Sunday, 4 February 2018
A few miles north of Stroud there are a series of interesting roads. Small but steep hills are spread over a couple of miles. Reaching the crest of one hill gives a view far enough ahead to safely make the apex of the next rise. The roads are narrow country lanes, just wide enough for two cars to scrape past in certain spots. The Triumph would leap over the crest, the revs momentarily flicking into the red as the back wheel lost contact with the road. Then it was a full throttle thrash down the slope, the game to make the ton before the bottom. The stomach lurches as the Triumph starts up the vertical, like an out of control Big Dipper. The momentum and torque carry the bike forward up the steep incline, until the top is attained and the whole cycle starts again. At certain points in the route, the road swings violently to the left or right. Before the Triumph has a chance to recover from the effects of rapid gradient fluctuation, it's heeled over, scraping sidestand and footrest, braking deep into the corner. Bumps, gravel and cow shit try to push the narrow patches of rubber off line. The Triumph takes it all in its stride. The combination of light mass, stiff frame and taut suspension are difficult to catch out.
And that, folks, is just about all the Bonneville is good for. On tricky country lanes when feedback from the road and agility are the main concerns, the Bonnie's other failings are momentarily forgotten as the good times roll. As the speedo flicks past an indicated hundred miles an hour at the bottom of one of those hills, the vibes have bypassed the rubber mountings of tank, footrests and handlebars. Everything gets the shakes. Good training for pneumatic drill operators, but a kind of hell for long distance speed freaks. Between sixty and seventy five, the engine is relatively smooth, with top gear touring just about plausible. Any extended excursion into higher speeds is met with nasty vibration and short engine life. The ancient engine design is sabotaged by restrictive noise laws and emission controls. It's power output is little better than Jap 400/450 twins. The later bikes are rather more usable due to lack of vibes and inherently strong engine designs. They also have better fuel economy. This all tends to leave the poor old Bonnie out in the cold.
Back in 1937, Triumph produced the first commercially successful vertical twin. A design severely restricted by manufacturing techniques, that has been updated over the past fifty years with hardly any innovation or creativity. The result, a 750cc engine that cannot stand up to the levels of neglect and abuse normally inflicted on modern motorcycles. And an engine incapable of safely delivering enough power to satisfy the market
The 744cc long stroke (76x82 mm) engine has a 360 degree crank supported by two small bearings. Crankcases are vertically split, the main machining restriction on the possible number of bearings. Triumph used to have enough trouble making two holes line up, let alone three or four. Thus the crankshaft is unsupported over most of its length. Stressed by both combustion forces and the unbalanced primary vibes, it's in for a tough time.
A set of gears drives two camshafts mounted either side of the crankshaft. The camshaft operate tappets, which control pushrods acting on the valves through rockers with screw and locknut adjustment. The effects of vibration on crankshaft and valvegear are rather horrific. If ever a design needed a four bearing crankshaft then the big vertical twin is a suitable case for treatment. Engineering horrors are continued with chain primary drive that has a self destruct tensioner. Again, the same old story of a lack of machining integrity stopping the use of a vastly superior gear primary drive.
The engine becomes dangerously unreliable if tuned beyond the stock 50hp, a figure little better than sixties Bonnies. In those old days few bikes could see off the Triumph 650. Only the brave and tough aspired to ownership, which shows just how times have changed. The 750 needs regular attention every 500 miles.
Adjust valve clearances, contact breakers, carbs and primary drive tensioner. Tighten up all the engine and chassis bolts. Mildly run bikes go until around 20000 miles before needing a rebore, reground valves, new primary chain and new clutch plates. Hard used bikes can need attention by as little as 10000 miles.
If the Triumph engine shows little improvement over the 650, twenty years older, then at least the rolling chassis shows some kind of definite progress. Until the early seventies, Triumph frames had quite a nasty reputation for high speed wobbles. True, each year often saw a mild improvement, the most noticeable occurring when additional swinging arm support was provided by plates between engine and frame. But the Triumph never came near to the precise and stable nature of its Norton rival. Under the influence of the BSA management, Triumph introduced an oil in the frame unit that offered a huge leap in handling ability. Initial models had a seat height suitable only for giants, but by the time the 750 was introduced it was down to a more reasonable level. The large diameter main tube flows around the engine, from steering head to swinging arm mount. Two smaller tubes go from steering head to cradle the engine and provide extra support for the swinging arm, eventually ending up supporting the seat rails.
Suspension is stiff, limited in movement and gives plenty of feedback from the road. The end result gives the Triumph handling ability almost up to Norton Featherbed standards. The Bonnie needs a little more muscle to change direction, but it feels just as stable and precise. The security afforded in wet weather, when the chassis never holds any nasty surprise and the rider is always aware of the tyres' reaction to the road, is ahead of any other modern bikes, except perhaps certain Wop exotica. The Triumph is aided by having only 400lbs to shift, and by the narrow engine allowing a low centre of gravity. These allow steering geometry that gives high speed stability without having to worry over flickability in traffic. The suspension may be too hard, but fitting some of the ultra trick items now employed on modern fours would probably wreck the whole feel of the Triumph.
The disc brakes at each end work quite well in the dry. The front loses feel in the wet, while the rear tries to lock the wheel. Lever pressure on the front brake is excessive, but there's enough feedback to avoid locking the wheel. Better than early seventies Jap bikes, they are hardly up to modern standards. Indeed, a decent set of drum brakes would give just as good stopping power, while fitting in with the character of the bike. Front disc pads go for 6000 miles, while the rear last for 10000 miles.
The multi-plate plate clutch has inherited clutch drag from the original 1937 machine. It's necessary to free the plates by kicking the machine over before it is started. Failure results in a bike that lurches forward and stalls when first gear is engaged. Clutch lever pressure is far too high and very tiring in town. Clutchless changes are possible, but lead to early failure of the five speed gearbox. Gear selection is precise, but can't be taken too quickly. Problems may occur with changes between first to second and third to fourth, as well as selection of neutral. The gearbox is one of those complex devices that sprays tiny bits all over the garage floor when stripped down. Chain final drive needs adjusting every 750 miles, lasting for 7500 miles. Running on a worn chain is dangerous because it likes to snap, locking up the transmission and breaking into the engine compartment. Nasty.
Starting the Triumph is no problem if you're used to British twins. Neither the cams nor compression ratio are radical enough to cause the motor to kick back. Some later versions of the 750 had an electric start, but this never managed to work from cold. It was useful for starting a stalled engine. Third or fourth kick is the normal point at which the engine rumbles into life. Fitting electronic ignition reduces this to first or second kick.
Fuel consumption varies around the 50mpg average. The best I ever achieved was 60mpg, when gently pottering down some A roads . Trying to cruise above 80mph returns as little as 35mpg. The single carb Tiger is hardly any slower but improves economy by as much as 10 mpg. Electronic ignition helps the Bonnie to average 55mpg. Neglect of the regular 500 mile servicing increases consumption to an average of 40mpg. Continual neglect of maintenance will send the ignition timing far enough out to burn holes in the pistons. And exhaust valve clearances will tighten up until clouds of smoke start coming out of the exhaust due to burnt exhaust valves. Out of balance carbs increase vibration throughout the rev range. It can get so bad that mudguards, exhausts, petrol tanks and headlamp brackets all start to fracture.
Lights are standard Lucas units, which blow frequently and don't provide sufficient illumination for fast riding on unlit roads. Budget for new front and rear bulbs every 1200 miles. Indicators are a sick joke. They either fall off due to excessive vibration or flash in a random enough manner to give epileptics severe convulsions. The Lucas rectifier and zener diode have a 5000 mile life before they are affected by vibration or the erratic nature of the electrical supply. The alternator suffers from overheating, vibration and poor mechanical support. Life varies between five and fifteen thousand miles. The switchgear lacks a positive feel and can short out when subjected to wet weather. The wiring loom is a terrible mess. It is usually too short and wears away at the steering head when the front wheel is turned. The stoplight switch for the rear brake is situated to pick up the maximum amount of road debris. It lasts for around 5000 miles. The speedo reads 110mph, and all the idiot lights blow with too predictable regularity. The rev counter gyrates wildly whenever the motor enters one of its periods of bad vibration. Both instruments need replacing after as little as ten thousand miles. Most owners will have uprated the electrical components and rewired the bike.
The overall appearance of the Bonnie is of an awful mess. There are so many small components that look out of place, that even the beautiful lines of the engine can't salvage the character of the bike. But the potential is there. Of all the British bike builders, Triumph have always built the most suggestive looking engines. The lines of the frame do not interfere with the styling possibilities, and - properly done - enhance the image of the bike. It takes very little to clean up the Triumph's act. For street credibility it cones second only to those stripped down Harleys. It just needs a little attention to detail.
The Triumph's main Jap rival is the Yamaha XS650. The Yam is a hundred pounds overweight and fitted into a dangerously flimsy frame. Where the Yam is limited to 70mph cruising by its road manners, the Triumph is limited to the same speed by its production of excessive vibration. The Yamaha also has pistons that move in unison, but there the similarities end. The XS has horizontally-split crankcases and a massive four bearing crankshaft, with central chain drive to a single overhead camshaft. The Yam engine is just as simple to maintain, but needs this maintenance at infrequent intervals. Vibration and oil leaks are drastically reduced. Performance and economy of the XS is slightly better than the Triumph. Dare I suggest that this is the kind of engine that Triumph should have produced. And that Meriden missed a great opportunity by not fitting XS650 engines into their rolling chassis. Sure, it wouldn't have been truly British. But it would have been jolly good fun.
It's somewhat ironic that the first of the British twins is now the only British bike on sale. As a practical motorcycle the Bonnie has nothing to offer. Under limited circumstances it can be fun. If all the effort is worthwhile depends on if you can view the Triumph as a poor man's Harley. Triumph 750s can be picked up for around £500. Strip the thing down. Throw away all the junk (indicators, filters, silencers, disc brakes, cast wheels, etc). Fit one of the small tanks, drum brakes and loud exhausts. Use a single carb and mild cams. Then, and only then, you might just have a practical and interesting British classic.