Cast of Four
“Everything that has been will be again.” Some learned type, probably Greek, said that. It’s a statement that applies quite well to motorcycle engine configurations. With the current rash of V-four Hondas just appearing in the showrooms, it’s worth remembering that Matchless produced a V-four (the Silver Hawk) long before Sochiro Honda had mastered the art of making reliable piston rings.
And when you’re talking about in-line fours mounted across the frame, you could modify that opening statement to read “Everything that has been last year will be again this year but smaller, lighter and more clever.”
The new CBX750F bears little resemblance to the CB750 that preceded it apart from the fact that both machines are motorbikes and both have in-line four cylinder engines mounted transversely.
Why they built the bike is not a hard question to answer. Honda know that most people still regard the across-the-frame-four as the best engine configuration available. Not everyone will be seduced (as I have been) by clever V-Fours and when the name of the game is sell large quantities of two-wheeled hardware, you must appeal to all of the people all of the time, or attempt to, in order to survive.
How they built the engine is a more interesting question which we’ll be taking a close look at later. First, we can deal with the basics – it goes well (it’s on the pace for its class), it handles well, onece you’ve spent some time setting it up, and it’s comfortable. It’s styled carefully for maximum visual impact – aggression being the keynote here as elsewhere.
My first impressions of the bike were gathered in South Africa and they were not altogether favourable. Maybe it was the altitiude of maybe the fact that the bikes we rode only had a few miles on the clock from new. Whatever the cause, the CBX felt unbearably peaky with all the power between 8,000 and 10,000 rpm. On reflection, it was perhaps simply that I was riding in the company of other bike journalists whose main preoccupation, like mine, was to go as fast as possible whenever possible.
Heading up the A1 from Honda’s UK base in Chiswick on a cold, wet Thursday with a mountain of luggage bungeed to the seat, our test bike felt and was a much different animal. It pulled well from about 5,000 rpm through the slippery roundabouts and out of the long wet curves, feeling remarkably safe and controllable in the most appalling of conditions.
The only time you notice the peakinessis when you’re going for it either at a race track (good excuse to get out of the office for the day) or down the back lanes. In those cricumstances, it’s like riding a two-stroke. You’ve got to be in the power band and in the right gear if you don’t want to end up halfway round your favourite corner either running out of revs or falling off the power curve altogether.
It depends on what you like. For me, having to balance the gear ratios and the revs while scratching added to the enjoyment. Other people may find it annoying, in which case I’d recommend Kawasaki’s GPz750.
But if you want to take a lazy cruise two-up down the same lane, lugging top gear at 5,000 rpm (about 70 mph) and rolling, not hurtling, through the curves, the CBX will accomplish that with ease and feel relaxed as well.
The chassis holds no surprises. It is what we have all, by now, come to accept as conventional. Light, low frontal area, acres of ground clearance, rising-rate rear suspension, air-assisted forks, anti-dive, 16-inch front wheel – in short, all the current fashions that no modern 750 with a ‘sporting’ tag would dare to be seen without.
I was not initially pleased with our test bike’s handling so spent an hour before the photo-session setting it up. The tyre pressures were wrong anyway, be away too low.
With 42 psi in the back and 36 in the front (recommended maximums) it started to feel better.
The rear Pro-Link unit has an air-preload range of 0-57 psi, adjustable through a Schraeder valve sitting on an extention hose behind the righthand side panel. Again, I opted for the maximum and set the threeway adjustable rebound damping on the middle position. Better still; the steering was just how I wanted it for hard riding – ultra sensitive with plenty of feedback.
The range of air-assistance in the forks just 0-6 psi, not enough to be worth bothering with, so I didn’t. Similarly, the anti-dive didn’t seem to have much effect how ever it was set, so I left that alone too. More useful was the variable rebound damping on the forks which suited me and the rear-end settings best in the middle position.
An hour’s work added up to a taut, if slightly twitchy handling. Anyone who ends up with one of these bikes would do well to spend some time setting it up. There’s a reasonable amount adjustment to be had particularly at the back end. Some day soon, maybe we’ll see front forks on street bikes with the same range options.
For lazier riding, when you want to allow yourself a bit of leeway to change slovenly lines through bends and the like, I found that dropping the rear pre-load down to between 30 and 35psi gave a more comfortable ride and slowed the steering down noticeably. However, even if you carry only a light pillion passenger, you need to keep the air pressure in the rear unit up around the 50psi mark to stop the back wheel getting twitchy.
The details are good in places and not so good in others. The seat, for example is difficult to refit properly. It’s too easy for the two sharp catches at the rear to miss their holes and scratch the tail panels in process. The imitation plastic bellmouths on the two outer carburetors won’t fool anyone and can only be described as tacky. The left-hand frame downtube also acts as an oil pipe. Sensibly, it carries a warning sticker to say that when the engine’s hot, the frame tube willbe also. The sticker doesn’t lie.
Not so good is the fact that there is an oil pipe coming from the sump just below the front lefthand engine mounting. Thgere is very little clearance between this oil pipe and te lower, left hand frame rail.
Still in poorish details, the oil filter is awkward to get at. It’s of the disposable, spin-off cannister type and site snugly behind the closely tucked in downpipesof the exhaust. OK, until you come to drain the oil for which, as you know, you need the engine to be hot. Even with the bellypan off, it doesn’t look terribly easy to avoid burning your hands unless you’re either kitted out with a thick pair of gloves or want to wait for the motor to cool down before you remove the filter.
Let’s talk about the good details. The air adjustment and rebound damping adjustment for the rear Pro-Link suspension are intelligently sited. No chance of trapped or even dirty fingers in that area. The rebound is set by a push-pull knob mounted on the righthand rear frame rail just below the seat.
Passengers are thoughtfully provided with an adequate, well-postitioned grab rail. The new clocks with their bright yellow lettering are well placed and easy to read and, guess what, you can actually rely 100 percent on the large fuel gauge.
The small fairing, with its tinted screen keeps most of the high wind away from the vital areas so there are no screaming neck muscles during or after sustained high speed blasts. This component comes in two halves: another good idea; if you damage one side in a crash you only have to fork out for half a fairing, not the complete unit.
The twin headlights are worth a mention too. On full beam they pump out 120 watts, giving you more light on the road than any other production roadster we have tested so far. Other manufacturers please note.
And finally, before moving on to more fundamental matters inside the engine, the exhaust is considerably more raucous and good to listen to than the muted whispers that we are just beginning to accept for most bikes. Good for the owner; maybe not so good for his neighbours.
The engine itself is not altogether new but it’s the first time it’s been seen in this country. It’s a variant of the CBX 650 powerplant but that bike was not imported into the UK.
Two facts are immediately apparent when you take your first close look the CBX750 engine: one, it’s very narrow and two, it’s very short. The narrowwness has been achieved by the increasingly common expedient of mounting the alternator behind the cylinders rather than on the end of the crankshaft. It’s driven by a short Hy-Vo chain from a pinion in the centre of the crankshaft. The alternator turns at 2.24 times the engine speed, so achieving a high output despite its small physical size. It needs to be powerful because as already mentioned, the twin headlights soak up 120 watts on main beam. Maximum output of the alternator is 320 watts at an engine speed of 5,000rpm (11,200 alternator rpm). It is kept cool by a pair of fans integral with the construction and, because of the high speeds involved, the roller bearings on which it runs are fed oil under pressure. A shock absorber is incororated in the driveline which reduces the shock load on the alternator during sudden changes of engine speed and also gives the drive chain an easier times.
The shortness of the engine has been achieved by means which are quite conventional on smaller engines but which, so far, appear on just a couple of multis. Instead of the drive being taken by chain (or gear) from the crank to a primary shaft from which the transmission is then driven (as well as other items like the oil pump an occasionally the alternator), one web of the crankshaft is made in the form of a gear pinion and this pinion then drives the clutch direct. Obviously, without a primary shaft (often referred to as a jackshaft) to accomodate, the engine can be made that much shorter.
The pinion which forms part of the clutch basket assembly is made in two pieces spring loaded against each other so that, when detached from the engine, the teeth are offset from one another. This offset between the teeth is taken up as the drive is transmitted, eliminating backlash that can occur when two sets of gear teeth do not mesh exactly.
The clutch itself is a two-way item so arranged that when a heavy backload caused either by brutal downshifting or sharp deceleration in a low gear is transmitted through it, more than half the plates slip. This avoids locking the rear wheel and also prevents the wheel from hopping as the suspension attempts to extend itself to the limits of its travel.
This effect is achieved by having a two-piece clutch centre (see photograph) the outer part of which is equipped with a sprag clutch that allows the outer portion to rotate in either direction. Under normal operation, the sprag clutch is engaged allowing drive to be transmitted in the usual way. Put a backlaod through it and the sprag clutch disengages allowing the plates to slip.
Don’t worry if you still don’t understand, we’ll be dissecting the particlular piece of engineering in Mechanics in the near future.
If you don’t know, this two-way clutch is also a feature of the V four Hondas.
Up the top, there are four valves per cylinder driven by the now compulsory twin overhead overhead camshafts. The camchain tensioner is similar, but not identical, to the one used in the V-four engines. The adjuster arm is connected to the tensioner body by two pins and when assembled, can best be described as a parallelogram of variable geometry (remember your maths lessons). The camchain runs behind the tensioner arm as that the chain is pulled in towards the adjuster body to keep it taut, rather then pushed away from it. Honda are not alone in having an unhappy record with camchain tensioners and we can only hope the they’ve got this one right.
The valves themselves are operated via hydraulic tappets. It would take at least a whole page to describe in words how these items work so refer to the three illustrations accompanying this test for a full explanation. Hydraulic tappets, incidentally, are not all new (What is?). They’ve been used in car engines for many years and on Harley Davidsons for almost as long.
As the illustration shows, the engine oil is the hydraulic medium used. Oil, of course, is non-compressible but get any air in the system, and the tappets would not work. Therefore, the lubrication system has to have a means of supplying air-free oil to the tappets. Oil is pumped up from the crankshaft main bearing gallery into a separator chamber mounted in the top of the crankcase half. Any air bubbles rise to the top of the chamber. The bubbly oil is then pumped to the alternator bearings. Air-free oil is drawn from the bottom of the chamber and pumped to the camshaft bearing surfaces. It then drains down into defoaming chambers mounted above the tappets (see photograph) and neat, air-free oil is finally fed to the hydraulic tappets.
The advantages of all this is that hydraulic tappets require no routine maintenance, so the tedious task of setting valve clearances will be a thing of the past for CBX750 owners.
One final detail is worthy of mention before redirecting our thoughts to the bike as a whole. The gearbox output shaft oil seals are held in a separate plate that bolts on to the side of the gearbox. That means that when the seals need replacing, you simply have to drain the oil, remove the plate and replace the seals. You don’t have to mess about trying to fit seals into the housing ruining them on the shaft and you don’t have to pull the whole engine to pieces to do the job.
We need some more thoughtful touches like that on production motorcycles.
Right then, what does all this mean? Well for a start, the hydraulic tappets as well as being maintenance free make the top end of the engine uncannily quiet at all points throughout the rev range. The setting up of valve clearances when buckets and shims are used is costly, tedious and a job that many people are shy of tackling anyway. So maybe the absence of this task will make CBX750 owners more inclined to do their own servicing once the warranty period has expired.
Servicing generally would appear to be pretty straightforward on the bike as a whole and any moves to keep running costs down must be applauded.
We were impressed by the power output as we were by the overall design. On the LEDAR dyno, we measured 80.2 bhp at the gearbox sprocket, making the CBX the most powerful standard production 750 that Mechanics have ever tested. The only one near it was last year’s Suzuki GSX750 ES with 77 bhp.
On the road this gives a top speed of 132 mph give or take a couple of mph depending on the weather conditions. It also gives standing quarter mile times in the low twelves, which means that it is well up with other bikes in its class.
What you don’t get are the specatacular top speed and quarter mile figures that we recorded last year in the VF750F because you don’t get narrowness of the power unit or, we suspect, the same high level of streamlining. On the other hand, you get a much simpler engine, a slightly lower price tag and cheaper, easier maintenance.
Reasonable fuel consumption was a rare commodity during the test, mainly because I couldn’t resist the engine’s frequent invitations to rev the nuts off it. Restraint finally took a hand on the way back from the speed testing session at MIRA when I covered a hundred miles not exceeding 5000 rpm either cruising or through the gears. Those revs give a comfortable cruising speed just over 70 mph and trickling through Leicester city centre en route to a brief social call, I found that the CBX would pull away from as low a 1500 rpm smoothly if not spectacularly in top (sixth gear) – 43 mpg was the best figure recorded.
The gearchange was mostly so slick in operation that I hardly noticed it. The only problem, which occasionally occurred during rapid shifting, was a reluctance to engage top gear.
My personal preference in the 750 class still rests firmly with the VF750F which I regard as the most complete motorcycle on the road at the moment. But we’re not all the same and weighing the merits of the CBX750F against the other in-line offerings from Suzuki and Kawasaki, it has got to be worth a serious look from anyone who wants a straight four with lively performance, adequate touring ability and sound handling. After 1100 UK miles I wasn’t heartbroken to take it back but, on the other hand, if I found one in my yard with the keys in, I certainly wouldn’t be in a hurry to get rid of it.
Standing start 1/4 mile…12.20s/111.19mph
Fuel consumption average……………..37.4mpg best………………..43.58mpg worst……………….31.3mpg
Good points……………low maintenance
Bad points…………….pretend bellmouths
Performance……………par for its size
Handling………………good plenty adjsutment
Value…………………a touch pricy