Redline: It’s that crimson shading in the upper reaches of the tachometer that you’re not supposed to venture into. But what happens if you do spin it into the red, and why do motorcycles have rev ceilings to begin with?  

By: Ari Henning  Courtesy: Revzilla

Rev limits exist for the same reason that speed limits do, because beyond a certain speed bad things are more likely to happen. Out on the road, excess speed might make it sketchier to navigate a turn or stop in time for a traffic light, whereas in your engine too much component velocity can lead to catastrophic failure. Reciprocating engine parts like the piston and connecting rod have mass, and thus inertia and momentum, which means they will resist changes in speed and direction. So the higher the engine revs, the harder the piston tries to fly off the wrist pin and the more violently the connecting rod attempts to tear itself away from the crank. And at some point, they will. The metal will fatigue and the engine will turn itself into scrap.

It might be hard to imagine robust metal parts just giving up and breaking, but consider the pistons in a Honda CBR600RR. At redline, up near 15,500 rpm, each piston has a maximum speed of nearly 75 mph. It’s accelerating from a dead stop at the bottom of the stroke up to 75 mph somewhere around the middle of the 1.7-inch stroke, and it’s doing it in just 0.002 second. Then, it’s coming to a complete stop again at the top of the stroke in roughly the same amount of time. 

That instantaneous acceleration imparts over 1,700 Gs on the piston, so while it only weighs about four ounces at rest, when the piston is reversing direction at redline, which it’s doing 500 times per second, it has a load of roughly 450 pounds. That’s as much as the entire bike weighs! With that in mind, it’s easy to see how things might have a hard time staying together.   

Of course, bottom-end components can be strengthened and lightened to handle more revs, but there are other areas of the engine where inertia will rear its ugly head. Namely, the valvetrain. 

In a four-stroke engine, valves in the cylinder head control the flow of fuel and air into the cylinder and the flow of exhaust gasses out. The valves are pushed open by the camshaft and closed by a spring or a set of nested springs. The pressure the springs exert ensure the valve always follows the cam’s movement and returns to a closed position.  

If the engine is revved high enough, however, the momentum of the valvetrain components — that is the valves, rocker arms, buckets, etc. — may be too strong for the springs to overcome, and the valve’s motion may not follow the cam’s motion.

This is called valve float, and it’s bad news, because if the valve hangs open long enough it can contact the piston as it’s rising in the cylinder. And you can guess how that goes. So valve float is another important reason why engines have redlines.

There are engines, like Ducati desmodromic motors, that don’t rely on valve springs. In a Desmo engine a separate rocker arm forces the valve closed, so valve float is impossible. And two-stroke engines don’t even have valves to float, and yet, both Desmo Ducatis and ring-ding two-strokes still have rev ceilings, because their other reciprocating parts can’t escape the laws of physics.

OK, so that’s why every engine — four-stroke or two-stroke, desmo or conventional valve spring — has a redline. But why are the redlines so different on different bikes? For example, a Harley Street Glide has a 5,500 rpm limit while the CBR we’ve been discussing spins to over 15,000 rpm. 

Going back to the issue of valve float, there are significant differences in the valvetrain design of these engines. The Honda has an overhead cam design; the cams are up in the head and operate directly on the valves. There aren’t many parts and thus less weight so less inertia to deal with and the engine can spin faster before valve float is an issue.

On the other hand, the Harley’s pushrod valvetrain has more components; there are tappets, pushrods, and rocker arms, in addition to the valves and springs. That’s a lot more reciprocating mass to manage, which limits the speed the engine can turn before valve float occurs. 

Then there’s piston speed, which is proportionate to piston stroke. The longer the stroke, the higher the peak piston speed for a given engine rpm. So at redline, this Street Glide’s piston is maxing out at close to 64 mph, not that much slower than the CBR’s slug up at 15,500 rpm. That’s why sport bikes and all performance engines run shorter strokes, so they can rev higher and produce more horsepower.

There are other contributing factors that can limit an engine’s rev ceiling, like intake and exhaust design as well as combustion speed, but by and large the inertia of reciprocating parts is what determines any given engine’s safe operating threshold. 

If all this has you worrying about accidentally blowing up your engine by giving it too much throttle, don’t sweat it. Most modern bikes have rev limiters that cut fueling or spark to keep the revs at a safe speed, and the manufacturer usually bakes in quite a bit of margin. 

That being said, the rev limiter is there for the engine’s safety, and not as a party trick at stop lights or bike nights. So go ahead and enjoy the revs your engine has to offer, which is everything up to the rev limiter, but don’t keep it spinning that high unnecessarily. 

By: Ari Henning  Sources: Revzilla , Ducati

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