[Fluff]

Chris ,many thanks for a concise answer ,I expect never racing at tip level lead me to wonder why such a rim was developed .I fully understand that the increase in diameter for more speed ,this is used in drag racing but the concave bit in the corners I found a bit of a puzzle .I raced in the 60 ,s but only at club level and have never heard of injecting tyres with air before .We were still sticking tyres to the track with wintergreen .

Steve