & piston, head, carb and an angled intake to clear the tank. For that matter, you could opt for a 52mm (88c) bore-up kit for the same money. Your Z50 motor is, essentially, the same lower end used with every 6v/3-speed motor from 49cc up 89cc (Cub 90, which was never sold in the US).
 
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RadRacer203

Active Member
racerx said:
You only need a CT70 top end: cylinder & piston, head, carb and an angled intake to clear the tank. For that matter, you could opt for a 52mm (88c) bore-up kit for the same money. Your Z50 motor is, essentially, the same lower end used with every 6v/3-speed motor from 49cc up 89cc (Cub 90, which was never sold in the US).
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I know I can just swap the top end but I really want that 4th gear too if it'll fit in my frame
 
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69ST

Well-Known Member
Cases are the same. Thus, a 4-speed tranny will fit the same as it would in a CT70 engine. For that matter, you can pair either transmission with either clutch type. OEM 4-speed transmssions are a bit hard to find. High-end Japanese (Takegawa, Kitaco) are on the expensive side. AHP 4-speed is inexpensive, readily available. Quality isn't Japanese aftermarket, or OEM. It's way better than the super-cheap, Chinese stuff though and has been on the market close to 15 years now. IOW, it should be fine for your project, if you're looking to save some bucks.
 
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RadRacer203

Active Member
racerx said:
Cases are the same. Thus, a 4-speed tranny will fit the same as it would in a CT70 engine. For that matter, you can pair either transmission with either clutch type. OEM 4-speed transmssions are a bit hard to find. High-end Japanese (Takegawa, Kitaco) are on the expensive side. AHP 4-speed is inexpensive, readily available. Quality isn't Japanese aftermarket, or OEM. It's way better than the super-cheap, Chinese stuff though and has been on the market close to 15 years now. IOW, it should be fine for your project, if you're looking to save some bucks.
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Huh, I had assumed all ct70's were 4 speeds like my XL70. I never knew that most were 3 speeds. I guess I'll just be looking for an sl70 or xl70 motor then
 
6

69ST

Well-Known Member
May as well improve your chances just that much more and add CL70 to that list. Those are 4-speed/manual clutch too.
 
R

RadRacer203

Active Member
racerx said:
May as well improve your chances just that much more and add CL70 to that list. Those are 4-speed/manual clutch too.
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I'll put that engine on the list too! I found someone selling a couple sl70 motors today but they're about 200 miles away. I'm going to see if my buddy will pick one up for me
 
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RadRacer203

Active Member
Out of curiosity, when I eventually get one of these 4 speed motors, can I swap the z50 semiauto clutch onto it? I'm not sure I would, but maybe because full manual might be a giveaway if I want to try to sneak in registration as a moped
 
kirrbby

kirrbby

Well-Known Member
SL70s are 1down 3up shift pattern. They say a semi auto clutch makes it hard to hit neutral with that shift pattern.
A CT70H is 4down. Some CL70s are 4down. And some ATC70s are 4up, or 4down 4 speeds, with a semi auto clutch...but they are pullstart only.

XL70s are all 1down 3up.
 
6

69ST

Well-Known Member
RadRacer203 said:
Out of curiosity, when I eventually get one of these 4 speed motors, can I swap the z50 semiauto clutch onto it? I'm not sure I would, but maybe because full manual might be a giveaway if I want to try to sneak in registration as a moped
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Absolutely, transmission type is independent of everything else. As for the issue finding neutral with a 1-N-2 shift pattern, consider that Honda used that with some semi-automatics. ST70 Dax models that were sold in the EU, came with 1-N-2-3 shift pattern. They also came fitted with a heel/toe shifter.
 
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RadRacer203

Active Member
Awesome, thanks for the input. When I eventually get one of these motors I think that's what I'm going to do. I don't know how I forgot about the shift pattern considering I own an XL70 but I don't think it should be too much of an issue for what I'm using this bike for.
 
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69ST

Well-Known Member
I don't really buy the shift pattern being an inherent problem with a semi-auto clutch. Can you imagine Honda selling a problematic setup, for decades? A low-dollar transmission may be a large part of the problem, precision machining is critical when it comes to a trans that will reliably land in neutral, when desired. A lot of the Chinese engines went to an all-down shift pattern, the cheapest solution. Mostly, it comes down to proper idle speed & preload adjustment. A number of riders dislike a heel/toe shifter; I'm not among them. A toe-only lever provides less control, imo.
 
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RadRacer203

Active Member
Just got the new wheels on the z50r and I think they're looking really good on there! I have a new longer kickstand on the way and a new set of brake shoes because they fell apart when I swapped the brakes over... I figured out that I need a 42 tooth sprocket for it but I'm having a little trouble finding a sprocket that big that fits the ct70 wheels. Does anyone know offhand where I can buy one?

z50 new wheels.jpg
 
6

69ST

Well-Known Member
I've always liked the look of a softail fitted with CT70 wheels. With that CT70 front end, you've essentially got a short-wheelbase/lower mass version of a CT70. A longer, heftier, seat might help balance-out the visual "weight" of the bike; mostly, you'd buy yourself some added rider room & comfort. If you ever decide you'd like CT70 folding handlebars, K1-78 top tree/bar clamp assemblies are all the same. This bike is going to deliver a much nicer ride than it did with the 8-inchers & Z50 pogo stick front end.(y)
 
R

RadRacer203

Active Member
racerx said:
I've always liked the look of a softail fitted with CT70 wheels. With that CT70 front end, you've essentially got a short-wheelbase/lower mass version of a CT70. A longer, heftier, seat might help balance-out the visual "weight" of the bike; mostly, you'd buy yourself some added rider room & comfort. If you ever decide you'd like CT70 folding handlebars, K1-78 top tree/bar clamp assemblies are all the same. This bike is going to deliver a much nicer ride than it did with the 8-inchers & Z50 pogo stick front end.(y)
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Yeah, it rides REALLY nice now although the front end is a little soft. I'm thinking about what I want to do with the seat but I'm thinking I'll probably go with a stock replacement when I upgrade eventually but I kinda like it as is. Such a fun little bike, could use a little more power and a couple odds and ends but I'm really happy with how it turned out
 
6

69ST

Well-Known Member
RadRacer203 said:
Yeah, it rides REALLY nice now although the front end is a little soft. I'm thinking about what I want to do with the seat but I'm thinking I'll probably go with a stock replacement when I upgrade eventually but I kinda like it as is. Such a fun little bike, could use a little more power and a couple odds and ends but I'm really happy with how it turned out
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If you've already rebuilt the fork legs, then it's a good time to learn how "oil tune" one of these front ends. If you haven't rebuilt them, that only takes one new oil seal for each side, disassembly & cleaning. Then, it's onto refilling. 15W fork oil is usually a good choice, if only for the starting point. Refill quantity, per the manual is 3.5oz after draining, 3.8oz from a dry fill. If compression action is still too soft, you can gradually increase the oil fill quantity, adding 5cc, then retesting until you find the sweet spot. The caveat is that if you overfill, the fork will hydrolock before it can bottom out and you'll lose suspension travel; should that occur, you'd drain oil until full travel returns. There's a pocket of trapped air inside the fork leg, reducing its volume acts like progressively stiffening the spring. As a general guideline, optimal "tuned" oil fill level will be when the oil level, visible from the top of the leg (leg fully compressed) is ~100mm below the top. The amount of improvement possible with the K1-79 fork, via this type of tuning (oil viscosity & quantity) is pleasantly surprising.
 
R

RadRacer203

Active Member
I haven't rebuilt the forks but they aren't leaking at all so I'll leave well enough alone. I think I only had 5w oil on hand when I filled them last time so I'll definitely change that.

Any idea where I can get a 42 tooth sprocket for these wheels?
 
6

69ST

Well-Known Member
RadRacer203 said:
I haven't rebuilt the forks but they aren't leaking at all so I'll leave well enough alone. I think I only had 5w oil on hand when I filled them last time so I'll definitely change that.

Any idea where I can get a 42 tooth sprocket for these wheels?
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The fork internals will hold a surprisingly large quantity of abrasive garbage, likely some sludge too, that'll score the moving surfaces. New seals are cheap. 5W oil is thinner than the what Honda used back in the day.

As for sourcing a 42t cog, I don't know if dratv still has them. 44 & 45t are what's commonly available. You might be able to upsize the CS sprocket by a tooth, to get the same numerical ratio with one of those.
 
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