Sprocket Sizes ???????

[JoshRandall07]

Hey guys i am doing a 125 Lifan swap in my CL70. I would think my power should almost be double over the factory motor so i was thinking about going with a taller gear ratio to increase top speed. Can you guys throw out some ideas? The 125 came with a 14T on it so i would like to stay with that. I saw a suggestion on one thread to use a 39T but he didn't say what he was running on the front. What kind of speeds would i be looking at with this bike. It is a kick only 1 down 3 up BTW.

 

[69ST]

Your motor should pull gearing as tall as ~7mph/1000rpm and going much lower than what it can pull, in top gear, won't do much except have it spinning more rpm as you cruise down the road. The math is pretty easy...if you know the overall crankshaft-to-countershaft ratio and the tire circumference. For that, you'd either need to have the primary & 4th gear ratios, or the teeth counts of the two gearsets.

Short of that, it's guesswork for the start, followed by trial & error test rides. There have been many iterations of the L125 motor, over the years. With stock Trailwings, you'll probably need something between 15/35 and 17/33.

 

[JoshRandall07]

Thanks for the start. Any reason not to go with the 14T that is on it? If i did a 14/33 that would be the same ratio at the 15/35.

 

[JoshRandall07]

Racerx, i found the 4th gear ratio. Can you take me through a math lesson. 4th gear is .958:1 Other figures are 3.722 MAX net power & 6.5kw/7500r/min
Inflated dia 22.83 = 71.722cir.

 

[69ST]

Okay...

Here's the basic formula, once again, for those who may have joined relatively recently: (.006 x wheel radius)/final drive ratio = mph

With a little basic algebra: (wheel radius x 6) / final drive ratio = mph @ 1000rpm

For your specific engine & tire combo: .958 (4th gear ratio) x 3.722 (primary drive ratio) = 3.56. Tire radius = 11.415. Thus: (11.415 x 6 = 68.49), 68.49/(3.56 x sprocket ratio) = mph per 1000rpm.

Plugging-in your proposed 14/33 combo: 68.49/(2.35 x 3.56)=8.16mph per 1000rpm, ~1.13x beyond the suggested 7.00mph/1000rpm starting point. Now, you can easily work with percentages.

14/33 = 2.357
2.357 x 1.13 = 2.66

Sprocket combos that produce exactly 7.00mph/1000rpm:
14x2.66 =37.24
15x2.66 =39.9
16x2.66 = 42.56
17x2.66 =45.22​

Since no one has yet invented a viable sprocket with fractional teeth, you'll have to decide on the "best" combo. 7.00mph/1000rpm is a semi-arbitrary value I've used, for mathematical simplicity. The best actual sprocket combo, for your bike, may be different. I'd want to be within +/-.2mph per 1000rpm of what the engine can pull. Any taller and it'll feel really weak with light headwinds and when ascending slight grades. Any shorter, and the engine will spin a few hundred rpm faster than it really needs to...which can make more of a difference than you might think...especially as you accumulate saddle time.

 

[OLD CT]

Any reason not to go with the 14T that is on it?

Added chain wear. 15-35 will provide longer chain life.

 

[69ST]

OC is on the money. 16t would be even better. The bigger the C/S sprocket, the less chain wear.

 

[JoshRandall07]

Nice job racerX, thanks for taking the time to do that. Good stuff. Old CT thanks for the info also.

 

[Drew]

OC is on the money. 16t would be even better. The bigger the C/S sprocket, the less chain wear.

I used a 16/ 35 combo on my Lifan 125,anything smaller in the rear and it ran out of power in 4th gear,you should go 54 mph on a CT 70 with the Trailwing tires,with a good carb maybe 59 mph.

 

[69ST]

I am very careful when recommending specific sprocket combos. I've seen Chinese 110 & 125 motors run combos ranging from 15/35 to 17/33...and things may have changed even further in recent years. The explanation is differences in primary & tranny gearing. That's why I base my numbers of mph/1000rpm, in top gear. The L110/125 motors mostly run out of breath around 8Krpm, unless someone has spec'd them with higher static CR and more bumpstick.

For road use, I like going as tall as possible, to keep the revs as low as possible. As the miles go by, even a 300rpm difference in noticeable. FYI, I define the limit (of how tall you can gear the machine) as the point where top gear becomes noticeably weak. It only takes about 3% beyond what the engine "wants" to match its torque peak. You can be quite a bit further off, on the short side, before you'll lose speed but the motor will be a lot "busier". If you want to test different combos, keep going taller, one tooth at a time at the C/S until the bike stops gaining speed; that should get you within one tooth of optimal, at the wheel...where a 1-tooth change "splits the difference", ratio-wise.

 

[Drew]

Got my 18 tooth front sprocket installed on my YX 140. Was looking to lower my rpm at city cruising speed,and it did help quite a bit.I can cruise along at 45 mph and it only shows 3,450 rpm and at 35 mph it shows 2,640 and that is the most often speed limit around my town.The 18 tooth sprocket with the chain will rub on the stator/sprocket cover,with ALITTLE modification it works fine.A grinder or a pneumatic cut off wheel will work.

 

[69ST]

Something here doesn't smell right. 13mph/1000 rpm...?!!! With 140cc, optimal gearing should be in the 7.5-8.0mph/1000rpm range, unless there's something major, fundamentally, different about this motor. Assuming a power peak around 8000rpm, for a mild, OEM-level, tune that'd translate to a top speed of ~104mph; that ain't gonna happen, nohow! If you're happy with the setup, then go ahead, trim the flywheel cover and enjoy. Take the bike out on the road, someplace where you can safely take it up to, say, 60mph. Assuming that your tach readings are accurate, I'd expect the motor to fall on its face, under anything less than prime operating conditions, a.k.a. a decline plus a healthy tailwind. What size sprocket & rear tire are you running?

 

[Drew]

Something here doesn't smell right. 13mph/1000 rpm...?!!! With 140cc, optimal gearing should be in the 7.5-8.0mph/1000rpm range, unless there's something major, fundamentally, different about this motor. Assuming a power peak around 8000rpm, for a mild, OEM-level, tune that'd translate to a top speed of ~104mph; that ain't gonna happen, nohow! If you're happy with the setup, then go ahead, trim the flywheel cover and enjoy. Take the bike out on the road, someplace where you can safely take it up to, say, 60mph. Assuming that your tach readings are accurate, I'd expect the motor to fall on its face, under anything less than prime operating conditions, a.k.a. a decline plus a healthy tailwind. What size sprocket & rear tire are you running?

Yes I have several things that I can't figure out.Yes my tach is very cheap POS,but it seems to be pretty accurate at idle.But I can't hardly get it to go over 5,200 when in WOT and in 3rd gear.Im hitting 62 mph in 4th at 4,800 rpm according to my tach,if you wanna call it that lol.I use a 35 tooth rear.The bike runs out of breath,not hitting redline at top speed,but I'm also still using the carb that came with my Lifan 125 kit- l actually had to turn the air fuel mixture screw almost all the way closed for this motor to run right.This motor uses a lot less gas than the 125 engine.I wish someone could set me up with a real carb for my motor.I have bought 4 differnt cheap carbs,but the one I'm still using seems to work best.

 

[69ST]

Yes I have several things that I can't figure out.Yes my tach is very cheap POS,but it seems to be pretty accurate at idle.But I can't hardly get it to go over 5,200 when in WOT and in 3rd gear.Im hitting 62 mph in 4th at 4,800 rpm according to my tach,if you wanna call it that lol.I use a 35 tooth rear.The bike runs out of breath,not hitting redline at top speed,but I'm also still using the carb that came with my Lifan 125 kit- l actually had to turn the air fuel mixture screw almost all the way closed for this motor to run right.This motor uses a lot less gas than the 125 engine.I wish someone could set me up with a real carb for my motor.I have bought 4 differnt cheap carbs,but the one I'm still using seems to work best.

Thems is classic symptoms of being over-geared. Your mph/1000rpm numbers, in top gear, seem consistent enough that the only way I'd be questioning tach accuracy is in the base calibration, i.e. how it's reading the ignition pulses. IOW, if there's an error, it's a consistent error, across the rev band. And it's a big deviation from expected values. I'd have guessed optimal sprocket combo numbers in the 1.8-2.1 range...18/35 falls right smack in the middle, at 1.94(!). Figuring out what you actually have is the enigma. Short of doing a comparison with a known accurate tach, you're down to trial and error...unless the vendor can supply you with primary-through-4th gear tooth counts and accurate dyno numbers, showing the torque & hp numbers, with their respective rpm points. Where does engine rpm peak in first & second gears?

For now, I'd test 17 & 16t C/S sprockets. Based on what you've reported, thus far, one of those should be very close to optimal...close enough that a single tooth change (or the equivalent) of the wheel sprocket will dial-in the gearing quite accurately.

As for your carburation circus, this illustrates the difference between price & cost. A real VM20, with the necessary pilot & main jets one could reasonably expect to need is sub-$100. Ask yourself, "was there we enough potential cost savings to be worth all of that hassle?"

 

[kirrbby]

Thems is classic symptoms of being over-geared. Your mph/1000rpm numbers, in top gear, seem consistent enough that the only way I'd be questioning tach accuracy is in the base calibration, i.e. how it's reading the ignition pulses.

X2 You should read thru the instructions for your tach. There is something that has to be set, depending on your engine type.
Our motors...I think actually fire on every other revolution of the crank...maybe im wrong. BUT, I think your tach is only giving you 1/2 of the actual RPM's of your motor.

OR...you're way over geared.

 

[kirrbby]

I may have said that wrong.
Our motors spark once per revolution, but only fire on every other revolution.
Your tach doesn't know how many cylinders your bike has, etc, unless you tell it.

 

[Drew]

I may have said that wrong.
Our motors spark once per revolution, but only fire on every other revolution.
Your tach doesn't know how many cylinders your bike has, etc, unless you tell it.

The tach seems very accurate at idle about 1,200 rpm and much less and it don't seem to want to stay running.

 

[kirrbby]

Does your tach look like this one?


I know I had to change the setting on mine to get it reading right. I think it was those S1 S2 buttons.

If you rev your motor in neutral, to the point that you don't want to rev any higher, what says your tach?
9000? 10,000? Or 5000?

 

[69ST]

The only scenarios under which I can see getting 1/2 of actual rpm would be is the tach is set for "waste spark" (spark with each crank revolution) when there isn't one...or the tach defaults to multiple cylinder mode, with the trigger coming from the HT lead, which seems very unlikely.