The Vincent Girdraulic Fork Glitch

Innovation never sleeps: Decades later, racing improves the Vincent Girdraulic fork.

Vincent 1949 Girdraulic Fork ad

At today’s prices, there aren’t too many owners of classic Vincent motorcycles who are going in deep on the brakes, then flicking to maximum lean at the limit of tire grip. Fine wines are to be sipped, not swigged.

But there are a few who still do just that – vintage racers. Some of them have made an interesting discovery. Vincent's Girdraulic fork has a glitch – a design error which allows hard braking to top it out, to drive it to full extension. In that condition, with all suspension motion locked out, it skips over pavement irregularities, chirping the tire as it does so.

A friend and long-time Vincent Grey Flash (500 single) vintage racer put me in touch with David Dunfey, who had carefully graphed out the Girdraulic's axle motion. The stock geometry, with the high initial downward angle of the fork's lower link, translated strong braking force into downward rotation of that link, jacking up the front of the bike and locking suspension movement against its up-stop.

Dunfey found that, in trying to add a measure of anti-dive, the Girdraulic’s designer had overdone it, resulting in the fork topping that Vincent vintage racers have experienced.

When Dunfey sought to achieve the straight-up-and-down axle motion that former Vincent engineer the late Phil Irving speaks of favorably in his book “Motorcycle Engineering,” a less-angled lower fork link resulted. When he built a new steering stem and lower pivot to this classic model, it worked without topping, wobbling, skipping, or contact between the front tire and the magneto cowl on the front of the engine. And it received praise from riders racing these machines.

The spirit of individual inquiry is a good thing, even when it questions something as iconic as the widely admired and influential Vincent motorcycle. As news of Dunfey’s discovery spread, it turned out that at least one other vintage racer had made the same discovery, and had quietly fixed his problem in the same way.