[JFMNINJA400]

Originally Posted by DannoXYZ

If you want to get maximum grip out of 140 tyre, you must use 4.0" wheel. Otherwise tyre-profile will be pinched and more rounded than front. Meaning when you've reached edge of front, there's still unused rubber at edge of back. You're only using 130 of 140mm tyre and only getting 130mm worth of grip anyway.

Pinched rear tyre causes nervous , yet slower turn-in with unpredictable handling. This is why many, many 400 racers go down from stock 150 to 140 rear tyre. Better handling and more grip. Kawi dropped ball and didn't spec 4.5" rear rim which is better for 150 tyre. Using 4.0" rim just doesn't work well at track for performance with 150 tyre.

In both these 300 & 400 examples, it's best to use optimum tyre-size for rims you have. In our AFM races, fastest 250s use 90/120 tyres for factory wheels.

Guess I will be getting a 4 inch used 300 rim for my 250! I do see them for 100 and less on eBay. I was planning on getting a 140 for my 400 when the 150 wears out. I am pretty much on the edge of the front 110 and rear 150 on the 400. I haven’t noticed any handling issues yet, but I am no where near the top in the race series that I do. The price for the Michelin Cup I am going with next is the same for the 140 and 150.