Balanced outlook
Clive Fletcher emails from New Zealand to say that when his 1961 Triumph T120 crank was balanced some years ago, the factor chosen was 58% but subsequent research indicates that although Triumph specified factors up to 85% for their twins, the ’61 Bonnie was originally 50%. “Given that we have a 62mph speed limit here, will this be a problem, and is there a formula to work out the best factor?”
Well unfortunately not – and you can’t eradicate vibration, you can only move it around. You often see Triumph forks juddering in time with the engine at tickover, but it’s better to have the worst shakes there than at popular cruising
speeds. Triumph varied balance factors for a number of reasons; the change to unit construction increased power unit rigidity calling for a rebalance and reducing flywheel weight to improve acceleration in the late ’60s called for further adjustment. Even chassis redesigns played a part, because they change the bike’s harmonics.
So what about Clive’s 58% factor? I rang Jeff at SRM Engineering, who offer a balancing service. “The main thing,” he said, “is the rocking couple – the imbalance from side to side – this can only be measured by dynamic balancing, which Triumph didn’t do. You’ll recognise a dynamically-balanced crank by its having the balancing holes drilled offcentre. That’s the starting point for setting the percentage – but there’s no formula; factories did it by trial and error, building engines with different factors and getting feedback from the test riders. 58% should be fine – there’s a fair bit of leeway. After all, if you fit oversize pistons, they’re likely to be heavier than standard, but the factory manual doesn’t tell you to rebalance the crank every time you have a rebore…”