Sprockets

[georgeknight]

Hello all,
Just have a question. I had a 44 tooth sprocket that ran 32mph, but I wanted to cruise without being wide open at 30 mph. So it was suggested that I buy a 36 tooth. I did, now it won't run above 21mph. The engine is not in any bind but it won't get no where near wide open throttle. Any advice? Thanks George

[HoughMade]

40 tooth perhaps?

[stude13]

ill buy the 36. please pm me

[Creative Engineering]

georgeknight,

There is not even the slightest chance that a tooth reduction from 44 to 36 on your rear sprocket is causing this problem.

When it comes to mechanical things, coincidence can be frustrating...but coincidence this is.

Look further, grasshopper!

Jim

[Large Filipino]

What engine? Remember that low displacement makes for an unhappy engine with too tall gearing. The only way you're gonna be happy with that gearing is that you have to help pedal it up to speed.

[Bikeguy Joe]

Originally Posted by Creative Engineering

georgeknight,

There is not even the slightest chance that a tooth reduction from 44 to 36 on your rear sprocket is causing this problem.

When it comes to mechanical things, coincidence can be frustrating...but coincidence this is.

Look further, grasshopper!

Jim

I'll respectfully disagree...

If the engine can rev up high enough to get "on the power curve" he would suffer a lower top speed than with the 44T.

I have ridden several motorcycles that would go faster in 4th than they would in 5th gear.

[Bikeguy Joe]

To the OP- Try a 40T.

Depending on gross weight, and other factors, there will be a point where higher gearing won't improve anything.

[Creative Engineering]

Bikeguy Joe,

I may be wrong, let's run the numbers. I have not tried a 36T sprocket on the Point Beach bike I have, so the experience of those who have; will of course over-ride my thoughts on this.

I assume you meant to say "if the engine can't rev up high enough to get "on the power curve" he would suffer a lower top speed than with the 44T. Yes that's true, but looking at the numbers, I can't believe these engines have such a narrow power band!

The reason some motorcycles go faster in fourth than fifth is wind resistance. At 100+ mph wind resistance is huge.

Assuming a bicycle with a 26" diameter rear tire is used, the numbers work out as follows.

Pi X D = circumference, or a circle laid flat.

One revolution of a 26" Dia. wheel will make the bike travel 81.681 inches or 6.806 feet.

There are 5,280 feet in a mile. 5,280/6.806 = wheel revs per mile.

776 wheel revs per mile.

32 miles per hour will correlate to 24,832 wheel revs per hour.

24,832/60 = wheel revs per minute @ 32 miles per hour. 414 per minute.

414 x 4.4 for a 44T sprocket = 1821 jackshaft RPM
414 x 3.6 for a 36T sprocket = 1490 jackshaft RPM

Engine RPM through the 4:1 ratio clutch gearing...I'm not at the shop so I can't accurately count the gear teeth. from photos I'm sure 4:1 is close. It looks like 80T on the clutch gear and 20T at the crank from photos.

44T = 7,284 engine RPM
36T = 5,960 engine RPM

A difference of 1,300 rpm well into the powerband of this engine.

Working back through the numbers 21 MPH and 36T would require an engine RPM of 3,911. Seems like enough RPM for the engine to be making power?

Georgeknight: Have you tried a fresh NGK plug?

I don't know, the numbers seem to suggest the problem lies elsewhere.

Jim

[Creative Engineering]

Hello all,

I forgot to mention that the above assumes a 10T jackshaft sprocket.

Jim

[georgeknight]

Originally Posted by Creative Engineering

Hello all,

I forgot to mention that the above assumes a 10T jackshaft sprocket.

Jim

I've tried a new plug, no help. My bike has a 10 tooth front sprocket.
Weight should not be a problem as I weigh 140. It does seem to try to get to the power band but falls just short. The motor seems to be lacking enough fuel to get going but it runs fine with the 44 tooth. It just turns too many rpm's running 30 mph, but the stock carb may need to be modified somehow. I wish I had a carb that I could adjust the air fuel mixture