Friday, 22 June 2012

Yamaha XS500

The XS 500 now that was an ugly bike !!!!!
Bought this in 1984 thought I should get a big bike !
Found in Exchange and Mart just local to where I was living at the time in Takeley Essex.
The guy who owned it was a pilot at stansted airport just up the road and very rarely used the bike.
The main reason I bought it was it had never been out in the rain and was totally spotless..

Anyway had a great engine with counterbalance shafts and performed very well..

Although looking like the smaller XS400 and XS250 bikes, the XS500 has a different twin-cylinder engine which features twin-overhead camshafts and four-valves per cylinder. Although such an engine would be ideally suited to a sports motor cycle, Yamaha instead have used it in a state of tune for a bike they classify as a tourer with enough engine power to enable it to keep up with larger capacity machines.
The oversquare power unit displaces 498cc and with a compression ratio of 9.6:1 produces 49bhp at 85oorpm, just one brake horsepower down on the company's 650cc twin.
A four-valve-per-cylinder engine is used because four small valves use space more efficiently than two larger valves, so that, in fact, more head area can be utilised which in turn makes the engine more efficient. Also the lower reciprocating mass of small valves, means higher engine speed before 'bounce' is induced, and better ignition is helped by the sparking plug being central in the head, its most efficient position. The problem with the layout is that to get the valves in an efficient semi-hemispherical head, twin overhead camshafts have to be employed or a complicated system of pushrods and rockers which would negate the effect of the extra engine speed gained from the layout. The XS uses direct-operating overhead cams which make servicing more complicated and time consuming.
The Yamaha has pistons set at 180 degrees unlike many four-strokes with their 360 degree layout with the inherent vibration difficulties. However, Yamaha had the problem of the crankshaft rocking with the alternate reciprocation, so to encounter this they use their patented 'omniphase' balance shaft.
The top speed of this 425 LB bike is 105mph, while it will accelerate over a quarter mile from standing start in 14.3secs; fuel consumption is 46mpg. In all, the XS500 is deceptive for it is more advanced than it looks and 'four-stroke twin' specification suggest.

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