Motorcycle engines and high RPM's?

Question

Why are motorcycle engines able to operate at RPM's such as 12,000 and above? Whereas car engines are much lower. the highest you really see in production cars is 8500-9000.

Answer 1

Physics; moment of inertia. Try a little demonstration:

Take a gallon of milk and swing it as fast as you can back and forth at arm's length, then do the same with an empty bottle. It was a lot easier to swing the empty bottle faster, right?

Car engines have much larger displacement, which means that their pistons are larger and heavier than a motorcycle's pistons. Since pistons have to move back and forth like your gallon of milk, a smaller (and therefore lighter) motorcycle piston can naturally go faster without tearing the engine apart.

You'll also notice that a larger motorcycle engine, like an 1800cc cruiser, will only rev to around 5,000 RPM. Again, larger pistons mean more torque but lower engine speed. Since the throttle is an important control in a sportbike, engine torque and the gyroscopic effect of smaller pistons makes the bike easier to ride as well.

Answer 2

Because they can. That's not meant to be a smart a$$ed answer either. While there are some good sounding explainations, everybody missed the mark. The main thing is piston speed that determines how fast an engine can rev. To have any kind of longevity and durability, piston speed should not exceed 4,000 feet per minute. More than that and accellerated ring wear occurs. That's been a constant (and limitation) for the last 60+ years. Modern metalurgy, machining, oils, etc hasn't changed that value either. With the 4,000'/minute limitation, that means an engine with a 6" stroke should not exceed 4,000 rpm to have any kind of longevity. Since a piston goes up and down on each revolution, a 6 inch stroke equals one foot per revolution. An engine with a 3" stroke can safely turn 8,000 rpm, a 1-1/2" stroke 16,000 rpm and a 12" stroke 2,000 rpm. Race cars, pulling trucks and drag racers often exceed the 4k'/min rule, but many also install new rings before each race or pull. The number of piston riding on a crankshaft has absolutely no bearing on the max rpm. There are other things limit rpm. One is the weight of the pistons. Remember the old rule about an object in motion tends to remain in motion? The heavier the piston is, the more it wants to keep on going in one direction rather than come to a complete stop and reverse direction. Back in the 1930s-40s, the switch from cast iron pistons to aluminum alloy pistons allowed engine to rev higher. Something else is the connecting rod to stroke length ratio. Many engines have a connecting rod that is approximately 4 times as long as the length of the stroke. The longer the stroke ratio, the more gradually the piston accellerates and decellerates when leaving or approaching top or bottom dead center. The shorter the ratio, the longer the dead time is when the piston is not moving up or down and the quicker it accellerates and decellerates. This places a lot more stress on everything inside of the engine (con rod, rod and main bearings, crankshaft, piston pin, piston, rings). Abrupt changes in piston direction can easily break rings. Think of slamming a door repeatedly and you get the idea, eventually you bust something. Of course there are other things that limit top rpm such as camshaft timing, lift and duration, valve weight, valve spring stiffness, intake and exhaust breathing and so on, but the above are the main ones.

Answer 3

Many thing contribute to high RPM capability, the ratio of piston diameter to crankshaft stroke, the wieght of reciprocating parts, the balance of the reciprocating parks, the balance of the rotating parts, the wieght of rotating parts, the materials used, the type of valve train, this last one is very important because only one type of production valve train will acomedate 11,000-18,000 rpm redlines found on modern sport bikes. this system is called shim under bucket, which severly lightens the recirocating mass of the valve train, limiting the posibilty of floating valves, the primary cause of failure associated with high rpm, I've just scratched the surface here but hope it helps answer your question.

Answer 4

Now who says bikes run at 12,000+ RPM. My hog would blow up shortly before hitting 7 grand. (It's stock. The redline is at 6.) Thing is, the 'ol hog ain't quick, but I can carry lots of stuff. Stuff a 12,000+ RPM bike would not be able to leave the starting gate with before it's clutch exploded!

And when I say she ain't quick, that don't mean she ain't fast. Off the line is one thing, but at 90 MPH I'm pullin' about 4 grand and still got throttle left. If you figure I'm doing 55 at three grand, then I got about another 30-35 MPH at 5 grand. Never ran one of these puppies that quick. My pair ain't quite that big. But she's plenty fast as I will ever need.

Answer 5

They're smaller, so the pistons are lighter and they are less heavily stressed by high redlines. Also, sportbike engines aren't expected to last 150,000 miles like a car engine.

Answer 6

Shorter stroke pistons - they don't travel up and down as far as low rpm engines.
Crankshafts and flywheels are lighter - which allow them to spin faster without a lot of weight holding them back.

Answer 7

Smaller lighter valves don't float as soon.

Answer 8

Precision machining and roller bearing crankshafts :)