I'd start with reaching out to RacingBros directly and asking them for a recommendation. Tell them what issue you're trying to address, your weight in riding gear, the bike model, and what type of riding you do.
I have the
Ohlins fork spring kit which came with a set of 5.0 N/mm rate springs as well as 6.0 N/mm rate springs. The manual basically said to run both 5.0 N/mm springs for a soft feel, mix one 5.0 N/mm spring with one 6.0 N/mm spring for a medium feel, and run both 6.0 N/mm for a hard feel. I'm running both 6.0 N/mm springs for a hard feel and it's plenty comfortable for me. My CT125's OEM springs were not marked with a spring rate so I don't know what rate they were, just that they were a Showa part. The other differences with the aftermarket fork spring kits are that most include a PD valve, and preload adjustment at the cap which are the more significant upgrades from stock rather than spring rate. Other than RacingBros and Ohlins, YSS also have a fork spring kit for the CT125. There are a few reviews of people finding the YSS fork spring kit too soft on
YouTube and on
this forum. I think the YSS kit only comes with one set of dual rate springs, could be mistaken in my memory though. It was also
confirmed that fork springs from the crf100/xr100 fit the CT125. You can change the oil weight in the forks to change rebound speed - thinner oil for quicker rebound, thicker oil to slow it.
Pulling the springs can be done with a strong magnetic pickup tool, so if you wanted to try different springs, you don't need to pull the fork legs to change the springs which makes it a lot easier of a task, though still messy with oil.
