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1953 TWN BDG250 - Siamese Twn
A radical two stroker in 1953?
By : Kyle Pereira | Published : August 11, 2008 | Photos : Kyle Pereira
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In conventional two-stroke engines, some of the fresh incoming air-fuel mixture gets shunted straight across the combustion chamber and then out through the exhaust port along with the exiting burnt gases. In fact, the incoming fresh charge is used to drive out the exhaust gases in a process called scavenging. However, fresh charge used to scavenge results, obviously, in lower efficiency and raises fuel consumption too.This innate drawback in the two-stroke engine was taken care of to a certain extent by the concept of the split single. The split single two-stroke format was used extensively by Ing Zoller in 1931 to bolster the dominant position of DKW (another German motorcycle marque and far more famous) in the Lightweight and Junior catetories in pre-war racing.

Coming back to the BDG250, its split engine design employs the use of two individual pistons. The duo are mated to a Y-shaped connecting rod. And in a triumph of division of labour, they both have different jobs outside of absorbing the explosion and turning the crank. One opens and shuts the exhaust port, while the other one handles the transfer port. This allows the exhaust port to open and close before the transfer port, preventing the escape of fresh air-fuel mixture through the exhaust port. Net result were two-strokes with far stronger low and high rev performance and as much as 20 per cent savings on fuel consumption over peers.

However, just like a juicy steak is nothing without fries and copious amounts of mashed potatoes, the engine needs to be accompanied by an equally competent frame and other ancillaries. The BDG has its engine mounted onto a single downtube frame which is suspended by telescopic forks in the front and plungers at the rear. Half width drums at either end do sterling service in slowing down the TWN.A tool box adorns the right rear side of the motorcycle while the chain runs in a fully-enclosed stylish-looking chain cover on the right side. Deep mudguards hug the wheels and a chromed tail light sits below an emblazoned ‘TWN’ on the rear mudguard. A deeply chromed fish-tail tipped exhaust sweeps away the exhaust fumes from the engine. 

After taking in the sweeping lines of the bike, I swung my legs over the saddle. The saddle has special mention here because the seat is damped by a big block of rubber, and not with springs like the other motorcycles of its vintage. I bounced onto it a couple of times merrily till I realised that the owner, Colonel Shashidharan, was just an arm’s length away and could put me into a coma in the blink of an eye if I irked him too much. Oops, sorry about that, sir!When finally settling down onto the saddle, I noticed that the instrumentation is very spartan with just one bulb (headlight high beam indicator) and a very simple speedometer, in addition to the push-on-pull-off ignition switch. The meter itself is original but has been refurbished, albeit with some recent ‘Hyderabadi’ (as the Colonel put it) styled stickering within.
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