feel no pick up from 3rd to 4th gear on a Lifan 125cc motor

bvyka

bvyka

New Member
when running the bike and hitting 3rd gear, it runs good. hitting about 43 mph. if i hit 4th gear on it I feel no difference. i want to be able to hit 55 or 60 mph and mostly all my riding will be street. 8 mile to work and back home. right now its running 29/16 with the 10" wheel. I've been talking and asking and have been told to change my gearing. ok so I've been told to go 38/17(2.23), 36/16(2.25), or 35/15(2.33). since l am running a 29 tooth rear sprocket right now I've seen and been told its much easier to change out the front sprocket... right now it has a 29/16(1.81) why can't i drop the front to a 13 tooth and change my ratio to (2.23)? the final ratios are all most the same now with all the above mentioned. what will be the problems or issues with what i want to do? other than chain length what are the differences in all those combs if the final ratios are almost the same. thanks in advance for all the help!!!
 
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kirrbby

kirrbby

Well-Known Member
So your motor DOES go into 4th tho, right??

I've run 32 tooth rear sprockets, with 16 and 17 tooth fronts for big motors. I think if you get a 15 and a 14, you'll like one or the other. I'm thinkin 13 is too small.

29 tooth on the rear is a small sprocket IMO. I thought 32 was about the minimum to keep the chain from damaging the hub, on a CT70.
 
bvyka

bvyka

New Member
kirrbby said:
So your motor DOES go into 4th tho, right??

I've run 32 tooth rear sprockets, with 16 and 17 tooth fronts for big motors. I think if you get a 15 and a 14, you'll like one or the other. I'm thinkin 13 is too small.

29 tooth on the rear is a small sprocket IMO. I thought 32 was about the minimum to keep the chain from damaging the hub, on a CT70.
Click to expand...
Yes it does goes into 4th. I was trying to go with the 13t just because of the ratio. I don't think it matters what sprockets you use if the ratio is the same, right?
 
kirrbby

kirrbby

Well-Known Member
Yes I think that's right.
Different engines have different ratios built in too.
You need to start with your engine's specs. You need to know your target. Then choose the sprockets to hit that target.
 
kirrbby

kirrbby

Well-Known Member
I've always had someone who would give me my target. (racerx)

He would give me a MPH per thousand RPM target...7mph per thousand...6.5mph per thousand, etc.
this was based on his experience with similarly configured engines.
 
bvyka

bvyka

New Member
Those were some good articles. I'm going to get the model number of my L 125 so that I can start with the specs that it has. I'll post when I find out
 
bvyka

bvyka

New Member
This is what I found. The numbers are the same on my motor only difference is mines is one down 3 up. Other than size of sprockets and chain length what's the difference between these two set ups? 36r/16f=2.25 & 29r/13f=2.23? Thanks!!!
 

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preventec47

Member
Not mentioned is some theoretical targets. If you understand this then you will know
when you are approaching optimum.
1. Your max speed should be at the same rpm as max engine hp. if you are not running
the exact rpm when you are maxed out on level ground then your gearing is non optimum.
Get a spark plug generated RPM indicator and use a GPS for true ground speed
and take some recordings at different gear ratios. Like most ( and me ) we rarely
know what is the max engine rpm so we just fall back to see what particular setup
of sprocket sizes results in max speed. Be careful not to redline far above your peak
hp rpm as all you are doing then is wearing out your engine. In my BMW car, my top speed
is identical ( meaning hp is identical ) in both 4th and 5th gear. That means in 5th gear I
am way past peak on the HP curve. It should be noted however with the car the objective
is different and that the priority is max economy at hwy cruise speed in 5th gear.
 
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