it was adrian that changed the size of the rear cylinders. i have got 9 inch drums on mine and it went through without any modifications what so ever.

tyre diameter must play a large part in this as the larger diameter is going to lessen the braking effect, also tyre width is going to affect tyre lock out in the rollers. Is it the guys running the smaller tyre diameters that are getting the locking rears?
Bob