| Author | Message | ||
     Kowpow225 |
Can you tell me if I'm correct with these assumptions? A dyno works by taking a torque output reading of the motorcycle at evenly spaced rpm points. It calculates these torque figures into horsepower numbers based on the famous equation something over 5252 rpm. (I obviously don't know the famous equation.)  Is there any pre-setup things that go on before the measurements are taken, such as final drive ratio entered into the machine? It seems that in my understanding, if no gearing numbers are taken into consideration, someone could re-gear an XB with very short gears to achieve very HIGH torque and horsepower numbers. (With sacrificing a lot of top speed.) Unless a dyno actually measures m.p.h. and torque and gearing. Maybe I'm all wrong. I've always been kind of curious to find out how these things REALLY work. Anyone with information I'd really appreciate your help. Kyle | ||
| Aaron |
The most common type of dyno anymore is the inertia type. The bike accelerates a drum of a known mass. There's a simple optical sensor on the dyno that tells the computer each time the drum completes a revolution. Therefore, the computer can read the speed of the drum as well as it's acceleration rate. Knowing the mass of the drum, it has enough information to calculate horsepower. Horsepower doesn't vary with gearing. Horsepower fundamentally is torque times rpm (divided by 5252, but that's a nit, a scaling to make the number match what some horse could do a long time ago). Gearing shorter, or running in a lower gear, increases rear wheel torque and decreases rear wheel rpm proportionally. As you move up through the gears, you have less and less rear wheel torque and more and more rear wheel rpm, but the product of the two, i.e. torque times rpm, aka horsepower, remains the same. The horsepower at the rear wheel is the same as the horsepower at the crank, too, it's just made up of a different combination of torque and rpm, due to the gearing that lies in between the two. From the dyno's perspective, it sees a quicker drum acceleration rate when you're in a lower gear, but it also sees less drum rpm, so it's horsepower calculation comes out much the same. There's a caveat or two to that, but I don't want to digress. A Dynojet dyno doesn't actually show rear wheel torque, this is a common misconception. If it did, you'd see much larger numbers, and they'd vary radically with the gearing. It won't show torque at all unless you hook up the tach pickup and it gets an engine rpm reading. Knowing engine rpm and drum rpm, it can deduce the gear reduction between the crankshaft and the drum and calculate torque at the crankshaft, which is what it displays. Notice how on the dyno chart, the hp and torque cross at 5252 engine rpm, not rear wheel rpm. It's engine torque. This type of dyno is inherently accurate. So long as the mass of the drum doesn't change, and the friction in the bearings it rides on doesn't change, it remains accurate. Where error really gets introduced into the whole process, though, is the correction. The dyno takes in environmental conditions (temp, humidity, barometric pressure) and comes up with a "correction factor" that it applies to it's measured result. This is nice because it allows some degree of comparison in results taken in different conditions. But it's not a perfect process. The most common formula for this is the SAE correction, although you see others (some shops pick and choose the correction formula that makes their results look best, watch for this). Some dynos have an automatic weather station that makes these measurements and factors them in, others require the operator to make the measurements manually and type in the results. In either case, this whole process introduces some error. In fact, the dyno software will warn you if you try to compare results that were taken more than a certain temperature apart. I've noticed the correction doesn't do a good job with humidity. Many times I'll be tuning on a rainy day, and getting some result, and then I'll tune the same bike on a nice day and see something that's around a 2-3hp improvement. Another thing I've noticed is that pretty consistently, a bike I tune here, at 5300' elevation, gets the owner a better number when he checks it at a lower elevation. It's like low barometric pressure isn't fully factored into correction for some reason. | ||
| Kowpow225 |
Thanks Aaron!  | ||
| Blake |
Thanks for that excellent and comprehensive answer Aaron. Just one picky clarification... replace "mass" with "inertia"?  | ||
| Aztec12r |
Mass: the property of matter that manifests itself as inertia. Inertia: the resistance a body puts up to a change in its' state of motion. | ||
| Aaron |
You're welcome. re: the terminology ... from the dictionary: Mass: A property of matter equal to the measure of an object's resistance to changes in either the speed or direction of its motion. The mass of an object is not dependent on gravity and therefore is different from but proportional to its weight. Inertia: the tendency of a body to resist acceleration. Hmm, I dunno, seems like it's two ways of saying the same thing to me. You are being picky  | ||
| Alex |
@ Aaron "The horsepower at the rear wheel is the same as the horsepower at the crank, too, it's just made up of a different combination of torque and rpm, due to the gearing that lies in between the two. " Do I understand You correct that You assume thereīs no power loss within the primary chain drive, the gearbox and the secondary belt????? Our XBs show a difference of around 12hp between crank hp and rear wheel hp. Yours donīt? Can You tell me the secrets of building a zero power loss drive, I really would love to use it in our drag race bike Best regards | ||
| Blake |
Obviously a poor clarification on my part. If we were to be perfectly accurate we'd say "mass moment of inertia." Referring to the MMOI as "mass" is like refering to torque as "force." Just like how a torque involves a force, but also involves geometry (the distance from rotational axis that the force is applied), so to does MMOI involve mass, but also geometric properties as well. Consider that we could have two drums of exactly equal mass, but one with its mass concentrated inwards (like a solid cylinder) will have less rotational inertia than one with its mass concentrated at outwards (like a thin shell hollow cylinder or "drum" . Thus if we are to know a drum's resistance to angular acceleration (rotation), we need to know more than just the mass of the drum; we also need to know the geometric distribution of that mass about the axis of rotation. Combined, those properties are described as the "mass moment of inertia." I'm lazy and so often refer to it simply as "inertia." Maybe that's how that common type of dynamometer you described got its name, you think? Lots of good basic info on the subject... | ||
| Aaron |
Yup, that makes sense. The inertia goes up based on the radius, prolly as the square would be my guess. Alex: What you're describing is the hp measurement made at the crank with the rest of the drivetrain disconnected from it. Yes, if you disconnect it, a hp measurement made at the crank is no longer affected by drivetrain losses and you're likely to see a 15% or so bigger number. But I was referring to hp measured at the crank with the rest of the drivetrain hooked up to it. In this case, the drivetrain losses will affect the measurement even at the crank. You'll get the same hp result you get at the rear wheel, albeit with a different mixture of torque and rpm due to the gearing that sits in between them. | ||
| Blake |
You are right. With all else being equal, moment of inertia increases with the radius squared. | ||
| Alex |
Aaron, the power an engine delivers at the crank is affected by the internal combustion process and the engineīs efficiency. Not by a hooked up drive train. The fact that You canīt measure the crank power directly with a rear wheel dyno doesnīt mean itīs not there. Letīs assume an engine capable of producing 100 hp at 6500 rpm. It is coupled to a drivetrain eating up 20 hp at that rpm. Your rear wheel dyno will tell You that this bike delivers 80 hp rear wheel at 6500 engine rpm. Now letīs use a measuring device that is capable of measuring power at the crank without removing the drive train. Letīs assume weīre able to measure the crankīs torque output via shaft torsion and we use a rpm meter. You know that we can calculate power output from these two numbers. What would You think will power output be with the drive train hooked up: 80 or 100 hp at the crank? From my point of view Iīm sure we will find 100hp at the crank and 80 at the wheel, not 80 at the crank and at the wheel. By the way most inertia dynos are capable of measuring drive train power loss to be able to calculate crank power from rear wheel power. If these numbers were the same why would someone undergo the effort to additionally measure and calculate? Sorry but from a technical engineerīs point of view crank power and rear wheel power will never be the same as long as thereīs a drivetrain with less than 100% efficiency in between. Best regards Alex | ||
| Aaron |
Sounds like you're insisting that a crank hp measurement has to be power produced, instead of power available after drivetrain losses. My point is simply that in a running motorcycle with the drivetrain hooked up, the hp available at the crank is reduced by those drivetrain losses. It has to turn the drivetrain. If I were to hook up some device to the crank, say a supercharger or something, with the drivetrain still hooked up, do you not agree that the power available to run that supercharger is reduced by the drivetrain losses? If you agree with that, would you acknowledge that hp available at the crank, after losses, equals hp available at the rear wheel, after losses? But it's just made up of a different combination of torque and rpm due to the gearing that lies in between them? That was the original claim that you challenged. I'll readily acknowledge that power produced is greater than power available after losses. But if you keep it apples to apples, compare power available at the crank to power available at the rear wheel, they'll be the same number. The same losses affect them both. BTW, neither of the Dynojet inertia dyno's I've owned had a provision for factoring drivetrain losses into the results. You can do a crude measurement, called a "coast-down" test, but there's no way to feed the results back in to the software. I've seen engine dyno's with this feature, though, because some sources of loss are very difficult to mechanically remove. Hard to start a Harley motor without the primary hooked up. | ||
| Odie |
The way I always understood it is that say a car manufacturer produced an engine and ran it on a test stand. They then measured the horsepower actually at the crank without any drivetrain hooked-up at all. This would equal crank hp- the amount of hp said engine, all by it's lonesome, would produce. Then put said engine in a car and they measured hp on a inertia dyno which would be with the drivetrain attached and operating. Obviously hp would be used to drive said drivetrain components thereby resulting in rear wheel hp..........I can see the theory behind the hp at the crank being the same as hp at the rear wheel in this case as the engine is driving the drivetrain and hp should be the same regardless where in the loop you measure it......I could be wrong.........I'm just telling you how I understand it....Odie | ||
| Alex |
Sorry that it took a while to answer but I was a little busy. "Sounds like you're insisting that a crank hp measurement has to be power produced, instead of power available after drivetrain losses." Aaron, I guess thatīs the way crankshaft horse power is defined. It tells You how much power You can make use of at the crank. Calculating the drive train losses into the engine thereby reducing crankshaft power doesnīt make much sense for me. First driving the power train doesnīt reduce crank shaft output but makes partially use it (the power output is still dependable on how much power Your engine is able to transfom into torque at rpm from the energy stored in the fuel). Second the purpose of hooking up the drive train is to make the crankshaft power partially accessible at the rear wheel. So again we make partially use of the power available. Both the rear wheel and the drive train make use of the available power, they donīt reduce it. Aaron, I know what You mean but I think this is quite a funny sight of things. Itīs like offering a "whatever-is-left-after-You-hook-up-the-drive-train engine" instead of a "100 hp at the crank engine". I hope You donīt find my words offensive as they are in no way meant to be. I do honor Your knowledge no matter if we see things from the same point of view. All the best Alex |
