I was most interested to read Mr. Donald Cross’ letter about J.M.B.’s in your August issue, and particularly intrigued by the photograph, because although I can claim to own a J.M.B. I have no idea what it looked like when it was all up together.
Some years ago I designed and built myself a 3-wheeled cycle-car having a passion for such things, and knowing this, two young men who lived in this village persuaded me to take over the remains of a 3-wheeler which they had acquired with the idea of re-building it. The body had been removed in pieces and scattered, and the bare bones exposed to the weather for rather longer than was good for it, but even in that state it had an irresistible charm, and I managed to get the chassis (and works), one door and the tail home. Unfortunately, two curved metal girder-like structures which ran along either side of the chassis to support the body were beyond redemption, so I gave up any idea of re-constructing the vehicle in its original form, and concentrated on restoring the tubular frame, steering and front wheel suspension, brakes, gear change linkage and engine. I have fitted an angle iron “stool” to support a seat, and rigged-up a temporary dash-board for ignition switch and ammeter—and to support the steering column—re-wired the coil ignition and battery circuits and now have a chassis in running order ready for a body to my own design.
I am delighted to know the names of the three gentlemen responsible for designing and producing this little car. I suppose nothing is perfect, but as a design for 3-wheeling, this must be very near it. The basic frame is in stout metal tube with the rear transverse member housed at each end in rubber bushes in split metal cups. The cups can be parted like big-ends for renewing the rubber “bushes.” This arrangement allows swivelling movement—or rather rotating movement—of the transverse member. Welded at right-angles to the transverse member is a truly powerful tubular boom projecting backwards, and this carries the engine and gearbox mountings, and behind them the rear wheel and brake. The engine lies in a horizontal position with the cylinder head immediately beneath the driver’s seat. Comfortingly warm in winter!
Now, since all this heavy metal is fixed to the boom which is fixed to the rotating cross-member, why does’nt the whole contraption collapse with a broken back into the road? Because gripped by four large bolts in a bracket beneath the engine mounting is the root of a massive leaf-spring projecting forwards, and the extremity of the lower leaf of this spring slides to and fro in a slotted bearing plate held in a rubber packed housing fixed to the cross-member in front of the rotating one, and forming the centre cross-member of the basic frame. Of course if this spring broke, you would collapse in the road, but it won’t break; its not that sort of spring. By this cunning arrangement, engine, gearbox and wheel are all in fixed relationship each to the other—so no chain snatch or nonsense of that kind, and all weight is sprung.
As Mr. Trill says, each front wheel mounting is held by a top and bottom transverse spring—quite short, and with their roots clamped to a vertical tubular metal frame rising from the front of the basic frame—so independent front wheel suspension—and steering is by a split tie rod (two separate lengths) stretching out to each wheel from a metal peg protruding from a toothed sprocket held in a beautifully designed carrier, which also supports a smaller sprocket immediately above and turned by the driver via the steering column. The two sprockets are joined by a piece of bicycle chain to give geared steering of rack-and-pinion character.
Brakes are internal expanding, with all round inter-changeable wire wheels fitting over the brake drums. The brakes are Bowden cable operated through knurled brass adjuster units, and all brakes work from the foot pedal or the hand-brake lever.
My J.M.B. has a big side-valve J.A.P. engine, 497 c.c. or thereabouts and the Albion 3-speed and reverse gearbox mentioned by Mr. Trill. The air-cooled cylinder finning is unlike anything I have seen before, and must have been specially designed for the horizontal position. Like most of its kind, this old engine is virtually indestructible, and a delight to have anything to do with. I am interested to read about a 348 c.c. two-stroke engine, though. I have the impression that the finished car would have been rather heavy for so small a capacity, but no doubt the answer lies in the gears.
I could go on and on because everything about the J.M.B. has the stamp of considerate and intelligent minds, but this letter is already much too long. I must just tell you this, though. I got some help with the wiring from a mechanic in one of the big garages nearby, and I made some remark about the J.M.B.’s many virtues. He looked at it for a long minute and then he said “They didn’t know any better in them days. They were too bloody ignorant to do a bad job.” I think I know what he meant.