this is from the 2019 Torbay trial regs (out now). There's a few rules there I hadn't noticed before
3 MOTORCYCLE CLASSES
(A) Solo motor cycles of British manufacture. (Engine & Frame)(Pre 1965)
(B1) Solo motor cycles up to 225cc
(B2) Solo motor cycles over 225cc and up to 450cc.
(C) Solo motor cycles over 450cc.
(D) Motorcycles with touring or trials sidecars.
(E) 3 Wheelers (SEE APPENDIX)
(X) Motorcycles which don’t comply with Class A-D, and/or meet the descriptions below will run for Honours only (i.e. no
Championship influence).
Wheelbase less than 1320 mm Note1 (Pre 1965 bikes in Class A are exempt from this dimension)
Sidecar Outfits that do not meet the ACU Sidecar Outfit Specification. All Sidecar Outfits (Including those in Class X) Must
have a Nosecone and Padded Passenger Seat to the dimensions of that stated in the ACU Handbook for Trials Sidecars.
Note 1 - This dimension has been reduced from 1350mm to 1320mm in the ACTC Rules.
All Motorcycles must have daytime running lights, except Class A who are exempt.
PERMITTED MOTORCYCLE TYRES
Only tyres which conform to the specification of the ACU may be used.
Any motorcycle not complying with the ACU (ACTC) Tyre spec of Trials Tyres will not be allowed to start.
ACU Dimensions.
All tyres will be measured mounted on the rim inflated to a pressure of l4lbs/sq.in and must have a nominal aspect ratio of 100/100.
Overall width (O) measured at wheel spindle height must not exceed 115mm.
Refer to diagram:-
Tread depth (A) must not exceed 13mm. All tread blocks in the same circumference must be of the same depth. The space between the tread blocks (B) must not exceed 9.5mm across the tyre or 13mm in a circumferential direction (C). The space between the shoulder blocks (D) must not exceed 22m. The space across the tread (E) must not extend completely across the tyre measured at right angles to the tyre wall unless broken by a block. All main tread blocks must in principle be parallel with or at right angles in the tyre axis. (Tyre must have the same appearance if reversed and conform in principle with these diagrams). The tyre surface must not be fitted with any subsequently mounted elements such as anti-skid devices, chains, etc.
These dimensions will apply irrespective of Machine manufacturers or Tyre manufacturers statements.
In addition to above specifications Torbay Motor Club will accept tyres which have a non-square/rectangular block.
However, the space between the blocks across the tread must not exceed 12mm, and not exceed 16mm when
measured in a circumferential direction. The space across the tread must not extend completely across the tyre
measured at right angles to the tyre wall unless broken by a block
Examples of acceptable tyres - Bridgestone Trailwing 302, Pirelli MT43
Trials tyres for this event must be road legal.
Trailwing 302 tyres were considered by the MCC committee some time back but I think (I can't recall exactly and I may be wrong) they included the word "Enduro" in the description/branding so were not allowed.
Just for interest and comparison this was the ACTC rules for class X as issued in 2014
Class X: Any road-legal solo motorcycle or motorcycle sidecar outfit that does not comply with Classes A to
D above, and all motorcycles fitted with road legal tyres which do not comply with the dimensions
in ACU Regulation TSR8. Class X must run “hors concours” (ie engaged in a contest but not competing for a prize) in all ACTC Championship events.