Suzuki GT 750
Make Model
|
Suzuki GT 750 |
Year
|
1975-76 |
Engine
|
Liquid cooled, two stroke,
transverse 3 cylinder,
|
Capacity
|
738 |
Bore x Stroke |
70 х 64 mm |
Compression Ratio |
6.7:1 |
Induction
|
3x Mikuni VM 32 70 mph indicated, actually 63.8
|
Ignition /
Starting
|
Battery and coil Braking distance /
electric & kick |
Clutch |
Multi-disc, wet 70 mph, sec 6.8 |
Max Power
|
67 hp @ 6.500 rpm |
Max Torque
|
7.7 kg-m 51.7 ft-lb @ 5.500 rpm |
Transmission /
Drive
|
5 Speed / chain |
Gear Ratio |
5th 4.84 Standing one-eighth mile, sec. 8.38
/ 4th 5.90 terminal speed, mph 78.74 / 3rd 7.15
Standing one-quarter mile, sec 13.87 / 2nd 9.11 terminal speed, mph
93.55 / lst 14.92 |
Front Suspension
|
Telescopic forks |
Rear Suspension
|
Swinging arm fork with adjustable shock
absorber, |
Front Brakes
|
2x 295mm disc |
Rear Brakes
|
180mm drum |
Front Tyre
|
3.25-19 |
Rear Tyre
|
4.00-18 |
Seat Height |
32 in |
Dry-Weight
|
235 kg |
Fuel Capacity
|
17 Litres |
An American Perspective
December 1971
SUZUKI'S T500 "TITAN", largest of the twin-cylinder two-strokes in general
production, aroused the interest of the motorcycling press several years ago.
Critics of the design argued that because of cooling problems, it wasn't
feasible to put two large cylinders so close together. Spacing the cylinders
farther apart would add to the width of the engine, thus spoiling the lines of
the machine, besides adding to the bulk of the engine transmission package. But
with a sound basic design and Japanese technology, the T500 soon became a
machine to be reckoned with in all categories. As a touring bike it is
comfortable, reliable, economical, and it comes within a hairsbreadth of being a
true Superbike in terms of performance. The last T500 we tested turned in a
standing quarter-mile of 14.31 sec. with a terminal speed of 91.06 mph.
Once again Suzuki has introduced a paradox the GT750. Big, heavy, comfortable,
economical and extremely smooth, the GT750 is capable of high 13-sec. standing
start quarter-miles, effortless high speed cruising, hairline steering and
tremendous braking. It's a Superbike in every sense of the word. The most
interesting feature of the GT750 is the engine, and more particularly the method
used to cool it. Water cooling for two-strokes is not new, the British Scott,
which went out of general production just before WWII, was the last
large-capacity machine to use it. More recently, the horizontally opposed
Velocette "LE" 200-cc Twin employed water cooling, but it too is defunct.
Several advantages accrue to water cooling a two-stroke, where heat is an
inherent problem because a two-stroke fires twice as often as a four-stroke.
Water cooling more efficiently disperses this heat, which can cause piston and
cylinder distortion, with a consequent reduction of obtainable power. If
clearances between moving parts must be made large enough to avoid excessive
friction and possible seizures, this also leads to loss of power from blow-by
past the piston rings, and contributes to the mechanical noise of the engine. A
properly designed water cooling system can reduce these problems.
Among the negative aspects of water cooling is the need for a radiator,
thermostat, water pump (which can sometimes be dispensed with if a thermo-syphon
system is used), and a cooling fan, items which Suzuki has managed to skillfully
blend into a most attractive package. With the exception of the radiator, the
motorcycle is as cleanly styled and aesthetically pleasing as any model in
Suzuki's line-up. Moderately valanced chrome plated fenders blend well with the
wide dual seat, large instrument cluster and large, slightly bulbous gas tank.
Even the radiator, which is protected by a chrome-plated safety bar, finally
takes its niche in the design, although it looks somewhat out of place at first.
A transversely mounted two-stroke three is a design made popular by Kawasaki,
with their frighteningly fast 500-cc Mach 111. The GT750 engine features
horizontally split crankcase halves, which support the massive,
four-main-bearing crankshaft. The crank has throws spaced 120 deg. apart, giving
the same number of firing impulses as an in-line six-cylinder four-stroke.
Looking at the engine from the top, we find three sets of contact breaker points
located at the extreme left end of the crankshaft which are driven by a flexible
coupling.
Just inboard is an idler bearing and to the right is a gear, which drives the
tachometer and water pump. The water pump is a vane-type unit driven by a
horizontal shaft and located in the bottom of the crankcase. It has an output of
l6.2 gal./min. at an engine speed of 6000 rpm, which compares favourably with
many small cars. At the right is a one-way clutch for the electric starter, a
main bearing, the left cylinder's flywheel assembly, another main bearing and
the central cylinder's flywheel assembly. Still another main bearing follows,
which is adjacent to the helical gear for the primary drive. Grafted onto the
other side of the primary gear is the third cylinder's flywheel assembly,
another main bearing and the alternator. The main reason for taking the primary
drive from between the second and third cylinders was to keep the engine's width
behind the crankshaft to a minimum. This arrangement leaves more space between
the numbers 2 and 3 than numbers 1 and 2 cylinders, but with water cooling it
doesn't make any difference. The one-piece cylinder casting is fitted with
cast-in iron liners which are not removable. However, pistons are available in
two over sizes, and if the air cleaner remains intact and properly serviced,
piston wear should be negligible. Cast-in aluminum cooling fins don't help
cooling the cylinder and the head much, but are much nicer looking than a smooth
exterior.
All aluminum crankcase, cylinder and cylinder head castings are polished to a
high luster, adding a touch of class to this unique motorcycle. A single
cylinder head casting with relatively shallow combustion chambers is used, and
the outlet pipe from the engine's cooling system emerges from between the center
and right-hand cylinders. Conventional two-ring pistons are virtually identical
to those of the T500 (the bore/stroke dimensions for both machines are the
same), and, like the T500, needle bearings support the pistons, and roller
bearings support the connecting rods. It is interesting to note that while the
pistons and connecting rods are almost identical to those of the T500, the
flywheels are smaller in diameter and roughly approximate the size of those used
in the TR500 road racer. Water for cooling the engine enters the crankcase from
the radiator at the front of the engine. The pump picks up the water and pushes
it up through the water jackets around the cylinders and into the cylinder head
jackets. Until a temperature of 82 deg. C (180 deg. F) is reached, the
thermostat remains closed and the heated water is redirected to the pump in the
bottom of the crankcase. Complete opening of the thermostat occurs at 95 deg. C
(203 deg. F), and if the water temperature reaches 105 deg. C (221 deg. F), the
tiny cooling fan behind the radiator cuts in and keeps running until the water
temperature falls to 100 deg. C (212 deg. F).
During a series of tests carried out in the Los Angeles area by a team of Suzuki
engineers during the hottest part of the year, it was practically impossible to
get the water temperature high enough to activate the fan, so there's a good
possibility it will be dispensed with altogether. Total coolant capacity is a
generous 5.4 U.S. qt., and the system is pressurized as are those on
automobiles. Also included in the system is a header tank, which further
precludes boiling over. Drawing heavily from their water-cooled motorcycle grand
prix experience, and from their work with small water-cooled cars and trucks,
Suzuki engineers claim a temperature reduction of some 30 percent over the
machine's air-cooled counterpart, a six percent increase in torque and the
ability to reduce piston clearance from the 0.0026-0.0030 in. of the T500 to a
very close 0.0019 in. An added benefit of the close tolerances is reduced piston
noise, which is further lessened by the deadening effect of the water in the
jackets. Silence and smoothness are key words for the GT750. Rubber mountings
inside the crankcase attachment points to the frame allow the engine to shake
just slightly around 2000rpm, but above that figure the motor is completely
smooth up to maximum revs, Both the handlebars and the footrests are solidly
mounted, indicating no need to damp out spurious vibrations at those points of
rider contact with the machine. In a further effort to make the GT750 as quiet
as possible, Suzuki incorporated an air filter intake system to minimize the
intake roar characteristic of two-cycle engines. At the rear of the air cleaner
box are two rubber tubes with slightly flared ends through which air is admitted
to the filter.
These tubes do a remarkable job of silencing, and because they are pointed
downwards, it is more difficult for water to enter the filter. Wanting to
maintain as much symmetry as possible, but faced with the large physical
dimensions of each of the super quiet mufflers, Suzuki opted to split the center
cylinder's exhaust system into two smaller mufflers containing the same volume
as one large one. A connector (balance) pipe running between the three header
pipes not only aids noise reduction, but also increases the amount of torque at
2000 rpm by about 20 percent. Viewed from the rear, the GT750 looks like a
four-cylinder machine with its four pipes. The two large outer pipes are tucked
up out of the way, while the two smaller inner pipes are somewhat lower but
tucked in far enough to preclude dragging on sharp corners. Although the
machine's actual ground clearance is only a shade over 5 in., this lowness is
near the center line of the bike; the only thing we were able to drag during
vigorous cornering was the side stand. A particularly appealing feature is a
gadget to reduce the output of exhaust smoke under certain conditions. When a
two-cycle engine is operated at low speeds for a time, unburned oil collects in
the bottom of the crankcase where it stays until the machine is accelerated
hard, as when going on the freeway, at which time the bike takes on the
qualities of a crop duster! This device is a one-way valve in the bottom of each
crankcase, which is connected by a tube to an adjacent cylinder's transfer port.
Instead of collecting, this unburned oil is directed to and burned by the other
cylinder and is ejected from the engine at a more constant rate. The same amount
of oil is used, of course, but not so noticeably.
Three 32-mm Mikuni carburetors are connected to the twist grip through a
junction box with a single cable, and operate surprisingly easily. A starting
enrichment lever (choke) located on the left handlebar lever mount must be used
to start the machine on cool mornings. It makes starting so easy, we can't see a
reason for retaining the kick starter. Depressing the starter button instantly
fires the engine, indicating that less than one revolution of the crankshaft has
taken place. A negative feature of the starting system is that, as is, the crank
lever bumps the rider's right calf, transmitting the engine's vibrations to his
body. With the kick starter assembly removed, the entire engine unit could be
made smaller and lighter, with a subsequent reduction in the area occupied by
the transmission. The transmission design follows previous Suzuki practice with
constant mesh gears and one shaft located behind the other along the crankcase
horizontal centerline. All indirect, the ratios are ideally suited to the power
characteristics of the engine; they are not especially close, but making them
close would be foolish because of the broad, flat torque range of the engine.
Shifting action is very smooth and positive, but we feel that the GT750 is
"under geared" and think a two- or three-tooth-smaller rear sprocket would be
more appropriate. At 70 mph on the highway the engine is spinning at 4500 rpm,
many more than are needed to keep it up to speed with a low throttle opening. In
fact, the torque curve is so flat that pickup is possible from 2000 rpm in top
gear with almost no change in apparent acceleration. We were amazed at the small
amount of throttle needed to keep the GT750 moving, and at the large engine's
willingness to rev. Also impressive, if not outstanding, was the fuel economy.
Dividing our test riding as we do (70 percent highway, 30 percent in town), we
got gas mileage ranging from 41 to 47 mpg, outstanding for a four-stroke of
comparable size, and nothing short of amazing for a two-stroke.
Oil consumption was also low. The oiling system on the GT750 is virtually
identical to that of the T500 which employs a variable displacement plunger unit
that increases its delivery according to the engine's rpm and the amount of
throttle opening. A cam, connected to the throttle cable junction box, opens the
pump in accordance with the throttle opening. At cruising speeds, the pump is
almost shut off, but as soon as the throttle is opened to climb a hill, the oil
supply is increased. Conversely, when the throttle is closed, the pump’s output
is reduced to help lower the chances of the engine's loading up with raw oil.
Two additional outlets at the pump supply oil to the right-hand cylinder, but
the system works the same. Oil is injected to all the main bearings except the
one located next to the primary gear, which receives its lubrication from the
transmission's oil supply. Oil used to lubricate the other mains then travels
through the crankpin centers and lubricates the big-end bearings before being
flung upwards to lube the wrist pin needle bearings. It is then burned and
expelled out the exhaust. There is also an oil feed point in each inlet port to
supply oil to the wrist pins and cylinder walls.
At 524.lb. with a half-tank of fuel, the GT750 is just slightly heavier than the
Honda CB750 Four, but it feels deceptively light at any speed. In slow traffic
the only clues you have about the machine's bulk are the width of the gas tank
(which holds a moderate 4.5 gal. Because of a cutaway at the front for the
radiator header tank and filler), the broad handlebars and the size of the
instrument panel. At the top, in between the speedometer and tachometer, is the
water temperature gauge. Neutral, high beam and turn signal indicator lights are
located on the face of the tachometer, but there is no charging light. Our test
machine had a confusing array of thumb-operated switches on the left side, which
will be modified for production. From top to bottom were the headlight
switch/dipper assembly, turn signal switch, horn button and a headlight flasher.
The right side houses an on/off switch and the electric starter button. Later
models will not have a headlight flasher, and the headlight switch/dipper
assembly will be moved to the right side.Like the T500, the GT750 gets top marks
in the handling department.
A massive double cradle frame features a huge main tube with the radiator filler
going through it, running from the steering head back to the rear of the gas
tank. It butts into a transverse cross brace which doubles as the top attachment
point for the air cleaner. Two auxiliary top tubes extend from the steering head
rearward. They finally terminate at a point some 9 in. behind the top rear
suspension mounts to serve as mounting points for the rear fender. The down
tubes act as a mounting point for the radiator and continue under the engine
rearward to just behind the transmission, where they bend upward and attach with
the top tubes under the seat. Diagonal tubes branch from the bottom cradle and
extend back to the rear suspension top mounts. Strengthening triangulation is
evident here as in the steering head/main top tube area. The relatively short
swinging arm is supported at the pivot point by large bushings, and the tubes
are a healthy 1 5/16 in. in diameter. Thick steel gussets, which, provide
mounting points for the bottom of the rear suspension units and the rear axle
are uncommonly long and strong. Even under hard acceleration in high-speed
corners, the GT750 tracked as though on rails. In fact, the only part of the
suspension package which isn't quite up to spirited cornering are the front
forks. In keeping with the comfort-oriented idea behind the machine, the forks
are softly sprung, but they nonetheless provide excellent control on all but the
roughest surfaces. Notable also are the De Carbon-type sealed gas rear shock
absorbers featuring a five-position spring rate adjustment. The Suzuki's shocks
are very sensitive to small road irregularities, and have sufficient damping to
prevent rear wheel hop under heavy braking, or yawing in fast corners. At first
we had reservations about the brakes. With many of today's high performance
motorcycles going to disc brakes on the front, it seemed logical that Suzuki
would follow suit. They haven't, but the GT750's brakes are just short of
fantastic, capable of hauling the heavy machine down from 60 mph in 118 ft.
The front brake is little more than two T500 brakes in a single housing with
cooling air scoops. Twin cables fitted into a balance bar at the brake lever are
adjustable for length both at the lever and at the brake operating arm end.
Although the front brake heats up under hard usage, fade is minimal. The rear
brake is identical to the rear unit on the T500, but it, too, is reluctant to
fade. Riding the GT750 is a joy, which must be experienced to be fully
appreciated. The wide, ultra-soft dual seat is very comfortable, although we had
a heavy friend who complained that the padding was too soft and allowed his
buttocks to hit on the bottom of the seat. Footrests are slightly on the long
side, and do not fold rearward. This makes it easy to bump one's calves while
"walking" the machine from a seated position. The relationship between
handlebars, seat and foot pegs was deemed very comfortable by all our testers.
The main complaint stemmed from the upright seating position, which doesn't
allow the rider to lean forward into the wind at high speeds. The almost
complete lack of vibration and unnerving mechanical, intake and exhaust silence
give one the feeling of riding a steam turbine. The loudest noise, by no means
unpleasant, was the gear whine of the all-indirect ratio transmission and the
rear chain. Our duty pillion passengers also rated the seat comfortable and were
pleased with the lack of vibration through their foot pegs. Detailing is
exquisite. Heavily applied chrome and lavish use of the buffing wheel on
aluminum parts slightly outdid the paint, which was moderately less perfect than
on other Suzuki models. Welding on the frame is very good, and all engine parts
fit neatly together. Lighting is excellent, with what is possibly the largest
taillight on a motorcycle. All electrical components are first-rate, including
the neatly routed wiring. We also liked the location of the ignition switch:
right in the center of the instrument panel. Suzuki's most pleasing combination,
the GT750 is the most refined, and yet most awesome, two-stroke ever.
Dan Hunt tests the GT750
REAL COOL-THE WHISPERING GIANT
SUZUKI'S GT750 really moves out. And it handles quite well. But the fancy new
water cooled 750 Three will not be the favorite of many Café racers, because its
conception doesn't lend itself to the street racing mentality. It is a big
machine and at 524 lb wet, perhaps the heaviest two-stroke ever produced. As
such, it loses out if considered only in terms of the other Superbikes that have
preceded it. You must look elsewhere to find out why Suzuki has taken the
inherently lightweight two-stroke engine design and come up with a bike that is
heavier than the 750s of BSA, Triumph, Norton, Honda, BMW and Kawasaki. The
reason the GT750 is heavy? All that extra plumbing for the water-cooling system.
No more the simple cooling fins. In place of that we have water jackets, hoses,
radiator, thermostat, temperature gauge, and even an auxiliary cooling fan. But
for this extra weight and complexity, the compensations are considerable: Two
strokes have greater cooling problems than four-strokes. Firing twice as often
they produce more heat. Lacking the elaborate oil circulation system of a four
stoke , which handles as much as 60 percent of the cooling functions, they must
depend more upon fining to dissipate heat. Thus, in an in-line three-cylinder
designs, the centre cylinder will invariably run hotter than the outer two.
Kawasaki gets around the problem by wide cylinder spacing and careful
aerodynamics; modify the Kawasaki front fork and fender, wrongly and you may
have a heating problem. Suzuki, in using water-cooling, over comes the in-line
Three problem entirely. Cooling all three cylinders evenly with water jackets
reduces piston and cylinder distortion and therefore prevents loss of power.
Clearances may be tighter, preserving yet more power, reducing noise, and
squelching mechanical noise. And, the silencing effect of the water-jacket
itself is extraordinary. The G7750 is quiet! Fuel economy is improved as a
function of better heat dissipation and engine efficiency, so it is no surprise
that the GT75O gets good mileage. The figure we obtained in a combination of
town and motorway riding was 51 miles per gallon (Imp.), comparable to a few 750
four-strokes. Now we begin to see what the new Suzuki is all about. It is a big,
high performance machine for long distance touring-a new role for a large
displacement two-stroke. The innards of the GT750 are somewhat similar in design
to the in-line Three pioneered by Kawasaki: horizontally split crankcase halves;
massive four main bearing crankshaft with throws 120 degrees apart; five-speed
transmission. Then the other differences; a vane type water pump located in the
bottom of the crankcase, one-way clutch for the electric starter at the right.
The helical gear primary drive is taken from between the second and third
cylinders to keep the engine's width behind the crankshaft to a minimum. This
leaves more space between cylinders two and three than between one and two, but,
with water cooling it makes no difference. Upstairs you'll find a one-piece
cylinder block with cast-in iron liners, and a single cylinder-head casting with
shallow combustion chambers. The outlet pipe for the cooling system emerges
between the centre and right-hand cylinders. Conventional two-ring pistons are
supported on needle bearings, while the connecting rods ride on roller bearings.
The cooling system, pumping 19.2 gallons per minute at an engine speed of 6,000
rpm compares favorably with that of a small car engine. Water enters the
crankcase from the radiator at the front of the engine. The pump takes the
water, pushes it through the water jackets and up into the cylinder head
jackets. The thermostat remains closed until the water temperature reaches 180F
and the water redirected to the crankcase pump. At 203 degrees F the thermostat
is opened completely to fully utilize the power of the radiator. The little
auxiliary fan cuts in only when the water temperature reaches 221 F. In boiling
Southern California I never encountered a condition hot enough to make this fan
operate. The basic cooling system is so effective that Suzuki may dispense with
the , fan in subsequent models. Suzuki engineer's claim that the water-cooling
about 30 Per cent more effective than air-cooling, resulting in a six percent
increase in torque. Piston clearance on the GT750 is .0019 in. compared with the
.0028 in. clearance used on the air cooled 500 twin. Now to the riding. You are
straddling a hefty machine, but the 32.0 in. seat height is not to imposing to
an average size rider. Open the fuel taps, pull the choke lever and hit the
start button. The machine starts burbling in that smooth muffled idle
reminiscent of its ancient predecessor, the Scott. If you have time, wait for
the temperature needle to rise to the bottom of its operating range. The gear
lever, operating a one-down four-up pattern, is on the left. As soon as you're
under way, the machine begins to lose the initial impression of weight and mass.
It steers easily in tight going and U-turns easily on a narrow one-lane road.
Above 30 mph the feeling is one of relative lightness and quickness. In sweeping
turns over 65 mph, the bike "sets" well into a turn with little threat of
under-steer. In spite of the four balanced silencers, you get the impression
that they will not ground easily. The ride is typically Japanese, that is,
stiff, and I'd rate damping action excellent at the front and good enough at the
rear. The bike is a solid, stable motor-way runner-wiggle free and placid. At 80
mph in fifth gear, the engine is turning hardly more than 5,000 rpm. At 110 mph
it is geared to turn 7,000, rpm, which is quite slow in two-stroke terms.
Acceleration is scorching, as it turns the quarter mile in the high 13s. But the
effect is mild, because the power band is fat. In fifth gear with a solo rider,
the bike dawdles along easily at 2,000 to 3,000 rpm and pulls away with no
snatching or flat spots. The only objectionable traits the machine exhibits are
centred around the transmission. With so little noise coming from the engine
itself, and from the mufflers, you can hear the gearbox whine noticeably in the
higher ratios. And the act of shifting is' reward with a loose, clunky action,
after the manner of, but not quite as pronounced as the BMW or Moto-Guzzi. But
unlike these Continental tourer's, the inertia in the Suzuki gearbox is low. The
GT750 may be up shifted or downshifted quite rapidly. I was surprised that
Suzuki had not chosen a disc brake for the front, as all its Japanese
competitors have done for their big bore machines. The inside line here is that
the company would have nothing to update the machine in coming years if the disc
brake was featured this year. But you can't object, as the big drum brake is
superb and hauled the GT750 from 60 to zero in only 118 feet. It is the
equivalent of two T500 brakes in a single housing with air scoops . Both it and
the T500 sized rear brake, are reluctant to fade. Seating and peg comfort is
excellent for both rider and passenger. The handle bars sweep back a bit too far
which means a riding position that is too upright. Finish of the machine is good
- with clean welds and neatly fitting engine parts. The electrical wiring, too,
is quite tidy. The general styling and colour- cool purple and white, or aqua
blue and white-is bright but tasteful. Balancing complexity against the
benefits-smoothness, power and reasonable fuel economy-the Suzuki GT750 may be
the first big-bore two-stroke to win converts from the firmly entrenched long
distance four-stroke clan.