The "Two Wheels" Road Test of the RC17

Disclaimer: This article is used here without permission of ‘Two Wheels’ magazine. However, I fully recognise their copyright to all the words that they wrote.

SHAZAAM!

Like a bolt of lightning from the sky comes Honda’s CBX750, the fastest, brawniest 750 we’ve ever tested.

As any historian will attest, time has a habit of making events come full circle. Think back to 1969 when Honda stunned the world with the CB750, the most powerful 750 ever unleashed. Ever since, other Japanese manufacturers have joined Honda in a seemingly endless conflict to decide the performance king of the bunch. Now in 1984, another across-the-frame Honda four has rocketed to horsepower supremacy. And although the CBX750 might not be as far ahead of its opposition (chiefly the VF750 and Kawasaki’s GPz) as was the first Four, it’s without doubt the new leader in the three-quarter litre power derby.

The whispers are out that this may be Honda’s last fling at an across-the-frame four. If this is the case the concept is going out on a winning note. The CBX is arguably the best 750s currently available, a compact sportster package of immense stature – and doesn’t it look the part? The swoopy VF styling has been carried through to the new four and, in the opinion of many, improved as well.

It’s difficult not to get excited about the purposeful look of the CBX. The black Comstar wheels and motor, black chrome exhaust system, frame-mounted quarter fairing, twin headlights, aggressive bellypan and alloy clip-ons look very swish indeed. However, don’t be fooled by what, at a distance, seem to be ram tubes nestling behind the carburettors. These are very flimsy alloy covers over the airbox – a certain degree of overkill?

One good point is that these little falsies are the only blemish on the bike’s superb finish. The CBX is close to the best in the Honda stable in terms of finish quality and careful attention to detail. It seems that Honda has spared no effort to complement the performance and handling aspects with other less crucial but nonetheless quite important niceties. The twin headlight is a blinder, the taillight is almost as impressive, the oil cooler is tucked safely away under the nose of
the fairing behind a small air scoop, switches and instrumentation are excellent, controls well positioned, and accessibility of suspension adjustment controls very commendable.

The sportster styling is not an illusion either; the CBX is a veritable wolf in wolf’s clothing. It is faster and more powerful than a speeding VF and a notch better in the handling department as well. That’s no mean feat! It’s not really too surprising the the CBX is at least in the same ball park as the VF since the running gear is very similar. Up front there’s the 16-inch wheel, air-assisted forks with anti-dive and rebound adjustment, and at the rear an 18-incher, a rectangular section
swinging arm and rising rate monoshock system – all tied together by a very rigid but quite conventional frame.

Same family, same behaviour

It’s also not too surprising that the VF and the CBX have a degree of commonality in their road behaviour. Certainly the riding positions are very similar. Footpegs are quite high and set slightly rearwards so the rider leans forward, sportster style. However the forward attitude isn’t severe enough to induce wrist ache around town and in this regard the Honda is better than the Kawasaki GPz. At low speed the rider isn’t going to suffer from too much weight on the wrists and at high speeds the air blasts reaching the the unprotected upper body balance the forward lean
nicely. Tall riders may find that their knees are forced upward too much by the high pegs, but most will find that CBX to be a comfortable fit.

In other ways the bike feels quite different from its V-four counterpart. Suspension travel has been reduced by 10mm both ends and the motor sits very low in the frame; consequently the CBX has a lower seat and doesn’t feel as top heavy as the VF. Dry weights are identical so the bike is noticeably easier to control at a walking speed. It is very well balanced and if it wasn’t for the peaky power delivery would be distinctly superior around town.

The motor of the new model bears more than a passing resemblance to the CBX650E’s. Like the smaller unit, it uses a gear primary drive direct to the clutch rather than the chain-driven primary shaft more commonly found in Honda’s fours. Also in common with the 650, width has been reduced by positioning the alternator behind the cylinder block. This unit is chain driven from the crankshaft and cooled by a fan at the rear of the alternator shaft. Unlike the smaller motor the 750 has seen a drop in dimension in another plane. By cutting down the sump volume,
the height is reduced and the motor can be hung a lot lower in the frame without causing any clearance problems. To compensate for the smaller oil volume in the sump the two front frame tubes are used as a subsidiary reservoir. There are additional benefits to this measure:
better oil cooling and a neater way of routing the oil to the cooler.

Another of the important characteristics of the motor is low maintenance. Backing up the fully electronic ignition and the auto-tensioned cam chain are the hydraulic valve rocker pivots first seen on the 650. The ingenious system has been further refined by reducing the maze of oil lines used to supply the pivots on the 650.

Set and forget?

With valve clearances, cam chain tension and ignition looked after automatically regular maintenance chores will be restricted to oil changes, drive chain tensioning and the occasional tune-up. As a bonus the new CBX also offers far easier mechanical accessibility than Honda’s V-fours.

Although the bike has such predictable CBX features as double overhead camshafts, four valves per cylinder, and a forged plain bearing crank, a myriad of changes are involved in the transition from 650 to 750. The new model’s greater capacity results from a boost in bore of 60 to 67mm and decrease in stroke from 58 to 53mm. Valve size has been increased, the crankshaft strengthened and the 32mm CV carbs replaced by 34mm units. Surprisingly, the compression ratio at 9.3:1, is lower than the 650’s 9.5:1.

The end result of these modifications is a quite substantial boost in power. Honda claims a maximum output of 66.9 kW at 9500 rpm and a peak torque of 69.7 Nm at 8500 rpm, an increase of roughly 20 percent on the factory figures for the 650. That’s remarkable considering that capacity has been increased by a mere 91 cm*3, or less than 14 percent. Claimed output is even higher than the VF (66.3 kW).

Strapped to the dyno the CBX provided proof of Honda’s claim. The maximum reading of 51.3 kW at 9500 rpm was 2 kW more than the VF, and 2.7 kW up on the Kawasaki GPz, so the CBX can lay claim to the title of the most powerful in the 750 class.

However, peak power is only part of the story when it comes to useful on-road performance. While the CBX may win the numbers game it does not lose out on midrange compared with the V-four. In essence, its power delivery characteristics could be described in terms of the 750s tested in the 3-way comparo (TW, Sept ’83). The CBX has the bottom end of the Kawasaki, the midrange of Suzuki’s GSX750 and a top end which is a shade better than the VF and the GPz. It is a reasonably peaky power delivery, with no appreciable midrange sag, and a little boot in the pants above 6000 rpm. You need to use the gearbox a lot more than the VF on the open road but when you do the fireworks are released.

At the strip the CBX equalled but didn’t topple the V-four. A 12.0 second streak was on par with the VF and 0.15 seconds quicker than the GPz. The CBX pulled a better 400 m terminal speed (180 km/h) than the others, and also topped them with its 220 km/h maximum speed.

Pick a card, any card

The bike has yet another ace up the sleeve. As well as providing scintillating power characteristics the motor shows a great degree of refinement. It spins freely, has good engine braking and is very smooth right through the rev range. The mirrors may get fuzzy over 5000 rpm but the rider is almost totally insulated from annoying vibes. Starting from cold is a “full choke and one press” affair and warm-up time is very short. At idle and low revs the CBX sounds very much like a GPz750 and near redline emits a wail very much like the RSC racers campaigned
in Australia two seasons back. The Honda sounds and performs like a red-hot sportster.

There is only one significant spanner on the works: all this performance comes at some cost, for the CBX is very thirsty. Hard riding sees the economy plummet to 13.2 km/l. Around town the test bike returned 15.1 km/l and even at a steady speed limit cruise gave only 17.6 km/l. “Economy” is not a part of the newcomer’s vocabulary.

Complementing the superb going performance is an equally impressive stopping system. The twin spirally grooved discs up front have the usual Honda double piston floating callipers with sintered metal pads, and the rear disc is similarly equipped. We found their performance to be superior. Front stoppers were really powerful and offered great feedback. Feel is especially good at high speed and unless a rider is extremely careless front wheel lockups will be very unlikely to
occur. It is possible to get the front tyre to chirp during stops from very high speed if the road is a little rough but there is certainly no drama involved in these exercises. In fact, the CBX demonstrates beautiful stability during harsh braking from breakneck speeds (thanks to the effective antidive) and repeated applications didn’t induce any discernible fade.

The rear brake was powerful enough but not as impressive overall due to its lack of sensitivity at speed. At a low rate of knots the feel is acceptable but it’s a little too easy to lock the rear wheel during braking from insane speeds.

Trick clutches ‘n’ things

Not unexpectedly the bike incorporates all the latest transmission components that Honda can muster – notably the one way clutch which helps prevent the rear wheel locking up on sudden downshifts by disengaging half the plates when sufficient overrun force is applied to the clutch mechanism. The clutch has a very smooth action but unfortunately the engagement point is sudden and the take up zone too narrow for a bike with the 750’s moderate low rpm power.

The six speed close ratio gearbox was smoother than the VF we tested last year, but suffered from a hint of vagueness in the changing action and a longish throw. Successful changes called for a bit of muscle, which is a pity since downchanging especially could be feather-touch operation if the gate was shorter and the vagueness removed.

We have no complaints about the gear ratios. Though they’re evenly spread and although the CBX runs a slightly taller sixth than the VF’s fifth the bike has the extra power to compensate. The one serious bitch we have with the transmission is that the bike still betrays the Honda tradition of excessive drivetrain freeplay.

With such a high level of performance and braking it would be surprising if the handling didn’t rise to the same standard. No worries, it does. Steering is light yet very precise so swapping lines at high speed is super easy. The Honda is a delight to push through both tight bends or high speed sweepers thanks to the brilliant steering and the bike’s well balanced feel. Most surprising was the discovery that the front end didn’t display as many nasty habits over bumpy stretches as most other bikes with 16-inch front wheels. Sure, you can induce a modicum of head-shaking antics over big potholes and large corrugations, especially when the rear suspension working hard as well, but generally this behaviour doesn’t reach the same desperation levels as it occasionally can with the VF, for example.

At high speed the bike’s stability was top rate, again a shade better than the V-four. Why this should be so is a bit of a mystery, since the CBX has a shorter wheelbase (1465 as opposed to 1495 mm) and steering geometry is biased more towards quickness than high speed stability. Perhaps the long sloping nose of the fairing provides more downforce on the front end. Still, that’s the way it came out, rock solid over all but the roughest terrain.

Not quite best at leaning

Cornering clearance is excellent too, though not up to untouchable talent of the VF. However, only the most adventurous riders of the new four will clip the folding footpeg lugs in the tarmac with any regularity.

It’s not very surprising that the Honda is such a proficient handler considering the quality of the suspension and frame components. Forks have an unusual mix of adjustability pioneered with the VF. The right fork has three rebound damping settings adjustable by rotation of the knob at the top of the leg and the left one wears the four way adjustable TRAC antidive. Unlike the VF the antidive settings can be altered by turning a small knob on the front side of the fork, so there’s no need for a screwdriver. Each fork is air assisted, a balance tube is fitted and the
valve is situated on the top of the left leg. A strong alloy fork brace is used to counteract differences in compression and rebound damping incurred through the action of the antidive on one fork and the other.

Rear suspension is provided by Honda’s ProLink monoshock system. The valve for the air springing is located under the right sidecover and is sensibly sited on a long pull-out arm so the whole spectrum of different pumps will be capable of doing the job. Rebound damping force can be changed to either of the three available damping settings by yanking on a knob attached to the frame tube on the right side just beneath the sidecover. This knob is connected by a cable to an adjustment ring on the top of the monoshock. It’s possible, but difficult, to make this adjustment on the move.

At first sight the CBX’s frame may not look as impressive as it’s V-four counterparts but according to Honda the design is the result of exhaustive computer simulation with different load and stress parameters and represents the lightest and strongest unit possible. We’re not about to argue. The frame may be a fairly traditional oval and round section double cradle but rigidity has been enhanced by the huge single backbone, a cross brace between the down tubes under the exhaust ports and another huge brace running from each side to the backbone at the rear of the motor. Strength has been designed in where necessary.
The combination of rigid frame and quality suspension promotes superb roadholding, a feature aided by the very good Japanese Dunlops. These tyres worked very well in the dry, but unfortunately the weather was so good during the test that it wasn’t possible to ascertain wet weather behaviour. Tut, tut, such a pity…

One point on the handling remains. If your regular schedule includes a liberal sprinkling of dirt roads the CBX may be less suitable than other 750s with larger front wheels (the GPz for instance). The 16-inch front wheel makes the bike a bit of a handful in these situations and the low bellypan would be very easy to leave behind on any unyielding rock.

A tad tough on the toosh

The only criticism about suspension performance relates solely to comfort. Although the front forks are biased towards tautness the smoothness and compliance are beyond reproach. However the rear unit feels too harsh and hasn’t the initial compliance of Suzuki and Kawasaki monoshocks. Ride comfort also suffers a little because of the narrow seat. The pillion passenger is better served in this regard but tall passenger will have to contend with high-sited footpegs.

Don’t get the idea that the CBX is excessively uncomfortable on a lengthy cruise – it isn’t. The major factor that will limit the time spent in the saddle will be the relatively poor touring range if the bike is ridden in the manner its makers intended. High speed fanging will reduce the distance between fuel stops to much less than 300 km. That may not be horrible, but it’s not that good either.

In designing the new four Honda seems to have been very careful not to duplicate the few rough edges found on the VF. The gearbox is better, the lights improved, and the switches cleaned up as well. The on/off control for the headlight has been resited to avoid incidents where blinker activation can douse the headlight. At the same time, the excellent push button high beam switch and index finger operated pass switch of the VF have been retained.

Additionally, Honda has now fitted a manual cancel button for the indicators. It’s a good, but not perfect, system. You have to be careful not to apply any downwards force on the indicator switch or they’ll be cancelled before they start. After a time one adjusts to this foible and all is sweet, but it is something to watch in the initial association. A collar type choke control is handily located on the left switch block,
as with the VF750 and Yamaha’s XJ range. All things considered the layout and functionality of the switches is excellent.

The instrument console is quite different from the VF. The tacho is the centrepiece with a slightly smaller speedo on the left and a voltmeter and typically inaccurate fuel gauge on the right. Idiot lights are sited at the top of the console. The verdict? Excellent again.

In fact the only grumbles about the CBX ancillaries involve the side stand and the horn. Honda insists on using a minute foot to the stand so very solid surfaces are needed for guaranteed support; while the horn emits a very feeble beep. It’s definitely time Honda fitted twin horns to its sportsters.

It is impossible to walk away after a ride on the new CBX750 without thinking that this bike is a bloody ripper. Superb handling, a fiery top end and excellent brakes earn Honda the distinction of providing the best 750 sportster yet. Sure, the power delivery is peaky but you’ll find the bike’s other talents more than compensate for this arguably minor shortcoming. In addition, it offers an exalted level of overall
refinement, finish, and low maintenance mechanicals. If you consider yourself hot-blooded but discerning the CBX could well be singing your song.

Vital Statistics

ENGINE

Air-cooled transverse four-cylinder four-stroke, Chain driven double overhead camshafts, four valves per cylinder, automatic hydraulic valve clearance adjusters. One-piece forged crankshaft, plain main and big-end bearings. Wet sump lubrication.

Claimed maximum power 66.9 kW at 9500 rpm
Claimed maximum torque 69.7 Nm at 8500 rpm
Bore x stroke 67 × 53 mm
Displacement 747 cm*3
Compression ratio 9.3:1
Maximum engine speed 10,000 rpm
Carburation 4 × 34 mm Keihin CV
Air filtration Pleated paper
Starter system Electric only
Ignition Solid state battery/coil

TRANSMISSION

Gear primary drive through wet, multiplate clutch to six-speed, constant mesh gearbox. Left foot change, one-down, five-up pattern.Final drive by roller chain.

Ratios (overall:1) (Km/h per 1000 rpm in brackets)
First 15.02 (8.3)
Second 11.19 (11.2)
Third 8.76 (14.3)
Fourth 7.18 (17.5)
Fifth 6.21 (20.2)
Sixth 5.59 (22.4)
Primary reduction 1.708:1 (73/41)
Secondary reduction 2.813:1 (45/16)

FRAME AND BRAKES

Welded tubular steel double cradle frame. Air-assisted telescopic forks, three-way adjustable rebound damping, four-way adjustable mechanical antidive. Rear suspension by ProLink rising-rate system, single air spring unit, three-way adjustable rebound damping. Twin-disc front brakes, single disc rear, double piston floating callipers.

Front suspension travel 130 mm
Rear suspension travel 90 mm
Fork rake 27 degrees
Fork trail 93 mm
Front brake diameter 280 mm
Rear brake diameter 280 mm
Front tyre 110/90 V16 Dunlop K527
Rear tyre 130/80 V18 Dunlop K527

DIMENSIONS

Seat height 790 mm
Wheelbase 1465 mm
Ground clearance 140mm
Fuel capacity (incl. reserve) 22 litres
Fuel reserve 4 litres
Engine oil capacity 3.6 litres

CALCULATED DATA

Weight to power ratio (90kg load) 6.02 kW/kg
Specific power output 68.5 kW/litre
Mean piston speed a redline revs 17.7m/sec

PERFORMANCE

Acceleration
Standing 400m 12.0 secs at 180 km/h
Zero to 100km/h 4.5 secs
Maximum speed 220 km/h

Braking
From 100 km/h to zero 34.6 metres
From 60 km/h to zero 12.1 metres

Fuel consumption
Touring 17.6 km/litre
City 15.1 km/litre
Hard riding 13.2 km/litre
Average on test 15.8 lm/litre

Best points: Styling and finish are tops. Bike is very refined overall. Sports capabilities are enhanced by fiery top end, excellent brakes, light and precise steering, and excellent high-speed stability. Twin headlight is a blinder, and low-maintenance aspects of motor most very welcome.

Worst points: Power output is fairly weak at low speeds and into the midrange, while motor’s appetite for fuel is heavy. Rear suspension could do with better initial compliance, while behaviour of the front end over big bumps is still not perfect. Sidestand and horn need improving, seat could be wider.

This review was typed in by Mister_T (tedp@SPAMOFF.replicant.apana.org.au),
so all typos are his fault. Thanks for typing it all in for me, Ted.