BMW reveals R 18 Big Boxer details

BMW R 18 Big Boxer

Details of the Big Boxer engine in the new BMW R 18 cruiser have finally been revealed in Munich overnight.

The air/oil cooled boxer will have 1802cc of capacity and develop just 67kW (91hp) at 4750rpm, but a whopping 158Nm of peak torque at 3000 revs.

More than 150Nm will be on tap from just 2000 to 4000 revs.

Despite the enormous pulling power, BMW says the “generously sized flywheel mass” will provide “exemplary running smoothness”.

It will idle at a lumpy 950rpm and hit the limiter at a lazy 5750rpm.

However, we still don’t know what the final appearance of the bike will be.

Following two independent custom shop R 18 concepts, BMW Motorrad unveiled their Concept R 18 at the Concorso d’Eleganza Villa d’Este in May 2019 and a second concept, the more modern R 18/2, was unveiled at EICMA earlier this month.

There have also been some spy shots, but not a full reveal which is expected early next year.

Even before a production model R 18 cruiser has rolled out of the Spandau factory gates, BMW Motorrad Australia is taking orders and deposits and has set up a dedicated site.

Big Boxer detailsBMW R 18 Big BoxerBMW R 18 Big Boxer

BMW says the Big Boxer engine will have overhead valves and a separate engine and transmission housing, but otherwise the same structural features of their first boxer which had laterally controlled valves.

It is BMW’s highest-capacity twin-cylinder boxer engine in motorcycle series production.

The Big Boxer weighs a hefty 110.8kg and has a 107.1mm bore and 100mm stroke.

Other features are large ribbed cylinders and heads with a vertically split aluminium engine housing.

Unlike their classic air-cooled two-valve boxer engines, the crankshaft is forged from quenched and tempered steel.

It has an additional main bearing at the centre, which is necessary to prevent “undesirable bending vibrations of the crankshaft”.

Like the crankshaft, the two connecting rods with I-shaft are mounted on plain bearings and are also forged from quenched and tempered steel.

They accommodate cast aluminium pistons with two compression rings and an oil wiper ring. The running surface of the light metal cylinders is coated with NiCaSil.

Lubricating and cooling oil is supplied by a wet sump lubrication system with a two-stage oil pump via sleeve-type chain driven by the crankshaft.

Two camshaftsBMW R 18 Big Boxer

Modern features include four valves, dual ignition, a modern combustion chamber architecture, intake manifold injection and the BMS-O engine management system for optimum torque and to meet Euro emissions targets.

However, it uses the classic OHV configuration for its valve drive of the legendary R 5 to R 51/2 from 70 years ago.

2016 BMW R 5 Hommage
2016 R 5 Hommage concept

As in the historical role model, the two camshafts are positioned to the left and right above the crankshaft in the Big Boxer.

BMW says the advantage of this “twin camshaft boxer” is shorter pushrods which reduces moving masses, minimises deflections and lowers linear expansions for a stiffer valve drive with improved control precision and higher speed stability.

In the traditional BMW Motorrad boxer design, the two pushrods actuate one pushrod per cylinder side for the intake and one for the exhaust side, guided in a sealed pushrod tube on the top of the cylinders. The two intake and exhaust valves in the cylinder head are actuated in pairs via fork toggle levers.

In contrast to today’s widespread engine technology, valve clearance compensation is not effected by means of hydraulic elements, but – as was the case in most classic air-cooled BMW two-valve boxers for decades – via one adjusting screw with one lock nut for each valve.

As was formerly the case in the classic two-valve boxers, valve clearance adjustment (0.2 – 0.3 mm) in the R18 Big Boxer is also quick and easy. The valves are made of steel, with a disc diameter of 41.2mm on the inlet side and 35mm on the outlet side. The valve angle is 21 degrees on the inlet side and 24 degrees on the outlet side.

TransmissionBMW R 18 Big Boxer

It is married to a constant-mesh six-speed transmission and self-reinforcing single-plate dry clutch with anti-hopping function to prevent rear-wheel lock-ups on rapid downshifts.

The transmission is located in a dual-section aluminium housing and is designed as a four-shaft transmission with helical gear pairs.

The gearbox input shaft with lug dampers drives the two gearbox shafts with the gear wheel pairs. An output shaft is provided to bridge the distance and reverse the direction of rotation. A reverse gear is available as an optional extra. This is driven by an intermediate gear and an electric motor and can be shifted manually.

As in all BMW motorcycles with boxer engines, torque is transmitted from the gearbox to the rear wheel in the R 18 via a propeller-shaft or universal-shaft drive with universal joint, shaft and rear-axle drive with bevel and ring gear.

The propeller shaft and universal joint are classically nickel-plated and open, as used by BMW Motorrad in models up to 1955.

A so-called tripoid joint is applied on the gearbox side for length compensation.

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