IHRO 2026
Regulations
Race authentic. Race safe.
The 2026 ONK Classics regulations protect the character of historic Grand Prix racing while setting clear technical and safety standards for every competitor.
Eligibility at a glance
Can you race with IHRO?
The quickest check for the motorcycle and rider. The complete technical regulations remain decisive.
Historic Grand Prix racing motorcycle
- Built no later than 31 December 1972
- Engine capacity from 250 cc up to a maximum of 500 cc
- Drum brakes only – disc brakes are not permitted
The motorcycle must also meet the period appearance, construction and technical requirements in the complete regulations.
Internationally licensed and correctly equipped
- Valid international road racing licence
- Airbag racing suit or airbag vest is mandatory
- ECE 22.06 approved full-face helmet is mandatory
The licence must be valid for the event. All protective clothing must comply with the applicable KNMV and FIM requirements.
Official documents
Read before you race.
Download the complete ONK Classics regulations in Dutch or English. The full KNMV Road Racing Technical Regulations also apply, including the general technical, safety, rider-equipment, inspection, noise and fuel provisions referenced by the Classics section.
Nederlands reglement 2026
De officiele en juridisch leidende Nederlandse tekst.
English regulations 2026
Full English translation. The Dutch wording remains decisive.
Full KNMV regulations
Current general Road Racing Technical Regulations published by the KNMV.
Competition classes
IHRO 1, IHRO 2 and IHRO 350.
The three class descriptions clarify where each historic Grand Prix motorcycle belongs. They distinguish single-cylinder, multi-cylinder and 350 cc machinery while preserving the 1945-1972 Grand Prix character of IHRO competition.
Single-cylinder Grand Prix
For single-cylinder thoroughbred Grand Prix racing motorcycles constructed from 1945 through 1972 for the specific purpose of taking part in Grand Prix competition during that period.
Maximum capacity: 500 ccMulti-cylinder Grand Prix
For multi-cylinder thoroughbred Grand Prix racing motorcycles constructed from 1945 through 1972 for the specific purpose of taking part in Grand Prix competition during that period.
Maximum capacity: 500 ccUp to 350 cc Grand Prix
For single- and multi-cylinder thoroughbred Grand Prix racing motorcycles constructed from 1945 through 1972 for the specific purpose of taking part in Grand Prix competition during that period.
Capacity: 250 cc up to 350 ccImportant: these explicit class descriptions are included for classification clarity. Technical eligibility and compliance remain governed by the official Dutch 2026 ONK Classics regulations and the applicable KNMV regulations. No other provisions from the 2025 document have been carried into this page.
Key regulations
The essentials at a glance.
These points make preparation easier, but competitors remain responsible for reading and complying with every applicable article in the complete regulations.
Eligibility and appearance
- Four-stroke and two-stroke motorcycles up to and including 31 December 1972.
- Capacity from 250 cc up to a maximum of 500 cc.
- The complete motorcycle must retain an authentic period appearance.
- Fairings, seats, frame construction and external engine appearance must correspond to the period and original design.
Frame and suspension
- Period-style steel tube or plate frame construction.
- Conventional swinging arm with each side constructed from one tube.
- Two rear shock absorbers are mandatory; external reservoirs are prohibited.
- Maximum external front-fork leg diameter: 35 mm.
Engine and intake
- Original-type crankcase, cylinder, cylinder-head and gearbox castings.
- CNC-made copies are permitted only when identical to the original.
- The factory-original number of valves and transfer ports may not be increased.
- Period-type round-slide carburettors; flat slides prohibited except Gardner.
- No airbox, power jets or accelerator pumps. Injection only with period proof.
Wheels, tyres and brakes
- Wire-spoked 18- or 19-inch wheels, maximum rim width WM4, unless an original exception applies.
- Factory-treaded tyres only; maximum front and rear width 120 mm.
- No hand-cut slicks or dedicated racing wet tyres.
- Conventional drum brakes only. Disc brakes and modern brake-lever guards are prohibited.
Safety and fluid retention
- Airbag suit or vest mandatory from 1 January 2026; helmet and clothing rules follow KNMV/FIM requirements.
- Oil filler and drain plugs, specified connections and brake anchors must be visibly lock-wired.
- An oil catch tray is mandatory beneath the engine and gearbox and must meet Article 2.3.7.
- The general KNMV rules also cover controls, chain protection, transponder mounting and technical inspection.
Noise, fuel and weight
- Maximum noise level: 103 dB(A), measured under KNMV Article 2.16.
- Open megaphones only when permitted by the event organiser.
- Commercially available unleaded petrol or permitted biofuel under the applicable fuel rules.
- Minimum weight: 100 kg for 250-350 cc and 110 kg for 350-500 cc.
Race-number identification
Class colours.
Numbers must be clearly legible, non-overlapping and finished in matt colours. The front number is positioned centrally; side numbers should preferably be placed on the fairing.
Before scrutineering
Final machine check.
A quick preparation reminder. The scrutineer and the complete regulations remain decisive.