DOLMETTE

the chainsaw-powered motorcycle

what it looked like on paper...

nearing completion

to the finished product - in the metal

Despite the fact that each 79cc motor produces a mere 5.3kw (barley enough to turn a real bike's wheel in anger), together they churn out a whopping 127kw, able to propel the screaming, fire-belching Dolmette to a top speed in excess of 260kph.

It takes about 15 minutes to start the monster (24 pull-starts), so riding down to the cafe on the Dolmette for bread and milk could be a bit tedious.

What makes the Dolmette unique in engineering terms is its 24-cylinder 2-stroke power plant. Nothing quite like it has been seen before. The combined power of 24 Dolmar chainsaw engines is fed into to a 5-speed Harley-Davidson transmission by a series of twelve toothed belts.

The engine in question is the one used in the PS-7900 professional chainsaw, which currently boasts the best power-to-weight ratio of any professional petrol chainsaw in the world – just 1 kg per PS. In standard trim each of these high-performance air-cooled 79 cc units generates 4.6 kW of power (6.3 PS) at 9500 rpm. A specific output of 59 kW per litre is outstanding for a chainsaw, and the secret lies in Dolmar’s High Performance Combustion (HPC) technology. This term refers to the special design of the six gas exchange ports in combination with the asymmetrical combustion chamber head and the crankcase configuration: the result is a highly efficient combustion cycle with low exhaust emissions.

For use in the Dolmette the engines were tuned and tweaked to deliver 5.2 kW of power (7.1 PS) at 10,000 rpm. Maximum torque delivery is 5.5 Nm at 7,750 rpm. When all 24 are combined, the result is a 24-cylinder power plant with a total displacement of 1.9 litres, a power output of 125 kW (170 PS) and 130 Nm of torque at the centrifugal clutches.

The heart of the combined power plant is the central drive belt casing of aluminium construction. As well as encasing the eleven belts used to couple up the individual engines, it also houses the exhaust ducts for the cooling air. Twelve engines are mounted in two rows of six on either side of the drive belt casing. Each engine is held in place by means of two threaded studs that normally serve to secure the guide bar to the chainsaw power head. The individual motors are coupled together in sets of three by means of a double-sided drive belt, making up eight modules in all. The individual engines are not rigidly connected; instead they deliver their torque through the centrifugal clutch that is a standard feature of every chainsaw.
Through a sequence of three more toothed belts the power of the eight modules is transferred to a single output shaft, which in turn drives the clutch for the 5-speed transmission via a twelfth belt. The toothed belt drive also serves to gear down the speed of the high-revving chainsaw engines in the ratio 3.45 : 1, producing a manageable maximum of 4,500 rpm at the transmission unit. The friction loss that is inevitable with belt drives is less than 10%, which means that the maximum torque available at the transmission input shaft to power the bike is a massive 400 Nm.

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As a consequence of the non-rigid coupling of the individual engines via their centrifugal clutches, the combined power plant has a self-determining – i.e. totally random – firing sequence. In this case, because of the way a 2-stroke engine works, there are 24 ignition points during each engine revolution, or roughly one every 15º. In other words, this power plant fires four times more often per revolution that a 12-cylinder 4-stroke engine!
The very high maximum engine speed of over 15,000 rpm and the much lower clutch engagement speed of 4,000 rpm at the centrifugal clutch leaves the engines with an extremely wide usable rev band of over 11,000 rpm. On the higher gearing the Dolmette reaches a speed in excess of 120 kph in first gear, and on low gearing it reaches around 80 kph.
Because a chainsaw is a hand-held tool, keeping weight down to a minimum is naturally a key consideration. So the cylinders and pistons are made from aluminium, while the crankcase is manufactured from the even lighter metal magnesium. This was already a very good start, but the Dolmar engineers were able to find additional weight savings by machining down the standard magnesium crankcase and stripping out a number of components not needed by the engine in drag-racing mode. By this means they trimmed the weight down from 6.3 kg to a mere 4.5 kg. The combined power plant as a whole, including the transmission, weighs in at a relatively light 188 kg, despite an overall length of 2495 mm.

the belt drive system

Another cutting-edge technological feature are the twelve drive belts manufactured by the ContiTech Power Transmission Group. These high-tech components ensure that the power of the 24 chainsaw engines is transferred to the road as efficiently as possible.

Eleven CONTI SYNCHROTWIN® SUPREME double-sided drive belts incorporating a special high-tensile core reinforcement are used to link the 24 chainsaw engines in a serpentine configuration. A 100 mm wide single-sided
CONTI SYNCHROFORCE® SUPREME high-performance drive belt takes care of the final drive.

Because the engines are coupled by means of centrifugal clutches, resulting in a random ignition sequence across the 24-engine array, the engineers at Dolmar GmbH found themselves entering uncharted territory. No such drive configuration had ever been attempted before. Together with their colleagues at ContiTech they rose to the challenge by developing a drive belt system that transfers the asynchronous power delivery from 24 engines to the back wheel through several transmission stages running at different relative speeds.

Given the asymmetrical nature of the forces acting on the drive belts, the latter need to be engineered to cope with massive loads. The development process was correspondingly lengthy and laborious. In order to reduce the mechanical shocks associated with the asynchronous absorption of stresses, the belts needed to have teeth that were exceptionally resistant to abrasion. Normally this is achieved by making the belt exceptionally rigid and stiff: but in the case of the Dolmette the specification called for a drive belt that was as flexible as possible. So the engineers at ContiTech modified the existing polychloroprene belt and strengthened the bond between the material compound and the core reinforcement, or tension member, pushing the technology close to the limits of manufacturing capability. And in order to make it possible to transfer all this power within the limited space available on a motorcycle, where only very narrow drive belts can be used, the engineers had to push the envelope well beyond the limits established by existing applications.

The central element of the frame is the self-supporting drive belt casing with an overall length of 2,495 mm. Bolted onto this is the front section of the frame, constructed from 32 mm tubing. Sitting in the steering head is the Techno Plus front fork, machined from the solid, with telescopic dampers. The bolted-on back fork assembly is likewise constructed from 32 mm tubing and accommodates the back wheel with no suspension travel. The back wheel is a massive 8½” solid disc wheel fitted with a 240/40 × 18” tyre. At the front is a more conventional 3½” aluminium spoked wheel shod with a 120/90 × 18” tyre.

The fully assembled bike measures 3,870 mm from front to back and is 590 mm wide. As the power plant is largely built from aluminium and magnesium, and consequently weighs a modest 188 kg despite its 24 cylinders, the overall weight of the Dolmette has been held down to a touch over 300 kg. This has resulted in a very favourable power-to-weight ratio of just 1.8 kg/PS.
A machine that goes this fast also needs to stop fast. The front wheel is fitted with 292 mm twin disc brakes with 16-piston calipers. The back wheel has a single 292 mm disc teamed with a 4-piston caliper.

Andi & Roetger Feldman built the Dolmar chainsaw-powered Dolmette

You saw it in REDLINE