Letters to 'The Motorcycle' Magazine

 The public's reaction to the announcement of Sunbeam's new model

From March 28th 1946


The Motor Cycle

Founded 1903

Circulates Throughout The World

No. 2242. Vol. 76 Thursday, March 28th, 1946


Will the Cost be a Snag ?

I WAS most interested in reading about the new shaft-driven Sunbeam, especially in the small-diameter wheels and large tyres. The mudguards are sensible and should be most efficient in service. Anyway the machine looked quite clean after the road test, although perhaps it was used on dry roads. [The photograph on page 177 was taken following a journey of over 100 miles on roads which in places were covered with slush.-Ed.]

Another thing I noticed is that quite a few makers are turning to coil ignition, and if it can be as well made and efficient as the B.M.W. ignition the magneto will die out. After all, a dynamo is a form of magneto, it generates electric current and the magneto points are transferred to the distnbutor. Mr. Poppe is to be congratulated on his ingenuity and courage, the only snag now is the cost of the machine. May I hazard a guess it will probably be over 200 at present-day prices. This will restrict sales to a comparative few wealthy enthusiasts I am afraid.

Birmingham. PANTHER.

No. 2243. Vol. 76 Thursday, April 4th, 1946

The Question of Uneven Torque

WHY on earth did not Sunbeams squeeze in a third cylinder and thus give us an engine with uniform torque at high speeds? The middle cylinder might get a hit warmer than the other two, but it is more likely to stay truly circular, and the crankshaft would not cost much more and could be quite stiff enough with only two bearings. In fact, I see no snags at all except the draughtsman`s headaches at getting the mixture evenly distributed and oil to the middle crank-pin.

To anticipate crabbing by the ultra conservative : the always indirect gear box was introduced by Napier's 40 years ago. I should imagine the reason it was abandoned was 90 per cent prejudice and 10 per cent the rough gears of those days. Actually an ordinary motor cycle has double reduction on top gear and quadruple on all other gears, Sunbeam has double reduction on all gears.

The best place for the ammeter is as close as possible to the live terminal of the accumulator, even if it cannot be conveniently seen there. Only one short piece of wire then has to be perfectly insulated against shorts and corrosion, the ammeter will show shorts elsewhere. A good point is the nearly straight spokes. Every back wheel and all but one of the front wheels on machines I have owned (1907-1917 and 1922-42) have had to be rebuilt before they were five years old, solely owing to the sharp bends in the spokes, irrespective of accidents or rough riding.

I would like to see the contact breaker running at crankshaft speed for two cylinders, or 1 1/2 crankshaft for three cylinders, there would then be one lobe on the cam and perfect synchronism of the ignition even when worn, a useful point to a chap who rarely scraps his machine before it is 10 years old. But at last we have the weak three-piece magneto shaft eliminated in a high-class machine.

I am sorry to see that 30 patents were considered necessary, this means a minimum expenditure of 4,500 spread over 15 years, a considerable addition to the price of each machine.

My point about the uneven torque of the twin cylinder is derived as follows. The same argument applies to all the machines ever marketed in hundreds or more, even the Square Four, Brough Dream, Wooler beam all have the same fault. All the pistons are moving as their maximum speed simultaneously and half a stroke later "stationary" together. I do not think this point has been sufficiently emphasized at any time, even in the long discussion which took place in your Correspondence columns.

In the case of the new Sunbeam I do not know the weight of the piston so will assume the weight (mass really, m) of one piston complete with proportion of little end of connecting rod, is 1 lb. Adjustment for the correct weight is simple because m only appears to the first power in the calculations

The stroke being 2 3/4 in the circumference of the crankpin circle is o,72 ft. The output peaks at 23-24 h.p. at 6,000 r.p.m. = 100 r.p.s. Crankpin speed v = maximum speed of both pistons and is 72 ft/sec. The pistons start from rest at either dead centre and reach maximum speed in 1/400 sec (a bit less on down stroke, a bit more on up stroke).

The ft-poundals of energy in two pistons at maximum speed is 1/2 mv2 = 2 x 1/2 x 1 lb x 722 = 5,188. They acquire this kinetic energy in 1/400 sec and ft pdls per sec = 2.07 x 106. 1 h.p. = 550 x 32.2 = 17,700 ft pdls per sec. The power taken up by pistons in first half of stroke and given out again in second half = 118 H.P. averaged over 1/4 revolution.

Torque peaks at 4,000 rpm, at this speed the h.p. over 1/4 revolution is 118 x (4/6)3 = 35 h.p. still not negligible with respect to the output froin the explosion, and this speed will constantly be used with the evenly quiet gear box.

Variations in speel of the crankshaft and hence of torque transmitted at least as far as the gear box are not noticeable to the human body because the frequency is about twice that of the flicker of some discharge lamps, but the transmission feels it.

Bromley, Kent. B. M. VENABLES.

Support a Manufacturer Who Has Ideas

MAY I reply to the letter from "Anxious Don R." 'regarding the new Sunbeam ? He shudders to think of the possibility of present-type machines being replaced by similar models to the new 'Beam. I often wonder if motor cyclists really know what they do want, for so many letters have been written of the "ideal" mount, multi-cylinders, springing fore and aft, shaft drive, and better mudguarding. I should say the new 'Beam is very definitely a step in the right direction, but why too big a stride ?

Surely we should progress with the times, and support a manufacturer who has ideas and the ability to execute them.

Tarporley. Ex-R. N. R.

A Reply to the Col. Blimps Who Live in the Past

I HAVE had only 17 years' experience of motor cycles, so perhaps I am not qualified enough to question "Anxious Don R's" opinion of the new Sunbeam.

As a motor cyclist who has had enough of noisy, dirty machines, and who longs for a nice, quiet, clean, designed-as-a-whole machine, maybe I ought to retire to an armchair.

Although I would like to say that if that is what the "diehards" want, it is high time they died and let the manufacturers get on with their own designing and give us modern bikes, not improved old ones. It is such people as "Anxious Don R." who keep the motor cycle game stagnant. Do away with the noise, filthy chains and the bits, and many more will enter our ranks.

There can always be a Rapide and an "Inter" Norton for those who want them, but let Mr. Everyman have his bike also. Personally, I want to ride for pleasure, not thrills. If I could afford it I would now be writing an order for a new Sunbeam "Eagle," which would be an apt name for such graceful lines combined with speed and silence as the new "'Beam" has.

I should then feel sorry for those diehard Col. Blimps who insist upon living in the past.

My congratulations to Sunbeams; keep up the good work.

London, N.1. 1946 VETERAN.



E-mail: bsmc@classicglory.co.uk