While you lot are talking up tire sizes.........On this Norvin 1200 race bike I'm re-building, we can only go to a WM-3 in width of rims for that class of racing here in Australia, If I go with 18's front/rear should I run both WM-3 or a 3 on the rear and a 2 on the front.....? I prefer the looks of a 2 on the front but have been advised that for a heavier bike to go to the largest size front and rear, this given it is way heavier than a Manx. Regrettably I've had to ditch the beautiful and also new 250 mm Magnesium Fontana that was in the front end for a smaller replica 230 mm Ceriani..........Oh well, them's the rules.........Cheers.............Greg.
John Renwick, Carletom Palmer and I all ran 18" WM-3 rims front and rear on Girdraulic Vincents, which are a 6" wide fork. The big wheels certainly work fine, but provide a lot slower steering.
The width of the rim is important for the actual profile it imparts to the tire. With the Dunlop K-124 I should have used a WM-2 because I flattened out the profile enough to run at the edge of the tire too quickly. However, the Avon tires are quite round and I would have had no problems with a WM-3 front.
With race tires I would advise you to find the tire you want and and build the wheel to the tire. I like the 18's because of the responsive steering and quick line changes you can make without any drama. On the Comet, I would be happy to run the same tire size front and rear. I have no experience on a twin, but I would probably do it. If I wanted a larger tire on the rear, then you have to check to see what is available in race rubber.
Both the AM22 and the Road Rider have same size front and rear tires available in good compounds, the A49 and the 308 on the AM22 and the A49 and the 633 (633 being sifter than the 308) on the RR.
If available, and you want a bigger rear tire, try the AM22 front with the AM23 rear. Avon makes one tire that is designed to run on an narrow rim, which they call "Cantilever". It is designed for a WM-3 and is available in two compounds, the 914 being harder than the 633 and softer than the 308.
The 18" wheels, being a little more popular in racing tend to have more compounds available also.
Here is the compound scale:
I would see what is available along these lines. The compounds are quite good and personally, I would not be concerned with the grip of a same size rear, but it depends upon Tom's skill level and confidence.
David