Gear reduction for 125cc lifan?

[TheNecromancer13]

So I am building a 125cc lifan bike, and I am trying to figure out what a good gear reduction would be. The engine itself has internal gear reduction, 4 speed. 1st gear is 2.833:1, 4th gear is 0.958:1. I was thinking about 12.5:1 gear reduction after that, which if my calculations are correct would give me a top speed of a little over 60 mph at 9500 rpm in 4th gear. This all sounds good on paper, but I got to thinking that my single speed 79cc predator bike runs a 13:1 gear ratio, and still has lots of torque. So I did the calculations for first gear and that would give me a ratio of 35.4:1, which sounds like the kind of gearing that would make me wheelie as soon as I gave it a little gas. So I guess my question is; am I gearing too low?

 

[WECSOG]

You can't equate that engine to a 79cc HF engine. The 79cc engine is designed to make peak power around 4000 rpm and peak torque around 2700-3000. The 125 Lifan makes peak power at about 7500 rpm and peak torque around 5500.
Gearing it to do 60 mph at 9500 rpm sounds like a good idea.

 

[Tony01]

Decide what speed at what rpm you want to run in 4th gear and figure from there. Well the speed is figured already, it won't exceed 60 maybe it will do 65 at most. So whatever rpm you wanna run at 65 in 4th gear.. 6500rpm it's a 7.7:1, 9000rpm be around a 10.7:1.

Also you probably wanna run only one chain to the back with no jackshaft, so consider doing a 6:1 like 10 to 60t and a smaller rear like a 24". Just sounds like with that 4th gear overdrive with a 26" tire you'll never get out of 3rd gear. And that's fine too.

Btw here's a similar build I found on the other popular mb site:

 

[WECSOG]

Those engines have primary reduction, too. Probably 3.72:1, because that's what the Hondas they are based on have. That would make it 3.56:1 in high gear. The engine likely comes with a 14 tooth countershaft sprocket. I would probably try one of the 44t sprockets that MB engine kits come with; that will be close. If it's too low you could put in a 15t or 16t counter sprocket. Cheap and easy.

 

[Tony01]

Well with that new info I suggest a top gear ratio of 9:1. I thought about it some more and that'd be about a 35t rear sprocket for 7000rpm @ 60mph.

 

[WECSOG]

That sounds about right. A Honda CT70 uses a 35t rear sprocket for about 40 mph with a shorter tire, approximately 17 inches.

 

[TheNecromancer13]

Those engines have primary reduction, too. Probably 3.72:1, because that's what the Hondas they are based on have. That would make it 3.56:1 in high gear. The engine likely comes with a 14 tooth countershaft sprocket. I would probably try one of the 44t sprockets that MB engine kits come with; that will be close. If it's too low you could put in a 15t or 16t counter sprocket. Cheap and easy.

The gear reduction for the engine I ordered is as follows: 1st 2.833:1, 2nd 1.706:1, 3rd 1.238:1, 4th 0.958:1. It has a 14t sprocket.

So if I understand correctly, you and tony are suggesting that I do a direct drive from the engine to the rear sprocket with something like a 44t rear, with no external jackshaft? Would that give me a ton of torque in first gear, as in considerably more than I get from my 79cc predator geared 13:1?

 

[WECSOG]

The gear reduction for the engine I ordered is as follows: 1st 2.833:1, 2nd 1.706:1, 3rd 1.238:1, 4th 0.958:1. It has a 14t sprocket.

So if I understand correctly, you and tony are suggesting that I do a direct drive from the engine to the rear sprocket with something like a 44t rear, with no external jackshaft? Would that give me a ton of torque in first gear, as in considerably more than I get from my 79cc predator geared 13:1?

Yes. These engines are designed and set up to directly drive the rear sprocket. There is absolutely no reason to run a jackshaft. Just look at the small motorcycles that use them, and duplicate that.

 

[TheNecromancer13]

Yes. These engines are designed and set up to directly drive the rear sprocket. There is absolutely no reason to run a jackshaft. Just look at the small motorcycles that use them, and duplicate that.

Upon doing some further research I discovered that I had forgotten to include the primary gear reduction of 67:18 before the transmission in my calculations. Upon redoing everything, I have discovered that using a 44t rear sprocket should give me a top speed of about 65 mph at 9500 rpm.