(technicalities)
(TECHNICALITIES)
GORDON H. JENNINGS
REMEMBER THE British "World-Beater" scheme? Some of the proceeds from the Manx Hospital Lottery were to be diverted into a fund, and the fund would finance the design and construction of a road racing motorcycle. The motorcycle, according to plan, was to lift Britannia from the competition doldrums. Mayhap, even to beat the Dread Orientals at what has become their own game.
Curiously, something has actually come of this scheme, but I am not much convinced that the end result will be what the schemers had in mind. Part of the problem is that the Manx Lottery supply will only be able to place in the fund $56,000, and that is simply not enough to get the job done. So, nothing happened until someone could be found who would underwrite the remainder of the development costs (underwriter, translated from the British, would seem to be "patsy").
The necessary someone materialized in the person of Mr. Alfred Owen, an English industrialist who heads the Owen organization of which BRM is a part. Now it should be understood that neither BRM (British Racing Motors) nor the Owen Organization are connected with Britain's motorcycle industry. But, Alfred Owen apparently feels that building Grand Prix racing cars is not enough; that there is a lot to be learned in the development of a GP motorcycle engine. In that, he is more right than he knows.
Nothing official has been said, but strong rumor has it that BRM's efforts will be devoted to an engine for the 250cc class. Very courageous. In that class they will be running headlong into the massed strength of Honda and Yamaha, and those companies have had some several years of building ultra-high output 250s.
It will be pointed out that BRM has had years of experience in GP car racing, too, and that their recent racing engines have been very successful. All very true. Unfortunately, none of these "successful" engines is operating in the same specific output range as either the Yamaha or Honda motorcycle engines.
Arguments that the "scale effect" will put the racing car engine at an automatic disadvantage, compared to a racing motorcycle engine, are partly correct. However, that is not the whole story. It is a matter of record that when Honda Motor Company produced their first automobile racing engine, that engine had a very noticeable edge in power over all rivals — and that included BRM. Various problems with handling and braking, in addition to a tendency toward overheating, kept the Honda GP car from victory until the last race of the 1965 season (which was also the last event of the 1.5-liter formula) but sheer power was never lacking.
Considering all of the factors, BRM's chances of success are exceedingly slender.
(Continued on page 24)
The Owen Organization faces not one, but two solidly-entrenched adversaries — who are, not incidentally, locked in a struggle that is sure to keep their development moving briskly ahead. The prospects for an outsider, with ground to gain before even reaching "scratch" are very poor.
And then there is the rest of the motorcycle to consider. Velocette has contracted to build everything but the World-Beater engine, which implies that they will be responsible for the transmission as well as the frame, suspension, etc. My personal feeling toward Velocette is quite warm and friendly, for one of the first motorcycles I owned was a '47 girder-fork Velo and over the years I have come to have a great respect for the Velocette's reliability. Even so, I cannot but wonder how a company that has not built a racing motorcycle in the past 15 years, and one that is comfortably settled into a state of middle age (bordering at times on senility) will rise to meet this challenge.
The real capper is that those concerned are making noises about design, construction, and development spanning three years before the World-Beater is race-ready. Three years! By that time, the Japanese GP engine will have so many cylinders the mechanics will be making head-counts just to be sure none have been forgotten, and Japan's 250cc displacement motorcycles will have shattered every existing 500cc lap record. The very best of British luck to Alfred Owen and Velocette's Bertie Goodman.
Enough of that; on to things more constructive. A reader, M. H. Patenaude, writes to inform me that he has checked the compression ratio on his Yamaha YDS-2, and finds that the nominal figure, using the full piston stroke, is 12:1. The advertised compression ratio of 7.5:1 is obtained when only that part of the stroke from the top edge of the exhaust port and upward is employed in the calculations.
In reality, only that upper part of the stroke means anything. There can be no compression of mixture (at least, none that means much) until the piston has closed the exhaust port. Thus, in a twostroke racing engine, with its high exhaust port (for long-duration exhaust timing), the nominal compression ratio must be very high just to achieve a moderately high "true" compression ratio. At present, the various manufacturers are divided about the rating system to be used. Logic would indicate that the compression ratio should be given as measured from the point of exhaust-port closing, but that too can be misleading. Racing engines may be given a low compression-ratio rating of their exhaust timing, when in fact their good high-speed breathing results in quitehigh combustion pressures. An appropriate example of this is provided by Yamaha's TD-1B, which has an 8.1:1 compression ratio (measured from exhaustclosing) and is right on the verge of detonation with that seemingly very low ratio.
A similar problem in compression-ratio rating exists in four-stroke engines. There is always some overlapping of the intakeopen period into the compression stroke, so the true compression ratio should be calculated from the cylinder volume after the intake valve closes. But then you have the same difficulty with breathing, and it is if anything even more confusing than is true of the two-stroke engine. A long-duration cam may give a low effective compression ratio at low engine speeds, but when the speed is high and the usual ram-charging effect takes over, detonation is likely to occur.
Precisely the opposite effect is found in touring-type engines. In these, the best breathing occurs at low speeds and there is a tendency to run out of air as the revs go up. We found this to be all too revoltingly true with our Bonneville Triumph TT Special. With the standard carburetors, the "showroom" 12:1 compression ratio created few problems. A bit of detonation as the revs would pass through the torque-peak, but this would die away at higher speeds and was entirely absent at the power peak. Subsequent modifications (bigger carburetors, valves, ramtuning, etc.) improved the high-speed breathing and it was no longer possible to use 12:1 pistons. Detonation began as the revs approached the power peak, and would increase alarmingly as combustion chamber and piston became overheated. It was painfully obvious at that point that detonation is a self-aggravating condition. Eventually, we settled on 10:1 pistons, and that cured the problem. It is my opinion that a 10.5 - 11.0:1 compression ratio could be used in most other forms of racing-, but at Bonneville, where the engine is singing at full throttle for so long, heat build-up makes really high compression ratios dangerous in an engine that really "breathes."
(Continued on page 26)
Another reader, Arthur Fernandez, remarks that he was so satisfied with a set of "radial-cord" tires (presumably Michelin-X or Pirelli Cinturato) on his automobile that he has been trying to find them in a size suitable for his new Honda 450. I know of no such tire.
A word of explanation regarding radialcord, or "belted," tires is appropriate here. These tires have a band of cord laid under the tread that extends around their outer diameter. This band resists deformation under cornering loads, and makes high-speed expansion (due to centrifugal force) virtually impossible. All of which has the effect of making steering more precise, increasing cornering power and extending tread life. The only penalty is in slightly higher manufacturing cost, and a tendency toward ride-harshness. Apart from the comfort aspect, there is no car that would not benefit from the fitting of radial-cord tires.
The same does not apply to motorcycles. An automobile's tires are always
more or less squarely perpendicular to the pavement. That condition exists for the motorcycle only during straight-line running. During cornering, the tire is presented to the pavement at anything up to a 45° angle (or a bit more) from the perpendicular. Thus, a radial-cord "belt" under the central part of the tread would be useless.
What does work, and works well, is low cord-angle construction. As you probably know, the cord in a tire starts at the bead on one side and wraps around to the opposite bead. If the cord led right straight over, taking the shortest path around the carcass, it would (seen from the side) be at 90° to the bead. Cord laid parallel to the bead would be zero-angle. High cord angles lend a maximum flexibility to the tire, giving a soft ride. Low cord angles make the carcass resist distortion, and that produces both a harsher ride and a higher degree of cornering power.
Racing tires have low-angle cord, and it is that, as much as special "cling" super-stick rubber compounds in their tread, that gives their grip in the turns. You will appreciate that this implies a certain futility in any attempt to re-tread a touringtype tire with a "racing" rubber compound or tread pattern. It would make more sense to put one of the harder "touring" compounds on a racing carcass. That would provide characteristics roughly parallel to those obtained with radial-cord tires on a passenger car.
