5. Transmission and Gear Selection Systems
5.1. The transmission system is free including gear selection system which may be modified or replaced by an alternative system.
5.2. The use of active front and rear differentials i. e. any system acting directly on the differential adjustments is forbidden except for cars originally homologated by the FIA. Only entirely mechanical systems are allowed. None of the parameters of the front and rear differentials can be modified except with the help of tools when the Vehicle is stopped.
5.3. Passive viscous differentials are considered mechanical and are permitted.
6. Suspension
6.1. Except for Vehicles accepted under Category 2 and Vehicles with modifications conforming to their homologation papers( Art. 2.3 above):
a.
b.
The suspension must retain the operating principle and utilise the mounting points as provided by the vehicle manufacturer.
Strengthening of the sprung parts of the chassis and bodywork is allowed provided that the material used is the same material as the chassis / bodywork and follows the original shape and is in contact with it.
c. Springs shock absorbers anti-roll bars and control arms and links may be uprated. d. Only mechanically operated anti-roll bar systems are authorised. e.
The anti-roll setting can only be adjusted directly by the driver using an exclusively mechanical system without external power.
f. Any connection between dampers is forbidden. g. Any connections between front and rear anti-roll bars are forbidden. h. The ride height must be adjustable only with the use of tools and when the car is immobile. i.
Any servo control activating a power circuit acting directly or indirectly on the suspension parts is forbidden.
7. Wheels and Tyres
j. Any electronic control system for the shock absorbers is forbidden.
7.1. Not be fitted with spiked or studded tyres or any non-skid attachments other than chains if permitted in the Official Documents.
7.2. Have all spare wheels securely fastened in position.
7.3. Have complete wheels( flange + rim + tyre) free providing that they can be housed within the original bodywork( including wing extensions) and that they do not exceed the widths in relation to the cubic capacity of the Vehicle:
a.
Up to and including 1600 cc: 8in b. Over 1600 cc: 9in. 7.4. Note: Wheel widths relate to the capacity of the Vehicle inclusive of the forced induction coefficient.
7.5. Rim diameters may be increased or decreased by up to a maximum of 2in but the rim diameter must not exceed 18in.
7.6. Wheels do not necessarily have to be the same diameter. 7.7. The complete wheel shall be measured with the tyre at normal running pressure. 7.8. The measurement of the width will be taken at the widest point of the complete wheel above hub level.
Motorsport UK 2025 National Competition Rules Version 11
Chapter 13 Appendix 16- Special Stage Rally Cars 505