transmission ratio too low

VMaxer

New Member
Jul 28, 2020
12
9
3
50
I recently installed a 49cc 4-stroke china kit on my bike, it seems like I was really revving just to go 15mph. I opened the transmission and I have 7 teeth on the primary and 41 on the secondary. On the output shaft its 10 and then the rear wheel is 44. By my calculations, that means 5000 rpm=15mph. Did they change the gearing on these transmissions for some reason? Even if I put a 32 tooth sprocket on the rear, 5000 will only get me 20mph. Maybe I should find a larger sprocket for the output shaft? Any suggestions?
trans1.jpg
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,661
237
63
This transmission was made for high-winding 2-stroke engines.

As such, your final drive now is 25.77:1.

There's not much room to change sprockets inside the box.

Changing the rear sprocket to 36t only raises gearing to 21.09:1.

Just changing the output shaft to 18t raises gearing to 14.32:1.

If that doesn't suit you, change the output shaft higher or lower.

Good luck.
 

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,468
306
83
59
Rockwood, TN
Most of the 49cc 4 strokes max horsepower rpm is around 7000. It'd really pay for you to get a techometer/hour counter. Around 5000 rpm is where the max torque is. The tech can keep you from over reving especially going down hill. If you're pulling a heavy load or going up a steep hill and aren't at least able to reach 5000 rpm then you'll experience engine strain. This is why many of us who choose to use small cc engines use a shift kit. The hour counter is helpful for maintaining maintenance schedules.

Not sure what size wheels you're using but with your current ratio at 7000 rpm you should have a speed of 21 mph on 26" wheels. Using a 32 rear wheel sprocket on 26" wheels will give you a speed of 28.9 mph.
 
Last edited:

VMaxer

New Member
Jul 28, 2020
12
9
3
50
So, it sounds like this transmission wasn't meant for a 4-stroke, but, for what I paid, I will make it work. I found a 17 tooth sprocket that I can fit on the output shaft. That will certainly help. I actually put this motor on a 700 hybrid bike, so the wheels are a little bigger than 26", that will help , too. I really need to get a tach and keep the rpms at 5000 or below. One of the pads came unglued from one of the clutch shoes, so I'm waiting on a new clutch and the 17 tooth sprocket before I can experiment any further. Thanks for the information...
bike1.jpg
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,661
237
63
So, it sounds like this transmission wasn't meant for a 4-stroke, but, for what I paid, I will make it work. I found a 17 tooth sprocket that I can fit on the output shaft. That will certainly help. I actually put this motor on a 700 hybrid bike, so the wheels are a little bigger than 26", that will help , too. I really need to get a tach and keep the rpms at 5000 or below. One of the pads came unglued from one of the clutch shoes, so I'm waiting on a new clutch and the 17 tooth sprocket before I can experiment any further. Thanks for the information...
View attachment 105341
Your ratio will rise to 15.16:1.

That should solve your problems.

5000rpm will then net you almost 26mph.
 
  • Like
Reactions: Tom from Rubicon

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,468
306
83
59
Rockwood, TN
So, it sounds like this transmission wasn't meant for a 4-stroke, but, for what I paid, I will make it work. I found a 17 tooth sprocket that I can fit on the output shaft. That will certainly help. I actually put this motor on a 700 hybrid bike, so the wheels are a little bigger than 26", that will help , too. I really need to get a tach and keep the rpms at 5000 or below. One of the pads came unglued from one of the clutch shoes, so I'm waiting on a new clutch and the 17 tooth sprocket before I can experiment any further. Thanks for the information...
View attachment 105341
I'm trying to figure out what 49cc 4 stroke engine you have that limits you to 5000 rpm. Most of the small 49cc 4 strokes the max horsepower rpm is 7000 and the max torque rpm is around 4500-5000 rpm. The centrifugal clutch normally engages around 3500 rpm. Idle rpm is around 2800-3000 rpm. You can get a techometer/hour counter on eBay fairly cheap.

On 700's at 5000 rpm the 44 should give you a speed of 15.9 mph. At 5000 the 32 should give you a speed of 21.9 mph. At 7000 rpm the speed on the 44 would be 22.3. At 7000 the speed on the 32 would be 30.6 mph.

As you can see the reductions would be appropriate for an engine whose max horsepower is around 7000 rpm. Only 3 things comes to mind as to why your engine is only going to 5000 rpm where max torque is. It's a very poor quality clone and/or you have lots of steep hills and/or (you're like me) a large person.
 
Last edited:
  • Like
Reactions: Tom from Rubicon

VMaxer

New Member
Jul 28, 2020
12
9
3
50
I guess I'm just not that familiar with small engines, so I figured for constant cruising I'd shoot for 5000rpm. I guess with the vibration and everything, maybe it just seemed like it was really reving high. I ordered that tach and that will give me a better idea. I just didn't want to blow this thing up before I get to have some fun with it!!! My only reference is my VMax and its a just a little faster than this bicycle....shft.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,661
237
63
I'm trying to figure out what 49cc 4 stroke engine you have that limits you to 5000 rpm. Most of the small 49cc 4 strokes the max horsepower rpm is 7000 and the max torque rpm is around 4500-5000 rpm. The centrifugal clutch normally engages around 3500 rpm. Idle rpm is around 2800-3000 rpm. You can get a techometer/hour counter on eBay fairly cheap.

On 700's at 5000 rpm the 44 should give you a speed of 15.9 mph. At 5000 the 32 should give you a speed of 21.9 mph. At 7000 rpm the speed on the 44 would be 22.3. At 7000 the speed on the 32 would be 30.6 mph.

As you can see the reductions would be appropriate for an engine whose max horsepower is around 7000 rpm. Only 3 things comes to mind as to why your engine is only going to 5000 rpm where max torque is. It's a very poor quality clone and/or you have lots of steep hills and/or (you're like me) a large person.
Max torque is not at 5,000rpm.

More like 2500rpm.

Who wants to run their 4-stroke engine steadily @ 7000rpm?

Not me.

On my 4-stroke, max hp is at 3400rpm governed, and higher ungoverned
Max torque is at 2500rpm.
 
  • Like
Reactions: Tom from Rubicon

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,468
306
83
59
Rockwood, TN
Max torque is not at 5,000rpm.

More like 2500rpm.

Who wants to run their 4-stroke engine steadily @ 7000rpm?

Not me.

On my 4-stroke, max hp is at 3400rpm governed, and higher ungoverned
Max torque is at 2500rpm.
Exactly what 49cc 4 stroke engine are you talking about? The Robin Subaru' s 25, 35 Honda's 35, 50 and the clones all have max hp around 7000 rpm, max torque around 4500-5000 rpm, clutch engagement around 3500 rpm and recommend idle speed around 2800-3000 rpm. It's recommended they not be pushed beyond 8000 rpm. So therefore 7000 rpm is a safe operating level for these engines. If you can't obtain 4500 rpm minimum then the engine will experience engine strain. This information is printed in the engines owner's mannuals.
 
  • Like
Reactions: Tom from Rubicon

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,661
237
63
I stand corrected.

The 49cc max hp at 6800rpm, max torque @ 4500rpm.

However, I stand on the idea that no one with a 49cc 4-stroke engine wants to run STEADILY at 7,000rpm.

Maybe @ 4500rpm.
 
  • Like
Reactions: Tom from Rubicon

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,468
306
83
59
Rockwood, TN
I stand corrected.

The 49cc max hp at 6800rpm, max torque @ 4500rpm.

However, I stand on the idea that no one with a 49cc 4-stroke engine wants to run STEADILY at 7,000rpm.

Maybe @ 4500rpm.
I have a Honda 50 and a Robin Subaru 35, I change the oil every 25 hours and clean the air filter every 10 hours of use. Both have held up for 10 years now. I shift to a gear where full throttle gives me 7000 rpm. Thus I maintain the highest average speed possible. All I've ever had to replace is the fuel pump bubble on the Robin 35. On the Robin Subaru 35 I don't have to manually shift that bike shifts 7 gears automatically.
 
  • Like
Reactions: Tom from Rubicon

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,661
237
63
With 7 speed, your engine is not spinning steadily at 7000rpm, like a single sprocket would be turning.

My first engine was a Subaru 35cc on F/D. Its fastest ride was 26mph on a slight decline.

I don't think I was spinning 7000rpms.
 
  • Like
Reactions: Tom from Rubicon

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,468
306
83
59
Rockwood, TN
With 7 speed, your engine is not spinning steadily at 7000rpm, like a single sprocket would be turning.

My first engine was a Subaru 35cc on F/D. Its fastest ride was 26mph on a slight decline.

I don't think I was spinning 7000rpms.
With the auto shifter if the tech drops below 6000 rpm it down shifts. If it goes above 7400 rpm it up shifts. So at full throttle it stays around 7000 rpm unless going down hill (have to throttle back) or going up 30% grade hills (have to pedal assist). I'm 6'2" and 250 lbs. The bike weighs around a 100 lbs. Which is why I must use max horsepower.
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
2,026
4,290
113
70
Rubicon, Wisconsin
I stand corrected.

The 49cc max hp at 6800rpm, max torque @ 4500rpm.

However, I stand on the idea that no one with a 49cc 4-stroke engine wants to run STEADILY at 7,000rpm.

Maybe @ 4500rpm.
I don't know but I expect there is a correlation between these tiny engines and Post war Harley engines. The Harley manual dictates regular periodic closing of the throttle to pull oil up into the cylinder and rings. Pretty much applies to any air cooled engine.
 
  • Like
Reactions: Sidewinder Jerry
May 22, 2020
68
42
18
37
Are you still working on this? As I most likely have the same engine/transmission I did have a similar issue on my latest build. I have a 40 tooth rear sprocket and was able to get it to 20mph. When first taking it out, I was struggling to stay at around 16 mph. The fix? I adjusted the high-speed:confused: needle on the carburetor and gained a good amount of speed on it. I have another bike to compare it to so i noticed the sluggishness immediately.