[technicalities]

[TECHNICALITIES]

GORDON H. JENNINGS

WHATEVER ELSE. the Isle of Man Tourist Trophy races may be. they are a little bit of heaven for the nuts-and-bolts enthusiast. All of the very latest Grand Prix machines are there, and with a few secretive exceptions (Honda being one) the teams do not mind too much if one visits their garages for a bit of polite snooping. There are, of course, some features of the bikes that they do not particularly want to discuss, but when the equipment is there, in pieces on the work benches, all of the basic information is supplied automatically.

Technically, the most interesting machine at the island this year was the 250cc Suzuki “four.” This motorcycle has a frame and suspension layout much like that of the Norton Manx (only smaller, of course), which everyone seems to be copying. However, its engine is like nothing that has gone before, being a “squarefour” two-stroke, water cooled and with 4 separate rotary intake valves. This layout sounds exceedingly complicated, but in point of fact it is quite a straightforward bit of design work. A single casing, split along the horizontal plane, serves as both crankcase and transmission casing, and when it is split, the crankshafts and transmission shafts can be lifted right out. One interesting design point is that the coupling gears for the two crankshafts are located in the middle of the engine, so that there arc in effect four crankshafts, bolted in pairs to central gears, and with the gears for each pair in mesh. Power is transmitted to the clutch by a gear drive taken from the “rear" crankshaft gear.

The ignition system is rather remote from the engine proper. Instead of being connected directly to either of the pairs of crankshafts, the ignition is driven from

a pinion in the gear train that couples the engine with the clutch. And, there are two separate elements to the ignition system: a low-tension spark generator (actually an alternator): and a sort of “distributor,” with breaker points, that feeds the low-tension electrical pulses to a bank of coils, which provide the high-tension spark for the plugs. Considering that the low-tension alternator must be timed to the distributor, and both timed to the engine. and that ignition lead is very critical in two-strokes, one cannot but have a great deal of respect for the Suzuki crewmen who must be more nearly full-fledged engineers than mechanics.

The Suzuki four's cylinders are cast in blocks of two — and are paired longitudinally. rather than transversely (as one might expect). This has apparently been done in the interest of getting the most effective flow’ of cooling water. Water is fed from the radiator into the gallery cast in the side of each cylinder block (presumably with orifices aimed at the hot exhaust ports) and returns from the forward end of each cylinder head. Although the piping is separate to each of the two cylinder blocks, there is a single wide, low radiator with large capacity tanks at the top and bottom to which the 4 coolant pipes are connected. No water pump is employed; thermosyphon provides the necessary water circulation.

It goes without saying that each of the Suzuki’s 4 cylinders is equipped with its own carburetor — and each has its own float chamber as well. The float chambers are rubber mounted, to free them of any vibration effects, but oddly enough, the carburetors are bolted solidly to the engine. Another oddity is that the carburetors (except for their remote float chambers) closely resemble an ordinary Amal Monobloc. Their metering needles are centered in their throats, and there is a “choke” slide that drops right down in front of the throttle slides — where it cannot be used w’ithout restricting the passage of air through the throat. The Suzuki is, however, known to be extremely sensitive to mixture strength, and it may be that a finer mixture control is obtained with these semi-touring pattern carburetors.

At the Isle of Man. the weakest point on the 250cc Suzuki was its drive chain. A very light chain is used, and virtually all of the reduction gearing is in the transmission, so the “countershaft” sprocket and rear wheel sprocket are quite nearly the same size. This arrangement places exceptionally heavy loads on the drive chain, and in the case of the Suzuki the chain showed its displeasure by breaking several times. Probably, the chain is strong enough to take engine torque, but the engine, because of its small-diameter, ultra-light flywheels, will gain revs at an astonishing rate when the rear wheel is bounced free of the road surface and this creates shock loadings, when the wheel again returns to the road, that break the chain.

With all its problems, the Suzuki four is still one of the most promising machines in the 250cc class. When it does run properly, which is admittedly seldom, it has more sheer speed than almost any of its rivals. Handling is something else. The engine’s narrow power range (from 10.500 to 12,500 rpm) and its tendency to “come in” with a sudden surge make things rather difficult for the rider. Moreover, the gearbox has been redesigned to give 5 speeds, with gears wide enough to take the power, instead of the original fragile 6 speeds. This, of course, makes it all but impossible for the rider to keep the engine working in the middle of its power range and as a consequence he must gather speed in a series of sudden surges as he changes up through the gears. Even so, the Suzuki is occasionally terribly fast (as we have said) and if the project en gineers can tame the beast to the same level of tractability and reliability as their 50cc race-winning single, the fabulous four could well become a real force in the 250cc class.

(Continued on page 14)

While the Suzuki might become the machine to beat in the 250cc class, Honda has a bike that actually is the undisputed master of the 125cc class. Hondas new 125cc four, which appears to be a scaled down version of their very successful 25()cc four, made its debut at the beginning of this season and immediately proved itself to be virtually unbeatable. We saw a trio of these machines in action at the island, and it would be no exaggeration to say that they simply ran away from everything else.

Actually, the 125 four is something a bit more than a scaled-down 250. In the first place, it is not possible to simply

reduce all dimensions; the job requires a complete re-engineering, and we may assume that the only thing carried over from the 250 was the basic layout. So, Honda has produced an all-new machine in their 125, and it speaks volumes for their racing design department that these new machines should be so fast and so reliable in their first outings. Having so many cylinders to fire, at such a rapid rate, Honda has, in common with others in GP motorcycle racing. gone over to a transistorized ignition system. They still use an engine-driven alternator, to produce low-tension current for the ignition system, but instead of the breaker points feeding current directly to the high-tension coils, the points merely control a series of transistors, and the transistors actually “valve” current to the coils. Some of the other manufacturers have experienced difficulties with transistorized systems, and have abandoned them in favor of a return to magnetos or conventional energy transfer systems. Honda has made the transistors work, and there is

(Continued on page 16)

no doubt in my mind but that by so doing they have secured yet one more advantage for themselves — as if they did not already have enough.

With speeds going higher and higher, streamlining is becoming increasingly important and it is obvious, in inspecting the better equipment, that this point has not been overlooked. Regulations dictate conditions that produce great similarity among race fairings, but there is nothing to prevent a narrowing of the fairings, and that is what is being done. As much as is practical, power units and frames are being narrowed, and fuel tanks are shaped to fit down between frame tubes so there will be nothing to prevent riders from getting down flat on the machines. Rules regarding minimum handlebar width

are circumvented by placing grips and controls right in against the fork crown, leaving inches of bare, unused handlebar protruding out beyond the fairing. In this quest for minimum frontal area, the nowuniversal two-stroke carburetor layout (standing straight out from the sides of crankcase rotary valves) has become an embarrassment. Room must be left between the side of the fairing and the carburetor mouth for air to flow, or power will suffer. Yet, the carburetor mouths extend out from the sides of the engine to such an extent that to make the fairing that wide would greatly increase drag. The solution seems to be to narrow the fairing, and have blisters on the sides of the fairing to give room for the carburetors. As yet, no one has made these blisters into air-scoops, but it would appear to be only a matter of time before this is done.

Talk among the riders indicates that Dunlop's new RMT-1 tire, which has a triangular section to place a lot of rubber on the road when banked over in a corner, is not without disadvantages. They (the riders) say that the RMT-1 is fine when a bike is upright, or when cranked well over in a turn. However, there are situations when the bike will be cornering at not quite the maximum angle of lean, and in such situations the RMT-1 tire tends to be a trifle unsteady. It does not slip or slide, nor indeed place the rider in any sort of peril, but it does make the machine waggle just a bit, and that makes the rider somew'hat nervous whether or not he is actually in any danger.

Hondas filled the first 3 places in the 125cc races at the Isle of Man, and (quite justly) got most of the attention, but there was a little CZ twin that did extremely well, too. It averaged 5 mph slower than the Honda fours, but it beat a whole gaggle of Honda “production” -¿wins, and that takes some doing as the Honda CR-93 is an extremely rapid motorcycle. Basically, the CZ is a very straightforward motorcycle, but it has some attractive and unusual features. The frame is of the duplex tubular variety, and one of the upper frame tubes serves as part of the crankcase breather system. A rubber hose, leading up from the combined crank/transmission case, feeds crankcase fumes into the frame tube, which is open back by the seat and gives a vent to the atmosphere. There is, incidentally, a very deep wet sump under the crank/transmission case to hold the oil supply, which lubricates both engine and gearbox.

The CZ’s camshaft (there are two, in the head) drive is through shafts and bevel gears. A set of bevels at the righthand end of the crank drives a shaft which leads up to the exhaust camshaft, and another shaft, with appropriate sets of bevel gears, takes the drive back to the intake camshaft. There are two valves per cylinder — and two spark plugs. A pair of breaker points, inside a housing on the end of the exhaust camshaft, take care of the spark timing, and each set of points connects with a double-lead coil, similar to the ones used by Honda and HarleyDavidson, and each coil provides the high-tension spark for the two plugs in each cylinder. Unlike most others, the CZ does not employ any sort of enginedriven spark generator; instead, there is a whole bank of small storage batteries, which lasted the duration of the race (almost an hour and 20 minutes) without faltering and without contributing unduly to either bulk or weight. It would appear that so long as not too many sparks per minute are required, the old battery/ points/coil ignition system will still do the job very well.

(Continued on page 18)

For the time, all of the first-line racing machines have brakes that are satisfactory, and most of the brakes follow the same pattern. At the front wheel, everyone is using dual drums, with pairs (both “leading” in most cases) of shoes in each drum. A single drum, usually with one leading and one trailing shoe is more or less standard at the rear wheel — which carries a small percentage of the braking load. There is some interest in discs, and in fact there was to be one machine at the island with American Airheart/Gunter disc brakes (it failed to appear) but for as long as drum brakes will continue to do the job, the discs will probably remain an oddity. The only notable exception to the brake fashion we

have described is seen on the new Honda 50cc twin. On that machine, braking is accomplished by mechanically-actuated calipers pressing against the wheel rim. The arrangement is quite similar to that employed on lightweight racing bicycles, and although it is not very elegant, it appears to be doing the job.

Hydraulic steering dampers are very much in vogue these days. These devices have been used on the Japanese racing machines for some time, in various configurations, and now they are beginning to appear on all the rest. Honda uses a telescopic damper that resembles a small suspension damper; Suzuki uses much the same thing, but on theirs the actuating rod goes all the way through the cylinder and out the other end (with a piston in the center) to avoid oil displacement inside the damper. Yamaha uses a damper built into the bottom of the steering head, actuating the pistons by means of a helix. For the rest. Girling has a reworked version of their latest suspension damper, with softer valving. In the Girling damper, as well as Honda’s, there are bladders filled with nitrogen to accommodate displacement of the oil as the piston actuating rod moves into the cylinder. A conventional suspension damper, which has an air space to make room for the displaced oil, will not function correctly if mounted sideways. The air will be pumped into the working spaces inside the damper, which renders it quite useless.

Among the several workshops we visited while on the island was Tom Kirby’s lair. Kirby, a super-enthusiast if there ever was one, fields a team of the most immaculately prepared AJS 7Rs and Matchless G50s you will ever see. These machines are, of course, quite badly outclassed by current works racing bikes, but they add greatly to the show for all of that. The present batch of Kirby-prepared bikes have nothing unusual about them; except of course that they are so well prepared. But, a radically reworked AJS 7R is nearing completion, and it promises to be the fastest 7R (if not the fastest road racing 350) of them all. The (Continued on page 20) new 7R has an engine that will be able to turn a few more revs than has previously been possible with the 7R. Bore and stroke dimensions for the 7R have been 75.5mm x 78mm; the experimental engine has a 81mm bore, so the stroke is reduced to 68mm to get the displacement back down to 350cc. very much less expensive and are available.

I had a long chat with Jack Williams, until very recently the racing design wizard for AMC and now with EMC. about the possibilities and problems of a short-stroke 7R. Williams knows the 7R engine better than anyone on earth, and he says that there is only limited gain to be had. The difficulty is partly in the rpm limit imposed by the existing lower end, but Williams says that the real barrier is the valve gear, which will not permit many more revs than the engine is already turning. By lightening the valve gear as much as possible, which includes the use of titanium valves, they hope to boost the rev limit by perhaps 1000 rpm, but to get any more would require a complete redesign of the cylinder head. And, even if things go as planned, it is not reasonable to expect that more than 44 bhp can be obtained. This may make the 350 AJS more nearly competitive with the Continental twins, such as the Jawa and Bianchi, but it is still woefully short of the power needed to defeat the 350 Honda four.

I had a nice chat with Paul Dunstall, who had brought a very impressive looking Norton “Domiracer” over to the island for the Senior. This machine was equipped with, among other things, a Schafleitner 6-speed transmission, and I was looking forward to seeing it in action against the Norton Manx singles. On paper, it is faster. Unfortunately its pilot dropped it almost within sight of the starting line on the first lap, so the Dunstall Domiracer had no chance to prove anything — one way or the other. In our pre-race conversation. Dunstall said he didn’t know if the Domiracer was faster than the Norton singles, but he was sure it would at least be competitive. He also remarked that the Domiracer, which is a race-prepared version of the Dominator twin, had the special advantage of being currently in production, which means that parts are

(Continued on page 22)

When I remarked that it was rather surprising, to an American, that so little was being done with the Triumph twin (in road racing), Dunstall said that he could not offer any reason, and that it was his opinion (especially after Daytona) that the Triumph had considerable potential. In fact, he plans to start working with the Triumph 500 twin in the very near future, and will probably develop a line of speed equipment for it. At present he makes special camshafts, exhaust systems and aluminum cylinder blocks for the Norton. The cylinder block particularly caught my attention, as it should provide better cooling and a reduction in weight. Dunstall does not employ any sort of iron liner in this block; the bores are chromium plated, and to date there have been no problems with bore wear. Chromed bores have acquired a rather shaky reputation in some circles, because of the occasional peeling encountered in two-stroke engines. In a two-stroke, it is exceedingly difficult to plate the bore properly, because the sharp corners in the port opening will not take the plate evenly. In four-stroke engines that problem does not exist, and chromium, if properly applied, will give phenomenal life to the cylinder bore even under racing conditions.

As many of you already know, I have been doing a bit of road racing with a Cotton Telstar, and because of this I had a personal stake in the 250cc class at the Island. Derek Minter was entered on a works-sponsored Telstar, and had lapped at nearly 90 mph, which tends to confirm my belief that the Cotton has a lot of potential — even after due credit is given Minter for his forceful riding. The Cotton is light, and agile, and there is a lot of power to be had from its Villiers Starmaker engine — even though, it seems, the power has been rather long in coming forth.

Minter’s works Telstar differed from my own in several respects. The Wi" racing carburetor (to replace the original pair of \Vs" Monoblocs) I had already added, but Minter’s bike also had a remote breaker-system ignition, with a flexible drive to the breaker cam to eliminate the effects of crankshaft flexing on ignition timing at engine speeds circa 9000 rpm. It also had a special 6-speed transmission and, of all things, Norton Roadholder forks. Although the standard Armstrong forks on the Telstar work very nicely on the smooth British circuits, the choppy road conditions on the Isle of Man produced some peculiar handling, and so the switch to Norton forks. Similar difficulties on the home circuits had been solved by changing to softer dampers on the Armstrong forks, but the Norton forks were required to set matters right for the TT. The 6-speed gearbox, of which a halfdozen have been made on an experimental basis by Villiers, was a mixed blessing. It made possible the most effective use of the engine’s now rather narrow power band, but proved unreliable and far, far too bulky. The 6-speed transmission is no larger in height or width than the standard 4-speed; but it is nearly twice as long, and hangs out the side of the Cotton so far that there is some danger of grounding

(Continued on page 24)

it on right-hand bends. Ah well! Back to the drawing board.

There was evidence that the long-range trend in two-stroke exhaust systems (bigger expansion chambers and smaller tailpipes) is continuing. This was especially apparent with the Suzukis, which had absolutely enormous expansion chambers, and final outlet pipes that could not have been more than 5/8" in diameter. These outlet pipes were, incidentally, also very long, and their length and small diameter must combine to impose quite an appreciable restriction to the escape of exhaust gases from the expansion chamber.

While at the Island, I searched in vain for anyone who would admit that they actually knew anything concrete about two-stroke exhaust systems. Phil Irving is in the final stages of writing a book on racing two-strokes, and about all he had to offer was that insofar as general dimensions were concerned, whatever looked right probably was near enough to be made right with a little judicious hacksaw experimentation. He may be right, at that. However, the best answer was from a two-stroke expert (who shall forever remain anonymous) who said that you calculate the whole thing, using exhaust gas temperature, port timing, cylinder capacity, etc. Then if what you have designed cannot possibly be fitted on the bike without dragging the ground, burning the rider’s leg, or requiring the relocation of several major components, then it must be correct. •