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Crankshaft Vibration Dampers 101

BY J.C. BEATTIE OF ATI PERFORMANCE PRODUCTS
REVISED OCT 29, 2013

I’ve been around dampers for a long time and have traveled to numerous engine shops around the country to physically test crankshaft twist over the past 16 years. Throughout this time, I have collected considerable data that allows me to determine “how much damper” a certain engine needs. When given the crank weight, peak normal operating RPM, horsepower, rotating system materials, rules about the damper specifications (if racing), and the application of the engine (road racing, oval or drag), I can make a very good prediction of the amount of inertia weight and the type of device your engine will need.


Within a motor, something has to be off the centerline of the crankshaft so that as the crankshaft turns one revolution, a piston is pushed to the top and then pulled to the bottom
Let’s take a moment and think about the way a crankshaft works. On one end, you have your flywheel, torque converter or clutch. On the other end, there is a timing chain / belt / gear drive, and then a small “snout” sticking out, on to which a damper and any needed accessories are bolted. In between the front and rear, there are main caps and bearings that hold the crankshaft in place in the engine. The number of main caps can differ from two to as many as six. These main caps go over the crankshaft and bolt to the engine block. Attached to the crankshaft, you have the rest of the rotating assembly which consists of connecting rods and pistons with wrist pins and rings. This is where all of the crankshaft twist and harmful “harmonics” truly begin.

The pistons and components travel up and down, to the top of the cylinder and then back to the bottom: one cycle drive, one cycle driving. Think about that motion within an engine: something has to be off the centerline of the crankshaft so that as the crankshaft turns one revolution, a piston is pushed to the top and then pulled to the bottom. If this is a power stroke, where fuel is compressed and combusted, that piston is then forced downward. That is what actually produces your power.

The pedals on a bike act like the pistons and the crank arm between the pedal and the chainring is just like your connecting rods.
Think about riding a bicycle and the way you pedal the bike to move. The pedals themselves are like the pistons and the rod between the pedal and the crank sprocket is just like your connecting rods. The pedal arms have to be off the centerline of the crank in order for you to make a circle with the pedals and move your bike forward. Your crankshaft and pistons can be viewed in the same light. Because something has to be off the centerline of the crankshaft in order to function, the leverage of that connection to the crank is very high. That is why the crankshaft will twist as the system is forced to rotate when the engine is fired.

While your engine is running, some pistons are being pushed downward on a power stroke, some are being pulled down by the crankshaft, and some are being pushed upward by the crankshaft. Now envision this entire system happening 8,000+ times per minute! Even further, all of these different actions are happening to the same piece of metal - the crankshaft. These actions make the shaft twist in one direction away from its natural home location, and when it tries to come back to that home location, its momentum makes it travel past its original location and farther in the other direction.

The measured magnitude of that action is called “Degrees of Twist – Peak to Peak” or crankshaft twist. This is what I measure when I am damper testing. It is this action that breaks parts and robs you of horsepower when there is nothing to counteract and eliminate the twist. In this system, the worst torsional vibrations, or twist, will always occur at the farthest point from the greatest load, or the heaviest mass. A torsional twist is defined as a twist without a bend. If you get too much of this twist, you will have a bend and this will cause engine and/or crank failures. Think about twisting a piece of rope over and over; you can make one or two revolutions and nothing happens. After that it starts to get a wave in it, and then as you twist more, the rope will pull your hands closer together.

Once torsional vibrations get to the front of the engine, something there needs to counteract that motion. This is where the damper comes into play. The damper’s job is to absorb and counteract as much of the twist as possible. With the right damper on your engine, the majority of the twist can be eliminated. However, with the wrong damper, virtually all of the twist can remain. A damper’s job is to rebound like the recoil of a spring. In this case the spring is your crankshaft twisting and when it tries to rebound past that natural state we discussed earlier, that is when the damper needs to stop it.

You can read more about the Dangers of Power Pulleys here.

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