You could stack them as long as everything is captured by the oem spacer. I think that is the issue. Just a wag. And, the oem spring has different size each end. The small end is 2.25 but the larger is 2.4ish so going with the 2.5” spring allows it to fit the larger ES adjuster end and the fabbed spacer is at the other end. Don’t think the 2.25 will fit over the adjuster end. Again, just a wag. And we are talking ES?
Ok, that makes sense thanks, and yes we are talking about the ES model.
Although maybe somebody that has the old direct fit RT spring could chime in. I'm fairly certain the id was 56.5mm, and it seemed to fit very well, although it seemed a little bit too long at 8.75" (222 mm) and did result in high initial preload which in my mind is not really desireable. I think that's why some of the guys that went that route used a spring which was only about 10% stiffer than stock. The extra preload raised the sag to the right level even though the spring wasn't really that much stiffer, and it makes sense that the guys that went with the 8" spring usually chose something at least 20% stiffer than stock because with the custom spacer the initial preload is less than with the longer spring.
My take on it is that if you can keep the initial preload close to the stock amount then the stronger spring will ramp up quicker through the travel, which should result in better control and better sag values while retaining a fairly plush ride on the road.
An 8.25" spring would be perfect but it doesn't seem like that exists. I will likely end up going with the tried and proven method of the 2.5"id 8" spring with a custom spacer, but I'd RATHER use a spring that can go in directly and not need the custom spacer if that makes sense.
Sorry I'm late to the party, I only got my new ST about a year ago so I'm a noob with these bikes.