‘NEVER ANY DOUBT THAT IT WOULD BE A PARALLEL TWIN’
“The very first thing you do when designing a new bike is decide on the engine, it’s the key element. You design the whole package together and the heart of any bike, especially ones such as the Bonneville or Thruxton is the motor.” “There was never any doubt that it would be a parallel twin. The capacity was also set from the start, we had a lot of discussion and could have gone bigger but 1200cc was a nice balance to create a logical step up from the 900 and make a family of Bonnie models.” “The motor is smaller and narrower than the old air-cooled engine and has a pre-unit Triumph look like the original Bonnie. Locating the drive train on the right allowed us to use symmetrical covers and that mimics the 1959 bike.” “We used a single overhead cam but it is driven by a chain and gears. The crank has a gear on its centre and this turns a sprocket on another shaft, which then turns the chain that links to the cam. If you didn’t have this arrangement, and ran a direct chain from the crank to the cam, you would need a large sprocket on the cam.” “The T120 uses finger followers instead of a bucket and shim valvetrain to reduce the head’s height. This also allows the head to be very narrow and meant we could expose the engine studs on either side, which keeps the engine looking authentically retro, and also exposes the spark plugs.” “The cam is slightly concave, unlike a traditional cam which is convex, to optimise combustion efficiency. It also contains a de-compressor system. A bob weight on the cam sprocket is forced out by centrifugal force when the engine is turned over on the starter motor and this activates two rods on the exhaust cams which open the exhaust valves to decompress the engine – so we can use a smaller battery.” “The engine uses two balancer shafts. You can get away with just one, adding the second completely balances the engine and removes all vibrations. The crank has a 270-degree offset to give it more character while the clutch is a ramp-style slipper clutch which gives a lovely light lever action.” “The water pump is under the gearbox and the thermostat is at the top front of the engine, so the cooling system only needs one small entry pipe into the top of the radiator and one small exit at the bottom. The fins not only look good, they help cool the motor, meaning the radiator can be smaller.” tyres, which look great and work well in the dry, are a bit limiting when the road is damp, but thankfully the T120 comes with traction control and ABS should the British summer hold true to form. Which brings me to the one part of the Bonnie I feel owners won’t gel with – its brakes.
I rode the T120 on both dry and wet roads and didn’t like the dead feeling I experienced through the lever from the two-piston sliding calipers. The rest of the Bonnie is so reassuring and accomplished that the brakes stand out as being under par. I know steel braided lines are a neat feature, but they don’t have that slight bit of reassuring give you get on rubber hoses. On a sportsbike such as the Thruxton you want instant stopping power, but on the Bonnie I was hoping for refinement, and it just isn’t there. It could be a pad issue, but I’m more inclined to point an accusing finger at the lines or master cylinder. It’s a small blot on an otherwise clean copybook, but one that I think could rile buyers. But the new Bonnie is certain to be a bike owners will grow to love, especially if they dip into Triumph’s accessories catalogue.
If you look at the new Bonnie and think ‘that’s for me’ then it’s pretty certain that you’ll love it, especially if you are stepping up from one of the older air-cooled models. As well as being refined to ride, comfortable, and beautifully styled, it has a gorgeously smooth motor and overall is a very practical retro for the modern world. It’s a grown up Bonneville aimed at riders who will appreciate and exploit this new-found level of maturity. However, if you favour performance over practicality and aren’t quite ready to slow down just yet, the Thruxton is a far more satisfying ride, while the T120 Bonneville could leave you feeling a little short-changed.
Bonneville Chief Engineer, reveals the internal secrets of the new engine