Re: 42 to 32-tooth sprocket

 


    That's a 31% increase, I think you'll find it difficult to maintain
speed in top (as a first guess).  If there's a street equivalent, check what
gearing it uses.

    On a practical note, it may be easier and cheaper to change the
front sprocket.  One tooth larger will yield about 6% increase, so I'd suggest
trying two larger if available (and it fits).

--
Ivan Reid, Electronic & Computer Engineering,     ___     CMS  Collaboration,
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        KotPT -- "for stupidity above and beyond the call of duty".

 



It doesn't sound like that little mouse motor would be able to push
the bike that much faster.  You'd be going from 43mph at redline to
56.4mph.  Thing is, it takes just over double the total horsepower to
go that much faster (I calculated ~2.14x as much).

Chances are, the motor Yamaha designed around that bike wasn't blessed
with twice the horsepower it needed to reach top end... I'd wager
you'd get at best a mph or two out of a sprocket change (and very
likely, you'd lower the top end).

Mark "or use a 130 tooth sprocket and set a land speed record" Hickey

 


Mark Hickey wrote:


to go that much faster (I calculated ~2.14x as much).

It's true that it takes 4 times as much power to go twice as fast. But,
I suspect that the DT-80 was probably geared very low to climb hills
off road, and that's what limited its top speed...

65 mph out of a DT-80 is very believable to me, if the bike is running
on level ground, with no head wind, and the rider lays down on the gas
tank...

I had a Honda 50, bored out to 55cc and I had it up to 52 mph, going
downhill. My buddy had a 60cc Yamaha streetbike, I disremember what it
was called, (the DT-80 probably shared parts with it) but he rode that
darned thing all the way from Lancaster, CA to Torrance---and back---in
one day. He was on the bike from dawn to after dusk...

My DT-1 250cc single cylinder bike had about 15 to 20 horsepower,
depending on which magazine tested it, but it would only go 71 mph, due
to its mild port timing and gearing. I think it should have been able
to go up to 15% faster, with appropriate re-gearing...

Typical gear splits at redline in a motorcycle transmission are about
15%. I think that if the OP geared his Yammie 15% higher with a
36 tooth rear sprocket, he might be able to cruise at the speeds he
wanted...

But he'd have to redline the engine in the next lower gear to keep
pulling in high gear, and lay down on the gas tank, on level ground, in
still air, otherwise, he'd never do it...

 

krusty kritter wrote:

Hmmm, that doesn't sound very good.

My dilemma is that the shop i'm ordering from only has the 32-tooth rear
  sprocket or a 14-tooth (+1 tooth) front sprocket. I am worried that
will not make a big enough effect.

 


Masospaghetti wrote:


am worried that will not make a big enough effect.

Check some of the chain and sprocket manufacturer's websites. I'm sure
you can get any number of teeth you want on a rear sprocket, as there
are only a limited number of bolt circle diameters and number of bolts
in the circle...

And, so far as worries about harming a small bore short stroke Japanese
engine are concerned, lots of RPM won't hurt the engine as much as
failure to keep your air cleaner element clean and oiled If it's a foam
type) with no leaks around the element...

If you don't maintain that air filter, a small bore engine will eat up
the rod bearing...