both the R2/4 and R5 use the same rim 18 inch 4 inch wide. I have the BT090 150 on there now and I feel it sits too flat. I can't use all the tyre even scraping fairings and pegs at the track. Getting hold of a 140 from Bridgestone seems next to impossible but is listed occasionally. My Brother had a 140 BT090 fitted to a 17/4inch rear and it profiles much better than the 150.

My next guess was the GPRa10 which list a 140 and 150 for the 18inch rear. What are guys choosing? How does the 150 profile look and work on a 4inch. Both the KR1s, MC18, and VJ21 all use the same rear wheel dimensions so must be some collective experience.

I have emailed Sean from the Tuning shop to see what he sells as the most common size so will come back to this.