C70 Gearing

Phyrphlyer

New Member
Tinkering with the idea of changing the front driven sprocket from the stock 14 tooth to a 16 or 17 tooth sprocket in order to nudge the top speed closer to 48-50mph. As is the bike (1980 C70) is stock with 14T front/36T rear sprockets. The bike will get up to about 42-43 indicated as is. I'm willing to sacrifice a bit of low end torque and acceleration since, if I'm not careful, a can pull the front wheel off the deck in 1st gear.
1. Wondering if this is a workable approach.
2. Wondering if I can get away with using the existing chain. It's in great shape, and I have about 5/8" exposed threads on the chain adjusters at the rear so the tension adjustment is in about mid-range as is.
Any thoughts out there?
 

69ST

Well-Known Member
A lot of guys at least go through the phase of thinking there are big mph gains to be had from a sprocket change. I know, on a gearing table it looks easy. That said, just because a given sprocket combo equates to 80mph at an rpm the engine can reach (when not overgeared) that doesn't not mean it has enough grunt to pull gearing that tall. In fact, I can save the suspense...it doesn't.

At most, you might be able to go up 1T at the countershaft. Give it a shot, CS sprockets are cheap, no real work to change.

A 72cc engine can pull 50mph, with the right tuning parts. But, the added hp and top speed will come from increased rpm. Gearing is limited to ~4.8mph/1000rpm, in absolute terms. Much more than that, you'll exceed the available torque and the motor will fall on its face.

If the chain is worn, may as well replace it now and get it over with. A worn chain will cost you in sprocket wear and may cost you a few mph up-top.
 

Phyrphlyer

New Member
A lot of guys at least go through the phase of thinking there are big mph gains to be had from a sprocket change. I know, on a gearing table it looks easy. That said, just because a given sprocket combo equates to 80mph at an rpm the engine can reach (when not overgeared) that doesn't not mean it has enough grunt to pull gearing that tall. In fact, I can save the suspense...it doesn't.

At most, you might be able to go up 1T at the countershaft. Give it a shot, CS sprockets are cheap, no real work to change.

A 72cc engine can pull 50mph, with the right tuning parts. But, the added hp and top speed will come from increased rpm. Gearing is limited to ~4.8mph/1000rpm, in absolute terms. Much more than that, you'll exceed the available torque and the motor will fall on its face.

If the chain is worn, may as well replace it now and get it over with. A worn chain will cost you in sprocket wear and may cost you a few mph up-top.
I have this theory: The Smartest Man in the World doesn't know everything - he knows whom to ASK. Your points are well taken. As you say, the parts are cheap and it's the work of minutes, not hours. Might be worth experimenting with, since the motor seems to have more than enough spunk at the low end. Not looking for an order-of-magnitude improvement, just a bit of a nudge. Think I'll give it a bit of a go and report back here..
 

condensedhan

New Member
I have this theory: The Smartest Man in the World doesn't know everything - he knows whom to ASK. Your points are well taken. As you say, the parts are cheap and it's the work of minutes, not hours. Might be worth experimenting with, since the motor seems to have more than enough spunk at the low end. Not looking for an order-of-magnitude improvement, just a bit of a nudge. Think I'll give it a bit of a go and report back here..
howd it go
 

chuck_fyn

New Member
I have this theory: The Smartest Man in the World doesn't know everything - he knows whom to ASK. Your points are well taken. As you say, the parts are cheap and it's the work of minutes, not hours. Might be worth experimenting with, since the motor seems to have more than enough spunk at the low end. Not looking for an order-of-magnitude improvement, just a bit of a nudge. Think I'll give it a bit of a go and report back here..
Hey phyrphlyer! I'm considering doing this exact thing to my c70 to get my top speed closer to 50. Report back if you can!
 

airblazer

Active Member
It’s now been 4 years since the original poster started this thread, AND since the last time they visited this site. @chuck_fyn I’d say the only way to solve your dilemma is to do it yourself. And then report back😏
 

chuck_fyn

New Member
Hah, okay fair enough, I think you're right. I've been thinking about how to get that last few mph for too long, time I did something about it! I'll report back, in phyrphlyer's place, in a few weeks
 

c70noob

New Member
this is a funny thread. I have a 1981 passport. my engine no work anymore. it's in pieces right now. I took the head apart (thinking it's not running because if my timing) and need help getting flywheel off. halp
 
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