42 to 32-tooth sprocket

I have a 1982 Yamaha DT80 and it has a 42-tooth sprocket from the
factory. I'm trying to increase its top speed so I can use it on more
roads - right now it only can go about 35 mph, 43 mph at redline.

Would a 32-tooth sprocket sap too much power from this little motor to
be practical?

 

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

 

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...

 

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.

 

Also, It is not important for me to rear 56 mph, just to have the bike
cruise more easily at around 45 mph. Right now, the thing is howling at
40 mph, and downright _screaming_ at 43.

 

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...

 

The question is - does it need to be screaming to make enough
horsepower to go over 40mph? If there seems to be a reasonable amount
of throttle response in the high 30's - that is, you crack the
throttle and something happens - maybe there's enough power at a lower
RPM to go 40. If you measure your 35-40mph time with an egg timer,
chances are good the little motor's doing about all it'll do.

My own "top end story" was an early 70's Kawasaki 125. I had an
expansion chamber on it and could get 70.0mph out of it laying
(literally) on the tank (with my feet out behind me). How do any of
us ever live to adulthood?

Mark "overworked guardian angel somewhere" Hickey

 

What teacup sized engine *doesn't* have to "scream" to make power?

Answer: a 2-stroke. It just *sounds* busy, as it fires twices as often
as a 4-stroke engine of the same size...

I remember a discussion I had with a 4-stroke lover back in the days
when 2-strokes were everywhere and the 4-stroke owners *hated* the
"ring dings"....

The guy actually believed that 2-strokes were "all RPM", while
4-strokes had "low end torque". He didn't believe me when I told him
that the average Honda 4-stroke was turning twice as fast as the
average Suzuki or Yamaha to make the same power...

An acquaintance used to tell me about riding his illegally-small
Kawasaki 90 on the freeway (during the 55 mpg speed limit era). He said
that he would be doing 55 mph, and when the CHP looked at him, he would
accelerate to prove he still had something left...

Now there's an original bit of instrumentation...

Were you wearing a bathing suit and bathing cap, like Rolly Free did,
on his Vincent,going 150 mph on the Bonneville salt flats?

By wearing a helmet, to avoid head injuries...