Dot 250 Scrambler
CYCLE WORLD ROAD TEST
THE DOT CYCLE & MOTOR MANUFACTURING COMPANY was the first concern to come up with the idea that two-strokes could devastate their big-bore counterparts by dint of superior handling, lightness and a bit of power. In fact, British rider Bill Baraugh in the early fifties ran his Dot so well against larger machines that the other riders had no choice but to protest that he was riding out of class. More recently, competitors to this small British company caught on and Dot was somewhat eclipsed by machines with engines that put out more frightening amounts of power.
Things promise to be different, judging by the prototype of the new 250cc scrambler to be sold on these shores. Called the Demon, like its predecessors, Dot looks graphically different this year, although this is only a result of its being fitted in fiberglass trappings instead of the familiar alloy.
The real changes, however, are on the inside and the first external clues to these are the tuned expansion chamber exhaust and the name, “Alpha,” appearing for the first time on the crankcase.
Dot previously employed the Villiers 36A engine, but it had been drawn out to the end of its string in the course of year-in, year-out improvements. The new engine is based on the beefier Alpha bottom half — including crankcase, crankshaft and connecting rod assembly. Dot adds its own aluminum alloy cylinder, into which is pressed a liner thick enough to take several rebores.
The exhaust port has been reshaped, and widened. As a wide port has a tendency to break piston rings, the port is bridged, and the makers claim that all piston ring breakage troubles have been eliminated with this new system.
An aluminum cylinder head, with centrally placed spark plug, gives the Demon 12:1 compression. In accordance with good mud plugger practice, the finning on both head and cylinder is widely spaced to prevent clogging. The finning is slotted forward of the intake to reduce heat transfer to the incoming fuel charge; if there was an interruption of metal in the finning at this point, heat from the hottest area of the cylinder, near the exhaust, would be conducted back along the fins to the intake.
When one tries to extract power from a two-stroke by means of an expansion chamber, port timing becomes more critical. So the new Dot piston is fitted with a higher ring belt to give more precise timing to exhaust and transfer ports. The ends of the piston rings overlap their locating pins to help prevent gas leakage.
The engine is carbureted by a 1 and 3/16-inch Amal Monobloc. A small Swedish Stefa magneto — smaller, lighter and more trustworthy than the Villiers item it replaces — takes care of ignition worries.
The expansion chamber tapers out from 1 and 3A inches at the exhaust pipe to 3'/2 inches at its widest section, but then does not taper back in. Instead, the chamber is stoppered by a flat plate, from which issues à rectangular pipe 114 by 2 inches. This has been done to achieve maximum top-end power.
Dot thus has assumed more firebreathing characteristics than in the past, although it is no low-rpm stump puller. The engine must be brought up “on the pipe” before it begins to pull, and then it starts, in smooth progression, to get quite vigorous.
DOT 250 SCRAMBLER
A Real Square Deal
This year the makers have switched to the close-ratio Albion gearbox, which bolts solidly onto the back of the Alpha crankcase, giving sort of a unit-construction effect. The engine-gearbox is cradled in the frame.
The frame is Dot’s usual, unusual design, employing square tubing at points where great stress occurs. The 1XA'" square tube configuration runs back from the steering head, behind and under the engine and up to the steering head again. The engine cum gearbox is bolted to the frame in four places — two forward and two aft.
Round tubing departs from the top and bottom of the main frame to go back and up to the rear suspension units. Square tubing appears again in the swinging arms, where it shows to best advantage, lending itself to a better weld than round tube does, and therefore firmer and more rigid attachment to the frame.
Front suspension remains the same well-proven Dot version of the leading link system. While it does not offer the most travel we have ever seen on a competition machine, the damping is excellent. The rear units seem to have about the right spring load for a 150-pound rider on their low setting. Every time we approached a bad section, we gritted our teeth, fearfully anticipating the worst sort of jarring and bottom-whacking imaginable. But it never came. Dot leveled the bumps in an easy rolling motion, which was made even easier by the small-looking but surprisingly comfortable seat (an all too rare item on European-style scramblers).
On smoother ground, the machine will lay over and slide nicely, although the 21-inch front wheel and skinny tire provided some hairy moments. Most dry-country riders will probably change to a 19-inch front wheel and a fatter tire, which is better for rocks and sand, while the riders of the Eastern loam may stick with the 21-incher.
Riders in the dusty parts of the United States will appreciate the high capacity paper-element air cleaner. This item used to consist of two bells, one on each side below tlie seat, feeding into a gigantic breather box. Because of the expansion chamber exhaust, there is now only one bell, on the left, but it appears slightly larger (500 square inches of filtering area) and feeds into the same big box. It is recommended that the rider wear full coverage boots to avoid rubbing his calf against the bell when he stands up or lies way back on the seat.
The gear shift lever on the prototype was overly long, and thus so susceptible to oscillation during normal progress over rough ground that it would shake itself out of gear. The U.S. distributors of Dot, Van Wyck Enterprises, say that the lever on the production models will be two inches shorter. At any rate, it’s easy enough to cut it down to fit one’s own boot or shifting style.
All in all, the increased power output combined with good handling results in a frisky little charger. Indeed, one feels more secure going over a given section as fast as possible, for dawdling gives an incorrect impression of things, as well as bogging one down. Gearing was a bit high on the test bike and made it necessary to rev the engine in extremis and slip the clutch to get rapid starts. Once we were underway, however, the close spacing of the Albion transmission kept things buzzing nicely through the gears.
In spite of raised performance, the Dot is an easy starting machine and refuses to be fouled out in slow going.
Its forte will continue to be in rough scrambles events, but the new power potential will come in handy for an occasional bash on fast, graded TT tracks.
DOT 250
$895
