スプロケット!

Replacing the Isolastic is something I knew would be a fair bit of work, and since removing the entire primary chaincase and all the wonders it contains would expose the gearbox sprocket, I thought this would be the right time to switch to a smaller size.

When this bike was rebuilt over a decade ago, I had intended it to be able to cruise at low RPMs and high speed. Those are of course relative terms, but in practice I wanted to be able to cruise at around 130Km/h with the engine well below 5.000rpm.

The combination of the primary drive belt conversion, and a 21 tooth gearbox sprocket have definitely delivered and the bike was a capable long-distance tourer that could go all day. I mention the belt primary because the ratio is higher compared to that of the standard triplex chain set-up, so coupled with a 21 tooth sprocket, it made for fairly tall gearing overall.

This is all well and good, but it meant that the camshaft was somewhat underutilized. I have a 4S camshaft, which delivers a very satisfying bucket of mid-range torque that is plenty to keep you entertained; the problem is, that tall gearing meant you would often run out of road before hitting that spot in the rev range (around 4.800) where the camshaft really does its thing. Lowering the gearing by one tooth should allow me to be "on the cam" more frequently on the type of roads we like to ride (see example here), but still leave me able - when needed - to tackle stretches of autostrada without feeling like the engine's out of breath.

So, since the Fastback has effectively lost a tooth, I left the old sprocket under the Norton's pillow, and the tooth fairy left a fresh quart of motor oil in exchange. Nice: