How Triumph reinvented the triple
We go inside the new Tiger 900 triple’s T-plane crank design
● ‘It will be better at finding grip off-road’
Triumph have been using three-cylinder engines since the firm’s revival almost 30 years ago. The droning super-smooth triple is their signature, but for the new Tiger 900 it’s undergone some serious architectural changes. In your engine, when a piston reaches the top of its travel (top-dead centre, or TDC) or the bottom (bottom-dead centre, BDC) the reciprocating masses generate a force as they slow. It’s an upwards force at TDC, downwards at BDC, and makes the engine try to leap up and down. It vibrates. This is called primary vibration.
Put three pistons in a row and you can cancel out these forces. Imagine viewing the crankshaft end-on and spacing the crankpins evenly with 120 degrees between each, creating a Y-shape. The even spacing means the force as a piston hits TDC or BDC is countered by smaller forces as other pistons accelerate or decelerate. So the 120˚ triples Triumph make are rotationally balanced, fire consistently (a four-stroke needs two crank revolutions per cycle so there’s a power pulse every 240˚, in the order 1, 2, 3 across the engine), and therefore extremely smooth. ‘Ah,’ says a man at the back. ‘My Triumph triple has a balance shaft.’ Yes, it does. A rocking couple caused by the distance between the crankpins tries to rotate the engine; the balancer is there to tackle this. The new Tiger 900 breaks convention by using a T-plane crank. As the name suggests, it’s T-shape viewed end-on: cylinder 1 sticks out to the left; cylinder 2 is 180˚ out to the right; cylinder 3 is another 90˚ round, pointing down. And rather than the firing order 1, 2, 3, the new order is 1, 3, 2 to give intervals of 180˚, 270˚ and 270˚ – compared to the usual Y-shape crank, the first two power pulses are closer together, with bigger ‘gaps’ between the second and third pulses.
This makes the Tiger sort-of ‘big-bang’. Remember when Honda closed-up the firing on Mick Doohan’s NSR500 GP bike? Delivering power pulses in a bundle followed by a big gap gave the tyre more time to ‘recover’, increasing tractability, improving feel and making the bike easier to ride. Same with the Tiger. With a 120˚ crank there are ‘gaps’ of 60˚ of crank rotation between each firing where there’s no power stroke; with the new crank, these gaps are 0, 90 and 90˚. It’s less extreme than a GP bike, of course, but with firing intervals closer to a twin the 900 will be better at finding grip off-road and give a greater sense of throttle connection. The only problem is the pistons and conrods are not evenly spaced around the crank, so primary vibration rears its head. No bother – Triumph will just have revised the balance shaft they already fit.