Yamaha
XS500 Gallery
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 1800 unlike many
four-strokes with their 3600 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 4251b 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.
| Bike |
Image |
Description |
| 1976 Yamaha XS500C |
 |
Air cooled, parallel twin cylinder, DOHC,
4 valves per cylinder. |
| 1976 Yamaha XS500 |
 |
Highl;y modified cafe racer, street legal.
|
| 1977 Yamaha XS500C |
 |
|
| 1978 Yamaha XS500C |
 |
|
| 1978 Yamaha XS500C |
 |
|
| 1978 Yamaha XS500 |
 |
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