The ISDT was originally intended to be a reliability event, with the motorcycles running for six days, and repairs could only be done by the rider with tools he carried. This made a great deal of sense back before World War I, as roads were in pretty rough shape. After the end of World War II the event had been altered to fit a more modern format, with the course being run mostly on dirt roads and trails, with a bit of pavement in order to keep everybody honest as to lights and braking. Many manufacturers took winning deadly seriously, but not the boys designing the TR5T.

Tough, but not too tough
The Triumph TR5T Trophy Trail has a 490cc, over-square engine (larger bore than stroke, in this case 69mm bore by 65.5mm stroke), detuned a bit from the hot-rodded, twin-carbed T100R Daytona version in order to give the machine more tractability. Compression was a moderate 9:1 and carburetion was by a single 28mm Amal Concentric. This provides satisfying throttle for the lady or gent wishing to plonk along disused roads in the wilderness, with a claimed 30hp at 7,500rpm. A wise rider will keep the tach between 3,000 and 5,000rpm, which offers sufficient torque at one end and acceptable vibration at the other.

The key to its climbing and slogging abilities is the fact the bike came stock with an 18-tooth gearbox sprocket and 53-tooth rear sprocket, a combination that worked well with the bike’s 4-speed gear box. Claimed top speed when the bike debuted was a teeth-rattling 75mph, predictably slower than its sporting street-savvy Daytona sibling, which was good for over 100mph.

Ignition is by coils and points, with a 12-volt battery and an alternator. Of note is the exhaust system, with header pipes coming out of the cylinders to be siamesed as they disappear behind a small skid plate and run below the engine into an unobtrusive, fabricated muffler that mounts under the right-side swing arm. There is nothing round, smooth and chromed about this muffler. Instead it is an efficient, flat-black, elongated box that was quite acceptable to the U.S. Forestry Service. Some people didn’t like the aesthetics when the bike came out — aren’t dirt bikes supposed to have pipes lifted high in the air? — while others thought the muffler was clever, as its location kept the rider safe from burning a leg in the event of a fall-over.

A new frame of mind
The chassis is more interesting. It was the bright lights at Umberslade Hall, the R&D division of Triumph/BSA, who had come up with the oil-in-frame design in 1970. In principle the idea was sound, avoiding the need of a separate oil tank for the dry sump engine, but the downside reality check was that British engines occasionally had a tendency to implode when hard pressed, causing little particles of metal to float through the lubrication system. Blow your engine, and even if you flushed the frame thoroughly all it took was one little scrap to dislodge from its hiding place after the rebuild and SCRUNCH! ... there goes another engine. The system holds just four pints, which is fine for modest efforts, but a trifle short when keeping the engine on the boil for a long period.