Q. Which contra spring should I use?
First off, let's look at the duties a contra spring (a.k.a. torque spring, main spring) has. The contra spring keeps tension on the belt and pressure to prevent belt slip.
Pressure is the simplest of the two ideas. The spring presses against one half of the rear pulley, pushing it toward the other half of the rear pulley and applying pressure to the sides of the belt. Stiffer springs put more pressure on the pulley and ultimately the belt which can be useful if you're experiencing issues with belt slip. Belt slip symptoms may include excessive belt wear and dust as well as black markings on the pulley drive faces.
The contra spring creates tension on the drive belt by pushing the rear pulley closed and trying to keep it closed. This keeps the belt in the outermost position in the rear pulley that it can be in given the variator's state of opening. As the variator closes to allow the belt to travel toward it's outer edge while accelerating, the rear pulley is opening to allow the belt closer to it's center. The opposite occurs on deceleration. It's a balancing act. Too much spring pressure or not enough roller weight and the variator won't be able to counteract the contra spring to open the rear pulley so the CVT can "shift" ratios and RPM would rapidly rise. This would be sort of like what would happen if you left a car in 1st gear and never shifted. Not enough spring pressure or too much roller weight and the pulleys will open and close prematurely during acceleration, "shifting" into higher ratios too quickly causing low RPM and poor performance. This would be sort of like trying to take off in 5th gear in a car instead of 1st gear. The 1st gear and 5th gear analogies are a bit extreme related to what most will experience with minor tuning problems, but hopefully they help to get the idea across.
Conversely, when you decelerate the contra spring pushes the belt back toward the outer edge of the rear pulley and therefor to the center of the front pulley. Basically it's "downshifting" the CVT. Again this is a balancing act, but a stiffer contra spring may allow the CVT to change to a lower ratio more quickly when you let off of the throttle or RPM drops from some type of load such as climbing a hill. This is why stiffer contra springs are often recommended when someone has issues related to going uphill, though sometimes lighter roller weights will resolve the issue without a need for changing the spring.
In addition to pulley opening and closing issues, soft springs can cause the belt to flop around because of the lack of tension. If you see your belt has excessive play (not caused by improper belt length or other pulley issues) then your contra spring is too soft. This can happen over time so it's not necessarily a sign that you need to put it a very stiff spring, but may be telling you it's time for a fresh replacement contra spring. Service manuals sometimes list spring heights with service limits to determine when they should be replaced. It's not a bad idea to measure a new spring before installation and keep a note of it's length for future comparison.
Briefly summed up :
Stiffer Contra Spring = More Belt Tension & Pressure + Higher RPM
Softer Contra Spring = Less Belt Tension & Pressure + Lower RPM
In many cases a stock contra spring will work fine along with swapping rollers. This is often the simplest solution to basic CVT tuning because it's easier to remove/install the variator than to remove/install the rear pulley and disassemble/assemble it. If you aren't experiencing belt slip or excessive slack, I would try changing roller weights first to see if your problem is resolved.
If an upgrade to a stiffer spring is needed a 1,000 or 1,500RPM spring usually does the job. I prefer to use the softest spring that keeps enough pressure and tension on the belt to avoid slip and slack. If the spring is stiffer than it needs to be you may have trouble getting full belt travel out of the CVT even with heavier rollers without affecting acceleration. Using heavy rollers to counteract a very stiff contra spring, the engine may not have enough output to overcome the effects of both. Stiff springs also make servicing the CVT a little more difficult because the rear pulley is harder to compress.
Keep in mind that it's very likely you will need to do some roller/slider tuning if you swap the contra spring due to their dependency on one another to create a balanced system.
Related Info :
CVT Operation Video
Full Service CVT Inspection
Rear Pulley Disassembly Technique
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