1971 Triumph Bonneville

1971 Triumph Bonneville Background & History

The 1971 Triumph Bonneville was a totally new bike. Only the engine carried over from 1970. Not since the 650 twin went unit construction in 1963 had so extensive a redesign taken place. The 1971 model year started for the Triumph Bonneville with Engine #NE01436. While the new frame and running gear were getting all the attention, the engine received some updates also.

The 1971 Triumph Bonneville was a handsome machine.  They modernized the look and yet kept it true to its Triumph character.

1971 Triumph Bonneville Engine Mods

Most significant were the changes made to the cylinder head, head bolts and rocker boxes. These changes were engineered at the last minute by the Meriden factory when they found that the engine wouldn’t fit into the new oil-bearing frame. Their approach was brilliant and also yielded some added benefits. The head bolts were reworked so that they took the load off the already over-burdened rocker boxes and made the top end easier to torque down. Otherwise, there were minor changes to the pushrod tubes and their O-rings, a new crankshaft flywheel and bolts, and a new timing-side (right) main bearing. Changes were intentionally kept to a minimum because virtually every other part on the bike was new. Late in 1971, an optional 5-speed Quaife gearbox became available, originally an aftermarket item adopted by the factory race team. It would be next year before they would designate these 5-speed bikes differently, with a “V” suffix (ie: T120V). Gearing was lowered with a 47-tooth rear sprocket, in the interest of acceleration.

The 1971 Triumph Bonneville engine was too tall to fit into the frame on the assembly line.  The rear rocker box had to be removed then reinstalled once the engine was in the frame.  Nice work, Umberslade Hall.

A New Frame for the 1971 Triumph Bonneville

Of course, all the hub bub was over the new oil-bearing frame and all its accompanying new running gear. Very little was carried over from the pre-1971 650 Twin family. The old brazed-lug frame, as good as it was (they handled great, were tough and rigid) was replaced by a brand-new welded frame, created in a whole new way by aerospace engineers instead of motorcycle geeks, from BSA’s posh new ‘technology center’, a mansion nestled in the English countryside called Umberslade Hall. Big things were expected to come from this new brain trust, important new products, leading-edge technologies, that would save Triumph and BSA. This new frame was just the first of many great things to come…right?

Unfortunately, for all the fuss, it really wasn’t a great frame, it had been ill-conceived, poorly designed, badly executed, and perhaps more telling, was completely unnecessary. The 1971 Triumph Bonneville needed many things, but a new frame carrying its own oil was not one of them. Vast resources were spent that could better have been invested in a good electric starter. But alas, we got the oil-bearing frame. Such was the wisdom of the leadership at BSA in those days.

The 1971 Triumph Bonneville ditched last year's 'pancake' air cleaners, and for the first time routed them into a modern-looking air cleaner housing.

The new frame was all welded, no more brazed cast lugs. An enormous 3″ diameter round backbone tube extended from the steering head back under the tank to the nose of the seat, where it bent straight down to a small sheetmetal sump at the bottom of the frame. The oil filler cap and dip stick were located just under the nose of the seat, where the backbone curved down. This meant that all the backbone’s interior volume from there forward to the steering head was not being used, which limited oil capacity in this lower section of the frame to just 4 pints. Triumph 650s generally have had 5- to 6-pint oil tanks. This again was due to bad planning on the part of the ‘brain trust’. The geniuses at Umberslade Hall originally thought that the entire volume of this 3-inch tube would be filled with oil, and that the oil filler cap would be located just behind the steering head. However, prototypes that were configured like this suffered from 'oil foaming' that they couldn't seem to solve any other way than to abandon over half of the volume, filling only the vertical portion at the back, with the filler cap relocated to just behind the tank. There was a removable cast aluminum finned cover plate on the bottom of the frame sump with a filter screen inside of it. The gold “Made in England” decal appeared for the first time, on the right front downtube, just south of the neck.

The 1971 Triumph Bonneville's new frame had twin downtubes and a cross-over pipe designed to equalized the pressure between the two pipes.  It was a noise-reduction thing.  Note the "Made in England" decal on the downtube.

Harking back to the pre-unit days, this new frame had twin front downtubes (just like the dreaded ‘duplex frame’, 1960-1962, but no such problems here) which cradled the engine, tied in with the swing arm pivot, then angled up to the top of the rear shocks, all in one unbroken piece of steel tubing. Plenty of crossmembers and bracing made the frame rigid and tough. There was a tendency of the left tube to bend at the kickstand. This was caused by the abrupt loads placed on it during kickstarting. It was therefore advisable to kickstart the bike on the center stand. An aftermarket fix was retrofitted years later. Of course, a new frame meant that all the ancillaries had to be changed also. Engine plates, swing arm, shocks, battery box, rear fender, air filters, tank, seat, footpegs, chain guard, everything was new for 1971. As part of the ‘forward thinking’ of BSA’s plan, this new frame would be shared by the Triumph 650 twin & the BSA 650 twin. Of course, BSA production stopped barely one year later.

This is the 'business side' of the conical brake.  The air inlet scoop looks awesome and it aided brake cooling.  This 1971 Triumph Bonneville shows how short the brake arms were, and rather than being mechanically linked, they were both pulled toward each other by the cable and its casing.  It was changed for 1972, and replaced by a disc brake in 1973.

When introduced, the new frame pushed the seat height up to 32-1/2″, putting many riders up on their tippy-toes at traffic lights, another blunder on the part of the ‘Whiz Kids’ at Umberslade Hall. By 1972, the seat rails were relocated to bring the seat back down to a more normal 30″.

The new front forks were quite attractive, with sort of a “Ceriani”-look, very popular at the time. Now with internal springs, the chromed fork legs were exposed (no gators or sheetmetal covers), with alloy sliders with 4-bolt caps on the bottoms to secure the front axle. Early ’71s suffered premature fork seal failure, but improved seals in production solved this. The new black-enameled steel yokes turned on tapered roller bearings now, giving smooth, precise steering. Wheel and tire sizes were now 3.25 X 19″ front and 4.00 X 18″ rear with Dunlop K70 tires as standard equipment.

The 1971 Triumph Bonneville had conical brake hubs front and rear.  The 8-inch front brake looked fantastic but didn't stop as well as the 8-inch TLS it replaced.

1971 Triumph Bonneville Conical Hubs

Another prominent feature of the new 1971 Triumph Bonneville and the 1971 Triumph TR6 also, were the conical front and rear brake hubs. They looked very cool, racer-inspired, but alas they didn’t work as well as the units they replaced. The front is a 8-inch TLS (Twin Leading Shoe) set-up with an aggressive-looking front air scoop all in an attractive conical hub. This is still one of the coolest looking front drum brakes you can find and today are favorites of builders and customizers for cafe racer, choppers, bobbers and street trackers. Part of the problem with the poor performance of the front unit was the odd way the cable actuated the cams. Instead of anchoring the outer cable casing to the backing plate, then attaching the inner cable to an actuator arm, which operated a linkage to a second arm (the way most TLS brakes are set up), the boys at Umberslade Hall thought it would be a better idea to throw out decades of brake development (much of it gleaned on the racetrack) and invent a whole new way to actuate the brakes cams. They attached the outer cable casing to one actuator arm and the inner cable to the other arm and had them both pull against one another when the brake lever was applied. It had poor leverage and one cam always seemed to pull harder than the other. The rear is a 7-inch SLS (Single Leading Shoe) set up and worked well enough.

The 1971 Triumph Bonneville introduced this gorgeous 7-inch conical rear brake hub.

1971 Triumph Bonneville Styling Changes

It boggles the mind to consider why BSA would have lavished it’s scant resources on the development of an exotic, risky new front drum brake when they knew that a disc brake was right around the corner. Perhaps the better question is, why wait? Why didn’t they just come out with the disc brake in 1971? Honda had been putting them on the front of their big bikes for a couple of years by this time and it was increasingly becoming the norm in the industry.

Besides all the mechanical changes, the new 1971 Triumph T120 Bonneville 650 had a new look to it also, one that put off some people, who thought it wasn’t as handsome a machine as the 1970 Triumph Bonneville. But it was still a nice-looking bike and as such it soldiered on for another 13 years.

The seat was new and all the bodywork were new. The massive 3″ backbone required a complete redesign of the fuel tank, now 3-1/2 US gal for all markets. Late in the year, a larger 4 Imp. gal tank was fitted to UK and Export models. The new tanks rested on foam rubber pads on the backbone and were secured with a single bolt that ran through the center of the top of the tank into a bracket on the top of the backbone, all rubber-mounted and concealed by a handsome pop-in cap with a Triumph logo on it.

Standard colors for the 1971 Triumph T120 Bonneville were Tiger Gold and Black separated by White pinstripe (although the ‘gold’ looks more like ‘root beer brown’ to me). The steel fenders were again painted gold with a black stripe & white pinstripes. The front fender was quite short compared to 1970, with spindly new fender braces.

The headlight was an odd pancake-shaped affair held in place by flimsy chromed wire loops that looked interesting but didn’t hold up well to vibration. Since the frame held the oil, there was no need for a separate oil tank. In its place was a large black cover on both sides to house the air cleaners which were now plumbed to the carburetors with rubber hoses (gone were the classic pancake filters).

1971 Triumph Bonneville Specifications

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Triumph Bonneville Books

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More Triumph Bonneville Pages

1959 T120 Bonneville

1960 T120 Bonneville

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1964 T120 Bonneville

1965 T120 Bonneville

1966 T120 Bonneville

1967 T120 Bonneville

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1970 T120 Bonneville

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