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1.  Let me first start with the fact that I am NOT a mechanic.  I have been working on motorcycles and scooters for a long time and know a little about what I am doing.  I simply take pictures for other people to use as a guideline to help them modify their own motors.  Any modifications you perform are done completely at your own risk and liability.

2.  Tampering with your motor in any way may void your warranty.  Make sure this is a sacrifice you are willing to make before you begin working on your own vehicle.

3.  Before you begin, read this ENTIRE article from start to finish so you know exactly what is involved and what steps are required to properly complete the procedure.  Also make sure you have a nice clean area to work in.

4.  This example was performed on a 2006 Sachs MadAss 50cc automatic.

5.  ALL bolts and socket head cap screws are in metric! Don't try to work on your vehicle if you don't have metric tools, you will just wind up messing up the heads.

The stock front sprocket (Pinion) on the MadAss is an 11T 420 and the rear sprocket is a 53T 420.  If you do the gear ratio, the stock ratio is 4.8 : 1  That means for every 4.8 revolutions on the front pinion, the rear wheel will turn 1 time. 
Our New Lifan 150 came with a 16T 420 gear on it.  When we first rode the bike with the new setup, it was very obvious that the gearing was WAY too low.  The front end came up all too easy and 1st gear was very short.  10 feet and you had to shift...   Which pretty much rendered 1st gear useless.  I was actually starting out in 2nd gear most of the time.  The 16T on the front and 53T on the rear gave us a ration of 3.3 : 1
Well we searched for a new rear sprocket all over the internet.  The OEM sprocket is 108mm center hub with 123mm bolt pattern.  The only place we could find anything that would fit was from our friends over at OO RACING which they are having custom made by Talon Sprockets.  They featured a sprocket kit for the MadAss which included a 15T 428 sprocket and a 43T 428 sprocket.  So now we had to also upgrade to the larger 428 chain.  The new 15T 43T combo dropped our ratio down to 2.86 : 1

When we installed the new set, things started to get very interesting.  Tons of low end power still available, but made 1st gear much taller and very usable.  Gears were nicely together and all around had a great feel to the bike.  However, once we had the bike broken in and we started inching toward our top end range, we soon found that the bike was still geared a bit too low.  Not bad, but not quite right.  We found that running at about 60MPH, we were revving the bike to about 9000 RPMs or more.  It also felt like the bike had a bit more to give, but was simply out of our reach due to the gear setup.

So we went and purchased a 16T 428 for the front, bringing our ratio down to 2.68 : 1

Now we are getting closer to optimal performance!  Cruising at about 55-60 MPH, we are running at about 7500 RPM...   Considering our peak HP is is rated to be ~ 15BHp @ 7500 RPM.  Now if you want to push the bike a bit harder, you can achieve speeds up to ~ 70 MPH at about 8500-9000 RPMs.  But I wouldn't advise holding those revs for too long.

UPDATE! 3/15/2009
After riding the bike for a while and having a few discussion with our friends at Fast Trails Performance, they decided to get a 39T rear sprocket made.  Considering that we still seemed to have plenty of low end power, and knowing that we are running at about 55MPH at our peak BHp RPM, we decided to aim for our peak torque rating instead.  When shifting into 4th gear, you can throttle the motor and it pulls great.  Our peak torque value is in the 4000-4500RPM range.  So we wanted to aim for 55MPH in the 4500RPM Range instead of the current 7500RPM range.

So we went with the 16T/39T combo bringing our ratio down to 2.44 : 1
All we can say is WOW!  It now feels like we are in the optimal operating range for this motor!  First gear is even more useful, still able to pull wheelies with no problem.  But all the gears seem to come together in a much smoother and more pull than ever before.  The lower gearing definitely brings out the bottom end torque of this motor and and it is fantastic.  So in our opinion,  the 39T rear sprocket from Fast Trails is definitely the way to go.  you are still able to drop the ratio more by going to a 17T front, but you are also able to increase it quite a bit by going to 15,14, or 13T on the front.  So there is still plenty of customization for people out there who want to tweak it to there exact liking!

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