CVT Drive Tuning

CVT Drive Tuning


Tuning your automatic scooter

So you just bought a new scooter and people keep telling you about changing variators, roller weights and clutch springs. Well, what is all that stuff and why should you care? You should care because you are missing out on a whole bunch of performance. If you want to leave shifter scooters and automobiles in the dust this is the best place to start. The goal of this tech article is not a step by step “how to” but rather to help you understand what’s going on under all that plastic and covers. It will serve everyone best to do it this way because anyone with a variable belt drive scooter can benefit from it. Next time you visit your favorite mechanic you can chat about drive modifications with confidence.

The variable belt drive is very simple in the way it works. You have the variator, it mounts directly on the crankshaft which rotates as your engine runs. The faster your engine runs the faster the variator spins. The belt drives the driven pulley/clutch assembly. The faster the belt spins the driven pulley and clutch, the more the clutch grips the clutch bell. The clutch bell is mounted directly to the final drive and drives the rear wheel. It sounds confusing right? Keep reading because it will start to make more sense as we go.

Here you can see a side view of the drive unit. We removed all the plastic covers so you could get a better idea of what’s going on. Scooters like this one have plastic covers over a support cover. This cover actually has bearings that support the weight and stress of the shafts. Not all scooters are like this some just have one main cover and no support.With the cover removed you can see the variator on the left and the driven pulley/clutch assembly on the right.

 Now things will start making sense. You can see we removed the outer face of the variator assembly to the left. If you look at the variator you can see the belt is tight around the crankshaft. From a standing start the belt rides low in the variator near the crank. The higher the RPM the higher the belt will ride on the variator. To the right is the drive pulley clutch assembly. If you look close you can see the clutch shoes, this scooter has three. They are held in place by springs. As the RPM raises the clutch shoes swing outward by centrifical force. When they reach the RPM the clutch springs are set at the shoes engage the clutch bell and off you go.

 Now you can see the backside of the variator and the parts that make the assembly. The big piece to the left is the variator and you can see the roller weights in place. We set some extra weights next to the variator just so you get a better idea. The roller weights are just that, weights. Roller weights are measured in grams. The lighter the weight the longer it will take the belt to get to the top of the variator. The weights push on the variator and ramp plate (on the right) this causes the belt to raise from near the center of the variator to the top as RPM raises. In the next picture you can see the cone shape of the driven pulley assembly. It’s just like the cone shape of the variator assembly when it’s installed.

 With the driven pulley assembly stood on its side you can see the cone shape of the faces. The variator faces have a similar profile. The operation of the faces is a little different than on the variator. The faster the driven pulley spins the lower the belt rides in the “V”. When you take off the belt is at the top of this pulley and as you accelerate the belt gets closer to the center. The two pulleys do have an effect on each other because the belt never changes its length. If the belt is at the top of this pulley it has to be near the center of the variator. The driven pulley is tunable. A stiffer pulley spring will keep the belt riding near the top longer. This keeps the RPMs up and aids in acceleration. The problem is aftermarket springs are not available for all scooters. Now we get to the fun part. I hope you are still with us.

 Remember a mention of roller weights? This is the pay-off for all your reading. If you are lucky you have a scooter that manufacturers make aftermarket performance variators for. An aftermarket variator is well worth the extra money because the roller ramps are designed with performance in mind. If you are riding a scooter with no variators available you are still OK. You can get great gains by just changing to lighter roller weights.

Here you can see the variator drive face. When you start trying lighter roller weights one thing you need to keep in mind is the RPM for the speed the scooter in traveling. Lowering the roller weight will raise the RPM over what it was at the same speed. That’s fine and it is how accelerates faster. The thing you need to watch out for is making the roller so light that the variator can’t push the belt near the top of the pulley as it should. If the belt doesn’t ride high enough your top speed can be reduced. Get the weights set correctly and you’ll cruise with less throttle and at the same or higher top speed. To help explain this we marked the face with a green Sharpie and went for a ride. You can see the belt didn’t quite get to the same height as the wear marks show it was reaching before. When you mark your variator do it from near the center all the way to the top as shown by the narrow green line.

 The scooter pictured in this article is a 400cc Yamaha Majesty (YP400). In stock form it had eight 15gram roller weights. The acceleration seemed uneven and slow. It did the quarter mile in 17.03 seconds and 0 to 60mph in 9.74. We installed eight 10gram weights and pulled off a 16.01 second quarter mile and 0 to 60 in 7.37 seconds. Our quarter mile speed climbed about 10mph! However, the 60 foot and 300 foot times were about 2/10 of a second slower than stock. From 600 feet and on the 10gram weights caught up and passed the stockers. In the end we used a combo of four 10gram with four 13gram rollers.

The result was faster acceleration across the board and a 9/10th’s faster quarter mile speed. All of our numbers were downloaded from a GPS calibrated Data logger. The runs were made in the same direction and on the same straight track minutes apart. The best run of three was used for each example. With the stock weights the Majesty had a hard time hitting 70mph in the quarter mile and with the eight 10gram rollers it hit 78.8mph!