Balancing Shaft
Let's talk about the balancing shaft, the voodoo part, the magical anti-vibration device.
“WhY dOn’T yOu JuSt TaKe It OuT aNd FrEe Up SoMe Of ThAt LoSt Hp?”
You see, removing the balancer shaft is a bad —
very bad — idea. It will end with cracked engine mounts, a broken crankshaft and a crying session. Our GS twins are 180-degree parallel twins: the two crankpins are positioned exactly 180 degrees apart, so whenever one piston is at TDC, the other is at BDC.
At first glance this arrangement seems rather well balanced. As one piston accelerates upward, the other accelerates downward, so their primary reciprocating inertia forces largely cancel each other out. The crankshaft counterweights take care of the rotating mass and provide the chosen balance factor for the reciprocating assembly. There is, however, an important catch. The two equal and opposite piston forces do not act along the same line: the cylinders are spaced apart. As a result, instead of simply cancelling each other, they create a
rocking couple that tries to rock the engine from side to side once every crankshaft revolution.
That rocking couple is one of the characteristic primary imbalances of a 180-degree parallel twin, and it is one of the main reasons Suzuki fitted a balancer shaft in the first place. The balancer shaft generates an opposing couple, greatly reducing the load transferred into the crankcases, engine mounts and frame.
There is also another, slightly less obvious, source of vibration. Because the connecting rod is constantly changing angle as the crank rotates, a piston does not move perfectly evenly between TDC and BDC. It accelerates harder near TDC than it does near BDC. In our 180-degree twin both pistons produce this effect at the same time, so these forces add together instead of cancelling each other out. The result is a secondary vibration that happens twice per crankshaft revolution.
Another way to consider the rocking couple is like this: "Think of two people standing a meter apart, one pushing up while the other pushes down with exactly the same force. The net vertical force may be zero, but you've still created a torque that wants to rotate whatever they are standing on. That is essentially what is happening inside a 180-degree twin."
If you compare our GS twins with other twins of the era, you'll notice that they have remarkably high redlines, with the GS500E going all the way up to a whopping 11,000 RPM. The 180-degree crankshaft layout, combined with a balancer shaft, gives Suzuki a very good compromise between high-RPM capability and relatively low vibration. And high RPM, provided the engine can breathe at those speeds, means the potential for more power. Remove the balancer shaft and you may save a small amount of friction and rotating mass, but you also remove an important part of the engine's vibration control. The resulting increase in vibration puts considerably more cyclic loading into the crankcases, engine mounts, frame and other components. In practice that reduces how comfortably — and, more importantly, how reliably — the engine can spend its life at high RPM. The balancer therefore consumes a little power, but it is very much a necessary evil if you want the engine to survive while producing power at high engine speeds.
Looking at the basic design of the GS400X balancer shaft, it is essentially a straight shaft carrying two eccentric weights, positioned roughly in line with the two cylinders. Because the two pistons of a 180-degree twin have opposite primary inertia forces, the two balancer weights are also phased 180 degrees apart. It is important, however, not to look at the balancer shaft in isolation. The crankshaft counterweights already compensate for part of the reciprocating mass, but because those counterweights rotate, their balancing force acts not only vertically but also fore and aft as the crank turns. The remaining primary imbalance therefore behaves as a rotating couple rather than simply an up-and-down force. The balancer shaft produces an opposing rotating couple. When its weights are above and below the shaft, their vertical forces counter the vertical component of the engine's rocking couple. A quarter-turn later, when the weights are pointing forwards and backwards, their horizontal forces counter the corresponding horizontal component created by the crankshaft counterweights. Taken together, the crankshaft and balancer shaft can therefore greatly reduce the primary rocking couple.
The GS500E balancer shaft is, in essence, based on the same principle as the GS400X balancer. The shaft itself is considerably larger in diameter, making it stiffer, while the counterweights are wider to provide more balancing capacity for the heavier reciprocating assembly. The GS500E does have a slightly shorter stroke, which reduces the required balancing effect somewhat, but the considerably heavier pistons still require a more substantial balancer. Another major difference is that the GS500E shaft runs in plain bearings, whereas the GS400X shaft runs in roller bearings. The GR650 balancer shaft takes the same principle a little further. Its counterweights are much narrower, but considerably larger in diameter, and extend into the available space between the crank webs. With its 70 mm stroke and very heavy pistons, the GR650 requires substantially more balancing capacity. Moving the counterweight mass farther away from the centreline of the shaft increases its unbalance, since the centrifugal force generated by a rotating mass is proportional to both its mass and its radius. The taller GR650 crankcase and its different gearbox layout also leave considerably more room in front of the crankshaft, allowing Suzuki to use a much larger-diameter balancer than would physically fit inside the GS400 crankcase.
Why is this important, you ask? Well, in my quest to build this Frankenstein engine, I obviously want the finished engine to be properly balanced, which means I need to decide which balancer shaft to use.
I could, of course, retain the stock GS400X balancer shaft, but it was designed around a much lighter reciprocating assembly. To make it suitable for the considerably heavier pistons I intend to use, a substantial amount of additional counterweight would have to be added. Adding weight in exactly the right place is difficult; removing excess weight is comparatively easy. The GS500E balancer is therefore a much more attractive starting point. Its counterweights are already substantially wider than those of the GS400X, and the shaft itself is considerably more substantial and rigid. The drawback is that it was designed to run in plain bearings rather than the roller bearings used by the GS400X.
Fortunately, the bearing journals are located in essentially the same axial positions as the GS400X bearings. My plan is therefore to machine the GS500E journals down to accept the stock GS400X roller bearings and adapt the shaft to the GS400 crankcase. Once fitted, I can increase its balancing capacity further, either by adding tungsten inserts to the counterweights or by adding material around their outer circumference and machining it back to the required profile. At the moment I measure roughly 4 to 4.5 mm of radial space before the balancer weights would begin to interfere with the crankshaft or crankcase, although obviously not all of that clearance can safely be used.
Balancer Shafts
| Engine Type | Weight Width | Weight Diameter | Distance Between Weights |
| GS400X | 45.10 | 55.00 | 74.65 |
| GS450E | | | |
| GS500E | 54.00 | 55.00 | 67.65 |
| GR650 | 22.40 | 82.40 | 97.90 |
**Under construction**
