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TechTranscript
00:00Have you ever wondered about the red line
00:02on your bike's tachometer?
00:03It's that red shading in the upper reaches of the gauge
00:06that you're not supposed to venture into.
00:08But why do engines have a red ceiling?
00:10And why do some motorcycles have a low red line
00:13while other motorcycles have a high red line?
00:15Well, let's open up the shop manual, find out.
00:18♪♪
00:27Engines are kind of miraculous.
00:29I mean, there are so many parts spinning
00:31and reciprocating up and down between these cases.
00:34But it turns out there's a limit
00:36to how fast things can move before bad stuff happens.
00:39-♪♪
00:44Reciprocating engine parts,
00:46like the piston and connecting rod,
00:48have mass and thus inertia and momentum,
00:50which means they're going to resist changes
00:52in speed and direction.
00:54So the faster the engine revs,
00:55the harder the piston is going to try
00:57and fly off the wrist pin,
00:59and the more violently the connecting rod
01:00is going to try and tear itself away from the crank.
01:03And eventually, they will.
01:04The parts will fatigue,
01:06and the engine is going to turn to scrap.
01:08It might be hard to imagine robust metal parts
01:11just giving up and breaking,
01:13but consider this piston out of a CBR600RR.
01:16At red line up near 15,000 RPM,
01:19it's accelerating from a dead stop
01:20at the bottom of the stroke
01:21up to nearly 75 miles per hour midway through the stroke,
01:25and then coming to a complete stop again
01:26at the top of the stroke.
01:28And it's doing all of that
01:29in just four one-thousandths of a second.
01:32That instantaneous acceleration
01:34creates a force of 1,700 Gs on this little piston.
01:38So while this thing only weighs
01:40about four ounces in my hand,
01:42when it's changing direction at red line,
01:44which it's doing 500 times per second,
01:46it develops a load of roughly 450 pounds.
01:50That's literally more than the entire CBR weighs.
01:54So with that in mind,
01:55you can kind of see
01:56how things might have a hard time staying together.
01:58So engines have a limit to how fast they can spin
02:01because beyond a certain RPM,
02:02the effective weight of reciprocating parts
02:05will cause them to fatigue and break.
02:07And even if you lighten and strengthen bottom-end parts
02:10to cope with the RPM,
02:11there are other areas of the engine
02:13where inertia is going to rear its ugly head,
02:15namely the valve train.
02:17In a four-stroke engine,
02:19valves in the cylinder head
02:20control the flow of fuel and air into the cylinder
02:23and the flow of exhaust gases out.
02:25The valves are pushed open by the camshaft
02:28and closed by a spring.
02:30The pressure the spring exerts
02:31ensures that the valve always follows the cam's motion
02:34and returns to a closed position.
02:37If the engine is revved too high, however,
02:39the momentum of the valve train components
02:41may be too strong for the spring to overcome.
02:44So the valve's motion
02:46might not follow the cam's motion as it's closing.
02:50This is called valve float and it's bad news
02:52because if the valve hangs open long enough,
02:54it may make contact with the piston
02:56as it's rising in the cylinder.
02:58And you can imagine how that goes.
03:00So valve float is another important reason
03:03why engines have red lines.
03:06Now, there are engines like Ducati Desmodromic motors
03:09that don't rely on valve springs.
03:11In a Desmo engine,
03:12a separate rocker arm forces the valve closed,
03:15so valve float is impossible.
03:17And two-stroke engines don't even have valves to float.
03:21And yet, both Desmo Ducatis and Ringding two-strokes
03:24still have rev ceilings
03:25because their other reciprocating parts
03:27can't escape the laws of physics.
03:30All right, so that's why every engine,
03:32whether it's a two-stroke or a four-stroke,
03:34a Desmo or conventional valve spring has a rev ceiling.
03:38But you might be wondering,
03:40why do different bikes have different red lines?
03:43This Harley-Davidson Street Glide
03:44maxes out at just 5,500 RPM.
03:47Meanwhile, this Honda CBR600R spins to over 15,000 RPM.
03:53Once again, it has to do with inertia.
03:56Going back to the issue of valve float,
03:58there are significant differences
03:59in the design of these two engines.
04:01The Honda is an overhead cam design.
04:03That means the cams are up in the cylinder head
04:06and operate directly on the valves.
04:08It's got fewer parts,
04:09so therefore less mass and less inertia to contend with,
04:12so the engine can spin faster before valve float
04:15is a concern.
04:17On the other hand,
04:17the Harley's pushrod valve chain has more components.
04:21It's got tappets, pushrods, and rocker arms
04:24in addition to the valves and springs.
04:27That's a lot more mass to manage,
04:28so it limits the speed the engine can turn
04:30before valve float occurs.
04:33Then there's piston speed,
04:35which is proportional to piston stroke.
04:37The longer the stroke,
04:38the higher the maximum piston speed for a given engine RPM.
04:42So at red line,
04:43this Harley Street Glide's piston
04:45is maxing out at 64 miles per hour,
04:48not that much slower than the 75 miles per hour
04:50we saw for the CBR up at 15,000 RPM.
04:54It's for this reason that sport bikes
04:55and other performance engines run a shorter stroke,
04:58so they can turn higher RPM and make more horsepower.
05:02There are other contributing factors
05:03to how fast an engine can spin,
05:05like intake and exhaust design,
05:07or even how fast gasoline can burn in the cylinder,
05:10but by and large,
05:11it's the inertia of reciprocating parts
05:13that determines the operating threshold for a given engine.
05:16If all of this has you worrying
05:18about accidentally blowing up your engine
05:20because you gave it too much gas,
05:21don't sweat it.
05:23Most modern bikes have a rev limiter
05:24that will cut the fuel or the ignition
05:26in order to keep the engine revs at a safe speed,
05:29and most manufacturers bake in quite a bit of margin.
05:32That being said,
05:33the rev limiter is there for your engine's safety,
05:36not as a party trick at stoplights and bike nights.
05:39So go ahead,
05:40enjoy the revs your engine has to offer,
05:43which is everything up to the red line.
05:45You just don't want to keep it spinning
05:47that high unnecessarily,
05:48because it will wear your engine out faster.