Jerry from Denver asks… I’m getting ready to buy my first big twin Harley-Davidson motorcycle and don’t understand the difference between the Twin Cam 96 and 96B (Balanced). Can you provide me with some insight?
Harley-Davidson motorcycles use a single-pin crankshaft and both connecting rods ride together (one inside the other) on that pin. Because of that, you have a very large amount of mass (two pistons, wrist pins, rings and connecting rods) changing direction twice (once at bottom, the other at top) each time the crankshaft makes a revolution. It is the direction change of the mass that causes the majority of the engine vibration we are going to talk about.
As you noted, Harley-Davidson makes two different versions of its Big Twin engine. The TC-96 (Twin Cam – 96 cubic inches) engine is used in all H-D Dyna and Touring models. It is mounted in the motorcycle chassis using an arrangement of carefully engineered rubber mounts and metal linkages.
The metal links allow the engine to rotate freely around the same center as the crankshaft, but prevent any additional movement in other directions. Because side-to-side movement is eliminated, the rubber mounts are installed in the area where the engine will vibrate to dampen the rotational shaking effect. Many Harley riders refer to the motorcycles using this system as "rubber-mounted".
Touring and Dyna model motorcycles tend to shake quite a bit at idle-not as much as the engine does, but the vibration is still noticeable at the seat, floorboards/foot pegs and handlebars. As the engine rpm rises, the vibration becomes far less intrusive and the genius of Harley’s mounting system comes into play, as the motorcycles are very smooth and comfortable at highway speeds.
Harley-Davidson’s Softail models are an entirely different animal. The engine in this chassis is the TC-96B (Twin Cam – 96 cubic inches, Balanced). Although this engine shares pistons, cylinders, heads and connecting rods with other TC-96 mill, the feel of the powertrain is significantly different when compared to other Harley-Davidson models. The difference is evident right at idle because even though the engine is bolted directly to the frame (no rubber mounts or linkages), there is minimal vibration.
In comparison, the TC-96 shakes back and forth when idling, reminiscent of a paint mixer at your local home improvement store. The "B" engine appears instead to be sitting calmly in the Softail chassis. A quick touch of the rider controls will confirm this, yielding almost no significant feeling of vibration.
So where is all the shaking going? Well, it isn’t as difficult to explain as you would think. Consider the photograph of the counterbalancer mechanism from a TC-96B. We are looking at the right side of the engine, so the crankshaft it will be rotating clockwise when the engine is running.
As you can see, the counterbalancer drive chain passes under the crankshaft gear then continues rearward to the rear counterbalance weight and continues around to the front counterbalance weight before returning to the crankshaft.
Because of this arrangement, the counterbalancer weights rotate in their bearings in the opposite direction of the crankshaft’s rotation, and at the same speed as the crankshaft (all three sprockets are the same size). The Motor Company engineers use two smaller weights equidistant from the crankshaft to eliminate any secondary vibration that might come from a singular weight that was mounted farther away from the crankshaft.
If you have any doubt about how effective the Softail’s counterbalancer system is on the Twin Cam engines, a quick ride on a pre-2000 Evo-powered Softail, with its solidly mounted non-counterbalanced engine, will illustrate the difference far better than anything we can write on this page.
In the end, Harley’s counterbalancer system used on the "B" engines has been virtually trouble-free, so this should take that aspect out of your consideration. Instead you should focus on the way the engine feels in the motorcycle.
If you commute or ride predominately in urban or suburban areas, you may find that the smoothness of the Softail suits you because of the regular starting, stopping and idling you will encounter. On the other hand, if you regularly get out on the highway or on rural roads where you can stretch your legs a bit, you may find a Dyna or Touring model more enjoyable.Google+