Everyone knows singles rock because they’re small and lightweight. KTM can even make them superbly powerful too, but the trade-off for 690 owners has always been lots of vibes through the contact points. But with the two new 690s, this is a thing of the past.
Jerks not welcome
To understand how KTM have eliminated the problem, we first need to look at what causes the vibration in the first place. All four-stroke singles only fire every other full rotation of the crank.
To stop the engine stalling, a large flywheel is needed to keep momentum and the bigger the capacity, the bigger the flywheel. On multi-cylinder engines this is less of an issue, as when one piston is coming down, another is usually going up.
Without anything to counteract the jerkiness of a big single, the vibrations would be unbelievable. On most singles, and on the previous generation of 690s, the manufacturers fit a balance shaft.
The balance shaft effectively moves out of phase with the crankshaft/conrod movement, helping to cancel out the vibrations they cause. However, there is a limit to how much vibration a single balancer shaft can cancel out.
To eliminate more vibrations you can fit a second balancer shaft, which adds weight and cost as well as stealing horsepower, or you can just fit rubber dampers to all the contact points, which is cheap but limited. To sort out their big single, KTM had other ideas.
Smooth like butter
Instead, KTM have worked on the head to turn the camshaft into a secondary balance shaft. The layout goes some way towards mimicking a DOHC cylinder head with the inlet valves driven straight off the cam lobes, while the exhaust valves are operated by a pair of rockers.
KTM have then added an eccentric weight to the end of the camshaft, which complements both the movement of the crankshaft and of the primary balance shaft.
This helps balance the weight between the engine’s top and bottom, as well as calming down the temptation of the bottom end of the engine to leap up and down.
The result is one of the calmest road-going singles available, without adding a great deal of weight or expense. This means KTM have been able to expand the 690 SMCR and the Enduro R into genuinely all-day comfortable bikes.