No sure how Harley spec'd the frame rake. Between my buddies and me, I have access to about fifteen FXR frames from the shovel years to the 2000 FXR4, and a variety of models. About an hour ago I had an 83 FXR, 91 FXRP and my own FXR Conv in my garage.
I measured the fork angles but then realized it didn't mean $hit because none of the bikes were stock. All them had different suspensions, wheels, etc, that would affect the measured neck angle.
What I did measure was the the angle between the neck and the bottom frame rail. With the bikes on my table lift with the front wheel clamped in the vice I measured the neck angle and the angle of the lower frame rail and then figured the difference.
All three frames had the same angle between the lower frame rail and the neck.
Logic tells us that the differences in claimed factory specs betwen the various FXR models can be attributed to the differences in suspension packages. My science tells me the frames are the same geometry.
What Ed is saying, I believe, is that if you raise the front end (taller front tire, longer downtubes) or lower the rear (shorter shocks), you increase the effective rake, as measured from vertical. He's saying that the frames were all the same, and the FXR's listed as having 31 degrees of rake simply had a different suspension geometry, with the front slightly higher than the rear.
Both of my FXRs have stock size rubber and stock length suspension.
I wonder is the there is a difference between the two rake wise.
I know that I still can't believe they are the same model bike. Completely different personalities, yet interchangable parts, the best of both worlds.
People think the FXLR is a custom wideglide, and mine must just be some "odd ball sportster".
So the FXRS-SP, FXRS-CONV. are 31 degrees, so I would assume the FXRP is also 31 degrees since it's based off the Convertible. However, it doesn't make sense that by lengthening the fork it would change the rake since the rear shocks are 1" taller too, which I think would cancel out the longer fork.