Increasing CVT cooling

Hi,

Through a friend I have access to a fully equipped workshop and pretty much any part for chainsaws and cut-off saw there is (my friend works with developing and testing them).
And so we have been discussing for some time how to improve the rotten CVT cooling capacity of a 139QMB engine.
A number of ideas where discarded, our first idea was to use a electric blower fan to force more air into the CVT case (would most likely have worked), then we thought about simply opening up the CVT case by milling larger air inlets and outlets but eventually we decided to try and swap out the existing cooling fan.

Here is what the original looks like. The fan blades are between 10 and 6mm high, not very impressive:



The original CVT cooling sucks the air in from the perimeter of the fan through a hole in the side of the case, a very in-effective solution. This type of fan should suck the air in at the center, the centrifugal force then "flings" it outward inside the case.


Now, the CVT case is full of what we deemed to be non-essential stuff such as kick-start gears and various badly designed baffles that limits the already mediocre air flow. Here is the original:



And here is the new CVT case, from the same angle after some work in the milling machine:


Much better!

Here is a comparison between air outlets on the the old and new CVT case:


Significantly larger outlet, and no obstructions!


To replace the fan we started out with a cooling fan/magneto for a cut-off saw, and then modified it. Here is the unmodified original:



Other side


After a lot of work with lathe, drill and milling machines we ended up with this:


The back of the fan is shaped exactly to the same angle as the belt drive wheel, to get it as close as possible to it. It will also help cool the actual wheel via heat transfer. The ramp angle is 15 degrees on a standard 139QMB drive wheel, as measured by us.
Interestingly enough, the variator angle wasn't the same as the drive wheel angle on my scoot, it was measured to 11 degrees. Possibly some kind of restriction...



Here is the final result, mounted on my scoot:


Closeup. The starter does not hit the fan, even tough it looks like it might:


And here is a closeup showing how the fan fits against the drive wheel once everything has been tightened down:



After trials, the conclusion is:

• The CVT is much colder now, in fact I can easily put my hand on it after stopping from a long run at WOT, something that was impossible to do before the mod. The smell of hot rubber after stopping has disappeared.
  
• The engine seems to be running a little colder too, thanks the the improved heat transfer capabilities of the CVT and svingarm.
  
• I have lost no acceleration, but I seem to have lost about about 5km/h in top end. Engine rews remains the same as before the mod, except at maximum speed where it is slightly lower.
  
• Even though I have no air filter whatsoever on the inlet, and the CVT now acts like a vaccumcleaner on steroids, I have had no problems with belt slip because of it sucking in water when it is raining. I have ridden in pretty bad weather with no problem at all.
  
• I have a funny whining sound (almost like a turbo) coming from the air inlet I have milled in the CVT case. :-)
  
• This mod removes the kick start completely, so one have to make sure the battery and starter is in good condition, always. So far I have not been stranded...
  

Here is a picture of the final result:

Eventually I'll get around to chrome it so that it looks like the original CVT case, and get rid of the Baotian logo...

All things considered, I am completely satisfied with the mod.
Anyone looking to do the same mod on their scoot should know that machining the fan was the hardest part.
However, with a cheap angle grinder and some determination anyone could modify the CVT cover the way I did, and gain an increase in the air flow with the stock fan. At the expense of the kick-start of course.
I would not recommend a Dremel or similar tool though, unless one is prepared to spend an awful lot of time on the job (and a lot of cutting wheels). It is simply too much metal to remove...

/Otto