Originally posted by Grover
It is the various types of bikes..
Heavyweight is pretty much liter class bikes
Middleweights are 600s
LIghtweights are I4 up to 400cc, 250 cc 2 strokes and 650cc VTwins, and all buells
UltraLightweight is I4 up to 400cc, 125cc 2 strokes and buells up to 1000cc (which is damn near cheating).
Then there are the 'fun' classes, like supertwins (personal favorite), thunderbike, formula 40 (for the more seasoned folks).
The rulebook has the breakdown, but it's not the most fun reading.
Actually to correct that a bit,
Heavyweights are 750cc I-4s and 1000cc twins
Unlimiteds are literbikes
MIddleweights also include 750 twins.
And Ultralightweights are superbike classes only.
Also the 4cyl rules are as follows for ULSB,
4 cyl liquid cooled pre 1987 up to 570cc
4 cyl liquid cooled 1987 to 1992 up to 500cc
4 cyl air cooled up to 750cc
BTW you can ask Kevin Elliot for clarifications on legality of certain bikes. Thunderbike states in teh rules that 600cc I-4s from 1988-1992, but he OK'd all F-2s and older F3's and up yo 1998 FZR600s due to them being virtually unchanged over the years.
And if you do get a ruling from him, make sure you print it out and take it with you. (I keep my rulebook at teh track with me at all times when racing along with a copy of that email).