AMCN RC17 Road Test

The “Australian Motorcycle News” road test of the CBX750

Disclaimer: This article is used here without permission of ‘Australian Motorcycle News’ magazine. I fully recognise their copyright to all the words that they wrote.

Overshadowed somewhat in the pre-release publicity by the bigger machinery from Honda, Kawasaki and Yamaha, one would be forgiven for thinking there was little to be excited about on Honda’s latest 750, the transverse four cyclinder CBX750. The truth of the matter is a little different, as Bob Maron found out…

Honda’s CBX750F is a stunning piece of engineering which will almost certainly achieve what the factory has planned for it. It will, without doubt, eat into the sales of transverse fours from Kawasaki, Suzuki and Yamaha. Believe it.

To look at, the CBX is, with it’s twin 55/60 watt square headlights mounted in an angular adaptation of the VF’s fairing, it’s belly pan thrust arrogantly forward from underneath and it’s wheels in deep black, one very aggresive motorcycle. Far from being a sports-touring complement for the VF750’s all-out racer image, the CBX sits alongside – a third generation across-the-frame four with an almost identical brief.

Since it’s introduction in 1969 as an SOHC machine delivering 67bhp, the Honda Four has progresssed through the 77bhp 1978 K and F DOHC models to the present astonishing 93bhp variant – a continuity which may be evident more in sales brochures than in fact, but which nonetheless is of paramount importance to Honda.

With the birth of the Vee-four, Honda found that, in spite of what it felt was a clear-cut superiority in it’s own machine, many riders continued to purchase the ‘old fashioned’ transverse fours from Kawasaki, Suzuki and Yamaha. So began the project which culminated in the CBX750, a blown out and heavily modified version of the CBX650 manufactured by that company in 1983.

The end product is a 747cc air cooled engine, mildly over-square at 67mm x 53mm and running 9.3:1 compression ration whose power output is higher that that of any comparable street bike, including the performance 750 of recent years, Honda’s own VF750F. Keep in mind too, that the claimed dry weight for each of these is identical at 218kg.

Powerplant

It is generally recognised among designers of small, high-revving engines that bore:stroke rations should be kept over-sqaure, the bore dimension being in the range of 1.1 to 1.3 times the stroke of the crankshaft, for optimum combustion chamber filling and evacuation.

The reasons are many, prime among them the benefits of lower piston speed at a given rpm (at 10,000 rpm the CBX is turning a mean piston speed of 17.66m/sec compared to the VF750’s heavily over-square 16.2m/sec), the opportunity for larger valves and the fact that gasses have a shorter distance to travel in order to fill or evacuate the cylinder.

In recent motors from Japan, where performance has grown from a paltry 89bhp/lt in 1969 to over 120bhp/lt in the CBX, such considerations are becoming increasingly important. So why has Honda opted for a bore:stroke ratio which is only marginally over-square?

Simple: piston speed, while relevant, is not an over-riding consideration on street bikes, carefull attention to exhaust and inlet design can improve cylinder filling substantially, and finally, four cylinders across the frame are wide enough without increasing the bore. Any increase here will ultimately interfere with either cornering clearance or centre of gravity height. The same philosophy which applied with the DOHC CB750/900 is still relevant.

Valve train design has contributed greatly to the CBX’s improved power potential. Four valves are arranged at 19 degrees to the bore axis (fore and aft only, unlike the radial splaying of Honda’s RFVC engines), keeping the pent-roof combustion chamber flat for reduced heat loss. For the same reason, the pistons only have valve clearance eyelids to disturb their flat tops.

Each valve stem has been pared down to a mere 5mm in diamter, cutting reciprocating mass and allowing the valve springs to be softer (less power loss) for the same degree of control. Of course, such thin stems are fragile and it is only the use of hydraulic tappets which prevents premature damage or breakage.

Hydraulics, by their very nature, tend to damp some of the stresses being fed into the valves by the camshaft. Also, because the system keeps a working clearance between rocker and valve stem at practically zero, impact stresses are virtually eliminated.

The workings though, are quite simple. Oil is fed into hydraulic pivots which are connected to the main oil gallery at the front and rear of the head. On these sit the rockers (one per valve), a small cup covering the pivot so that oil flowing from the passage in the pivot is trapped by the rocker. This trapped fluid uses up any clearance, holding the rockers against both the cam lobe and the valve stem. Lateral movement of the rocker arm is controlled by a small framework in the head.

Gas flow through the engine is assisted by an air cleaner box with a mammoth volume of 6.2 litres. So large is it that removing the filtration and re-jetting the four 34mm carbs accordingly will only provide an additional 2bhp – sure sign of a well-designed system. At the other end, the exhaust pipes are also seemingly less restrictive – outlet diameter is up and if sound is any indication (it’s not!) gas flow is freer.

The bottom end of the motor is largely responsible for the bike’s almost total lack of flywheel effect. Blipping the throttle gives immediate and rapid engine acceleration, shutting down seems to almost stop the motor dead – a characteristic which takes some getting used to when gear changing, but more later.

Power is fed from the crank by a straigh cut gear doubling as number four piston’s inboard flywheel and from there it is fed into a conventional two-shaft constant mesh gearbox.

Narrowness has been accentuated by taking the altenator (now fan cooled) from the end of the crank and driving it (like the camshafts) from the crank centre by hyvo chain in it’s new position behind the cylinders.

The clutch has two features worthy of special note: it’s compact width and hydraulic actuation. The first contributes to keeping the cases narrow and the second makes adjustment virtually unnecessary.

Chassis & suspension

The final engine design is closely inter-related with the choice of chassis layout. Because such care was taken to minimise width, the engine could be mounted closer to the ground, thereby improving centre of gravity height without compromising cornering clearance.

Such a move would , however, result in a reduction of clearance between the sump and the ground – a situation which could be a problem when negotiating badly pot-holed roads street curbs. Honda overcame this by reducing sump size considerably while in the process of cutting down external engine dimensions in other areas.

To do this, a reservoir had to be added and oil pumped out of the cases to prevent a too-high level from causing heavy oil use and increased internal pressure. The lower frame rails were chosen for the job, incidentally contributing to cooling by increasing surface area exposed to fresh air flow. At the same time, total oil cpacity had to be increased to cope with the engine’s higher heat output, so the diameter of the tubing chosen is quite large.

Further strengthening the frame, the single-spine backbone (which was chosen for easy engine removal) is of enormous diameter.

Front suspension appears little changed from the VF750 we rode last year, although the anti-dive is apparently more forceful. When braking over rippled ground, one can easily detect anti-dive release as the blow-off valve responds to sudden overload. The fittings here are also different, using an alloy pivot block (like on Honda racers) to transfer brake torgue to the forks rather than the black cast unit of ’83’s VF.

At the back end, the engine has played a role in the final dimensions again. Because the engineers shortened the crankcases and kept the engine forwards to improve weight distribution (48.6 percent at the front) the swingarm is exceptionally long to avoid shortening the wheelbase too much – even so, wheelbase is 30mm up compared with the long-engines VF.

Naturally, Honda has fitted a single shock Pro-Link. A nice touch here has been the relocation of the three position damping adjuster which now sits under the rear frame rail where it is accessible without fiddling. Unfortunately, the same hasn’t been done with the air pressure nozzle which still hides under the sidecover and only accepts some service station air hoses. The stem is too short and can make adjustment difficult.

The same applies to the fork valve under its black cover on top of the left leg, but on the other leg the damper knob can be rotated to any of its three positions quite readily.

Braking is supplied by a new set of Nissin twin piston calipers, these ones being rough cast in grey metal (and looking almost identical to the racing units) rather than polished and coated in black enamel. Two calipers control the front spiral grooved discs while a single identical unit handles the rear.

All this braking force is transmitted to the ground through a 130/80 V18 rear tyre and a 110/90 V16 front. Interesting – while the rear is the same as that on the VF750, the front is narrower with a higher profile for the same rolling radius. The VF displayed a tendency to shake its head on sharp bumps, something which seems to be a side-effect of 16-inch wheels.

However, the same symptoms can be had from too-wide a front tyre. The reason is simply that a wide tyre shortens and widens the contact patch, reducing the directional stability of the rolling wheel. On a smaller wheel the problem is exaggerated by the wheel’s diameter which shortens the contact patch even further.

The answer? Alter the contact patch shape, preferably by using a narrower tyre. That’s exactly what Honda has done: the VF front tyre is a 120/80 while the CBX has a 110/90.

Instruments and controls

Although the CBX750 is similar to the VF in may styling respects, there are a few differences which make the CBX a more luxurious street mount.

For a start the seat is wider, and more comfortable both for the rider and the pillion who has the added benefit of a much larger and more practical grab rail.

The seating position sets the rider in a fairly upright stance, although the rearset of the footpegs puts enough weight on the hands to balance wind pressure and give a nice sporting feel to the bike. Controls are almost replicas of those fitted to the VF, but a few detail changes have taken place.

The indicator slider is now a press-to-cancel type which works well, although signalling left turns with a heavy gloved thumb takes some care as one tends to press at the same time. Beam selection is by press-on, press-off with a flasher switch set into the rear of the block and the horn operated by a large button at the lower edge of the front. The right switch block has the electric start, kill switch and headlight on, park and off slider.

Instrumentation is housed within a fairing of similar shape to that of the VF, preserving the family resemblance, but in this case with a more rakish, aggressive look and blending smoothly into the tank. Small black plastic flares are attached to the outer edges below the handlebars for no immediately apparent practical reason – they do however, enhance the bike’s appearance.

The dash panel is mounted much the same as that on the VF750, but instead of the VF’s fighter aircraft style flat facia, the CBX has domed lenses over tacho, speedo and the volt/fuel gauge. The panel itself is sectioned so that all three major instrument recesses are aimed at the rider’s eyes. Round idiot lights are set across the top.

Perhaps the most noticeable change (and decidedly one for the better) is the fitting of two square headlights in the nose cone. Each boasts 55/60 watt illumination, adding up to 110 watts on low and a satisfying 120 watts on high beam. Under these, and also mounted in the nose, is an oil cooler whose hoses reach back inside the fairing to join the front down tubes.

Overall, the finish on the test CBX was an improvement over most previous bikes (and not just from Honda!), with all the fairing parts fitting nicely and everything well layed out. The bike had a definite air of quality and attention to detail. Take for instance the steel rails under the sides if the seat and tail-piece: perfect for the odd ocky-strap or two, these also had lugs built in to locate strap hooks.

So, while the bike looks just as racy as the Vee-four stablemate, it has many more practical touches to improve comfort and make the bike more suitable for a sport/touring application.

Ahh, the riding!

From the first stab of the starter, the four pots grumble into life and although the engine emits some gear whine, the exhaust note is deep and rumbling.

In fact, it sounds much like the Kawasaki GPz and has the same lean throttle response when cold. Only a few seconds of idle are needed before the bike can be ridden away but it is advisable (as with any engine) to allow more thorough warm up.

Gear shifting takes some getting used to. The lack of flywheel inertia means the engine scrubs off speed very quickly and can mis-match road and gearbox speeds until one becomes used to it and compensates. The gearbox is also very finnicky about rear chain tension; if the chain is at all loose, shifting suffers. Action on our test bike was slightly notchy, particularly when the engine was hot after a long run, but this was never a problem in itself.

The only problem encountered during the test was that occasionally the lever failed to return to the neutral postition after an upshift into third, resulting in momentary difficulty when reaching for fourth. It is unfortunate that this should occur on a test machine because it’s the sort of thing that happens on one in a thousand bikes at the most, and can be easily rectified under warranty. Don’t think that this is a trait of the CBX generically, it isn’t likely – as I said, we were just unlucky.

That aside, riding the CBX is sheer fun. It appears almost as though Honda has taken a close look at everything in the 750 class and combined all the best points from each.

The handling is much like that of Suzuki’s brilliant GSX750ESD, giving quick, responsive steering without any compromise of stability. The horsepower is a cross between the top-end of the Kawasaki GPz and the mid-range grunt of the VF750, having performance on par with anything the class while still giving an acceptable power spread. The sound is as exciting and pleasant as the GPz’s low-rev burble and high-rev snarl. The brakes are as powerful as the excellent Kawasaki and VF Honda units – perhaps even more so.

Because the CBX takes much of its steering lightness from the VF750 it is an easy matter to zap, courier style, through peak traffic or flick the bike from one peg to the other on tight, winding hill roads. Line changing, just as with the Vee, requires miminal effort.

And yet, at high speeds this bike retains its composure well past 200kmh – I have had the bike verging on redline in sixth gear at an indicated 220kmh, and I’m sure it would go a little faster. The steering never becomes heavy but, unlike the slight unsteadiness which can be provoked on a VF at such speeds, the CBX tracks with confidence-inspiring stability and precision.

Choppy, rutted and undulating roads were all taken without misbehaviour and there was only the barest hint of the headshaking antics associated with the VF in these conditions. Excellent tyres and suspension must take a large slice of the credit here – though as supple and comfortable in compression as the VF, the CBX suspension doesn’t allow as great an attitude change under heavy braking.

The suspension is tunable both front and rear for air pressure and rebound damping but, in Honda’s recent tradition, only one fork leg gets a damping adjuster while the other carries the four-position anti-dive. These extra forces are balanced by an alloy fork brace whose rear lip doubles as an air deflector toward the cylinder head.

Honda brakes have rarely lacked power in the past, and these are no exception. Two fingers will haul the bike to an immediate halt, but that is by no means the best part – the feed-back to the rider is top notch and allows front tyre-screeching stops to be executed at will.

Sadly, the picture is not so bright at the back. Powerful it is, but unfortunately feel is almost non-existant and lock-ups were always on the cards during heavy braking until I grew used to it vagueness.

All-out sport riders will be immensely pleased with the bike’s acceleration, top speed, stability and general handling, but one minor flaw is the shortage of cornering clearance. I say minor because the ‘shortage’ is only relative; it can be leaned over quite a way before anything scrapes, but the ease with which this bike can be ridden fast calls for the same vast reserves of clearance as Honda’s own VF. However, since it is only the folding footpegs that touch (until you’re going as quickly as Ajay or Mal Campbell who also scraped the sidestand and right side of the belly fairing) this isn’t a problem and can be improved readily in any case by simply unscrewing the one-inch long peg-protectors.

Under the more sedate circumstances of town work the CBX is just as good. The engine is exceptionally smooth and vibration-free. At revs below 3500, a slight rumble through the bars and pegs, but that is only noticeable when thrown into relief by an uncanny smoothness above that speed. The lack of vibration (the mirrors stay crystal clear well past 160 kmh) had me peering once again at the engine mounts and dismayed to find these were solid rather than the flexible mounts used on Kawasaki’s fours.

The twin headlights evoked comment everywhere, not only for the striking visual effect which has rarely been seen in this country, but also on the question of their effectiveness. Most people were surprised to learn that each puts out a full 55/60 watts.

The effectiveness of this set-up ranks these among the best headlights I’ve used, giving a wide, powerful spread of light on low beam and lighting the road up for miles on high beam. In the past, I have always felt inferior when overtaking at night on the highway – the car flicks his lights low, expecting your single unit to switch on and blaze a path through the darkness. Invariably I find myself embarassed with the comparitively feeble glow I end up with. Somehow it has always left me feeling rather inadequate.

Not any more. When the Honda lights click into high gear they throw out as much light as the average sedan. Motorcycle lighting has at last reached an acceptable level.

So that’s it. That’s the third generation Honda Four. It’s a damned good bike and if Honda follows through with its apparent policy of providing a CBX and a VF for every capacity class, don’t be surprised to see a CBX900 or 1000 in the future. Mind boggling.

Just as I was contemplating the riding during the test, I was asked how the CBX750 would rate in the 750 comparison AMCN conducted in 1983. In my opinion, Honda has taken many of the points we praised on the VF, the GPz and the GSX and put them together on the one bike – sure recipe for success. I’d have to say Honda’s CBX750F would be my vote as the best Japanese 750 on the market.

Kawasaki, Suzuki and Yamaha had better think fast. The CBX is a real gun.

Specifications

Make and model Honda CBX750F

Engine:

Engine type…air cooled DOHC four cylinder four-stroke with four valves per cylinder

Bore x stroke 67.0 × 53.0mm Displacement 747cc Compression ratio 9.3:1 Induction 4 × 34mm CV Keihin Starting electric only Gears six constant mesh Clutch wet multi-plate Lubrication wet sump, forced Primary Drive gear Final Drive chain Redline revs 10,000rpm Air filter paper cartridge

Chassis and Running Gear:

Frame type 54mm diameter single spine with dual loop cradle of round section mild steel
Front suspension hydraulically damped, coil sprung 39mm telescopic forks with air assist and 3-way rebound damping adjustment.
Rear suspension single unit hydraulically damped, coil sprung shock with air assist and 3-way rebound damping adjuster in rectangular section steel swingarm
Front wheel 16” Comstar
Rear wheel 8” Comstar
Front tyre 110/90 V16
Rear tyre 130/80 V18
Front brakes twin grooved discs
Rear brakes single grooved disc

Dimensions and Capacities:

Wheelbase 1465mm Fork rake 27 degrees
Trail 93mm
Seat height 795mm
Weight (claimed dry) 218kg
Engine oil capacity 3.6lt
Headlight type 2 × 55/60

Performance:

Fuel consumption 15.5km/lt (43mpg)
Fuel tank 22 lt Engine oil consumption negligable
RPM at 100 kmh in top gear 4500rpm
Top speed after one km 208kmh
Maximum power (claimed) 93hp at 9500rpm
Maxmum torque (claimed) 7.2kg-m at 8500rpm

Test bike supplied by Honda Australia
Price as test $4299
Tested by Bob Maron