Without getting technical, here is how it goes for sway bars on the 2013-2017 model Spyders. This same principal generally applies to the 2008-2012 1st generation Spyders as well.
First, there are 3 ways to make a sway bar stiffer. Increase in diameter is just 1 way, so you can't necessarily go just by size. Of course, size does matter, but only in context with the other 2 parameters. Metal compound and geometry (read 'Arm Length') can also make a big difference. The Spyder sway bars are quite small in the world of sway bars. Both in diameter and length. Because of this, just a slight change in any of these 3 parameters can make a great deal of difference in strength and function.
So, here we go!
2013-2016 RS, RSS, ST (and all F3 models). The RS and ST have identical sway bars which are much lighter than the sway bar installed on the heavier RT models. The RSS and F3 have a slightly stiffer sway bar installed from the factory. But frankly, the difference is all but undetectable, visually or in handling. The real improvement in these 2 models comes from the upgraded shocks and the fact that you can adjust the spring pre-load to suit the conditions. You can purchase this upgrade package for your RS or ST. The shocks will make a much needed improvement. But the sway bar that comes in this kit is really a waste of time.
There is a misconception that improving the shocks will compensate for a weak sway bar. While better shocks will definitely improve handling and stability, the sway bar still has a role to play which no manner of shock absorbed can adequately deal with. In attempting to do so it can greatly deteriorate the ride (stiff/harsh). Shocks and sway bar work together to give you great handling while retaining that smooth ride. It's a beautiful thing!
2013-2017 RT models have a thicker, heavier sway bar than the models listed above because of the added size and additional GVW capacity. You should NEVER put one of the lighter sway bars from an RS, RSS, ST or F3 on an RT. It will make things much worse. My sway bar for the RS, RSS, ST and F3 models is stiffer than the OEM sway bar on an RT.
You can put an OEM RT sway bar on an RS, RSS, ST or F3 model Spyder and get a noticeable improvement. However, as stated above, the bar I make for the RS, RSS, ST and F3 is heavier than the stock BRP bar on the RT and will provide additional handling stability.
The custom sway bar for the 2013-2017 RT is the stiffest bar I make because the 2013-2017 RT is the heaviest Spyder BRP makes.
With sway bars, like many other things in life, you get into the 'Goldilocks' principal. You can go too light, and you can go too heavy. You really don't want either one. Please do not install one of my heavier RT sway bar on your RS, RSS, ST or F3. Having a sway bar that is too stiff can be dangerous. A too heavy bar may work fine in good conditions. But for evasive maneuvers, high speed maneuvers, wet conditions, or just pushing the limits of handling ability, you can get some sudden and unwanted results if the sway bar is too stiff.
Putting a better sway bar on your Spyder will make a significant improvement in handling and overall stability. If you ever have any questions about any of the products I make. Or just general questions about these kinds of products, please ask. I know that an informed customer is always a happier customer, whether they purchase something from me or not.
Have a great and safe riding season! :thumbup: