Post by Pardee on Jun 25, 2014 13:49:17 GMT -5
FOUR-STROKE PARALLEL TWINS
Yamaha by Mick Walker
'Someday you'll own a Yamaha' extolled the company's American advertising campaign in the early 1970s; but for many purchasers of Yamaha's four-stroke parallel twin models built in that era, it was very often a case of wishing that they hadn't taken this advice to heart.
But to Yamaha's credit they made amends in later years by playing harder, coming from behind and going for the four-stroke market that they had neglected for so long.
The very first of the series was the XS1 of 1969, which Yamaha saw as a way of cashing in on its similarity to the British-made vertical twins that had gained such a loyal post-war following, particularly in North America, where Triumph enjoyed world-record status from their speed exploits over the Bonneville Salt Flats of Utah.
As the British factories disappeared, by the beginning of 1969 only Triumph and, to a lesser extent, BSA and Norton survived in any real strength. Yamaha imagined that its 653cc (75 X 74 mm) overhead cam XS1 would simply take over as a readily available twin exhibiting the old British values of power and straightforward design engineering, plus one or two of its own -- notably, reliability and sophistication.
What Yamaha didn't realise was that the British bikes had character, even if less durable components such as bulbs, bolts and nuts would work loose or shatter with often clockwork regularity.
The Triumph's, Norton's and BSA's also had brand loyalty garnered over decades and handed down from father to son, and if all this wasn't enough the XS1 had major handling flaws; its frame, suspension and tyres were simply awful -- something which couldn't be said of British bikes. The result was a motorcycle which was not well received, even though Yamaha revamped, renamed (XS650) and constantly improved it, until finally, by 1980 it was almost perfect .. but by then obsolete.
Next came another less than successful design, the much heralded TX750.
If the XS650 took a long time to mature, the TX750 didn't even get that chance; being axed within a few short months of its 1972 launch.
It was designed as a larger, smoother, more powerful stablemate to the XS650. Larger it was, at 763cc (80 X 74 mm) and certainly smoother with its vibes tamed by a complex balancer system. Named 'Omni-Phase' by the company, the system consisted of two counter rotating weights housed at the rear of the crankcase and acting to cancel out the amplitude of the crankshaft. Unfortunately in achieving this task they also sapped much of the engine's power! This meant that the TX was slower than the smaller twin, struggling to achieve 100 mph, which wasn't part of the plan.
Next came another expensive failure, the XS500. This was launched in Europe during 1975 and its double overhead cam 498cc (73 X 59.6 mm) motor was capable of 48 bhp at 8500 rpm, which on the street meant 106 mph.
Inevitably compared with Honda's 500T, the XS was the more sophisticated design, with not only dohc, but 4-valves per cylinder and a balancer shaft (of different design to the ill-fated TX) operating off the crankshaft.
On paper it looked a fine bike, but acceleration was only moderate, not helped by its 209 kg (460 lb) dry weight. The much older, cruder and cheaper Triumph Tiger 100 had equal, if not better performance.
The first Yamaha four-stroke twins to sell in any numbers were the XS250/360/400 family which first appeared in early 1976 (360).
All these featured roughly the same single overhead cam motor with 180 degree crankshaft and six-speed gearbox. Displacement details are as follows: XS250 (248cc - 55 X 52.4 mm); XS360 (360cc - 66 X 52.5 mm); and XS400 (391cc - 69 X 52.4mm). It was simply a case of boring out the cylinders to achieve each increase in capacity.
The last of the series, the XS400 was totally revised for the 1983 model year with the engine capacity changed to 399cc, by increasing the stroke by 1mm. In addition it was given a dohc top end, totally revised bottom end, new frame (with no front downtubes), single shock rear suspension, new eight-spoke cast alloy wheels, new upswept exhaust system and completely revised styling with square headlamp and instruments.
Other features included YICS (Yamaha Induction Control System) and, by mounting the alternator up behind the crankshaft, the engine was considerably narrower than the earlier models.
But once again Yamaha misjudged the market and this expensive update was wasted thanks to poor sales and the subsequent axing of the model.
There then followed a period where this engine configuration was not produced by Yamaha, that was until the emergence of the XTZ750 Super Tenere, and the new TDM850 which appeared in 1991, made use of the slant-angle cylinders, liquid cooling and the 5 valve per cylinder technology first seen in the four-cylinder FZ750 back in 1985.
What was different was the fact that these were both twins -- the XTZ (749cc - 87 X 63 mm) and the TDM (849cc - 89.5 X 67.5 mm). If nothing else, the XTZ and TDM models prove that Yamaha does not give up easily -- if you don't succeed at the first attempt, try, try and try again.
Yamaha by Mick Walker
'Someday you'll own a Yamaha' extolled the company's American advertising campaign in the early 1970s; but for many purchasers of Yamaha's four-stroke parallel twin models built in that era, it was very often a case of wishing that they hadn't taken this advice to heart.
But to Yamaha's credit they made amends in later years by playing harder, coming from behind and going for the four-stroke market that they had neglected for so long.
The very first of the series was the XS1 of 1969, which Yamaha saw as a way of cashing in on its similarity to the British-made vertical twins that had gained such a loyal post-war following, particularly in North America, where Triumph enjoyed world-record status from their speed exploits over the Bonneville Salt Flats of Utah.
As the British factories disappeared, by the beginning of 1969 only Triumph and, to a lesser extent, BSA and Norton survived in any real strength. Yamaha imagined that its 653cc (75 X 74 mm) overhead cam XS1 would simply take over as a readily available twin exhibiting the old British values of power and straightforward design engineering, plus one or two of its own -- notably, reliability and sophistication.
What Yamaha didn't realise was that the British bikes had character, even if less durable components such as bulbs, bolts and nuts would work loose or shatter with often clockwork regularity.
The Triumph's, Norton's and BSA's also had brand loyalty garnered over decades and handed down from father to son, and if all this wasn't enough the XS1 had major handling flaws; its frame, suspension and tyres were simply awful -- something which couldn't be said of British bikes. The result was a motorcycle which was not well received, even though Yamaha revamped, renamed (XS650) and constantly improved it, until finally, by 1980 it was almost perfect .. but by then obsolete.
Next came another less than successful design, the much heralded TX750.
If the XS650 took a long time to mature, the TX750 didn't even get that chance; being axed within a few short months of its 1972 launch.
It was designed as a larger, smoother, more powerful stablemate to the XS650. Larger it was, at 763cc (80 X 74 mm) and certainly smoother with its vibes tamed by a complex balancer system. Named 'Omni-Phase' by the company, the system consisted of two counter rotating weights housed at the rear of the crankcase and acting to cancel out the amplitude of the crankshaft. Unfortunately in achieving this task they also sapped much of the engine's power! This meant that the TX was slower than the smaller twin, struggling to achieve 100 mph, which wasn't part of the plan.
Next came another expensive failure, the XS500. This was launched in Europe during 1975 and its double overhead cam 498cc (73 X 59.6 mm) motor was capable of 48 bhp at 8500 rpm, which on the street meant 106 mph.
Inevitably compared with Honda's 500T, the XS was the more sophisticated design, with not only dohc, but 4-valves per cylinder and a balancer shaft (of different design to the ill-fated TX) operating off the crankshaft.
On paper it looked a fine bike, but acceleration was only moderate, not helped by its 209 kg (460 lb) dry weight. The much older, cruder and cheaper Triumph Tiger 100 had equal, if not better performance.
The first Yamaha four-stroke twins to sell in any numbers were the XS250/360/400 family which first appeared in early 1976 (360).
All these featured roughly the same single overhead cam motor with 180 degree crankshaft and six-speed gearbox. Displacement details are as follows: XS250 (248cc - 55 X 52.4 mm); XS360 (360cc - 66 X 52.5 mm); and XS400 (391cc - 69 X 52.4mm). It was simply a case of boring out the cylinders to achieve each increase in capacity.
The last of the series, the XS400 was totally revised for the 1983 model year with the engine capacity changed to 399cc, by increasing the stroke by 1mm. In addition it was given a dohc top end, totally revised bottom end, new frame (with no front downtubes), single shock rear suspension, new eight-spoke cast alloy wheels, new upswept exhaust system and completely revised styling with square headlamp and instruments.
Other features included YICS (Yamaha Induction Control System) and, by mounting the alternator up behind the crankshaft, the engine was considerably narrower than the earlier models.
But once again Yamaha misjudged the market and this expensive update was wasted thanks to poor sales and the subsequent axing of the model.
There then followed a period where this engine configuration was not produced by Yamaha, that was until the emergence of the XTZ750 Super Tenere, and the new TDM850 which appeared in 1991, made use of the slant-angle cylinders, liquid cooling and the 5 valve per cylinder technology first seen in the four-cylinder FZ750 back in 1985.
What was different was the fact that these were both twins -- the XTZ (749cc - 87 X 63 mm) and the TDM (849cc - 89.5 X 67.5 mm). If nothing else, the XTZ and TDM models prove that Yamaha does not give up easily -- if you don't succeed at the first attempt, try, try and try again.