Monday, August 2, 2010

Variator Tuning

As I described earlier, the Shanghai Shuttle can't quite hit the power band of the new pipe. This is because the variator is opening too quickly, making the gear ratio too tall to allow the bike to get into the pipe's power band. In order for the bike to hit the power band in the highest gear ratio of the variator, the variator must gradually open throughout the range of the power band.

A variator works on the principal of centrifugal force. As the rpm of the crank (and thereby the variator) increase, three weights (in a Honda Hobbit) are forced apart on ramps which in turn force the variator to close. As the variator closes, the belt running between the two angled plates is forced further out on the pulley. This changes the gearing of the bike from short to tall, or from low to high.

The picture on the right shows the variator in the closed position during high rpms. Notice how the rear variator, described in the picture as the "driven pulley," is open. This variator does not contain weights but rather a spring which is tuned to take up the slack of the front variator or "drive pulley". The benefit of a variator is that you essentially have an infinite number of gears between low and high gear which means, on a well tuned bike, maximum torque throughout; from low to high end.

However, if a) the variator closes too quickly the bike will drop out of the power band and be unable to reach maximum rpms, or b) the variator changes too slowly the power band can only take place within a limited range of the variator, meaning that you could top out well below your top gear.

The solution to this is to tune the variator to close later by lightening the weights in the front variator or by putting a stronger spring in the rear variator. If done correctly not only will you shift later, but the shift or variation will occur over a much wider range. Today I started by lightening the weights. There are three weights in a Hobbit variator. I weighed each of them on a triple beam and the average was about 14.8 grams. I found a drill bit that looked about right and drilled out the center of each weight taking off approximately 5 grams. 

The disassembled variator with a weight on the scale.

Drilling out the weight

A comparison of the weights before and after lightening.

With the freshly tuned variator back on the bike there was a noticeable difference. The variator opened much more gradually and wound up into the beginning of the power band at around 35 mph, but once again, shifted too soon to carry the bike through the variator while in the power band. As the picture above shows, there isn't enough material to take out any more weight. The next option is to either find lighter aftermarket weights, or even better, to find a stiffer rear variator spring to keep the variator from closing so soon. The good news is that there was a definite improvement, and there is plenty of room for more modification. Upgrading to a larger and more tunable carb will also really help. I took the bike out today for a trial run, and even without hitting the power band I was able to make high forties. On the downhills I could get into the power band and that was fun. It will be interesting to see how this Cali pipe performs on a variated bike once it's dialed in.

Variator images courtesy Just Gotta Scoot.