The Norton Challenge P86 | Cosworth 746cc Twin

The Norton Cosworth Challenge

With Phil Aynsley

The Norton Challenge faced its share of challenges, eventually triumphing after a hefty investment

It can’t be all that often that a bike finds its first success a dozen years after it began development, but the Norton Challenge P86, and its descendants, is one!

The Challenge was a partnership between Norton and Cosworth to create a Commando replacement

In 1974 Norton entered into a partnership with Cosworth to develop a replacement for the Commando. Among the requirements were a 360º crankshaft and a single SU carburettor – factors that were to prove major problems.

Design requirements included a 360º crankshaft and a single SU carburettor and 50,000 mile overhaul interval

It also had to have a 50,000 mile life before overhaul and meet all foreseeable pollution standards without major redesign. And be capable of winning Formula 750 races!

Cosworth used the DFV V8 F-1 motor for inspiration

Cosworth basically used two cylinders from their DFV V8 F-1 motor for a 746cc parallel twin which, despite having a modern bore/stroke ratio and shallow combustion chambers, was flawed by Norton insisting the flywheel was placed between the conrods which meant the lack of a third main bearing.

The design was however undermined by additional requirements

This resulted in a fierce harmonic vibration at 4000rpm that would destroy the motor if held at those revs for any length of time – a bit of a drawback for a road bike! The only major change from the DFV (from the barrels up) was the use of a toothed timing belt rather than a gear-train to drive the cams. In fact the pistons and conrods were DFV parts!

The Challenge's engine weighed in at a hefty 88kg — The Challenge’s engine weighed in at a hefty 88kg

The crankshaft and carburettor specifications meant that the motor required a heavy (9kg) counter-balancer and that the advantages of the wide downdraught intake ports (designed for fuel injection) were ignored.

With two 40mm Amal MkII carbs equipped, 95hp at 9750rpm was claimed

Some idea of the scale of these engineering compromises can be seen from the remarkable figures of the motor’s total weight of 88kg, 35kg of which was rotating mass! Fitted with two 40mm Amal MkII carbs 95hp at 9750rpm was claimed. By contrast the chassis was without problems.

The bike raced in 1975 before being retired

The bike was raced briefly in 1975, then sporadically over the next few years before the project was abandoned.

Later a Quantal version was raced to success

A much modified version (the Quantal) was developed from the P86 in 1984, and after much re-engineering (823cc, fuel injection, £100,000 and John Surtees involved), was successful – finishing 2nd in the 1986 Battle of the Twins – BoTT – (with rider Paul Lewis) race at Daytona and winning the race in 1986.