Crankshafts : What, How, and Why

The crankshaft is the backbone of of any engine. The what and why are easy to understand. To put it simply, it’s what turns the downward reciprocating motion of the piston into a rotational force.

The main plane is where the crankshaft bolts to the engine block. The rod journals are where the rod ends bolt to the crank. The counterweights are on opposite sides of the the rod journals counteract the harmonic forces of the piston and rod assembly. Not noticeably pictured are the oil passages. The one thing that keeps the crank turning freely, what it rides on, is a wedge of oil pressure. Now you know why you need to check your oil weekly. (Note, too much is as bad as too low). Anyhoo, there are passages drilled through from the main journals to the rod journals that sends pressurized oil to the rods at the appropriate time (on the downstroke) and keeps the rod journals riding on a wedge of oil. At each end are the flange to bolt the flywheel and at the opposite a shaft stub where the pulley/ harmonic balancer attaches.

Basics over. Now we get to the good stuff. I’ll start with basic engine dynamics. Every ICE has a determined firing time. An inline four fires at every 180 degrees of crank rotation. A six, every 120. Eight, every 90. The closer the firing time, the smoother the engine output. This explains why a V8 feels much smoother than a buzzy four. For the rotary, it fires every rotation per rotor. So a two rotor fires every 180 deg.

Your typical road engine has what would be called cross plane crankshaft. If you look down it lengthwise like a gun, the rod journals are not on the same plane. A four cylinder has no choice but to be what is called a flat plane crank. Its inherent 180 degree firing time allows nothing else. For non fours the cross plane is the rule. For an engine in a V configuration it gets a bit more difficult. Rods are side by side on a common journal. A V-8 has an inherent balance in a 90 degree configuration. Its aligned perfectly for even firing pulses. For a 90 degree V-6 it gets difficult. Its either going to use a common journal and be noticeably rougher, or use a split journal to achieve the 120 degree timing.

Buick V-6 crank. Notice the split rod journals.

Where we get different for V type engines is the flat plane crank. Used mainly for performance, it has noticeably rougher power output but its not used in many road engines. And the ones it is used in are heavily performance based. The Lotus Esprit V8 and current generation of Ferarri V-8’s come to mind. A flat plane is more easily balanced and more durable in high rev applications.

The creation method of a crank is as important as the design. There are three main methods of creation. The first is cast. Simple enough, you take hot metal and pour it in a casting. Once its cooled, the rod and main journals, and other attachment points are machined to the required tolerances. Cast cranks are in 99% of production engines. They are relatively tough and durable. And very cheap and easy to make. But in general, they are the weakest of the bunch.

The next up is the forged crank. It starts life as a specially alloyed cast crank. Once formed, its heat treated and its journals are wrestled into place. Once cooled, the machined surfaces are taken care of. Its costs are pumped up by the heat treating process, as well as the special alloy.

The final version is a billet crank. It is cut from a solid block of specially alloyed steel. Because of its construction from a single material and its machined origin, it is much closer to an ideal balance than the others. Its cost are much higher than either. Due to the cost of buying a large chuck of special alloy, as well as the extensive machining time to shave away all the extra metal. Its the most expensive method, but some question if its still the best.

The metals used are usually comparable. But the fact that the metal has an internal crystalline structure. Cutting away the excess metal removes some of the internal structures while a forged crank retains all of its. Its a debate. There is no clear winner, but I’m in the forging camp.

There are a few performance modification available to the crank. The first is a balance. That reduces the harmonics during high RPM use. It also allows, with a lighter piston and rod, a lighter rotating assembly, which makes a more responsive engine. The next modification is the next step up used along balancing. Its known as knife blading the counterweights. The lead side of the counterweight is machined down to a taper to cut down on wind resistance as well as oil and oil mist in the crankcase. Its a slight reduction in resistance, a small increase that when your making an engine built to competition rules can be the difference for a victory. The last modification is the stroker crank. In most applications, it is a crank with a longer throw. When its combined with a shorter piston and longer rod creates more displacement within the cylinder bore. More engine displacement means more power. But there are some engines running a destroked crank. Its an effort to create an engine with a higher RPM capability. Most destrokers are used in certain classes of drag racing. The index times are a function of engine displacement, so a smaller one has a better time.

So that’s cranks in a nutshell. If you have any feedback or questions for another segment, leave them in the comment section. We’ll get to them.