Should I fit a primary drive belt to my
classic bike?

Triumph | BSA | Norton | Ariel | AJS | Matchless | Primary chain | Tony Hayward 

This question comes up time and time again, and understandably so. On the face of it, a primary drive belt is an attractive idea and a good upgrade for a classic motorcycle. Generally, belt systems are lighter, smoother, cleaner and quieter. They're usually easy to fit. And there's not always much difference in price between a complete aftermarket belt set-up and a factory primary chain set-up; certainly the price differential isn't as great as it used to be back in the 1970s and 1980s when belt drives systems were very expensive.

However, one of our chief complaints with these transmissions is that the belts can snap without warning leading to a total drive failure on the road. We've had that happen to us twice, both instances leading to a bike recovery.

With a primary chain, that's far less likely to happen. In fact, if you maintain a primary chain, keep it lubricated and replace it when you notice significant wear, it will NEVER fail on the road.

Also, some classic bike mechanics and riders feel that switching from a primary chain to a belt can alter the engine balance leading to greater vibes; or, at least, can shift the vibes to other areas of the rev range that might not be optimum given an individual's riding style.

To elaborate, the idea here is that the engine designers factored-in the weight of the primary chain and the steel clutch drum. Then, with these criteria in mind they worked out the crankshaft balance factor. But as belt drive systems generally use an aluminium clutch drum and a belt that's often a small fraction of the primary chain weight.

Clearly, the differential isn't so great with a single primary chain engine. But for duplex and triplex chains, the belt weight is almost negligible. Note that many belt systems use a steel engine pulley in place of a steel engine sprocket. Therefore, there's not such a huge difference in weight there. But there are plenty of aluminium engine pulleys on the market.

Is any of this balance stuff true? We don't know for sure, except to say that there probably is a difference. But we've never really been aware of it. In fact, on our own bikes we haven't noticed any difference on the road between a primary belt and primary chain. At least, not until the belt snapped. Twice.

Our advice is to talk to the belt drive manufacturers/dealers and ask about these issues. Make sure you also ask if changes will have to be made to the clutch bearing/s or clutch plates. Ask also (in the case of 650 and 750 Triumphs) how engine breathing will be affected. Ask about lubrication issues, especially when running a dry belt. Ask about fitment/alignment problems. Ask about engine shock absorber changes/revisions. And check the price of replacement belts.

Meanwhile, check our Triumph T140 Bonneville belt drive feature. You'll find it enlightening.

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